NBSIR 74-543

0CR-14-32-000M230

High Temperature MHD Materials

S. J. Schneider, W. Capps, H. P. Ro Frederikse, W. R. Hosier

D. A. Kauffman, E, M. Levin, L. McDaniel, T. Negas and

E. R. Plante

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

August 1974

Interim Report for Period

July 1, 1972- June 30, 1974

Prepared for

Office of Coal Research

Department of the Interior

Washington, D. C. 20240
1
NBSIR 74-543

OCR - 14-32-0001-1230

HIGH TEMPERATURE MHD MATERIALS

S. J. Schneider, W. Capps, H. P. R. Frederikse, W. R, Hosier, D. A. Kauffman, E. M. Levin, C. L. McDaniel, T. Negas and

E. R. Plante

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

August 1974

Interim Report for Period

July 1, 1972- June 30, 1974

Prepared for

Office of Coal Research

Department of the Interior Washington, D. C. 20240

U. S. DEPARTMENT OF COMMERCE. Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS. Richard W. Roberts. Director

PREFACE

For the past several years the Inorganic Materials Division,

Institute for Materials Research, National Bureau of Standards, has

been engaged in materials research appropriate to open cycle coal

fired magnetohydrodynamic (MHD) systems. Since July 1972 a portion

of the work has been sponsored by the Office of Coal Research under

Contract No. 14-32-0001-1230. The purpose of this report is to

present a s\ammary of the technical activities of the combined NBS-OCR

program on High Temperature MHD Materials.

TABLE OF CONTENTS

Preface i

List of Figures iv

List of Tables vii

1. INTRODUCTION 1

2. COAL SLAG CHARACTERIZATION 3

3. PHASE EQUILIBRIA 7

4. ELECTRICAL CONDUCTIVITY AND RELATED PROPERTIES 27

5. VAPORIZATION 35

6. VISCOSITY 47

7. MATERIALS TESTING 61

8. SUMMARY OF PUBLICATIONS AND TALKS 64

9. REFERENCES 66

iii
LIST OF FIGURES

Figure 1. Unit cell dimensions and densities of monoclinic and hexagonal

Figure 2. Scan of the monoclinic-hexagonal transformation (2nd order) of

K^CO obtained by differential scanning calorimetry. C has

not been calibrated for quantitative measurement.

Figure 3.

Figure 4.

Figure 5.

Figure 6.

Figure 7

Figure 8.

Unit cell dimensions and densities of orthorhombic and hexagonal

K^SO^.

Unit cell dimensions and densities of orthorhombic and hexagonal

Cs^SO^.

System K^SO^-Cs^SO^ after Dombrovskaya [22]

System K^SO^-Cs^SO^ after Levin, et al [23]

a^ and £ unit cell dimensions of the hexagonal solid solution

phase vs composition in the system K^SO^-Cs^SO^

110 plane of Cs SO based on the KMaSO (aphthitalite) structure,

P3ma.

Figure 9. X-ray density vs composition for the hexagonal solid solution

Figure 10,

Figiire 11.

phase in the system K^SO^-Cs^SO^

Phase diagram of the system '^2^^3~^2^^4

Condensed state phase diagram of the K^SO^-MgO system.

• - quenching o - DTA

Figure 12.

Figure 13.

Schematic activity plot of the K^SO^-MgO system at 1700°C.

Phase diagram of the system. K^SO -MgO for 0.01, 0.1, and 1 atm

total pressure K^SO^ species. (1 atm = 101,325 M/m2, N/m2 = 1 pascal)

Figure 14. Phase diagram of the system K^CO^-MgO in a closed system.

Figure 15. Phase diagram of the system K„CO_,-MgO for 0.01, 0.1, and 1 atm

2 3 o 9 total pressure K^CO^ species. (1 atm = 101,325 N/m^, N/m*^ = 1 pascal

)

Figure 15. Phase diagram of the system K^SO^-LaCrO^ in a closed system.

closed system - specimen containers were sealed Pt tubes

• - quenching o DTA

IV
Figure 17. Schematic of MHD-duct wall, (from: R. J. Rosa, 5th International

Conference on ^'IHD-Power Generation, Munich, 1971, Vol. I. - Public, de I'O.C.D.E., Paris, France)

Figure 18. Conductivity as a function of reciprocal temperature for slag

(sample MHD-12)

Figure 19. Conductivity as a function of reciprocal temperature for synthetic

slag (sample K 216)

Figure 20. Electrical conductivity of base oxide slag sample (K 257) and

of slag-seed mixture (b

at two oxygen pressures

of slag-seed mixture (base oxide with 20% K^SO^; sample K 266)

Figure 21, Resistivity as a function of temperature for the ceramics of

most interest as electrode materials (ref. [37]).

Figure 22. Conductivity versus 1/T of crystalline doped LaCrO^ , La^

9^''^o 1

CrO_ and La^ Ca CrO^ (0 . l>x>0 . 01 ) . Note donor exhaustion and

3 1-x X. 3 donor reserve regions.

Figure 23. D.C. conductivity of SrZr02-14 CeO^ (m.ol %) at different oxygen

pressures and in helium.

Figure 24. D.C. conductivity of SrZrO^-e SrTiO^-1.5 Ta^O^ (mol %) at

different oxygen pressures and in helium.

Figure 25. Conductivity of channel coating (ZrO^ + MgO) to 1750 K at 0.2 atm

and 0.01 atm oxygen.

Figure 26. Calculated partial pressures of gases and activities of K^CO^

at 1 atm for the combustion of CH^ ^ seeded with 1 wt % K as V°3'

Figure 27. Calculated partial pressures of gases and activities of K^SO^

at 1 atm for the combustion of CH^ ^ seeded with 1 wt % K as K^SO^.

Figure 28. Stoichiometric combustion of CH with 80/20 mole % N /O

ri - seeded wi th 1 wt % K as K^SO. at 1 atm total pressure

2 4 Figure 29. Saturation pressures of magnesium bearing species in an MHD

environment at a total pressure of 1 atm (solid MgO present)

P'igure 30. Saturation pressures of magnesium bearing species in an MHD

environment at a total pressure of 5 atm (solid MgO present)

Figure 31. Saturation pressures of cliromiiim bearing species in an MHD

environm.ent at a total pressure of 1 atrri (solid ^"^-^-^ present)

V
Figure 32. Saturation pressures of chromiiain bearing species in an MHD

environment at a total pressure of 5 atm (solid Cr^O^ present).

Figure 33. Ion currents versus time for isothermal evaporation of slag

at 2000 and 2130 K.

+ 4 Figure 34. IT versus 10 /T for evaporation of slag and pure SiO^.

Figure 35. Viscosity - temperature relationship for oxide layer of seeded

channel flyash slag (K257)

Figure 36. Viscosity-temperature relationships for some model coal slags.

Figure 37. Viscosity-temperature relationships of coal slags - K352, K360,

K384.

Figure 38. Viscosity-temperature relationships of coal slags - K257, K389.

Figure 39. Viscosity- temperature relationships of coal slags - K363, K385,

K391.

Figure 40. Viscosity-temperature relationships of coal slags - K358, K388,

K392.

Figure 41. Viscosity-temperature relationships of coal slags - K396.

Figure 42. Viscosity-tem.perature relationships of coal slags - K390, K394,

K398.

Figure 43. High temperature adaptation of Brookfield viscometer.

Figure 44. High temperature viscometer and furnace.

Figure 45. Schematic diagram of air-bearing for supporting viscometer spindle.

Figure 46. Position of test sam.ples in channel.

Figure 47. Sample holder with test samples.

vi
LIST OF TABLES

Table 1. Nominal Slag Chemical Compositions.

Table 2. Experimental Data for Compositions in the K^SC^-LaCrO^

System in Air.

Table 3. Experimental Data for Compositions in the K^CO^-LaCrO^

System in Air.

Table 4. Compositions of a Real Coal Slag (MJID-11) and Some Synthetic

Slags Which are Used for Comparison of Measured With Calculated

Viscosities

Table 5. Chemical Analyses of Some Simulated Seeded Coal Slags (weight %)

vii

1. Introduction

The successful development and eventual commercialization of open

cycle, coal fired MHD systems requires an accelerated effort to develop

and optimize materials which can withstand the rigors of the severe r4HD

environment. Materials must resist not only erosion through vaporization

and corrosion by alkali and slag attack, but also must possess the requisite

physical properties necessary for system operation. Under the auspices of

the Office of Coal Research, the National Bureau of Standards has underway

a program of materials research appropriate to MUD. The overall objective

of the work is to provide materials property information necessary for

the design, construction and operation of a coal fired MHD facility. The

program consists of several interrelated projects in the following areas:

Phase Equilibria

Condensed and noncondensed systems, melting behavior,

compound formation, phase assemblages, phase transformations.

Electrical Conductivity

Electrical properties as a function of temperature,

composition and atmosphere.

Vaporization

Mass loss and vapor species under reactive and non- reactive conditions.

Viscosity

Behavior of liquid slags as a function of composition,

atmosphere and temperature

Materials Testing

Durability, reliability, and operating characteristics

of materials tested under real MHD conditions.

1
Initial work has centered around the behavior of coal slag since there is

a definite lack of basic data in this area. The program also encompasses

investigations on other MHD process contaminants, particularly alkali

seeds , and on important structural and operating components such as

insulators and electrodes. The following sections summarize the important

technical results achieved thus far in the NBS-OCR program.
2. Coal Slag Characterization

Coal always contains inorganic materials which vary both in amount

and constitution depending on the type and locale of the coal bed. Major

elemental components consist of Si, Al, and Fe present as silica, aluminosilicates and iron pyrite. Smaller quantities of Mg, Ca, K and S in

various compound forms make up the balance of constituents. It is the

behavior of these inorganic materials, the residue of coal combustion

(commonly termed coal ash) , which will ultimately determine the success

or failure of MHD. Proper manipulation of the ash is crucial.

MHD coal combustion techniques are designed to remove a large percentage of coal ash as a liquid (slag) . A certain fraction of ash however will be vaporized during combustion only to condense again downstream

at cooler sections. Additionally, another fraction of ash will be

mechanically carried throughout the system as solid or liquid (flyash)

.

Consequently different components of the MHD system can "see" different

types and forms of coal ash, all dependent upon local conditions of gas

pressures (partial and total) and temperatures as well as the initial

combustion parameters. The condensed layers, which may be partly liquid

or solid, can have detrimental as well as beneficial effects. It may,

for instance, chemically tie up an unacceptable portion of the alkali

seed in the form of an insoluble compound or it may inhibit or short

circuit the electrical current between channel electrodes. On the other

hand, it may protect the walls from degradative processes and still

enhance current passage. Consequently, it is important to determine

the physical and chemical properties of the coal ash (slag)

.

3
Extensive information is available on the inorganic content for a

wide variety of coals [1] . These analytical data however only provide

a guide to the starting or limiting chemical compositions. The extent

to which these limiting or maximum chemical compositions are modified

as the ash moves through the MHD system is unknown. Thermodynamic

calculations of course give some insight as to possible gas, liquid and

solid phase compositions. Direct information however may be obtained

through the characterization of materials selectively sampled from

different segments of a MHD system. The combination of data obtained

from thermodynamic predictions and that from a characterization of real

materials allow the formulation of synthetic materials or model systems

appropriate to a wide range of environmental conditions of MHD.

In an effort to characterize coal ash behavior under real MHD

conditions several samples of slag and flyash were obtained from the

Bureau of Mines combustor facility at Bruceton, Pennsylvania and

from the University of Tennessee Space Institute, Tullahoma, Tennessee.

These slag specimens were selectively sampled from different parts of

the respective systems and generally represent combustor slag, channel

slag and condensed gas phase constituents at the exit of the channel

(probe deposit) . All samples were subjected to a variety of analytical

techniques: chemical analysis, microscopic examinations, x-ray diffraction,

differential thermal analysis and Mossbauer spectroscopy. From these

data several general conclusions can be drawn:

4
a. There is a significant difference in slag types found in different parts of the MHD system. However, all deposits regardless of

location are composed of largely Al^O^-FeO-Fe^O^-SiO^ mixtures. The

portions of each vary depending on location as does the. relative

amounts of glassy (liquid) and crystalline material.

b. Combustor slags contain a significant amount of alkali (up

to 2%) , indicating that at least a portion of the alkali content of

the coal is not vaporized during combustion. In addition, there is

little evidence of crystalline material and all samples were largely

glassy (including immiscible liquids)

c. Condensed gas phase constituents collected by a cold copper

probe contain large amounts of alkali (under seeding conditions) . The

alkali content is probably present as both salts (K^SO^-K^CO^ solid or

liquid solutions) and as alviminosilicate-rich liquids. These liquids

are immiscible.

d. The presence of large concentrations of alkali on copper surfaces

(e.g. electrodes) promotes diffusion of Cu to the slag layer. This is

evidenced by the presence of Cu (or CuO) in slag samples collected by

the probe and from the electrodes. In contrast, the probe samples did

not contain any Cu when seed was not present during combustion. It would

appear that long time operation with seeding would cause degradation of

copper surfaces.

e. The slag deposited on cold Cu electrode surfaces contained

both glassy and crystalline material. It would appear that the condensed phase first deposited as a liquid (s) and then cooled to a

5
glass phase (s) which slowly crystallized. The predominant crystalline

material is KAlSiO^ (K^O- Al^O^ • 2Si02) with little or no K^CO^ or K^SO^

being present. This is significant since most of the alkali is tied

up as a high temperature oxide phase (mp KAlSiO^ '^'1700°C) rather than

as water soluble salt.

f. The base oxide composition (excluding alkali) suggested by

the chemical analysis of the electrode deposits (channel slag) for

the UTSI channel are tabulated in Table 1 . For comparative purposes

the base oxide composition for the Bureau of Mines probe deposit is

included. The main differences between the two nominal compositions

are higher ^1^0^ contents and lower ^^2^4 ^^^^^^^ for the UTSI electrode

slags. A more important difference, however, is that in the UTSI

channel the alkali when combining with the base oxide slag forms a

crystalline phase KAlSiO^ + excess SiO^ containing iron oxide. In

the Bu Mines samples, the alkali combines at least initially to form

two immiscible liquids (K^SO^-K^CO^ liquid + base oxide liquid)

g. The data and conclusions stated above represent only a first

attempt at establishing slag behavior. It is evident that all such

data are very general and cannot be applied as of yet to all MHD

situations, because of the many variables. In effect, however the

data implies that slag behavior can be determined and possibly controlled by appropriate modification of the system parameters and

coal types

6
3. Phase Equilibria

In order to increase gas phase conductivity in magnetohydrodynamics

(MtID) , certain salts such as K^CO^ and K^SO^ are added to the gas stream

passing through the channel. However, these salts with relatively low

melting points (<1100°C) can react with the refractory oxides (mp >2000°C)

to form, low temperature eutectics, which have a deleterious effect on

structural stability of the insulators and elegtrodes and on their

electrical conductivities. A knowledge of the phase relations between

seed m.aterials and the possible oxide components of the MHD channel are

important in understanding the reactions taking place in a channel under

various conditions and could provide a basis for developing better performing materials.

Investigations of possible seed (e.g. ^^CC^, K2S0^)-coal slag reactions (silicates, aluminates , aluminosilicates) are of fundamental

importance in seed recovery technology and could provide additional

data which would aid in the evaluation of refractories (e.g. Al^O^)

in air preheaters and duct insulators. Phase equilibrium studies of

condensed phase reactions (liquids-solids) and vapor phase - condensed

phase reactions can provide invaluable information for the design and

operation of MHD combustors.

3.1. Materials and Methods

ACS reagent grade materials were used. Homogeneous mixtures in the

binary systems were prepared by several cycles of grinding, pressing

into pellets, and heating below the solidus. Liquidus and/or solidus

data were determined with a commercially available DTA instrument,

7
using 10-20 mg samples in Pt microholders , with ^12*^3 ^ reference

material, and at a heating rate of 10°C/min. The instrument was calibrated against one or more standard reference materials [2] . Subsolidus phase relations and unit cell dimensions were deduced from

x-ray powder diffraction patterns (using CuKtt radiation) obtained by

heating thin specimens of the pure salts or mixtures in air in an x-ray

dif fractometer furnace equipped with an improved sample holder [3]

The binary systems were also studied by conventional quenching

techniques using open Pt tubes for specimien containers. In some cases

sealed Pt tubes were used to avoid volatilization. Volatility of a

component such as K^o was also minimized by preparing materials at

elevated temperatures using a buffering method. Data were also combined with thermodynamic calculations of activities to estimate phase

relations under (1) different total pressures of dissociated K2^^3

or K^SO^ gaseous species and (2) in the presence of additional gases.

Phase relations and vaporization rates in systems containing K^O were

investigated using a commercially available, high precision, DTGA

apparatus. The TGA portion of the unit was used in combination with

a Kjiudsen cell to obtain phase equilibrium and vapor pressure data.

3.2. Polymorphism of Seed Materials

3. 2. a. K^CO^

2 3 Although a number of phase transformations have been reported in

K^CO^ [4], later work [5,6] by DTA indicated only one transformation

in a CO^ atm, at 422°C. Other thermal effects in air were ascribed

to impurities, such as H O, KOH and KC£ . Schneider and Levin [7]

8
studied the system both by DTA and by high-temperature x-ray diffractometry. Figure 1 shows a plot of the unit cell dimensions of K^CO^ vs

temperature obtained in air with the high temperature x-ray furnace.

It is seen that K^CO^ indeed shows only two polymorphs, a low temperature

monoclinic and a high temperature hexagonal form, with a transformation

temperature of 420±5°C, in good agreement with Reisman's value [5,6]

of 422 °C. Cell dimensions of the monoclinic form were determined by

x-ray patterns indexed by comparison with room temperature data reported by deWolff [8] . Indexing of the hexagonal form was accomplished

by inspection.

It is to be noted that no discontinuity exists in the cell dimensions

or in the density at the transformation temperature and that the 3 angle

reduces continuously to 90°. The DTA traces for K^CO^ showed a small,

almost imperceptible, but reproducible endothermal effect in both CO^

and air at the transformation temperature. These findings strongly

suggested that the transformation occurs with no discontinuity in enthalpy

or volume but with a discontinuity in the first derivative of the enthalpy,

i.e., in C^. Figure 2, a plot of vs temperature obtained with a differential scanning calorimeter shows a second order transition at 420±5°C.

No peak is evident, but an abrupt shift in base line was observed, corresponding to a change in and not in enthalpy.

3.2.b, K^SO^

Majumdar and Roy [9] in agreement with Bernard and Jaffray [10] , on

the basis of thermal anomalies in DTA traces and high temperature x-ray

results, proposed several new low- temperature , possibly second order,

9
transitions in K^SO^, e.g., one at 300±10°C. Pistorius and Rapoport

[11] did not observe any second order phase changes by means of high

pressure DTA, but by indirect calculation of the volume change at the

transition conclude that K^SO^ at '^580°C differs crystallographically

from the phase stable at room temperature.

Figure 3 shows unit cell dimensions and density data obtained by

use of the high temperature x-ray furnace. The orthorhombic and

hexagonal forms were indexed by referral to data, respectively, of

Swanson, et al [12] and Pannetier and Gaultier [13] . From the discontinuity at 588°C in the c_ dimension and in the density plots, it

is apparent that the transition is first order. Also, within the

limits of accuracy of the x-ray measurements, there are no other

transitions. The transition temperature of 588±5°C is in agreement

with previous values: 582±1°C [14], 583°C [9], 584°C [11], and

586±2°C [13] .

. 3.2.C. Cs^SO^

As a seed material Cs^SO^ has a greater electronic conductivity

and might even be superior to ^2^^4 ' cost is also greater.

Unit cell dimensions and density as a function of temperature are shown

in Figure 4. The transition temperature both by DTA and high temperature

x-ray diffraction methods was located at 718±5°C. The value is in

reasonable agreement with previous ones of: 710°C [15], 712°C [16],

and 713±2°C [17] ; but it is almost 60° higher than the earlier reported

value of 660°C [18] , which is the one listed in the Standard Reference

Data Series [19]. Rao and Rao [20] report a value of 698°C, corresponding

10
to the peak temperature in a DTA heating curve; and as late as 1970,

Denielou, et al [14] in a calorimetric study reported the transition

at '^^667°C. Detailed data were not given so it is not possible to reexamine their two enthalpy equations for a better fit of the data at a

higher transition temperature. To account for the very low values,

other than on an impurity basis, Plyushchev and Markovskaya [18b] suggested that Cs^SO^ might be trimorphic . The figure shows, however, that

Cs^SO^ is dimorphic, similar to K^SO^, possessing orthorhombic [17,21]

and hexagonal [17] forms.

Unlike K^SO^ , however, which undergoes a first order transition

and shows a definite discontinuity in the specific volume (1/d) at the

transition, Cs^SO^ resembles the K^CO^ transition, in that the cell

dimensions and density appear to be continuous curves. Also, the heat

effect at the transformation is small, estimated by Rao and Rao [20]

at 150 cal/mol or at '^I/IA of the heat of transformation of K^SO^.

In the present study, the ratio was approximated, from DTA data, at

'^^1/30. Thus the data for Cs^SO^ suggest a second order transition.

Rao and Rao [20] , in fact, postulate that the transformation has considerable second order contributions and that the measured heat effect

(by DTA) is small due to the occurrence of the transformation over a

wide temperature range.

3.3. Alkali Seed Equilibria

3. 3. a. The System K^SO^-Cs^SO^

A knowledge of the K^SO^-Cs^SO^ phase diagram could be helpful in

selecting a suitable seed mixture for MHD. The diagram reported by

11
Dombrovskaya [22] in 1933 (figure 5) does not include the polymorphism

of the components, and the indicated melting point of Cs^SO^ at 846°C

is over 150° below the value of 1004±5°C reported in the National

Standard Reference Data Series [19] and confirmed in the present study

[23] . Also an unresolved inconsistency exists in the literature.

Diogenov and Ermachkov [24] in relation to the ternary system Cs^SO^-

K SO -Na SO show the boundary Cs SO -K SO system as a complete

solid solution extending from 1069°, the melting point of K^SO^ to

1013°C, the melting point of Cs^SO^, with a minim;am at 930°C and 50

mol % ^2^'^4* '^^^ diagram of Dombrovskaya (figure 5), however, is cited

as the reference. '\ .

, ,;

;

Figure 6 shows the phase diagram for the system as obtained from

DTA and high temperature x-ray studies of the components and intermediate

compositions. The solidus and liquidus curves have been drawn close to

each other, as the DTA peaks were relatively sharp with no apparent differentiation. This region of the diagram is in agreement with that

referenced by Diogenov and Ermachkov [22] . The subsolidus shows a

eutectoid with slight mutual solid solubility of the components in the

low temperature orthorhombic forms and complete solid solubility in the

hexagonal phase. The hexagonal phase could not be quenched, and as the

DTA results were inconsistent and not interpretable , the high temperature

x-ray proved the decisive method of study. From the standpoint of seeding

a MHD channel, the 50:50 mol % composition with a minimum liquidus at 940°C

would be a most likely choice.

12
A plot of the unit cell dimensions vs composition for the hexagonal

solid solution phase is shown in Figure 7 . Whereas the a parameter shows

a small negative deviation from linearity, the £ parameter shows a large

positive deviation. It can be seen that the substitution of a (I.R.,

1.33A) for a much larger Cs"*" (I.R., 1.67A) on the Cs^SO^-rich side has a

relatively small effect on the c_ parameter. However, the substitution

of a larger Cs"*" for k"*" on the K^SO^-rich side has a pronounced effect.

It is unusual for complete solid solution to exist between two isostructural

molecules with such widely different cationic radii, and Vegard's law is

not obeyed for the c_ parameter.

These results can be rationalized by consideration of the general

crystal structure for the high temperature sulfates. Figure 8 shows a

schematic [110] view of Cs^SO^, based on the structure of KNaSO^, as reported by Wyckoff [25] . The structure contains two essentially different

types of Cs atoms: Cs{l) and Cs(2) forming columns through the structure

in the c_ direction, and Cs(3) forming parallel strings of alternating Cs

atoms and SO^ tetrahedra. The effect of the gradual addition of K atoms

4

may be considered. If the K atoms first substitute mainly for Cs(3) atoms,

the smaller atoms replacing the central larger ones within the rigid

framework will have relatively little effect, except for a slight contraction of the structure in the a_ dimension. After about 50 mol %

addition of K^SO^, however, the Cs(3) atoms will have been mostly replaced

and further addition of K^SO^ will result in substitution of Cs(l) and

Cs(2) atoms. Replacement of columnar Cs atoms will have a large effect

on the reduction in c dimension but even less effect on the a dimension

13
than for the Cs(3) substitution. If Cs^ and k"*^ were randomly substituted

over all of the sites in the structure, a much closer adherence to Vegard's

law would be expected. In Figure 7, deviation from linearity occurs

at about 50 mol % Cs^SO^. Thus this mechanism explains the curvature

for the a_ and c_ parameters. From the fan-shaped nature of the curves

for the c_ parameter, it is apparent that the thermal expansion increases

with increased Cs^SO^ content.

A plot of x-ray density vs composition at 500°C, 600°C, and 700°C

is shown in Figure 9. The plot shows a positive deviation from linearity

but not nearly so pronounced as for the parameter plot, because in the

2 density formula, 0.865a c, the a^ dimension has a much greater contribution

to the density than does c_. -

3.3.b. The System K^CO^-K^SO^

When a dirty fuel such as sulfur-containing coal is used in MHD,

K^CO^ seed acts in pollution abatement, as the volatilized seed provides

free K to form K^SO^ on condensation of the vapor. Therefore, a knowledge of the K^CO^-K^SO^ system might be useful in formulating seed

mixtures and in interpreting their behavior. Le Chatelier [26] and

Amadori [27] studied the liquidus by visual observation in thermal

analysis. They observed a continuous solid solution between the end

members. The liquidus was found to be relatively flat on the K^CO^

side and relatively steep on the K^SO^ side.

In the present study (figure 10) , the liquidus was determined from

the DTA results of eleven intermediate compositions. In approximate

agreement with earlier work it is almost horizontal (slight rise) up

14
to about 25 mol % K^SO^. Similar to the K^SO^-Cs^SO^ system, it was

not possible to differentiate the solidus from the liquidus. Subsolidus

phase relations were deduced with the use of the high temperature x-ray

diffraction furnace for a number of intermediate compositions. The low

temperature orthorhombic solid solution phase extends from pure K^SO^

to about 90 mol % K^CO^ and shows a maximiam along the hexagonal solid

solution boundary at 635°C. Thermal expansion data for the various solid

solution phases as a function of composition and temperature will be

reported separately.

3.4. The System K^SO^-MgO

3. 4. a. Condensed Phase Equilibria Diagram

The condensed state phase diagram for the K^SO^-MgO system is given

in Figure 11. Experiments conducted in open and sealed Pt tubes gave

the same results except the K^SO^ had a tendency to vaporize during prolonged heating in open tubes . No intermediate phases were detected in

the system at temperatures up to 1700°C. Data obtained by DTA in this

study indicated no change in the K^SO^ orthorhombic to hexagonal transformation at 588°C using the various K^SO^-MgO mixtures. The system is

a simple eutectic type with the eutectic located at 1067 °C and "^2 mol %

MgO. The liquidus was determined up to 1700°C, the maximum temperature

attainable in the quenching furnace. This work illustrates the harmful

effect the seed material K^SO^ would have on the refractory insulator

MgO. Under equilibrium conditions, MgO dissolution (liquid formation)

will occur at any temperature above 1057 °C in situations where condensed

K SO is in contact with MgO.

15
3.4.b. Vaporization Equilibria

The condensed state phase diagrain shown in Figure 11 does not take

into account the possibility that the K^SO^ (soln) in the system may

become part of the vapor phase. Although the cold wall concept in an

MHD channel has had some success, the problems of corrosion and

electrode and insulator characterization appear less formidable if

wall temperatures are maintained at .some value below that imposed by

vaporization restraints but above that at which condensation can take

place. The basis for this contention is that corrosion of the electrode

or insulator of the MHD duct leading to formation of a liquid solution

can only take place if the activity of the vapor phase component in

the liquid solution is significantly less than the activity of that

component in the vapor. This would happen only if the vapor could

form a stable condensed state compound with the insulator or electrode.

On the other hand, if the vapor is allowed to exceed its saturation

pressure, the pure condensed phase may form which can then interact

with and corrode the wall, even though the activity of the vapor component in the corrosion product is only slightly less than that in the

vapor.

An example of this can be illustrated through use of the activity

diagram shown in Figure 12. This shows the activity of K SO in the

2. 4

pseudo two component system K^SO^-MgO at 1700°C. The dotted line

running from an activity of 1 to 0.85 represents the activity of K^SO^

in the K^SO^ rich single phase region of the condensed state phase

diagram and shows the gradual decrease in activity of K^SO^ as MgO

is dissolved in it. This line need not be linear but its true shape

16
is not known. When the mole fraction of MgO in the melt exceeds a mole

fraction of 0.15, MgO containing an unknown amount of dissolved K^SO^

precipitates as a solid solution. The extent of solid solution is not

known and is not detectable by x-ray methods. A constant K^SO^

activity of 0.85, assuming its activity is equal to its mole fraction,

is then found all the -way across the two phase region until at some

composition close to pure MgO, the activity of K^SO^ drops quickly

to 0 over a narrow composition range. It follows therefore that if

the pressure is reduced, a vapor phase will form at some appropriate

pressure, and if the pressure is reduced to a sufficiently low value,

the composition will become richer in MgO as the K^SO^ evaporates and

eventually the liquid phase will entirely disappear.

These concepts are applied to the phase diagram shown in Figure 13

in which three isobars have been introduced. This diagram was constructed

by assuming that total pressures of 0.01, 0.1, and 1 atm consisting

entirely of K^SO^ (g) and its decomposition products are present in the

vapor phase.

All data for this calculation were from JANAF [28] except for the

thermodynamic data for K^SO^Cg). Several studies have indicated that

K SO (g) is an important vaporization product of K SO (s,Jl) [29,30],

Data for K^SO^ (g) was therefore based on JANAF data for K2S0^(s), the

heat of sublimation of K2S0^(s) reported by Halstead [30] and molecular

constant data modeled after that reported by Cubicciotti [31] for

Cs^SO^ (g) and Rb^SO^ (g) . For K^SO^ (g) , a number of vibrational

frequencies were adjusted so that the entropy of sublimation calculated from the thermal functions agreed with that reported by Halstead

17
at 1300K. Taking the case for a total pressure of 0.1 atm as an example

we can note that at 1697°C pure K^SO^{i) would exert a pressure of 0.1

atm. If it existed in contact with pure MgO at this temperature, it could

react to form a solution consisting of slightly less than 15 mol % of MgO

and the pressure would fall by somewhat less than 15%. If the temperature

were gradually raised to 1709°C however, the pressure would gradually increase to 0.1 atm and at a temperature slightly above 1709°C, the liquid

solution phase would disappear. The same arguments apply to the other

isobars at 0.01 and 1 atm.

3.5. The System K^CO^-MgO

3. 5. a. Condensed Phase Equilibria Diagram

One system of obvious interest is that between K^CO^ and MgO, seed

and insulator materials, respectively. Preliminary experiments revealed

that pure K^CO^ was very reactive with moisture in the air, forming

K^CO^' 1. 5H2O, as identified by x-ray powder diffractometry [32]. Likewise, the K^CO^ in the heated and quenched MgOrK^CO^ mixtures reacted

at room temperature to form the hydrate, during the exposure time of

grinding the sample for specimen preparation and taking the x-ray powder

pattern. In one experiment, a sample of K^CO^ was dissolved in H^O and

allowed to dry in air at '^50°C for five weeks. The x-ray pattern showed

only K^CO^-l.SH^O. Ignition loss at 130° for 18h was 16.19% (vs 16.34%,

theoretical)

The melting point of K^CO^ was found to be sensitive to the environmental conditions. Using the quenching technique for material sealed in

Pt tubes, the melting point was 902±1°C. By DTA in a stream of flowing

CO^, at a heating rate of 10°C/min, the melting point was 901±5°C. These

18
melting points are in good agreement with 901±1°C, reported by Reisman

[5] , and obtained also in a CO^ atmosphere. Using an open Pt tube with

the quenching technique the melting point was found to be 888+5 °C; and

by DTA (air atm) , 889±5°C. The depression of the melting point in an

air atmosphere of about 13° is consistent with a depression of the freezing

point of 5-10° reported by Reisman [5]

Figure 14 shows the phase diagram for the system K^CO^-MgO, as

determined by the quenching technique using sealed Pt tubes. The liquidus

was traced only up to 1656 °C (and 33 mol % MgO) , the maximum temperature

reached by the quenching furnace. The system is a simple eutectic one,

with the eutectic located at 895.5±5°C and '\'2 mol % MgO. This diagram

illustrates dramatically the deleterious effect of the seed material on

the refractory MgO insulator. Anywhere in the MHD channel that condensed

K^CO^ exists in contact with MgO, melting will start at '^^896°C.

3.5. b. Vaporization Equilibria

Figure 15 shows the vaporization equilibrium diagram for the ^2^^3~

MgO system. It is similar to the K^SO^-MgO system, the most notable

difference is the lower temperatures at which liquid formation occurs

in the K^CO^-MgO system.

3.6. The System K^SO^-LaCrO^

3. 6. a. System Under Its Own Vapor Pressure

Considerable research has been focused on SrO doped LaCrO^ as a

possible material for use as electrodes in MHD-generators . The system

K SO -LaCrO has been studied to obtain knowledge of the chemistry and

phase relations of the pure end members. This study will give insight

to the behavior of the doped material (SrO-LaCrO ). Single phase LaCrO

19
specimens were prepared from La(OH)^ and ^r^O^. Due to the fact that

La^O^ readily hydrates when exposed to air, La(OH)^ of high purity was

used. However, this study did not detect any hydrolysis (temperatures

up to 150°C) of the LaCrO^ compound when in contact with water or exposed

to air for periods up to one month. Pavlikov et al [33] reports the

LaCrO^ compound melts congruently at '\'2430°C. Ruiz et al [34] reports

an orthorhombic to rhombohedral transition in LaCrO^ at 277 °C, Geller

et al [35] reports the transition at 260°C. The present work based on

DTA data agrees with the latter. Several investigators [34,35] report

a rhombohedral to cubic transition in LaCrO^ (perovskite type structure)

at some temperature between 1200 and 2000°C. This study did not resolve

the temperature of the rhombohedral to cubic transformation.

Single crystal x-ray diffraction data from LaCrO^ was collected in

an attempt to determine the space-group symmetry. Data obtained from

x-ray precession photographs were not conclusive. Various sizes and shapes

of single crystals were studied and all indicated twinning which perhaps

occurred during the rhombohedral to orthorhombic transition on cooling.

The equilibrium relationships between the condensed phases in the

K^SO^-LaCrO^ system under its own vapor pressure (in sealed containers)

are given in Figure 16. Homogeneous mixtures (K^SO^, LaCrO^) were heated

at 900 and 1000°C (in open Pt containers) for 18 to 21 hours. Portions

of the previously calcined material were then heated in sealed Pt tubes

at various temperatures for different periods of time (up to 9 days) and

quenched. At temperatures above 1000°C, sealed Pt tubes were used as

specimen containers. The K^SO^-LaCrO^ system is a simple eutectic type,

with the eutectic located at 1063 °C and '^^2 mol % LaCrO^ . No intermediate

20
phases were detected in the system at temperatures up to 1700°C. Under

equilibrium conditions, LaCrO^ dissolution will occur at any temperature

above 1053 °C in situations where condensed K^SO^ is in contact with the

LaCrO^ electrodes.

3.6.b. System in Air

The system K^SO^-LaCrO^ under its own vapor pressure is given in

Figure 16. The equilibrium relationships between the condensed phases

in this system (in sealed specimen containers) indicate a simple eutectic

type system with no intermediate compound formation. However, specimens

heated at the higher temperatures in an air environment gave results

indicating LaCrO^ is not stable in contact with condensed K^SO^. Table

2 shows experimental data for the compositions studied in the K^SO^-LaCrO^

system in air. Limited experiments were performed on these mixtures due

to the fact that compound formation indicates the phases are represented

by the more complex quaternary-type system K^O-La^O^-Cr^O^-O^ • The data

in Table 2 indicate that liquid K^SO^ reacts with LaCrO^ at temperatures

of 1100°C and above. The electrode material decomposes leaving La^O^

as a condensed phase. Compositions rich in K^SO^ indicate the presence

of a phase or phases, as yet unidentified, over the temperature range of

1100 to 1200°C. The unknown phase is a compound that appears to contain

K, La, Pt, and 0. It is somewhat similar to such compounds detected in

the *^2^^3~''^^2'^3 ^^^^^^"^ approximately the same temperature range. These

phases contain Pt and form only when the specimens are heated in Pt

containers. It should be noted the K^SO^ tends to volatilize at the

higher temperatures thus reducing the amount of reaction with the LaCrO^.

21
Neither La^O^ nor K^SO^ react with Pt to any great extent. However,

when both are present there is obvious degradation of the Pt. It seems

possible that it is this type of K-metal oxide-Pt reaction which causes

degradation of Pt electrodes in the MHD environment.

3.7. The System K^CO^-LaCrO^ in Air

The K^CO^-LaCrO^ system is similar to the K^SO^-LaCrO^ system in

as much as both cannot be represented by a binary-type system. Compound

formation indicates phases that exist in the ternary-type system K^OLa^O^-Cr^O^. Table 3 shows experimental data for the compositions

studied in the K^CO^-LaCrO^ system in air. All specimens were placed

in open containers due to the decomposition of K^CO^ and release of CO^

gas. No attempt was made to contain the CO^ in a closed container. It

was found in earlier work that mixtures containing K^CO^ heated in sealed

specimen containers resulted in essentially no reaction. The CO^ gas

would reach an equilibriiam pressure until such temperatures were reached

where the sealed containers would rupture. The data in Table 3 indicates

that serious corrosion problems might occur when condensed K^CO^ is in

contact with the electrode material. For example, at the 50:50 mixture

K^CO^ : LaCrO^ , the electrode material decomposes completely at 700°C.

However, the decomposition products combine to reform the LaCrO^ at

1200°C. Various phases, as yet unidentified, occur in the system. Some

of the phases are thought to contain Pt, similar to those discussed in

the K^SO^-LaCrO^ system.

It may be concluded that LaCrO^ may exist in equilibrium with K

vapor at high temperatures and may be used in the MHD channel, but only

if protection is supplied to the LaCrO^ when subjected to K at temperatures

below ^1200°C.

22
3.8. The System "K^C'-Al^O^

This system was investigated using two sources of potash, K^SO^ and

K2CO3.

3. 8. a. ^2 ^"^4"^ ''2 "^3 ^"^^^^"^

The following conclusions can be made from a study of the K^SO^-Al^O^

system:

1. At temperatures below about 1400°C, the system is a simple eutectic

one, with the start of melting (solidus) occurring at 1055 °C. Thus below

1055°C, there is no reaction between K^SO^ and Al^O^, which means in a

practical sense that Al^O^ will be unaffected by K^SO^ at low temperatures.

2. The eutectic composition between K^SO^ and Al^O^ lies very near

pure K^SO^ (>99% K^SO^) , and the liquidus curve rises very steeply, i.e.,

almost parallel to the K^SO^ component. Thus, even between 1055° and

1400°C, in this two component system, less than 2% ^^2'^^' ^^'^ possibly

much less, is dissolved in the predominantly K^SO^ liquid. In this

temperature range, therefore, even though K^SO^ liquid is present, its

effect on the dissolution and degradation of ^12^3 s'^^^l^ minimal.

3. The orthorhombic to hexagonal transition of K^SO^ at 588°C is

unchanged in the two phase region. The transition is accompanied by

about a 3% vol\ime increase, as determined from the thermal expansion

data for K^SO^.

2 4

4. Above about 1400°C, however, the system is no longer binary as

BaI^O^ is formed in an open system, according to the reaction:

K^SO^ + llAl^O^ = K20-llAl20^ + SO^ + ^/^O^

23
A composition SOK^SO^ : VOAl^O^ when heated at 1300°C for 20h consisted

of K^SO^ and ^l2*~'3 gave a weight loss of 0.51%, probably mostly

water. When heated at 1400°C for 20h, however, the total weight loss

was 4.8% and x-ray analysis showed the sample to consist mostly of

BaI^O^ and K^SO^, with a trace of Al^O^ . In support of this conclusion,

the composition SOK^SO^ : SOAl^O^ when heated at 1500° for 1.5h showed

only K^SO^ and SaI^O^.

5. In a sealed Pt tube the formation of ^^12*^3 ^^^^^^^ between

1500 and 1550°C, as the increased vapor pressure of SO^ raises the

temperature of reaction.

6. One interesting conclusion is that K^SO^ and BaI^O^ are nonreactive up to at least 1650°C, the upper temperature limit studied,

and, therefore, ^^12^3 potential use in MHD systems, as for example,

a preheater material. Potassium - BaI^O^, however, is probably not

suitable as a MHD electrode or insulator material because of the ionic

conductivity. The vapor pressure of potash over B-alumina also may be

too high at high temperature.

3.8.b. K^CO^-Al^O^ System

K^CO^ reacts readily (800°-1000°C) with Al^O^ to produce KAIO^.

Compounds richer in K^O than KAlO^ have not been found and, hence, the

system can be described as the KAIO^-AI^O^ join. ^AlO^ shows a reversible

endothermic peak near 535°. High temperature x-ray diffraction confirmed

this transition. The high temperature form has cv±>ic symmetry with

a_=7.78A (600°C) . The low temperature form appears to be a distorted

variant, possibly having tetragonal symmetry. ^1*^2 extremely reactive

with atmospheric moisture and care must be taken in handling the material.

24
When KAIO2 or compositions containing KAIO^ are heated in air above

1225° C, vaporization of potash occurs and this tends to drive the initial

bulk composition toward progressively higher ^12^3 contents. This precludes the preparation of accurate starting compositions by normal calcining .

methods for a study of the KAlO^-Al^O^ join. It was found, however, that

if K^CO^-Al^O^ compositions were pelletized, wrapped in Pt foil, then

completely covered with a powder of like composition, potash losses can

be minimized at temperatures up to 1350°C and compositional integrity

could be maintained. Starting materials in the KAIO^-AI^O^ system were

prepared by this method and are now being used for high temperature

equilibration studies using sealed Pt containers.

During the course of the work above a new DTGA apparatus was obtained,

assembled and calibrated. It became apparent that advantage could be taken

of the vaporization of potash from KAIO2 such that phase relations and

vaporization data for the entire KA102-Al20^ join could be derived. Using

a Knudsen cell in the apparatus, the 1350°C isotherm in the system was

defined. These results are preliminary and will be reported later in

detail. Briefly, at 1350°C, potash vaporizes from KAIO2 bulk

composition enters the two phase KAl02-3-alumina field. With continued

vaporization (linear rate) all KAIO2 lost as the bulk composition enters

a single phase B-alumina solid solution region. The rate of vaporization

decreases curvilinearly until the bulk composition enters the 6-alumina

plus ^^2^2 phase region (vaporization rate linear) . This method,

used for a n\jmber of isotherms, should define condensed phase relations

and vapor pressures for the entire KAlO -Al 0 join. Additionally, this

25
technique permits a determination of the composition of 6-alximina

independent of quenching studies.

It is apparent from the TGA data that potash is mobile from potassium

aluminum oxides even at moderate temperatures. Although complete vapor

pressure data will be obtained for additional isotherms, the preliminary

data suggest that alumina will not be subject to degradation effects of

potash vapors, certainly above 1500°C.

3.9. K^O-SiO^ System

Condensed phase relations in this system were determined by Kracek

et al [36] . Vaporization data for this system, however, are not available.

As silica is one of the major constituents of slag it becomes important

to determine experimentally its affinity for potash. Such a study has

been initiated using the DTGA-Knudsen cell method.

26
4. Electrical Conductivity and Related Properties

4.1 Introduction

The objective of this project is to provide engineering data

on the electrical properties of MHD materials and to develop an

understanding of the parameters that determine the conductivity in

such solids. Attention is focussed on the electrical behavior of

promising solids for electrodes and insulators in MHD ducts and on the

characteristics of coal slag at high temperatures.

The amount of electrical data on these materials available in the

literature appears to be very limited and needs drastic augmentation.

The selection of materials to be measured and studied, and the setting

of priorities has been done in cooperation with the investigators of

other properties of MHD materials. Close contact has been maintained

with the technical and engineering staff of MHD-facilities in this

country. The collaborative effort with scientists of the USSR, aimed

at the testing of materials in the U-02 MHD channel in Moscow, is very

helpful in focussing attention on some of the major materials problems.

During the last 2 years our research work has emphasized the

characterization of coal slag . Electrical (and other) measurements

were made on a variety of slag and slag-seed samples.

The choice of electrode material is not yet fully decided.

Ca-stabilized ZrO^ has been used in a number of MHD-channels . However,

its conduction is ionic and deterioration is inevitable. Hence,

27
various other materials are being considered; we have made measurements

on a number of these compounds.

Several MHD-facilities have experimented with channel -coatings

to prevent arcing and/or to keep the wall temperature reasonably low.

The electrical conductivity of one of these materials (ZrO^rMgO used

in the AVCO channel) has been investigated.

Another crucial parameter of MHD-duct components is their

thermionic emission . During the last 8 months we have assembled

equipment to measure this high temperature property and have performed

initial experiments.

. Besides the characterization of MHD-mater ials in laboratory

experiments it is important to determine how they stand up in actual

operation. For this purpose we have started a testing program in

cooperation with the University of Tennessee Space Institute (see

section 7) . Thus far, 4 different specimens have been exposed to two

runs in the channel of the UTSI MHD-facility . These specimens have

been subjected to a number of analytical tests.

4.2 Coal Slag ' -

. . _ /

Coal contains about 10 percent in weight of inert material

(slag), which is not burned in the combustion process. Consequently,

the water-cooled walls of an MHD-channel will be covered with a thin

layer (1-5 mm) of slag. The surface temperature of this layer facing

the gas flow will depend on the degree of cooling of the walls. In

general, this temperature will be higher than 1500 °C, and hence,

the top layer will be in a vitreous state with low viscosity. It then

follows that some of the liquid slag will be "sheared-of f

" by the gas

stream and that a steady-state slag layer of certain thickness

28
will be established. This slag layer must have sufficient conductivity

to permit passage of the MHD current perpendicular to the wall. On

the other hand, the conductivity should be low enough not to cause a

short circuit between adjacent electrodes. (see Figure 17). It is

important therefore to know the electrical conductivity of different

coal slags.

Experiments were performed on a "natural" slag sample obtained from

the U. S. Bureau of Mines (MHD-12) and also on a few synthetic specimens.

The "natural" slag specimen contained the following amounts of oxides

(in wt. %) : 50 SiO^, 21 Al^O^ 14 (FeO + Fe^O^), 5 MgO, 1 CaO, 1 K^O

and 2 SO^. Electrical measurements were made on bar-shaped compacted

samples using the 4-probe technique. Results are shown in Figure 18. The

-2 -1 -1

conductivity is rather high (^10 ohm cm at 1700 K) and is

attributed to the transfer of electrons between ferric and ferrous ions.

Anomalies in the range 1450 - 1610 K are the result of devitrification;

below about 1510 K one or more crystalline phases begin to appear. The

electrical conductivity of these phases is rather high and is determined

by the ratio Fe^^/Fe"^^. In turn this ratio depends on the degree of

oxidation or reduction of the environment, on the presence of other

ions, and on the solubility of ferric and ferrous ions in the different

crystalline phases.

Similar experiments were performed on several synthetic slags

(4 component system) . Results of measurements on one of these samples

(K 216) , which contained a large amount of iron (36 wt. % Fe^O^) , is shown

in Figure 19. The conductivity is about 10 times higher than that

of sample MHD-12 and both the temperature and pressure dependence are

small.

29
Originally these measurements were made up to 1700 K (1427 °C)

Later, this range was extended to 2050 K (1777 °C) . At the higher

temperatures the conductivity was determined using a 2 probe technique:

one electrode was a cylindrical Pt "bucket" containing the liquid slag,

while the other electrode was a Pt-wire dipping into the center of

the liquid. The resistivity was "normalized" to the highest t'emperature

result of the 4 probe method.

One important question is how the conductivity of slag will be

affected by the presence of seed (e.g., K^SO^) . In order to investigate

this problem a base oxide (K 257) was prepared consisting of 43 ^^O^,

19 Al^O^, 30 Fe^O^ and 8 MgO (in wt. %) . The conductivity of this

synthetic slag specimen is shown in Figure 20.

The base oxide was mixed with K^SO, seed at 1550 °C. Three melts

2 4

were prepared: (A) 1 slag-4 seed, (B) 1 slag-1 seed, and (C) 4 slag-1 seed

(weight ratios) . Melts A and B showed strong separation while C appeared

to be homogeneous to the eye. Chemical analysis showed that melt C

contained 12.38 wt. % K^O and no SO^ or SO^ was detected. The conductivity

of the slag-seed mixture (melt C) is shown in the lower curve of

Figure 20. The presence of potassium apparently inhibits the conduction

rather than contributing to it.

4.3 Electrode Materials

One of the most interesting candidate materials for MHD electrodes

is LaCrO^ (doped with Sr or Ca) because of its high electronic conductivity.

This compound was first introduced by Meadowcroft [37] who found that

LaCrO^ containing 16 mole % Sr was extremely corrosion resistant and had

a nearly temperature-independent resistivity of "^0.1 ohm cm

(Electrical resistivities of several promising electrode materials are

plotted in Figure 21.)

30
Electrical and optical measurements were made on a number of doped

and undoped La-chromites aimed at clarifying the conduction process in

this compound. Single crystal growth of this material appeared to

be very difficult- W. S. Brower (NBS) succeeded in growing one small

crystalline sample of the undoped material. The electrical conductivity

of this specimen appeared to be about 3 orders of magnitude higher than

that reported by Meadowcroft [37] ; other samples were produced by

pressing the powdered material and firing at high temperatures (1500 °C

and 1800 °C) . Figure 22 shows the conductivities of a crystalline undoped

sample, a Sr-doped sample and a Ca-doped specimen. The temperature

dependence is surprisingly similar. Below 500 K the activation energy

is between 0.10 and 0.20 eV; exhaustion is reached at about 1000 K.

The activation energies of the heavily doped samples consistently

appear to be the smallest.

It was impossible, unfortunately, to obtain Hall data. If the

molar concentration of Sr or Ca is an indication of the number of

carriers (in the exhaustion range) one calculates a mobility of 0.04

2 cm /volt sec. Such a value is entirely reasonable for electrons in

oxides and explains the difficulties in measuring the Hall coefficient.

A rough measurement of the sign of the thermo-electric power showed

the carriers to be holes. It is possible that these holes can be

4+

thought of as residing on Cr ions at low temperatures and producing

an electric current, when the temperature increases, by transfer of

4+ 3 + the hole between Cr and Cr ions. Another interpretation is that

the holes are localized at vacancies or near Sr- or Ca-ions and that

transport occurs by excitation of the hole into the (oxygen ?) valence

band.

31
other attempts to produce an electronically conducting high

temperature material were made by doping the very stable compound SrZrO^.

In contrast with ZrO^ this material is not subject to so many phase

transitions (as a function of temperature and pressure). We have

prepared one sample consisting of 86 SrZrO^ and 14 CeO^ (mole %)

and another with the composition 89.7 SrZrO^ , 5.9 SrTiO^, 1.5 Ta^O^

3+ 4+

and 2.9 SrCO^ (mole %) . It was hoped to produce Ce and Ce in

3+ 4+

the former case and Ti plus Ti in the latter.

Results of d.c. conductivity experiments on these two samples are

presented in Figures 23 and 24. A.C. measurements performed at 100 Hz

duplicate the d.c. results within the margin of error. In both cases

the conductivity rises steeply with temperature; for the Ce-doped sample

the activation energy is 4.0 eV and for the Ti-Ta-doped specimen

E = 3.5 eV. The fact that the conductivity is nearly independent of

frequency and of the surrounding oxygen pressure suggests that the

conduction is electronic in nature. Considering the size of the

activation energies it is possible that we are dealing with intrinsic

excitation across the energy gap.

The magnitude of o at 1700 °C is 0.1 ohm cm an acceptable value

for an MHD-material at that temperature. However, the conductivity drops

off very rapidly towards lower temperature, posing severe problems

with respect to the current carrying capacity in the neighborhood of

the ceramic-metal contact.

4.4 Channel Coating

The conductivity of a sample of channel coating from the AVCO-Mark VI

MHD-channel was measured over the temperature range 900 to 2000 K.

This material consists primarily of ZrO^ with additions of MgO (10-20 %)

32
Results of this experiment are shown in Figure 25. Little or no polarization effects were observed. The slope indicates an activation

energy of "^1.4 - 1.45 eV which is equal to that of ZrO2-20% CaO

reported by Strickler and Carlson [38] . This is taken as proof that

the conduction mechanism in the MgO-doped material is the same as in

the Ca-stabilized ZrO^ :dif fusion of oxygen.

4.5 Thermionic Emission

The emissivity of the electrodes is of great importance for the

transfer of electrons from the cathode to the plasma and from the

plasma to the anode. Hence, it is useful to obtain some data

concerning the thermionic emission of MHD-wall materials. We have

started to construct an apparatus to investigate the emission from

slag. It was decided to use an external heat source, rather than a

heating wire. A 60 W CO^-laser was used to heat the slag, which was

deposited as a liquid drop on a small Pt-bead and then solidified by

cooling. The temperature of the slag surface was measured by means of

an optical pyrometer. The current from this emitter was collected

on a thin semi-spherical Pt cup at a distance of 4 or 5 mm from the

sample. The collector could be biased with respect to the emitter

(+ or - 50 Volt) . Measurements were performed in 1 atm. of air,

1 atm, of oxygen and 0.01 atm. of oxygen in nitrogen. It appeared

that sizable currents - negative as well as positive carriers - were

emitted even at relatively low temperatures (1200 - 1300 °C) . The

difference with common thermionic emission experiments on metals is

that the slag material is much more volatile than solids like W or

Pt. Hence, the measured currents are not pure electrons, but are mixed

+ ++ with all kinds of ions. (e.g., K , Fe , etc.). It is to be expected

that at higher temperatures the electron emission will take over.

33
Unfortunately, the reflectivity of the spherical samples is very large

and the power of the laser beam is not sufficient to heat the slag

sample to temperatures much above 1300 °C. Consequently, further

thermionic emission measurements will have to wait until a higher

powered CO laser will be available.

34
5. VAPORIZATION

The vaporization program is concerned with the transport of material

to or from the walls of an MHD device. Efforts to study this problem

fall into two categories.

The first consists of theoretical modeling of a) condensation of

seed material, b) evaporation of structural materials, and c) reaction

of structural materials with gaseous constituents of an MHD plasma to

form stable gaseous species. Such modeling utilizes existing thermodynamic data and assumes that thermodynamic equilibrium is attained as

the temperature and pressure change in the device.

The second category involves the experimental study of evaporation

properties of coal slag under non-reactive conditions and the interaction

of potential MHD construction materials with water vapor, which is

present in relatively large concentrations in the MHD plasma. The

latter process could lead to the formation of gaseous hydroxide vapor

species which can become important transporters of material. In the

following sections of the report, some of the results previously

reported during the term of this project will be discussed.

5 . 1 Condensation of Seed Material

A significant problem in development of MHD devices has

resulted from interaction of seed materials with the ceramic walls.

Since the stability of K^O-metal oxide compounds appears minimal with

respect to decomposition to the metal oxide and gaseous K and 0^ in the

temperature region of interest, it would appear that the observed interaction could be avoided if the device were operated with a sufficiently

35
high wall temperature. Two approaches to this problem were made. The

lowest temperature at which interaction of seed would take place would

be that at which pure seed material condenses on the ceramic in question

and this is followed by solution of the ceramic in the liquid seedSecondly, the activity of the seed in the melt in equilibrium with the

ceramic in question can be estimated from the phase diagram by assuming

ideal solution behavior. This will predict somewhat higher interaction

temperatures because the activity of the seed in the gas phase will be

less than unity when interaction takes place.

Two limiting cases have been considered. First, if clean "coal"

having nominal composition CHq.v is burned with a stoichiometric amount

of "air" consisting of N2/O2 in the molar ratio of 4, and seeded with

K2CO3 so that the K content of the combustion products is 1% by weight,

the condensation temperature of the K2C03(2^) can be estimated. Secondly,

if the same combustion process is carried out using K2SO4 as seed,

K2S0^(l) condensation will occur.

Figure 26 shows the variation in pressure as a function of temperature for CO2, K, 02/ KOH, KO, and K+ for a total pressure of 1

atm , the activity of K2CO3 in the melt assuming the activity is

equal to the mole fraction in the K2C03-MgO phase diagram, and the

activity of K2CO3 in a hypothetical liquid phase for total pressures

of 1 and 10 atm. The formation temperature of a stable liquid phase

consisting of MgO dissolved in K2CO3, occurs at the point of intersection of the activity curve for the hypothetical liquid and the curve

representing the activity of K2CO3 in solution (about 1385 K at 1 atm.

total pressure) . In the absence of this interaction, pure K2CO3 would

36
condense at the point where the activity of the hypothetical liquid

phase equals 1 (about 1380 K) . All of the pressure curves have been

drawn below the condensation temperature as though condensation did not

take place at 1380 K. At a total pressure of 10 atm condensation of

K2CO3 will occur at 1580 K as indicated by the intersection of the line

representing the activity of K2CO3 at 10 atm and the line representing

the activity of solution- If no interaction took place, condensation

would occur at about 1555 K. No pressure curves are shown in Figure 25

for the 10 atm case. Vapor species assumed to be present at equilibrium

were CO^ , N^, KOH, CO, OH, E , H^O, , 0^ , K, KO, O NO, H, k"*" , and

OH . These results were obtained using a multicomponent equilibrium

program based on the method described by Kandiner and Brinkley [39] and

using thermodynamic data from JANAF [40]

Figure 27 shows results for the case where stoichiometric combustion

takes place but the combustion products are seeded with K^SO^. The

interpretation is similar to that of Figure 25. The activity of K^SO^

in the hypothetical condensed state equals that in the MgO saturated

K^SO^ phase (derived from the MgO-K^SO^ phase diagram) at temperatures

of about 1575 and 1775 K for total pressures of 1 atm and 10 atm

respectively. Curves showing pressures of additional K bearing species,

K2S0^(g) , and KOH(s) have been shown. The pressure curves are for the

1 atm case. Below the condensation temperature, all of the pressure

curves have been drawn to indicate that K^SO, is being removed from the

2 4

vapor phase. The sharp peak in the curve depicting the pressure of

K^SO^ is due to the onset of condensation or exhaustion of K2S0^(Ji)

from the condensed phase.

37
Figure 28 shows essentially the same information as Figure 27, but

in addition shows pressures of all those vapor species whose pressures

are in excess of 10 ^ atm under equilibrium conditions and shows the

mole fraction of K2S0^(s,£) in the solid or liquid form. Not shown are

the activities of K^SO^ in the hypothetical condensed state nor the

activities of K^SO^ in equilibrium with MgO. Since the total pressure

is 1 atm, the mole fraction equals the partial pressure. In this

calculation 57 additional vapor species were considered but were found

to have pressures of less than 10 atm for the conditions and composition

specified. The computations shown in Figure 28 were made using a

program by Gordon and McBride [41] .

All data for this computation were from JANAF with the exception of

data for K2S0^(g). Several studies have indicated that K^SO^ (g) is an

important vaporization product of K^SO^Cs,^) [42,43] Data for K^SO^ (g)

was therefore based on JANAF data for K2S0^(s), the heat of sublimation

of K^SO^Cs) reported by Halstead[43] and molecular constant data modeled

after that reported by Cubicciotti [44] for C^SO^Cg) and Rb^SO^ (g) . For

K^SO^ (g) , a number of vibrational frequencies were adjusted so that the

entropy of sublimation calculated from the thermal functions agreed

with those reported by Halstead at 1300 K. Additional details on this

work can be found in the papers by Levin etal., [4 5] and McDaniel and

Plante [45].

These calculations show that in the case of MgO, wall temperatures

in excess of 1800 K will be sufficient to eliminate condensed state seed

interaction. Most ceramics which are considered potential condidates

for MHD application can readily withstand such temperatures. The

38
problem of seed loss to coal slag and interaction of seed materials with

other potential MHD materials is an important one and is being further

studied in conjunction with the phase-equilibrium group at NBS . Of

special importance is loss of seed to SiO^.

5 . 2 Evaporation and Reaction of Structural Materials

The maximum temperature at which an MHD device can operate is

dictated by the vaporization rate or vapor pressure of a material. Although the rate of loss of material through vaporization cannot be

accurately predicted because of the high gas flow rate and boundary layers

in an MHD device, experience seems to indicate that pressures in the

range 10 ^-10 ^ atm may be sufficiently low so that continuous operation

for hundreds of hours could be achieved. The relatively high oxygen

pressure in open cycle MHD will lead to enhanced stability of oxide

ceramics which primarily vaporize by decomposition, will have no effect

on those which vaporize to yield gaseous molecules having the same composition as the solid phase, and will lead to enhanced volatility of

oxides which can form oxide vapor species having a higher oxidation state

than the condensed phase. In addition, because of the relatively large"

hydrogen content of most coals, conditions will be favorable for formation of gaseous hydroxide species. Some of these principles are illustrated in the following discussion.

In Figure 29, saturation pressures of Mg bearing species that would be

present in an MHD environment at 1 atmosphere total pressure are shown as

a function of temperature.

In this calculation, it is assumed that coal (CH ) is burned with a

39
stoichiometric amount of dry air (80/20 molar ratio N^/O^) and that the

combustion products contain 1 wgt% K added as K^CO^. Gaseous species

considered in this calculation are CO^ , , KOH, 0^ , H^O, OH, CO, , K,

KO, O, NO, H and other gaseous atoms or molecules that can be formed by

interaction of a metal oxide with H^O or 0^ as indicated.

All data used for this calculation were from JANAF [40], In addition,

some MgO(g) data were taken from Brewer and Rosenblatt [47]. Two curves

shown for MgO(g) represent the pressure based on the JANAF [40] tabulation

and the pressure based on Brewer and Rosenblatt's [47] assessment of existing thermodynamic data. Although we believe that the assessment of

Brewer and Rosenblatt probably predicts the MgO(g) pressure more reliably

than the JANAF tabulation, this is not of particular importance for the

point being made here. The major point here is that in the temperature

range where the walls of a hot wall device would be expected to operate,

say 1800-2200 K, the importance of hydroxide species appears to be about

as great as other species responsible for transport and probably would become

more important as the pressure is increased because of the increase in

the partial pressure of water and the decrease in importance of reduced

species such as Mg(g). (Figure 30).

Additional calculations have been made for other potential MHD

materials. Figures 31 and 32 show partial pressures of Cr bearing species

under MHD conditions and at total pressures of 1 and 5 atm. In this

case pressures of Cr bearing species have been calculated as though pure

Cr^O^ is the material undergoing interaction in the MHD environment.

LaCrO^, a potential MHD electrode material has a heat of formation from

the pure oxides of about -10 kcal, and each of the gaseous species has

40
a single Cr atom so that the partial pressures of the Cr bearing species

over LaCrO^ will be reduced by about a factor of 4 at 1800 K below those

shown i.e. ; Alog P = -5000/4.576 (1800). This calculation shows therefore that the losses of material due to vaporization from LaCrO^ will be

much greater than losses from MgO due largely to formation of the gaseous

species CrO^OH. The oxidized species CrO^ and CrO^ will also be transporters of significant amounts of Cr and the utilization of LaCrO^ as an

electrode material at temperatures as high as 1800 K appears doubtful.

For calculations of the partial pressures of the chromium bearing

species heats of formation and free energy functions for CrO(g), CrO^ (g)

and CrO^(g) were from Schick [48], the heat of formation for Cr^O^Cs)

was from Brewer and Rosenblatt [47] while free energy functions for Cr^O^

were derived from the table given by Kelley [47]. The heat of formation

and free energy functions for CrOH were from Jackson [50] while the data

for Cr(g) were from JANAF [40]. The standard heat formation of Cr020H(g)

was derived from an equation given by Graham and Davis [511 and was

attributed to measurements of Belton and Kim [52] . Free energy functions

for Cr020H(g) were derived at NBS from a molecular model adjusted to

give the observed entropy change at 1500 K. Another value for the heat

of formation of CrO^OH has been reported by Bulewicz and Padley [53]

from flame studies and is about 30 kcal more negative than that based

on the equation given by Graham and Davis [51] . If the heat of formation

based on the work of Bulewicz and Padley [53] is the more nearly correct

value, use of LaCrO^ as an MHD electrode material would be extremely

doiabtful

41
other calculations carried out show that ZrO^ will not undergo

significant transport because of formation of species such as ZrOH or

Zr(0H)2, but that calcia stabilized zirconia will tend to lose Ca because of formation of CaOH(g) and Ca(0H)2(g) molecules in addition to

vaporization of Ca(g) or (CaO(g).

5.3 Vaporization of a Model Slag

Studies on the evaporation of synthetic slag have involved the

mass analysis of vapor effusing from an Ir Knudsen cell with a modulated

beam mass spectrometer. Beam modulation makes it possible to distinguish

between vapor molecules which come from the Knudsen cell and those which

are due to residual gases in the vacuum chamber. Use of the mass spectrometric method to study evaporation processes is well known and several

excellent reviews are available [54,55]. Vaporization pressures can be

determined from the relationship

P. = k. l"*" T

1 11

where is the pressure, is a constant, is the ion current, i

refers to a particular vapor species, and T is the absolute temperature.

The value of depends on geometrical constants, the cross section for

ionization of the molecular species by electrons and multiplier gain

factors. It must be evaluated by calibration experiments. However,

even if the value of is not known, some conclusions can be reached

by using ratios of the product T where the factor k^ cancels.

Four experiments have been run during this study. These have involved the vaporization of a 500 mg sample of slag as a function of

temperature, the evaporation of a pure SiO^ sample as a function of

42
temperature, the total evaporation of a 10 mg sairiple of SiO^ at constant

temperature and the evaporation of a 50 mg slag sample at 2000 and

2130 K. The latter two experiments serve the purpose of calibrating

the spectrometer for its sensitivity to each of the major evaporation

products

The synthetic slag initially had a composition consisting of 0.582

mole fraction SiO^, 0.152 Al^O^, 0.105 Fe^O^, and 0.161 MgO. Attention

has been focussed mainly on the vaporization of the iron oxide and

silicon dioxide from the slag. Ion currents due to their major vaporization products, namely, SiO(g), Fe(g), and O^Cg) have been measured.

The analysis of these results is not completely unambiguous because

changes in the concentration of the solution undergoing evaporation take

place, the pressures of the species have different temperature coefficients, and phase separation may occur. Nonetheless, an attempt to

understand the results in terms of ideal solution theory and presently

available thermochemical data [40] can be made.

In principle, the oxygen pressure would be controlled by the vaporization of the more volatile component. From thermodynamic data, and

assuming that the slag forms an ideal solution, we can expect that the

oxygen pressure would be governed sequentially by the following reactions

at temperatures up to about 2200 K,

Fe20^(Soln) = 3Fe0(Soln) + 1/2 02(g)

FeO(Soln) = Fe(g) + 1/2 0^{g)

SiO^CSoln) = SiO(g) + 1/2 02(g)

43
The decomposition of Fe^O^CSoln) to FeO(Soln) and 0^ (g) would give rise

to an oxygen pressure orders of magnitude greater than the other decomposition processes until essentially all the ^^^^4 (Soln) had decomposed.

The oxygen pressure over the solution would next be controlled by the

decomposition of FeO(Soln) and finally by SiO^ (Soln)

Figure 33 represents ion current data as a function of time obtained

during an isothermal evaporation experiment. An initial vaporization

period at 2000 K shows a decrease in the ion intensity of O^"*" and

corresponding increases in the Fe"*" and SiO"*" intensities, all intensities

apparently becoming constant after about 20 minutes. The decrease in

0^ intensity and increase in SiO^ and Fe"*" intensities are likely due to

the gradual decomposition of Fe^O^. The following flat portions

represent the interval during which FeO in solution controls the oxygen

pressure .

-

After this period, the temperature was increased to 2130 K. The

increase in intensity of the Sio"*" peak with decreasing oxygen pressure

represents that concentration region in which the oxygen pressure is

falling below the value controlled by FeO in the solution. The portion

of the Sio'*' curve having essentially zero slope corresponds to the

region of concentration where the SiO^ in solution is controlling the

0^ pressure. The flat in the O^"*" ion intensity curve at the same time

interval confirms this, while the continual decrease in the Fe"*" curve

during this interval indicates that no equilibrium with respect to an

FeO species in the solution is being obtained, or the concentration of

such a species is rapidly changing.

44
Figure 34 shows i"''t data for Fe(g), 0^ (g) , and SiO(g) above evaporating slag and some SiO(g) and 0 (g) data above pure SiO (s) . The i'''t

data for slag were obtained in order of increasing temperature after some

initial heating to temperatures of 1930 K had been made. During the

initial heating, O^"*" signals slightly higher than those shown at the

lower temperatures were observed. The most notable feature of the slag

data is the high i''"t for 0^ at the lower temperatures as compared with

the i'''t of over pure SiO^. In addition, it can be seen that the

ratio of ion currents of 0^ to SiO"*" are very much higher at the lower

temperature than those shown in Figure 33 at 2000 K or 2130 K. This again

is evidence that decomposition of ^^^^/^ controlling the oxygen pressure

at these temperatures and at this particular time.

The i''"t data for SiO(g) and 02^^^ over pure SiO^Cs) can be used with

the i"''t data for SiO (g) and 02^'^^ over slag to show that the activity of

SiO^ in the slag solution must be close to unity. This follows from the

stoichiometry involving evaporation of SiO^ and the equilibrium constant

for the process applied to both pure SiO^ and the slag. Thus, at 1850 K

it can be seen that the i"''t of SiO^ for the slag is about 1/2 that for

pure SiO^ while the i"'"t of O^"*" for the slag is about 4 times that for

SiO^. This can only happen if the activity of SiO^ in the solution is

near unity. At the highest temperature, about 2060 K, the i"'"t data points

noted as 1 and 2 were taken in that order, and show changes in the SiO,

Fe and 0^ pressures as the oxygen pressure over the slag begins to be

governed by the SiO^ decomposition rather than the FeO decomposition.

A preliminary conclusion reached from this study is that the decomposition of Yb^Q^ (Soln) is slower than it would be if thermodynamic

45
equilibrium were achieved. Equilibrium appears to be achieved, however,

with respect to evaporation of SiO^ from the slag solution with the

activity of the SiO^ (Soln) near unity. The behavior of FeO in the slag

remains to be determined. The possibility that the Ir cell can extract

Fe from the FeO in the slag needs to be exaunined.

46
6. VISCOSITY OF COAL SLAGS

6.1 Introduction

The goal of this study is the development of viscosity data on coal

slags over a temperature range that will be useful in the design and

operation of coal-fired MHD systems.

The reason for this study is that the flow properties of the slags

formed from the combustion of coal and from the reaction between these

slags and the seeding material will affect the removal rate of the slags

and they will affect the thickness of the slag layer which adheres to the

electrodes and insulating walls of the channel.

Because of the great compositional variations that exist in American

coals, it is considered paramount that equations should be developed for

the prediction of the viscosity from compositional data rather than

relying on making viscosity measurements on the slags from each batch

of coal reaching a generating plant.

Coal slags are, for the most part, oxides in various proportions.

The main impurities in the coal and therefore the main ingredients in

the slags are the oxides of silicon, aluminum,, iron, calcium and magnesixam

The other impurities are minor and can be ignored for the present. The

most useful viscosity data then would be that for the system which include

these five oxides. There is some viscosity-temperature data in the litera

ture on parts of the compositional system involved. The detailed experimental study of a five component system over a broad compositional range

appeared to be too complicated. Initial work, therefore, was designed to

fill in the gaps in the literature in the several four-component systems

47
that make up this five-component system. In order to put the experimental

work in perspective a brief mention of some pertinent literature follows:

Selvig and Gibson [56] reported the compositions of ashes of literally

hundreds of U.S. coals along with temperatures at which samples of their

slags or ashes either deformed, softened or became fluid. These three

points for each slag, although roughly related to viscosity, are rather

subjective and not useful in equations for determining flow rates.

Machin and co-workers [57,58,59] published a series of papers dealing

with the viscosity of melts in the CaO-MgO-Al^O^-SiO^ system.

Heywood and Womack [50] show the effect on viscosity-temperature

relationships of one five-component slag when 15% K^O is added. This

gives an idea of the effect to be expected when coal slags are exposed

to seeding materials. The authors also show the effect of alkali

additions on the viscosity of the composition BSiO^ * Al^O^ -CaO

Elliott, Gleiser and Ramakrishna [61] review the viscosity literature

pertinent to steelmaking and include material from Machin and others that

also pertain to coal slags.

Other studies could be referenced here but the complete data needed

for predicting the viscosity of coal slags is not available. For this

reason laboratory studies were deemed necessary.

6.2 Model Slag Studies

The logical approach to the development of equations that would permit

the calculation of viscosity from composition was to make a model slag study.

By melting a series of relatively simple compositions differing from each

other in some systematic way, it was believed that rules for flow behavior

48
could be formulated. Two systems not found in the literature were chosen

for the initial study. These were the silica-aliamina-iron oxide-lime and

silica-alumina-iron oxide-magnesia systems. Preliminary melts were made

in the former system to determine melting characteristics while awaiting

some changes in viscometer instrumentation. Attention was drawn to the

magnesia containing system, and most of the four-component studies dealt

with this system.

Viscosity values were determined over a temperature range which was

limited partly by the instrumentation and partly by the nature of the melts.

The upper temperature limit was approximately 1550° C in some of the earlier

melts; this was increased to about 1600° C by rebuilding the viscosity

furnace. The lower temperature limit for each melt usually depended on the

liquidus temperature of that particular composition. Most of the melts

encountered in this study crystallized rapidly as soon as the temperature

got below the liquidus. As crystals began to grow on the spindle or on

the surface of the melt, the viscosity would start to increase with no

change in temperature. Once viscosity determinations became time-dependent

they were considered useless and were discontinued. In these studies the

liquidus temperatures usually fell between 1250° and 1400° C. Figure 35

illustrates the singular nature of the viscosity datum points above the

liquidus temperature and the multiplicity of values possible below the

liquidus temperature

.

Figure 36 shows typical viscosity data for the four-component synthetic

slags. Although the viscosities of these melts differ from each other at

49
any given temperature, the remarkable feature is the similarity of the

temperature coefficients of viscosity for the different compositions.

During the course of the investigation of these simple melts, each

containing four oxides, attention was focused on a British publication that

offered the type of equations for calculating the viscosity that was the

goal of this study. If the equations should prove satisfactory for predicting viscosities of slags from American coals then further model studies

would not be necessary. The four-component study, therefore, was at least

temporarily terminated in order to test the validity of the new equations

with synthetic or model five-component melts. This new task is described

in the following paragraphs.

6.3 Model Studies to Test Watt-Fereday Equations

Watt and Fereday [62] measured the viscosity-temperature relationships

of the ashes of more than one-hundred different coals from Great Britain.

From this data they generated equations for the purpose of calculating the

viscosity at any temperature and composition within their specified limits.

The authors claim that the important constituents in coal slags with

respect to viscosity determinations are the oxides of silicon, aluminum,

iron, calcium and magnesium. They ignore all others as individual oxides

and lump them together with magnesium.

The Watt-Fereday equations are as follows:

50
log n = 10 M + C

(T-150)

where n is viscosity in poises*

M = 0.00835 (SiO^) + 0.00601 (Al^O^) - 0.0109

C = 0.0415 (SiO^) + 0.0192 (Al^O^) + 0.0276

(equivalent Fe^O^) + 0.0160 (CaO) - 3.92

T is temperature in degrees Celsius.

The oxides in parentheses are the concentrations of these oxides

in the melt expressed in weight percents.

* To convert poises to pascal seconds, multiply by 10

Therefore, to express ^o^-j^q viscosity in SI units, subtract

1 from

If these equations satisfactorily describe the viscosity-temperature

relationships for slags from American coals there will be no need for

further model studies.

When these equations came to light a series of five-component melts

was made and viscosities were determined. The experimental data were compared with viscosities calculated with the Watt-Fereday equations. Table 4

lists the compositions of the melts whose specific purpose is to test out

the usefulness of the Watt-Fereday equations over the range of slag compositions which might be expected from different U.S. coals. Figures 37

through 42 allow the comparison of the viscosity data of these melts

obtained in the laboratory with the calculated values using the Watt-Fereday

equations. The data points at 1700° C in Figures 37, 38 and 39 are

51
mathematical extrapolations of data at lower temperatures. In Figures 40,

41, and 42 the extrapolations are graphical. The mathematical method is

described in detail in the next section of this report.

In almost all of these comparisons of calculated viscosities with measured

viscosities the ability of the equations to predict the slope of the curves

(the temperature coefficient of viscosity) is remarkably good. It is also

noteworthy that the equations are satisfactory for predicting the viscosity

of a majority of the melts, but they are very poor in some cases. Those

melts where the agreement between measured and calculated viscosities is

poorest are K391 in Figure 39, K388 in Figure 40, K396 in Figure 41 and

K394, K398 and K390 in Figure 42. In these cases, there appears to be a

pattern that would indicate that the equations are not suited for melts

containing unusually large amounts of either iron oxide or the oxides of

calcium and magnesium. This discussion does not lead to any positive conclusions except that the Watt-Fereday equations are not entirely satisfactory

over the desired compositional range but they seem to predict satisfactorily

the change of viscosity with temperature.

The measured data on all of these melts will be used to redetermine

the factors associated with each of the four oxides determined by Watt and

Fereday, in an attempt to achieve a better overall prediction of viscosity

over the desired temperature and composition ranges. The form of the

equation will be changed to include a factor for MgO as well. Perhaps an

extra term will extend the capability of the equations.

52
Two other articles have been published recently by geologists that

are of great interest. Bottinga and Weill [63] analyzed viscosity data

from the literature on glasses, slags, lavas and magmatic silicate melts.

They formulated equations for calculating the viscosity of a melt from its

composition and temperature. Unfortunately, these authors are primarily

interested in magmatic melts in which they assume the Al^O^ to be

associated in stoichiometric proportions with the alkali and alkaline

earth oxides. They make no provision for ^12*^3 excess of the amount

required to combine with CaO and MgO in a 1:1 ratio. The excess alumina

present in many coal slags cannot be accounted for by their technique, yet

this material influences the viscosity very strongly.

Shaw [64] refined the method of filottinga and Weill but his method is

still restricted to alumina compounds of the alkalis and alkaline earths,

is possible that their techniques may be useful if a separate factor is

provided for Al^O^ by itself.

6.4 Extrapolation of Viscosity to 1700° C

Interest has been shown in obtaining viscosity information on slags

at 1700° C for purposes of estimating slag layer thickness in the channel

at this temperature. 1700° C is being considered as an optimum operating

temperature for the gas flow through an r4HD channel. There will be a

temperature gradient across the slag layer from gas temperature on the

inside to electrode temperature on the outside of the slag layer.

1600° C is the upper limit of the present furnace and viscometer and,

therefore, some sort of calculation or extrapolation of known viscosities

will be necessary over an interval of at least 100 degrees.

53
The Watt-Fereday equatiorxs described above make a direct calculation

from composition at any temperature. The data from which these authors

derived the constants for their equations were determined by actual viscosity measurements at temperatures up to 1800° C and should therefore

be valid at 1700° C.

One method for extrapolating data on actual compositions being

measured is to assume some type of equation form to describe the known

behavior of the liquids, use that data in the equation to determine the

constants and then to apply the equation to solve for viscosities

at new temperatures.

This has been done by Napolitano and Hawkins [65] for glasses

including some used as standard reference materials for calibrating viscometers. They used the Fulcher equation having the form:

. . "'°^10 '^'-^^^^-'^y = B + C

T-T

where viscosity is expressed in poises, T and T^ are temperatures in Celsius

degrees. They used the equation together with a large amount of carefully

gathered data to determine via a least squares solution the constants B, C

and T^. The constants then were piiblished and are furnished with a sample

of the material when it is purchased for calibration purposes. Its intended

use is for interpolation to any temperature within the range of the data upon

which the constants were determined. Extrapolation was not intended but

should be satisfactory over a short interval with increasing temperature

and decreasing viscosity.

54
Figures 37, 38 and 39 not only show the viscosity over the measured

range for the model slags, but they show extrapolation of each melt to

1700° C using the Fulcher equation based on data not exceeding 1600° C.

These figures show that in many cases there is a very close agreement between

the Fulcher extrapolation to 1700° C of real data with the Watt-Fereday calculation using composition and temperature values only.

Mathematically the Watt-Fereday expression for viscosity, in terms of

temperature, is only slightly different from the Fulcher expression. In the

former the (T-T ) term is squared, whereas the same term in the latter case

o is a first power term. The influence of this difference is to give the

Watt-Fereday log viscosity-temperature relationship a smaller temperature

dependence than the Fulcher expression. This effect is quite noticeable

when comparing the measured with the calculated curves, especially with

MHD-11 and K384 in Figure 37, K257 in Figure 38 and K363, K385 and even

K391 in Figure 39.

6.5 Phase Separation in Seeded Melts

The composition of coal slags which enter an MHD channel will be different

from the composition of slags tapped from the combustor and therefore their

viscosities and other properties will be different. The seeding material,

which is necessary for gas conduction, will affect the viscosity of the

channel slag and must be assessed along with the other constituents.

In order to get some idea of the composition to be expected of a seeded

channel slag, a cold probe was inserted in the exhaust gases of a Bu Mines

55
coal combustor into which a ^^CO^ solution was being sprayed. The material

which condensed on this probe was not necessarily a perfect representation

of everything in the exhaust stream of this unit but it should be somewhat

representative of that which would condense on a cold channel wall. This

material is identified as r4HD-12F. An analysis showed that the material

was a mixture of oxides, and a K^SO^-K^CO^ solid solution. Potassium

sulfate is used in melting operations in commercial silicate glass manufacture and is known to form an immiscible layer on the surface of the

melt. It could exhibit this type of behavior with coal slags and its

viscosity behavior should be understood. Consequently, a series of

melts were made in which the composition of the oxide portion was kept

constant and approximately the same as in MHD-12F. This oxide

portion contains 43% SiO^ by weight, 19% Al^O^, 30% Fe^O^ and 8% MgO.

This is identified as melt K257 in Table 4 and Figure 38. The proportions

of K^SO^ and the oxide mixture were varied, in order to determine the extent

of the imraiscibility . . '

;

Melts were stirred for an hour at 1550° C and then held at 1500° C

without stirring for another hour. In some cases, the melts separated into

two layers. The upper layer was fluid, much less dense, and, when decanted

from the remaining liquid, cooled to a tan crystalline mass. The lower

layer was viscous by comparison, heavy, and cooled to a black crystalline

mass

56
Mixtures were melted in the following proportions: 80% K2SO^-20% oxides,

50% K^SO -50% oxides, 30% K^SO -70% oxides and 20% K SO -80% oxides. Table 5

2 4 2 4 2 4 gives the analyses of the upper and lower layers. Very little K^SO^ was

found dissolved in the oxide layers (lower) , but amounts of K^O were

found in these lower layers. This indicates that the K^SO^ decomposed

upon reaction with the slag oxides releasing "SO^". The almost pure K^SO^

dissolved very little of the oxides.

The boundary between the one-liquid and two-liquid regions at 1500° C

is very close to the 30% K2SO^-70% oxides because the melt of this composition contained but very little of the upper or sulfate layer. The 20%

K2SO^-80% oxide mixture remained a single liquid; no separation was observed

at all. There is undoubtedly some temperature above 1500° C at which the

materials are completely miscible and on quenching no phase separation could

be seen.

This phenomenon of phase separation in the liquid state in these slags

can be important in designing and operating real-life MHD systems. From a

point of view of maximum seed recovery it might be desirable to operate on a

schedule that would permit phase separation. For other reasons, it might

be desirable to operate at higher temperatures where phase separation might

not occur. By understanding this phenomenon in relation to the slag compositions, it will be possible to control the slag behavior by means of

seed-slag proportioning and by temperature control.

57
6.6 Viscosity of K^SO^ Layer

In further exploration of the oxide-sulfate system representing seeded

channel slags, it was desirable to determine the viscosity of the fluid

K^SO^ liquid and the effect on this viscosity of a small amount of dissolved oxide slag. Two melts were prepared; one was pure K^SO^ (K330)

and the other was 95% K^SO^ and 5% of the oxide slag described in the

2 4

section above on immiscibility (K348) . Both melts had such low viscosities

that accurate values can not be reported. They are both estimated to be

between 0.1 and 0.3 poises. No change in viscosity could be observed

between 1318 and 1080° C for the pure K^SO, and between 1518 and 1080° C

2 4 for the melt containing the oxides. The lower temperature is only a few

degrees above the freezing temperature of the K^SO^. The presence of the

dissolved oxide had no observable effect on the viscosity of the K^SO^. ^ 2 4

6.7 Viscosity of Oxide Layer

The viscosity of the synthetic oxide slag used throughout the phase

studies (K257) is shown in Figure 35 and Figure 38. This represents the

pure oxide starting material. After melting this with K^SO^ it has been

noted that there is considerable K^O present but no appreciable "SO^"

in this layer. The K^O will have a considerable effect on the viscosity

of the oxide slag. Heywood and Womack [50] showed the effect of 15% K^O

on a five -component oxide slag. The effect of K^O additions to this oxide

slag used in the present studies (K257) will be reported in the future.

58
6.8 Instrumentation

Viscosity measurements were made throughout this study with a hightemperature adaptation of a Brookfield "Synchrolectric" viscometer unit.

Essentially, the unit rotates a cylindrical spindle or "bob" at a constant

rate and measures the torque required to maintain this rotation against

the opposing viscous drag of the test liquid into which the spindle is

immersed. See Figure 43. The torque-rpm relationship is uniquely related

to the viscosity of a Newtonian liquid (one whose viscosity is not affected

by the shear rate) . This is not an absolute measurement method; it is

calibrated with a National Bureau of Standards standard reference glass

intended for this exact purpose. The particular glass used was SRM 710,

although there are others. This was chosen because its temperatureviscosity relationships are similar to the silicate melts in these coal

slag studies.

The high temperatures encountered require that the cylindrical spindle

as well as the containing vessel are made of a refractory and inert material.

A rhodium alloy of platinum was chosen for both. The furnaces used for melting the batch materials and for viscosity determinations are heated electrically,

using silicon carbide resistance heating elements, in an open or oxidizing

atmosphere. The torque unit was mounted on special ways for maintaining

proper centering of the spindle with respect to the melt, while allowing

for coarse and fine vertical adjustment. The fine adjustment is an important

feature to insure controlled immersion depth of the spindle in the melt.

Figure 44 is a schematic diagram of this furnace-viscometer arrangement.

59
During the course of adapting the viscometer unit for high-temperature

use it was determined that the long platinum alloy spindle was too heavy

for the bearings in the torque unit. An air-bearing was constructed which

supports the weight of the spindle on a cushion of air which escapes in a

radial direction from the axis of the system. This provides support while

introducing no torque or drag to influence the viscosity determination.

The air pressure is from a 15 psi line through a reducing regulator to

provide low pressure and fine control. A schematic of this air-bearing

is shown in Figure 45,

The temperature limitation of the viscometer is that of both the

electric furnace and the platinum alloy vessel and spindle. 1600° C is

considered the maximum regular operating temperature of the system. The

temperature is determined and controlled by thermocouples, each having

one leg of a 30% Rh-70% Pt alloy and the other of a 6% RH-94% Pt alloy.

6.9 Viscosity-Composition-Temperature Calculations

The Watt-Fereday equations for calculating viscosities from composition

and temperature data are found to be inadequate for certain compositions.

Additional model slag data are needed to compute new coefficients to be

used with the general equations of Watt and Fereday. At present there

is no coefficient to be applied to the MgO concentration. An extra term

needs to be added to the W-F equations to include MgO directly rather

than by difference.

Shaw's method should be analyzed to determine if an Al^O^ factor

can be added to his equations for viscosity prediction.

60
7. Materials Testing

7.1. NBS-UTSI Program

Since Dec. 1973, NBS has engaged in a cooperative materials testing

program with the University of Tennessee Space Institute. The MHDfacility at UTSI is relatively small; however, it has the distinction

of being the only coal-fired MHD-generator in the world. The walls

of the channel consist of segmented copper electrodes that are watercooled; the entrance of the duct has a cross section of 2" x 4".

Copper sample holders were fabricated that could be inserted in cutout sections of the electrodes. Samples of test materials can be fitted

into these sample holders; a thin coat of plasma sprayed alumina was

applied between the holder and the specimens. Holes were drilled in

some of the samples and thermocouples fitted into these holes (1 mm from

top surface) (see Figures 46 and 47)

Four different materials were tested in 2 runs. In the first test

(March 1974) clean fuel (kerosene) was used and in the second coal

(April 1974); both tests lasted 12 minutes. During the March run the

thermocouples indicated temperatures of 900 - 1000°C; in the April test

the temperature was much lower C^^SOO - 700°C) , due to the coal-slag layer.

The four materials were:

1. Mo-metal - UTSI, m.pt. 2610°C.

2. La Ca CrO (x = 1%) - Nippon Kagaku Togyo Co., Osaka, Japan,

m.pT^25§0°c:

3. ZrB^ (+ 20% SiC) - ("Boride V") - Manlabs - m.pt. >2664''C.

4. Sic (hot pressed) - ("Noralide") - Norton Co. - m.pt. 2450''C.

61
Data on these materials were assembled (before the test) from the

literature and from some additional measurements. After the test runs

the samples were examined by means of visual observation under the microscope and by x-ray diffraction. The results of these examinations can

be summarized as follows

1. Mo metal Run #1: strong oxidation (MoO^?)

Run #2: Mo-metal plus K^SO^.

2. La, Ca CrO^ (x=l%) Run #1: essentially unchanged

1—x X 3 Run #2: drastic change, none of the original

material left, no K^SO^.

3. ^rB^ Run #1: strong oxidation to ZrO^.

Run #2: little change, K^SO^ present, no ZrO^

4. Sic Run #1: no change

Run #2: little change, K^SO^ present.

A chunk of the coal slag layer was examined also. The top half

millimeter was entirely glassy and hence the surface temperature must

have been as high as 1300°C and possibly 1500°C (depending on the K

content) . Deeper into the slag layer the materials appeared to be

strongly crystalline; the stronger x-ray diffraction lines correspond

to orthorhombic KAlSiO^.

4

Near the coal wall of the channel the K-concentration was very high.

It seems that the seed condenses first before the slag layer is formed.

Delayed seed injection can probably reverse this sequence.

7.2. US-USSR Program

As part of its regular work on MHD materials NBS has been participating

in the planning and related activites connected with the materials testing

program now being set up under the joint US-USSR Cooperative Program on

62
MHD. The materials testing effort involves reciprocal tests in both

the USSR and the United States MHD facilities. These tests are designed

to provide information relative to the durability and reliability of

high temperature MHD components, particularly electrodes and insulators,

under controlled MHD conditions. Emphasis will be placed on the effect

the design of the component has on the materials performance. Further

advanced in planning is the sub-program utilizing the USSR U-02 MHD

unit. Phase 1 of this effort consists of a lOOh test of a series of

ZrO^ base (with CeO^ or Y^O^ additions) solid electrodes under clean

fuel and hot wall conditions. Actual testing is scheduled for the

first quarter of 1975 (CY) .

NBS will continue to aid in the planning and coordination of this

joint program and in addition, participate in the conduction of various

tests and in the characterization of pre- test and post- test materials.

63
8. Publications and Talks

The information and data summarized in the preceeding sections are

given in greater detail in quarterly reports regularly submitted to the

Office of Coal Research. In addition to these, however, significant

aspects of the work are often published in professional journals and

presented orally at various technical meetings. These are tabulated in

the following list:

1. H.P.R. Frederikse* and W.R. Hosier, "Electrical Conductivity of

Coal Slag", Proceedings of 13th Symposium on Engineering Aspects

of Magnetohydrodynamics , Stanford University, March 26-28, 1973.

2. E.M. Levin*, S.J. Schneider and E.R. Plante, "Phase Equilibria

Involving Seed Materials in MHD", Proceedings of 13th Symposium

on Engineering Aspects of Magnetohydrodynamics, Stanford University,

March 26-28, 1973.

3. S.J. Schneider and E.M. Levin, "Polymorphism of K CO " , J. Am.

Ceram. Soc. 56 [4] 218-219 (1973)

4. E.M. Levin, "The System K SO -Cs SO " , J. Am. Ceram. Soc. 56 [8]

427-430 (1973)

5. J.B. Wachtman and S.J. Schneider, "Measurements and Standards of

High Temperature Materials in Energy Conversion and Clean Fuel

Production", Standardization News , STDNA ]^ [8] 16-23 (1973).

6. H.P.R. Frederikse*, "Materials for MHD Power Generation", Wayne

State University, Detroit, Michigan, May 16, 1973.

7. S.J. Schneider*, W. Capps , H.P.R. Frederikse, W.R. Hosier, C.L.

McDaniel, and E.R. Plante, "High Temperature MHD Materials", US-USSR

Colloqium on MHD Powder Generation, Moscow, Russia, Feb. 25-27, 1974.

8. C.L. McDaniel* and E.R. Plante, "Phase Relations Involving SeedElectrode-Insulator Materials in MHD", 14th Symposium on Engineering

Aspects of Magnetohydrodynamics, Univ. Tennessee Space Institute,

April 8-10, 1974.

9. W.R. Hosier, W. Capps and E.R. Plante*, "Some Physical and Chemical

Properties of Coal Slags", 14th Symposium on Engineering Aspects of

Magnetohydrodynamics, Univ. Tennessee Space Institute, April 8-10, 1974.

64
10. H.P.R. Frederikse* and W.R. Hosier, "Electrical Conductivity of MHD"

14th Symposium on Engeering Aspects of Magnetohydrodynamics , Univ.

Tennessee Space Institute, April 8-10, 1974.

11. L.H. Grabner*, W.R. Hosier and H.P.R. Frederikse, "Some Optical and

Electrical Properties of Undoped and Sr-Doped LaCrO^", 14th Symposium

on Engineering Aspects of Magnetohydrodynamics, Univ. Tennessee Space

Institute, April 8-10, 1974.

12. G.A. Candela*, R.A. Forman, A.H. Kahn, D.B. Meadowcroft, and J. Wimmer,

"Magnetic Susceptibility of Lanthanum Chromite Doped with Strontium",

14th Symposium on Engineering Aspects of Magnetohydrodynamics, Univ.

Tennessee Space Institute, April 8-10, 1974.

13. S.J. Schneider*, "Energy Conservation Through Improved Energy Conversion", Annual meeting-American Ceramic Society, Chicago, Illinois,

April 28 - May 1, 1974.

14. W. Capps*, "Viscosity of Some Molten Coal Slags", Annual Meeting - American Ceramic Society, Chicago, Illinois, April 28 - May 1, 1974.

15. H.P.R. Frederikse*, "Materials Requirements for MHD", ASM Forum on

on Materials for Power Generation, Seven Springs, Pa., June 17-19, 1974.

* Denotes talk by indicated author.

65
9. References

1. W, A. Selvig and F. H. Gibson, Bull. 567, Bureau Mines 33 pp (1456).

2. NBS-ICTA Standard Reference Material 759, DTA Temperature Standards

(295-675 °C) ; Ibid , 760 (570-940°C)

3. E. M. Levin and F. A. Mauer, J. Am. Ceram. Soc. 46 [1] 59-60 (1963).

4. S. Makarov and M. P. Shugina, Bull. Acad. Sci . USSR 5_, 691-700 (1940).

5. A. Reisman, J. Am. Chem. Soc. 80_, 3558-61 (1958).

6. A. Reisman, J. Am. Chem. Soc. 81_, 807-811 (1959).

7. S. J. Schneider and E. M. Levin, J. Am. Ceram. Soc. 5£, 218-219 (1973)

8. P. M. de Wolff, Techn. Phys . Dienst, Delft, Holland. PDF Card

No. 16-820, Powder Data File Issued by Joint Committee on Powder

Diffraction Standards, Swarthmore, Pa. 19081.

9. A. J. Majumdar and R. Roy, J. Phys. Chem. 69_, 1684-86 (1965).

10. M. Bernard and J. Jaffray, Compt. Rend. 240, 1078 (1955).

11. C.W.F.T. Pistorius and E. Rapoport , J. Phys. Chem. Solids 30 , 195-201 (1969).

12. H. E. Swanson, R. K. Fuyat, and G. M. Ugrinic, NBS Circular 539,

3_, 63 (1954); PDF Card No. 5-613.

13. G. Pannetier and M. Gaultier, Bull. Soc. Chim. Fr. 1966 , 188-194

(1966); PDF Card No. 18-1060.

14. L. Denielou, Y. Fournier, J. -P. Petitet, and C. Tequi , C. R. Acad.

Sci., Paris 270 [8] 1854-56 (1970).

15. L. Erdey, G. Liptay, and G. Sol, Talanta 12_ [3] 256-67 (1965).

16. V. E. Plyushchev, R. G. Samuseva, and I. F. Poletaev, Zhur. Neorg.

Khim. 7_ [4] 860 (1962); Russ. J. Inorg. Chem. (English Transl.) 445

(1962) ; Also, Fig. 2886 in Phase Diagrams for Ceramists, E. M. Levin,

C. R. Robbins, and H. F. McMurdie, The American Ceramic Soc. Inc.,

Columbus, Ohio (1969).

17. D. Tabrizi, M. Gaultier, and G. Pannetier, Bull. Soc. Chim. Fr.

1968 [3] 935-39 (1968)

18. (a) H. Muller, Kristallografiya 53_, 511 (1914).

(b) V. E. Plyushchev and N. F. Markovskaya, Dokl . Akad. Nauk

USSR 95, 555 (1954)

66
19. K. H. Stern and E. L. Weise, High Temperature Properties and

Decomposition of Inorganic Salts, Part I. Sulfates, National

Standard Reference Data Series - National Bureau of Standards 7,

39 pp (1966)

.

20. K. J. Rao and C. N. R. Rao, J. Matls. Sci. 1, 238-48 (1966).

21. H. E. Swanson, N. T. Gilfrich and M. I. Cook, NBS Circular 539 , Vol. 7, p. 17 (1957); PDF Card No. 8-462.

22. O. S. Dombrovskaya , Zhur. Obschchei Khim. , 3^ f^] 1017 (1933);

Also Fig. 2884 in Phase Diagrams for Ceramists, E. M. Levin, C. R.

Pjobbins , and H. F. McMurdie, The American Ceramic Soc. Inc.,

Coliambus, Ohio (1969).

23. E. M. Levin, J. T. Benedict, J. P. Sciarello, and S. Monsour,

J. Am. Ceram. Soc. 56^ [8] 427-430 (1973) .

24. G. G. Diogenov and V. I. Ermachkov, Zhur. Neorg. Khim. LI [12]

2825 (1966); Russ. J. Inorg. Chem. (English Transl.) 1519-21 (1966).

25. R. W. G. Wyckoff, Crystal Structures, Vol. 3, 2nd ed; p. 113,

John Wiley & Sons, Inc., New York (1965).

26. H. LeChatlier, Compt. Rendu 118 , 350 (1894).

27. M. Amadori, Atti della Reale Accad. Dei Lincei [5] 2]^, II 65 (1912).

28. JANAF Therm.ochemical Tables, the Dow Chemical Co., Midland, Michigan

updated through June 30, 1973.

29. P. J. Ficalora, 0. M. Uy, D. W. Muenow and J. L. Margrave, J. Am.

Ceram. Soc. 51, 574 (1968)

.

30. W. D. Halstead, Trans. Farad. Soc. 66, 1966 (1970).

31. D. Cubicciotti, High Temp. Sci. 3_, 349 (1971).

32. P. M. de Wolff, Techn. Phys. Dienst , Delft, Holland, PDF Card No.

11-655, Powder Data File issued by Joint Committee on Powder Diffraction Standards, Swarthmore, Pa. 19081.

33. V. N. Pavlikov, A. V. Shevchenko, L. M. Lopato , and S. G. Trevyatskii,

Chem. of High Temperature Materials, 57 (1969).

34. J. S. Ruiz, A. M. Anthony, and M. Foex, C. R. Acad. Sc. Paris 264 , 1271-4 (1967)

.

35. S. Geller and P. M. Raccah, Phys. Rev. B2, 1167-71 (1970).

36. F. C. Kracek, N. L. Bown , and G. W. Morey, J. Phys. Chem. £1 [9]

1183 (1937).

67
37. D. B. Meadowcroft in "Open-Cycle MHD-Power Generation, Ed. by

J. B. Heywood and G. J. Womack, Publ. by Pergamon Press, Oxford,

England 1969, Chapter 7.

38. D. W. Strickler and W. G. Carlson, J. Am. Ceram. Soc, 122 (1964).

39. H. J. Kandiner and S. R. Brinkley, Jr., Ind. and Eng. Chem. , A2_, 850

(1950).

40. JANAF Thermochemical Tables, The Dow Chemical Company, Midland,

Michigan.

41. S. Gordon and B. McBride, NASA SP-273 (1971),

42. P. J. Ficalora, 0. M. Uy, D. W. Muenow, and J. L. Margrave, J. Am.

Ceram. Soc, 5^, 574 (1968).

43. W. D. Halstead, Trans. Farad. Soc, 66, 1966 (1970),

44. D, Cubicciotti, High Temp. Sci., 3_, 349 (1971).

45. Eo M, Levin, S. J. Schneider, and E. R. Plante, 13th Symp. on Eng.

Aspects of MHD, Stanford Univ., IV 5, March (1973).

46. C. L, McDaniel and E. R. Plante 14th Symp. on Eng. Aspects of MHD,

Tullahoma, Tenn . , April (1974).

47. L. Brewer and G. M. Rosenblatt, in "Advances in High Temperature

Chemistry," Ed., L. Eyring, Vol. 2, Academic Press, New York (1969)

48. J. L. Schick, Ed., "Thermodynamics of Certain Refractory Compounds,"

Vol. II, Academic Press, New York (1966).

49. K. K. Kelley, U. S. Bur. Mines Bull. 584 (1960).

50. D. D. Jackson, UCRL-51137 (1971),

51. H. C. Graham and H. H. Davis, J. Am, Ceram, Soc, 5^, 89 (1971).

52. G. R. Belton and Y. W. Kim, Quoted by [51].

53. E. M. Bulewicz and P. J. Padley, Proc. Roy. Soc, London, A323 , 337

(1971)

54. J. Drowart, A. Pattoret, and S. Smoes, Proc. Brit. Cer. Soc, No. 8,

67 (1967)

55. R, T, Grimley, Mass Spectrometry, in "The Characterization of High

Temperature Vapors," J, L, Margrave, Ed,, John Wiley & Sons, Inc.,

New York (1967)

68
56. W, A. Selvig and F. H. Gibson, U.S. Bur. Mines Bull. 567 (1956).

57. J. S. Machin and D. L. Hanna, J. Am. Ceram. Soc. , 28./ 310 (1945).

58. J. S. Machin and T. B. Yee, J. Am. Ceram. Soc, 3_1' 200 (1948).

59. J. S. Machin, T. B. Yee and D. L. Hanna, J. Am. Ceram. Soc,

35, 322 (1952)

.

60. J. B. Heywood and G. J. Womack, "Open-Cycle MHD Power Generation,"

Pergamon Press (1969)

.

61. J. F. Elliott, M. Gleiser and V. Ramakrishna, "Thermochemistry

for Steelmaking, Thermodynamic and Transport Properties," Vol. II,

Addison-Wesley Publishing Co. (1963).

62. J. D. Watt and F. Fereday, J. Inst, of Fuel, 338 , 99 (1969).

63. Y. Bottinga and D. F. Weill, Amer. J. of Sci, 272 , 438 (1972).

64. H. R. Shaw, Amer. J. of Sci., 272 , 870 (1972).

65. A. Napolitano and E. G. Hawkins, J. Res. NBS, 68A (Phys. and Chem.),

No. 5, 439 (1964)

.

69
CO

96.0

94.0

92.0

90.01-

8ao

2.50

I

2.40

^2.30

- )9(deg.)

CO

2.00

i

.1111 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1. L J i... 1 i 1 1„.-L-J L 1 1 1 i 1_

100 200 300 400 500

TEMPERATURE,

600 700 800

Figure 1. Unit cell dimensions and densities of monoclinic and hexagonal K^CO^.

70
71
-^a^ oj cr> c\j oo co ^ en r-:- lo i—; co lo ^^a^ ro 00

ci> <cf> CO GO oo I

' lo lo lo 00 oJ oJ cvj oJ oJ

mmm 1139 iiNn
T—

I—I—rn—I—I—I

I I I I\ 1 rn r~i i i ' ^ ' r *n rn \ i \ i s

<o o o o o c:> o cs o c:> o o o ci> ci> cu cz> o czs cd en cNj c2> oo CO "^r cvj lo r-o —- ro 00 —; ct) 00 r—7 co lo —I. cui cri cyi 00 oo 00 00 co co co "=3- «^J- t-O ro ro ro r-O

N0ISN3Wia 11]9 lINn r^o/b 'AilSNHa

73
600

: . . ,^50. 20 ^0 60 80 ^5^504

Mol.%

• System C-.SO, K;S<

Figure 5. System K^SO^-Cs^SO^ after Dombrovskaya [22]

74
00

1004 ±5°

1000

900

800

ooUJ

ir

Z)

h-

<

tr

LjJ

Ixl

700

6 00

940"

HEXAGONAL SOLID SOLUTION

718 ±5°

500

400

3 00

430 ±6-=

/ Ortho.

CS2SO4SS

Hex. ss I

Ortho. K2SO4SS+ Ortho. CS2SO4SS

To~5%at 26° To -90% at 26°

J L K2SO4 10 20 30 40 50 60 70 80 90 CS2SO4

COMPOSITION, Mol %

Figure 6. System K^SO^-Cs^SO^ after Levin, et al [23].

i

77
3.80

K2SO4 10 20 30 40 50 60 70 80 90 CS2SO4

. _ COMPOSITION, Mo! %

Figure 9. X-ray density vs composition for the hexagonal solid solution phase

in the system K^SO^-Cs^SO^

78
Figure 10. Phase diagram of the system K CO -K SO

79
3000

I069j

1000 -

o—

500 -

2850

LIQUID + MgO

1067° *

HEX. KgSO^ + MgO

588'

ORTHO. Kg SO^ + MgO

0

K2SO4

20 40 60

COMPOSITION, Mol%

80 100

MgO

Figure 11. Condensed state phase diagram of the K^SO^-MgO system.

• - quenching o - DTA 80
UJ I—-

CO

/

T(/) (/) O

r>0

O

O

O

CO

CVJ

o

CO Q

1

O

CVJ

o

CO O CM

<

q:

o

CD LU

O

O

00

o

C30 CD OJ • • • • ^OS^>l AIIAIIOV

O

u

oO

O

P

(0g

0) +Jw

>1

CO o

CP s

I o

w

CN

J5

+J

op

o

>1

+Jo

m

o

•Hp

(0e

0)o

•H

81
3500

3000 -

2500

o

°

' 2027

S 2000

<

§ 1697

LU

»- 1500

1437

1069

1000

500

1 1 1 1 1 1 1

1 VAPOR

•

1

/

1

I

11

1

1

1

1 VAPOR + LIQUID

-1

-

1

1

1

1

r

J

i VArUK + iVigU f

1

1

1

1

1

J 2067^(1 Qtm)

"~/^(~377o)

1 -"1

1

1

1

1 LIQUID + MgO

^

1709'' (0.1 atm)

'l57o)

J /

Y(~5%)

I44r(0.0latm)

L

1

1

1067*

1

1

HEX.K2S04+MgO

1

1

1

1

1

1

ORTHO. K2S04 + MgO

1 1 1 1 1 1

1

1

B.R

-3600

M.R

-2850

0 20 40 60

K2SO4 COMPOSITION, Mol %

80 100

MgO

Figure 13. Phase diagram of the system K^SO^-MgO for 0.01, 0.1, and 1 atm total

pressure K^SO^ species. (1 atm = 101,325 N/m^ , N/m2 = 1 pascal)

82
Figure 14. Phase diagram of the system K^CO^-MgO in a closed system.

83
3500

3000

2 500

2000

1834'

1519' 1500

1285'

1000

M.P.902''

"1 T r

VAPOR

VAPOR + LIQUID

VAPOR + HgO

VAPOR + LIQUID 1945' (latm)

49%)

1300* (0.01 aim)

LIOUID + HgO

LIO f •4

-4

1

(-2%)

HEX. KzCOj + MgO

500 ± L

BR

-3600°

H.P -2850'

40 60

MOL. %

MgO

Figure 15. Phase diagram of the system K^CO^-MgO for 0.01, 0.1, and 1 atm total

pressure K^CO^ species, (i atm = 101,325 N/m2 , N/m^ = 1 pascal

84
3000

2500

2000

oo

LlT

Z)

< 1500

ir

UJ

Q_

LU

1069

1000

LIQUID 2500° ^

LIQUID + LaCr03

/

/

/

/

1063'

o o —oHEX. K2SO4+ RHOMB. LaCr03

588°

500

ORTHO. K2SO4+ RHOMB. LaCrOj

260°

ORTHO. K2SO4+ ORTHO. LaCrOs

0

0

K2SO4

Figure 16.

20 40 60

COMPOSITION, Mol %

80 100

LaCr03

Phase diagram of the system K^SO^-LaCrO^ in a closed system,

closed system - specimen containers were sealed Pt tubes

• - quenching o - DTA

85
X

METAL ELE.VEM5

Figure 17. Schematic of MHD-duct wall. (from: R. J. Rosa, 5th International

Conference on MHD-Power Generation, Munich, 1971, Vol. I. - Public, de I'O.C.D.E., Paris, France.)

86
I o

I E

>

o

Q

icr'

1667 K 1538 1426 1334 1250 1176 Mil

10

-2

8

10-3

10r4

OXYGEN PRESSURE

(ATMOS.)

oo 1.0

02

X X 1.0 X I0"2

4.0 X 10"^

6.0X I0~^

Figure 18. Conductivity as a function of reciprocal temperature for slag

(sample MHD-12)

87
o

-p

0)

+jc

>1w

o

B

ftJ O

O

a^ H

O

(U

O

c

oH

-pu

G

3

(M m

-P H

P M

U ^

3

TJ Cn C rtf O M

U to

•H

I _(UJ0-UJM0) AllAllOnaNOO

88
o^^ .<^^ #

ure 20. Electrical conductivity of base oxide slag sample (K 257) and of

slag-seed mixture (base oxide with 20% K SO ; sample K 266) at

two oxygen pressures.

89
CO

0

-3

10^

1000

eO'

en

ole 7c

Zr02 : 50 mole %

Ce02 IO"^Gtrr.

Oxyger.

Zr02:

12 mole %

r203

1200 1400 1600

TEMPERATURE, ^'K

1800

Figure 21. Resistivity as a function of temperature for the

ceramics of most interest as electrode materials,

(from ref . [37] )

90
0 I 2 34 5678 9 10 (I 12 13

10^

Figure 22. Conductivity versus 1/T of crystalline doped LaCrO , La Sr

CrO^ and L^2.-x'^^x^^°3 l^^^O- 0^ • Note donor exhaustion

and donor reserve regions.

91
10-';=

1 e

>-

tier-

§10-'

o

u

d

10-*

.0/

10r6

o I ATMOS. OXYGEN

•0.2 " " (AIR) - X 001

HELIUM

52 5.4 56 58 60 62 6.4 66 68

10^

TfK)

70

Figure 23. D.C. conductivity of SrZrO -14 CeO_ (mol %) at different oxygen

pressures and in helium

92
10-6 I 1 1 1 1 1 1 1 1 1 52 5.4 5.6 58 6.0 6.2 6.4 <o6 6.8 70

lO'^

T(K)

Figure 24. D.C. conductivity of SrZr03-6 SrTiO -1.5 Ta O (mol %) at

different oxygen pressures and in helium. ^

^

93
5 6 7 8 9 10 II 12

10^

W)

Figure 25. Conductivity of channel coating (Zr02 + MgO) to 1750 K at 0.2

atm. and 0.01 atm. oxygen.

94
1200 1400 1600 1800 2000 2200 2400 2600

TEMPERATURE, K

Fig. 26: Calculated Partial Pressures of

Gases and Activities of K2CO3 at 1 atm

for the Combustion of CHq -7 Seeded with

1 wt % K as K2CO3.

95
1200 1400 1600 1800 2000 2200 2400 2600

TEMPERATURE/K

Fig. 27: Calculated Partial Pressures of Gases

and Activities of K2SO4 at I atm for the Combustion of CHq_7 Seeded with 1 wt % K as K2SO4.
I

1200 1600 2000 2400 2800 3200

TEMPERATURE, K

Fig. 28: Stoichiometric Combustion of

CHq ^ with 80/20 mole % N2/O2 Seeded with

1 wt ^as K2S0^ at 1 atm Total Pressure.

97
1800 2000 2200 2400 2600 2800 3000

TEMPERATURE (KELVINS)

Fig. 29: Saturation Pressures of Magnesiiom Bearing Species in an MHD Environment

at a Total Pressure of 1 atm.

(Solid MgO Present)

.

98
1800 2000 2200 2400 2600 2800 3000

TEMPERATURE (KELVINS)

Fig. 30: 'Saturation Pressures of Magnesium Bearing Species in an MHD Environment

at a Total Pressure of 5 atm.

(Solid MgO Present)

99

li

i

^
_—I— I I I — I

1— r E ^2°

1800 2000 2200 2400 2600 2800 3000

TEMPERATURE (KELVINS)

Fig. 31: Saturation Pressures of chromium Bearing Species in an MHD Environment

at a Total Pressure of 1 atm.

(Solid Cr O Present)

.

100
J 1800 2000 2200 2400 2600 2800 3000

TEMPERATURE (KELVINS)

Fig. 32: Saturation Pressures of Chromium Bearing Species in an MHD Environment

at a Total Pressure of 5 atm.

(Solid Cr 0 Present)

^ ^ 101
10000

A SiO"^

o 0'

on<

5 «000

a:

<

+

H (2000K)

100

0 50 100 150

TIME(min)

200 250

Fig. 33: Ion Currents Versus Time for

Isothermal Evaporation of Slag at 2000

and 2130 K.

lo:
2100

TEMPERATURE, K

2000 1900 1800 1700

A 2

t

^'+""1 PURE Si 02 -

^ SiO

o 02'*'

Fe +

+ ^ ABOVE SLAG

10000

a:

<

crm

q:

<

+

H

1000

28

20

B

4

4

4

A

4

4

100

4.8 5.0 5.2 5.4 5.6

IO^/T( K"')

Fig. 34: I"''t Versus for Evaporation of Slag and Pure Si02.

5.8

1C3
in

CN

o

o

o

o

CO

Cm

CO

h42

Q

Q

W

o

Ui

w

Q

M

X

o

CO oCO

oo o

o

H

oj o o _ O CN CO O

•H .H (U 60

CO <: fn s

o

o

ow

w

(X,

//

^rJir Vat

o

•rl 4J

03 60 N d

•H -H

i-( to rH 03

C3 <U

CO 60O

U U

O P. O

oCO

CO CM

(sasTOci °"^6oi) AlISODSIA 001

104
3.0

2.8

2.6

2.4

2.2

2.0

1.8 U

1.6

1.4

1.2 L

1 ' 1 ^

1.0

0. T

0.8

0.1

0.6

Legend

-1- K302 60

^^2"3'

15

^^3'^4

5 20

0 K304 60 20 5, X

<27 K308 65 15 5 15

© K311 60 15 10 15

A K31j -0 20 10 i:;

X K320 65 15 lu 10

• 1 •

1 127: -350 1400 i 4;jU 15 00 15 5 0 16 OO

Temperature

Figure 36. Viscosit.y "omperature Rel^itionsh ips tor

Some Mo lei C'o^il Slags

105
J I I L

JL

J I I I I I L oo

U")

.4->

ft)

(D

'.0

<coo

(/) a,H

CO

Cl, u: e

0) Eh

iiw

^1ou

cn m

CP

fC 3 0)

tj> +-!O 0) T) ^> 0) W M M j:: 3 O f

(U fO 0) 6 rH -IJ O 15

0) O) - l M (0 3

C5

U '1

:i

X i! o 0) > • <

fC :3 a) Cm U,I ai 4-1 O

-^

U

0) 4J O M

(T5

O 1J

-1 in

Uo

4-1 C

-r40

Oi (/) 4-' TJ

T)

C -H

4-J -H 'H H O O Q CIi CO

l06
I I I I I I ' ' ' ' 1 JL_I I L

4-> m oo > :3U) >

to Sh <u

m rH e o

to> 4-1 •v O (\) -p

0) to to

Jh c iH < 0 o W -H o u nS 4J iH

dj fO to E iH o o o

>, o m to o ft u H -p 0) rH K x; u

COO OJ 0)

u g; Cm O <u u 1 H 0) -P

0) Eh -P < o to C rH S:

4-> OJ t< 0 U u M

dJ (U fO § o Sh D o to U)

0) Eh Q) o u C o H

LTl

W

p o .H

ft (0 o

TJ

U) rH •H

4J rH 1 c -P 0

>^ -H to U)

Eh o M U) LOO m c to u -U -H -p w nj 0 to H Q >

COn

(1)

•H o

(sasTod '^"^box) Aq.T:soosTA £ot:

107
J_l 1 I I I J

L

4J

(0 4J 'O

03 Q T(

(U • in [fi 0) (T3 (U rH e

CO • O > 14-1 in 0 >

CO

Q

u

S-l c

13 0 U

<, U)

H

O (fl -P T3 u 0) (0 0) e rH +J 0 05 o 0) a,

03 ;3 CO 0! u U

a, -p rH M X 03 , —.

:i:

O <u

u

CJ CO

<U !h >1 o 0) 03

G)

n

un

u (D

d < c r-H !h -p 0)

03 C^ U u 1 CD P o U +J E a o rd 03 Q) D VP s

r-t

U

0)

4-J

O !H

w ^13 03 s U) rH in M +J 0) Eh c +J C 1 •H OS •H

>i 0 rH

in

H -P TS

CO

mH

O -p

H rH

u fO 0 0

UiQ Oi CO

M>m

•H

(sasTod ^^ooj) X:;tsoosta dot

108
1300

Figure 40,

1400 1500

Temperature (°C)

1600 1700

VISCOSITY-TEr'-LPERATURE RELATIONSHIPS OF COAL SLAGS

Data points are r>easured values. Solid lines are Watt-Fereday calculated curves.

109
T T T

K396 \

\

\

\

K396

! ! I 1300 1400 1500 1600 1700

y Temperature (°C)

Figure 41. VISCOS ITV-TEMPEPATURE RELP.TIONSKIPS OF COAL SLAGS

Data points are rr.easured values. Solid lines are

.^ • Watt-Fereday calculated curves.

110
1300 1400 1500 1600

Temperature (°C)

Figure 42. VISC0SITY-TE;'PER-^.TUP£ P£]1hTI0::SHIPS OF COAL SLAGS

1700

Data points are measured values

Watt-Fereday calculated curves. Solid lines are

111
Torque Indicator

Radiation Shield

Water Cooled Table

Platinum Alloy Support Rod

Test Liquid

Platinum Alloy Spindle

Platinum Alloy ^Vessel

Furnace

B'igure 43. High Temperature Adaptation

of Brookfield Viscometer

112
113
air out

air xn

Top View of i lenum

air holes

air in

to bob

to viscometer

disc

plenum

Side View of

Disc and Plenum

Figure 45. Schematic Diagram of Air-Bearing for Supporting

Viscom.eter Spindle

114
Figure 46. Position of test samples in channel

Figure 47. Sample holder with test samples

115
Table 1. Nominal Slag Chemical Compositions

UTSI Bu Mines

channel slag (probe deposit)

wt. % wt. %

SiO^ -49 43

AI2O3 27 19

Fe^O^ . 19* 30**

RO (CaO & MgO) 5 8

2+ 3+

* Fe /Fe^ =2:1

** Fe^'^/Fe^^ =4:1

100 100

116
Table 2. Experimental Data for Compositions in the K^SO^-LaCrO

System in Air

Composition Heat Treatment

LaCrO,

mole%

95

mole%

5

90 10

75 25

Temp.

900

1000

1000

1060

1100

1200

1300

1500^

1700^

900

1000

1100

1200

1300

1500^

1700^

900

1000

1100

1200

1300

1400

1700

Time

hr

21

18

66

21

24

66

22

24

21

18

24

66

18

2

4

21

18

24

66

17

18

4

X-ray Diffraction Analyses

K^SO^ + LaCrO,

2.. 4 ..3

+ LaCrO^ +

+ LaCrO^ + La 0 + LaCrO

+ La 0

^^2°3 +

+ LaCrO

II K^SO + LaCrO^ + X

LaCr63 + K SO^ + La203

La 0 + LaCrO

K §0 + La O

La^O^ + K^SO,

2 3 2 4

K SO + LaCrO

LaCrO^ + K^SO^

3 2 4

LaCrO^ + K^SO^ + La^O^

^ „ ^3 ^2 4 2 3 LaCrO + La 0

50 50 900

950

1000

1050

1100

1200

1300

1400

1500

1700^

21

20

18

120

24

66

22

18

3

4

LaCrO^ + K^SO^

..3 2 4

LaCrO^ + K^SO^ + La^O^

LaCrO + La 0

II 11^

117
25 75

10 90

900 21 LaCrO + K SO

1000 18 II ft 1100 24 II 11 1200 66 LaCrO^ + K^SO

1300 22 LaCrO + La 0

1400 18 II II 1500^ 2 LaCrO^ + La^O

1700 3 LaCrO^ + La^O

900 21 LaCrO + K SO

1000 18 II ft 1100 24 II 11 1200 66 LaCrO + La 0

1300 18 II 11^ 1500 67 ti II

All specimens were heat treated at 800°C a minimiam 18 hrs unless

otherwise indicated.

The phases identified are given in order of the relative amount

present at room temperature.

Specimen had no previous heat treatment.

Unidentified phase.

118
Table 3. Experimental Data for Compositions in the K^CO^-LaCrO

System in Air

Composition

K2CO3 LaCrO.

Heat Treatment

Temp

Time

X-ray Diffraction Analyses'

mole%

75

mole%

25 700

800

900

900

1000

1100

1200

1300

1400

1500

1700

hr

23

21

20

22

22

23

42

67

18

3

2

La 0 + K CO + K CrO

K CO + La 0 + K CrO + b

X

K^CrO^ + y + La^O.

La^O^ "+ K CrO + + y~

La O + Pt + K CrO + LaCrO

LaCrO + Pt + La 0

50 50 700

700

800

900

1000

1100

1200

1300

1400

1500

1700

20

23

21

20

22

23

42

92

20

3

2

K CrO + La O + x

II ^ u ^ II La^03 + YrO^

^f2°3

+ LaCrO,

+ LaCrO"^

^ + LaCrO^ + y

+ LaCrO,

+ K^CrO^

25 75 700

800

900

1000

1100

1200

1300

1400

1500

1700

23

21

20

22

23

42

67

20

34

LaCrO,

LaCrO,

LaCrO;:

LaCrO:

LaCrO.

+ La^CrO^ + K^CrO^

2 „ 6 2 „ 4

b b

+ K^CrO^ + X + y 2k 4 + x^ + K CrO

The phases identified are given in order of the relative amount present

at room temperature.

Unidentified phases

119
Table 4

Compositions of a real coal slag (MHD-11) and some synthetic slags which are used for comparison of

measured with calculated viscosities.

Melt SiO^ ^^2°3 ^^3°4 ^2° ^^2°

MHD-11* ;

, 55 25 12 2 4

K257** 43 19 30 0 8

K352 45 20 15 15 5

K360 50 25 12 8 5 K363 50 30 10 6 4

K384 - 40 ' 20. 15 17 8

K385 " 40 15 . 20 17 8

K386 70 24 3 2 1

K387 \ 60 32 4 2 2

K389 30 30 20 14 6

K391 40 10 5 30 15

K358 45 30 10 10 5

K388 . 40 30 25 3 2

K390 30 15 15 30 10

K392 60 20 17 2 1

K394 63 13.5 10.8 1.8

k396 20 10 20 30 20

K398 50 20 27 2 1

Nominal composition of a real Bu Mines combustor coal slag.

Synthetic melt representing oxide portion of two-phase

Bu. Mines seeded, channel flyash (MHD-12F)

.

120
^able 5

Chemical Analyses of Some Simulated Seeded Coal Slags

(weight %)

Melt SiO^ ^^2^3 ^^3°4 ^2° ^°

K264U 0.03 0.03 0.051 0.43 54.00 45.90

K264L 34.06 16.03 22.04 5.30 18.80 1.39

K265U 0.01 0.02 0.041 0.36 54.00 45.70

K265L 32.39 13.98 21.94 10.68 18.20 2.47

K266 37.83 16.20 24.46 7.49 12.38 0.00

Notes: 1. Before melting, K264 contained 20% oxides and 80% K^SO^.

K265 contained 50% oxides and 50% K^SO^. K266 contained

80% oxides and 20% K^SO^.

2 4

2. Melt designation U means upper layer after separation;

L means lower layer after separation.

3. Melts were soaked at 1500° C without stirring to allow

equilibrium to be reached.

121
i.1
BIBLIOGRAPHIC DATA l- R^po" No. 2, SHEET NBSIR 74-543

3. Recipient's Accession No.

4. Title and Subtitle HIGH TEMPERATURi; MHE MATERIALS

5. Report Date August 1974

6. Performing Organization Code

7. Author(s) S. J. Schneider, W. Capps, H.P.R. Frederikse, W.R. Hosle

D.A. Kauffman, E.M. Levin, C.L. McDaniel, T. Negas and E.R. PI

r6, Performing Organization Rept.

" No ante

9. Performing Organization Name and Address Ms +" "i ("in ;^ 1 Rnyf^;^ii of" *^f~^nH^vHG

Washington, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No. OCR 14- 32-L(.(;l- 1230

12. Sponsoring Agency Name and Address Efepartment of the Interior

WJ-J-XV^C \J ^ dJ- X\C^ o C dX v_ 1

2100 M Street, N.W.

Washington, D.C. 20240

13. Type of Report & Period Covered Interim Report

14. Sponsoring Agency Code

15. Supplementary Notes Project Manager: Dr. William Jackson

16. Abstracts Under the auspices of the Office of Coal Research the National Bureau of Standards

has underway a program of materials research appropriate to magnetohydrodynamics

(MHD) . The overall objective of the work is to provide materials property

information necessary for the design, construction, and operation of an open

cycle, coal fired MHD electric generation plant. The program consists of several

interrelated investigations in the areas of phase equilibria, electrical properties,

vaporization, viscosity and materials testing. Initial work has centered on the

behavior of coal slag but also encompasses the physical and chemical characteristics

of other MHD process contaminants (alkali seed) and important electrode and insulator

materials. The report summarizes the important technical results obtained during the

period July 1, 1972 - June 30, 1974 under the combined NBS-OCR program.

17. Key Words and Document Analysis. 17o. Descriptors

Coal slag; electrical conductivity (MHD); electrodes; insulators; MHD; MHD materials;

MHD materials testing; phase equilibria (MHD) ; vaporization (MHD) ; viscosity (MHD)

17b. Identifiers/Open-Ended Terms

17c. COSATI Fie Id /Group

18. Distribution Statement Unclassified, Unlimited

19. Security Class (This 21. No. of Pages

Reportl

UNCLASSIFIED 128

20. Security Class (Ihis 22. Price Page)

UNCLASSIFIED

FORM NTIS-35( 10-70) US COMM-D ; 40329-P7 '


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