NBSIR 87-3561

Thermochemistry of Materials by

Laser Vaporization Mass

Spectrometry, Part II - Graphite

John W. Hastie, David W. Bonnell and Peter K. Schenck

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute fo[ Materials Science and Engineering

High Temperature Chemistry

Ceramics Division

Gaithersburg, MD 20899

June 1987

Final Report

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

NBSIR 87-3561

THERMOCHEMISTRY OF MATERIALS BY

LASER VAPORIZATION MASS

SPECTROMETRY, PART II - GRAPHITE

John W. Hastie, David W. Bonnell and Peter K. Schenck

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Institute for Materials Science and Engineering

High Temperature Chemistry

Ceramics Division

Gaithersburg, MD 20899

June 1987

Final Report

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

THERMOCHEMISTRY OF MATERIALS BY LASER

VAPORIZATION MASS SPECTROMETRY, PART II GRAPHITE

John W. Hastie, David W. Bonnell and Peter K. Schenck

Ceramics Division

Institute for Materials Science and Engineering

National Bureau of Standards

Gaithersburg, MD 20899

Presented at the IUPAC Fifth International Conference on High Temperature

and Energy-Related Materials, Rome, Italy, 25-29 May 1987.

Abstract . In an earlier study (Part I of this series) an experimental

approach, which couples laser heating of refractory materials under vacuum

with mass spectrometric detection of the vapor plume species for

thermochemical determinations, was demonstrated using the BN refractory system

at 2900 K. This paper describes the results of a similar study on

polycrystalline graphite at temperatures around 4100 K and total species

pressures in the vicinity of an atmosphere (1 atm = 1.01325 x 10 5 Nm' 2 ). A

Nd/YAG laser system, focused to power densities in the region of 10 9 W/cm2

, was found to be a convenient energy source for producing thermally controlled

vapor plumes and with negligible post-vaporization perturbation of the neutral

species identity and concentration. The principal vapor species were found to

be Cn (n = 1 - 9) and their relative intensities were found to be more

consistent with the JANAF Thermochemical Tables than with other literature

assessments and results. No reliable means were available to directly

determine surface temperatures tinder the experimental conditions of very short

heating times (7 - 20 ns) and luminous vapor plumes. Therefore, various

indirect methods were used, including comparison of the total and partial

pressure data with literature results and time-of -flight analysis of the

1
plume species. Surface temperatures were found to be in the range of

3800 - 4100 ± 200 K, depending on laser fluence, and the sum of the

corresponding Cn species partial pressures was determined to be approximately

in the range of 0.1 - 1 atm.

1 Introduction

In the preparation and application of high temperature and energyrelated materials, much higher temperatures and more reactive conditions are

being encountered. New or improved measurement methods are required to

establish the basic thermochemical properties of materials under these

aggressive conditions. For this purpose, we have developed a Laser- Induced

Vaporization Mass Spectrometer system (LIVMS) which, in principle, allows for

containerless measurement of species partial pressures over a wide range of

temperature and pressure (Hastie et al 1984) . In an earlier study on boron

nitride [Part I of this series (Hastie et al 1987)], comparison of LIVMS data

with JANAF (1971)-based partial pressures for B, B2 , B3 , BN, and N2 , at a surface temperature of 2900 K, indicated the presence of local thermodynamic

equilibrium even for 7 ns laser pulse times. In the present study, the LIVMS

technique has been further developed and applied to the graphite system.

One of the goals of this ongoing study is to develop a molecular- level

understanding of the vaporization and related ionization and aggregation

processes which occur when high-power laser energy is dissipated in carboncontaining materials. These highly complex processes can be expected to

include the thermodynamically controlled production of Cn (n = 1 - 9)

molecular species, and the non- equilibrium production of cluster species

(typically, n = 10 - 1000), in addition to the formation of electrons, and

2
positive and negative ions. Inorganic carbon-based materials, such as

graphite, metal carbides, and carbon-carbon composites, have intrinsically

high resistance to thermal degradation. This behavior results from the

combined effects of an inherently high sublimation energy, high melting point,

and high thermal conductivity. The development of effective laser-resistant

materials and obscurants is expected to rely mainly upon such carbon-based

materials (Stickley 1985).

Despite considerable research over the past several decades, the basic

thermodynamic, kinetic, and mechanistic description of the thermal

decomposition of graphite, and related materials, is still in considerable

doubt. In fact, at least three widely used but discrepant sets of

thermodynamic tables exist for the carbon vapor species, the JANAF (1971,

1985) Thefmochemical Tables, the so-called Livermore Tables (Leider et al

1973) , and the Russian Tables (Gurvich, et al 1979) . Even the melting point

of graphite is a subject of considerable controversy (3800 - 5100 K range).

Since the initial JANAF tabulation (1971), an appreciable number of new

studies have been carried out on the graphite vaporization system.

Experiments have been conducted on the effect of moderate power C02 and Nd-YAG

lasers on the vaporization of various forms of graphite (Baker et al 1983,

Covington et al 1977, Lincoln and Covington 1975, Lundell and Dickey 1977,

Lundell 1982, Meyer et al 1973, Milne et al 1972, Zavitsanos 1968). Most of

these studies, however, have concentrated on the macroscopic, non-molecular

processes such as crater formation, material removal rate, plume dynamics, and

phase equilibria.

3
We have reviewed the recent literature and find that there is

considerable disparity among various workers, particularly concerning relative

and absolute partial pressures of the various carbon polymer species (Hastie

et al 1984) . Analysis of the results of these literature studies indicates

that the equilibrium distribution of various carbon species is still

uncertain, and particularly at temperatures greater than 2800 K. Despite

recent suggestions to the contrary, the JANAF (1971, 1985) tables still appear

to be the most consistent with the available data. However, one cannot

eliminate from consideration the possibility that this apparent dataconsistency may result from offsetting errors. This could be the case, for

instance, with the C3 species which exhibits unusual entropy and vaporization

coefficient characteristics. Also, the JANAF (1971, 1985) evaluation relies

heavily on the Knudsen effusion mass specbrometric data of Drowart et al

(1959) which were obtained at relatively low temperatures of 2700 K, and less.

These workers used an ionizing electron energy of 17 eV which should reduce

substantially any fragmentation effects present. However, this energy is

still 2 or 3 eV above threshold for possible fragmentation pathways and recent

photoionization studies of Cn (n = 3 - 20) cluster species indicate

significant fragmentation of the parent positive ions near threshold energies

(Geusic et al 1986). Likewise, electron impact fragmentation of

Sn (n = 1 - 8) cluster species was found to be significant near threshold

(Arnold et al 1986). Because of spectroscopic data uncertainties, JANAF

(1971, 1985) heavily weighted the Second Law data of Drowart et al (1959), the

reliability of which depends on the absence of systematic temperature error,

and also on negligible electron impact fragmentation and temperature

dependence affects in the ionization process.

4
Application of the LIVMS technique clearly provides a means to test the

above concerns and the reliability of entrapolating the JANAF (1971, 1985)

thermodynamic functions well beyond the experimental data limit of 2700 K

imposed by the conventional Knudsen effusion mass spectrometric technique.

Higher temperature access also increases the prospect of detecting the higher

molecular weight Cn (n > 5) species predicted to be significant by the

Livermore Thermochemical data analysis (Leider et al 1973). Likewise, the

influence of non-unity vaporization coefficients known to be significant in

the 2200 - 2700 K temperature range (eg, see Hoch et al 1974), should be

greatly reduced or eliminated at higher temperatures (eg, see Lundell and

Dickey 1977, and Baker et al 1983).

2 Apparatus and experimental procedure

The apparatus developed for this work has been described in detail

elsewhere (Hastie et al 1984, 1987) and the essential features are as follows.

2.1 Laser interaction and beam sampling geometry

Figure 1 shows a simplified schematic of the basic technique. Note that

the laser, molecular beam orifices, and mass spectrometer are configured to

reduce the possibility of laser interaction with the portion of vapor plume

directed toward the mass spectrometer. Such an interaction could lead to

ionization and plasma effects not characteristic of the vaporization process,

as noted in earlier studies, e.g., see Lincoln and Covington (1975). The

laser beam enters the continuously evacuated sample chamber at right angles to

the mass spectrometer beam line, as shown in figure 1. The minicomputer shown

5
controls the signal averager and records completed scans of time-resolved mass

spectral ion intensities.

After initial test runs at angles varying from 45 to 15 degrees, the

angle between the laser beam and sample surface (see figure 1) was chosen as

15 degrees for the data reported here. This geometry represented a compromise

between the laser -entrance and mass spectrometer sampling angles. The reduced

mass flux for the slightly off-axis sampling was not expected to be

significant, as shown by the earlier study of Covington et al (1977). ' From

post-run examination of deposit patterns around vacuum chamber apertures, the

vapor plume- to -sample surface orientation was found to be approximately

independent of the laser incidence angle. The plume axis was normal to the

surface to within a few (~ ± 5) degrees for sample- surface to laser-beam

angles ranging from 45 to 15 degrees.

Sample positioning in real-time is achieved by a computer controlled

kinematic two-dimensional stage. This stage permits laser-vaporization either

from an essentially fresh surface with each pulse or from a selected single

spot.

2.2 Laser system

The laser system basically consists of an externally mounted Nd/YAG

pulsed laser (10 or 20 Hz frequency using two individual lasers), which

operates at several discrete wavelengths from 1060 nm to 350 nm. Earlier

studies, for instance those of Ohse et al (1979), Lincoln and Covington (1975)

and Olstad and Olander (1975), have indicated the utility of short pulse time,

high power lasers (e.g., Nd/YAG) for controlled surface vaporization studies.

To allow for ready visual alignment of the beam, the 532 nm doubled line

6
(green) of the laser fundamental was chosen as the working wavelength for the

present studies. The laser pulse times used were in the range 7-20 ns, which

convert to thermal pulses of < 100 ns. This time scale is negligible compared

to that for the vaporization and beam-transit process (> 100 ps ) . The laser

beam was focused onto the sample by a 500 mm focal length lens , mounted on a stepper motor-driven stage. Sample surface spot sizes of about 250 ^m

diameter were used and the laser was maintained at a fixed power level for

each multi-pulse data -averaging series.

2.3 Sample preparation

The sample mounts used were constructed either of stainless steel or tantalum. Samples were taken from two sources: (1) a rod of spectroscopic

grade polycrystalline graphite, selected by the NBS Center for Analytical

Chemistry for spectrographic analyses on the basis of its freedom from

contaminants; (2) a rod of medium- grain dense graphite similar to HLM grade.

The hydrogen content is unknown but is probably insignificant although

relatively weak mass spectrometric signals were observed corresponding to C 2 H2 and other low hydrogen- content hydrocarbon gas species. Possible hydrocarbon

contributions to the Cn

+ (n = 1 - 9) ion signals are estimated to be less than

a few percent and can be neglected. Sample preparation consisted of partingoff a short section of a 1/4 inch (6.4 mm) rod, and milling a flat of the

desired angle using carefully cleaned steel cutting tools.

2.4 Vapor plume formation, beam sampling, and analysis procedures

According to Afanasev and Krokhin (1967), at flux densities above about

10 8 W/cm2 the laser interaction mechanism changes from a thermally controlled

7
vaporization process to one where thermal pressure ejects material into the

laser plasma. However, this theoretically-based argument is for a general

case, ie non-material specific. For the highly refractory materials of

graphite and BN (reported on elsewhere, Hastie et al 1987) it is still

possible to be within the thermal vaporization regime. No evidence was found

for a physical ejection process in the present studies using nominal flux

densities of - 108 - 10 9 W/cm2

. The actual fraction of this incident flux

that is absorbed by the sample is not known.

In a typical laser- induced vaporization experiment, a laser fluence of

about 20 J/cm2 is focused to an area of - 10" 3 cm2

. The deposited energy is

converted into thermal excitation at the surface and penetrates to a depth of

only a few tens of atomic diameters. Rapid vaporization occurs, leading at

peak- temperature to the formation of a gas -dynamically stabilized plume. The

visible region of the plume extended about 1 cm into the first vacuum chamber.

This plume undergoes rapid free- jet expansion to yield a collisionless gas

stream and a molecular beam. The molecular beam is collimated by three

circular, differentially pumped, apertures (not all shown in figure 1), and

travels a distance of 26 cm between the sample surface and mass spectrometer

ionization chamber. This distance effectively determines the time scale of

the measured time-resolved ion- intensity profiles as the transit time in the

quadrupole mass filter is negligible. Each aperture is sized to just allow

clear passage of the beam through the 4.8 mm diameter entrance aperture of the

quadrupole mass filter ion source and, hence, also through the larger (8 mm)

ion source exit aperture.

8
Species are ionized by electron impact at adjustable electron energies,

and mass analyzed by the quadrupole mass filter. In the present work, most of

the data were obtained with 26 eV ionizing electron energy. This is about

15 and 10 eV above the molecular and dissociative ionization thresholds,

respectively. These mass selected ions are then collected and amplified by an

electron multiplier. The signals are averaged using 5 /is/channel dwell time,

with the sweep synchronized by a trigger generated from the onset of each

laser pulse or from the flash lamp. This data-collection procedure gives a

time- resolved mass spectral ion- intensity profile of an individual preselected mass peak. We designate the profiles as time -of- arrival curves,

abbreviated as TOA. Sufficient signal time- sweeps are collected (typically

between 100 and 1000) to provide reasonably good statistics in the leading

edge and peak centroid area. The data-collection process is repeated for each

individual mass peak selected. For a fixed sample spot position it was

observed that, dependent on the laser beam to surface angle, the initial

signal intensity would decay during the first few tens of pulses and then

stabilize at a value about half that produced by the initial pulse. For this

reason, each series of intensity- time profiles (for individual masses) was

collected after a pre-conditioning period, and successive series were taken

from a new spot on the surface. To check for possible changes In laser flux

for a number of laser shots, a major peak (eg C 1 or C3 ) was sampled both at

the beginning and end of a data-collection series.

9
3 Results and discussion

•3.1 Vapor species assignment

Table 1 summarizes the species detected, in the form of singly charged

positive ions, by mass spectrometric analysis of the graphite laser plume with

26 eV ionizing electron energy. No higher molecular weight species were

detectable. Particular attention was given to the detection of C 11 which is

known to be a prominent species under non- equilibrium cluster- forming

conditions (eg, see Rohlfing et al 1984). The absence of detectable C 1

, or other species at n > 9 , is indicative of the likelihood of the sampled beam

having a frozen equilibrium composition representing that at the vaporizing

surface

For the two representative data sets given in table 1; A was obtained

under a static (stationary sample) mode condition, and B under a dynamic mode

where the sample was moving continuously, thereby providing to a new

vaporizing surface location about every 20 shots. About 3000 laser shots were

used for each species in set A, thereby creating a crater of similar

dimensions (~ 250 /xm diameter) to those produced in an earlier study on BN

(Hastie et al 1987). For set B, 1000 shots per species were used across the

surface without detectable crater formation. Two different graphite supply

sources, commercial laser systems, fluence levels, and focus conditions were

used to obtain data-sets A and B. At the sample surface, the laser fluence

levels were estimated to be in the range of 20-30 mJ

During initial heating, hydrocarbon impurities were observed for data-set

A. Similar observations have been made by other workers, eg, see Zavitsanos

and Carlson, (1973). In the present study, this effect was observed primarily

in the form of C2 H

+ (25 amu) , C2 H2 + (26 amu) , and C4 H2 + (50 amu) mass

10
spectral ions during the initial first few laser shots onto a fresh surface

location. These signals tended to vanish, with an increasing number of laser

shots, much more rapidly than those for the Cn species, suggesting the

presence of surface hydrocarbon contamination. The acetylene species (C2 H2 ) was the dominant impurity. It is perhaps pertinent that laser heated graphite

in the presence of hydrogen or ethylene also produces acetylene as the major

product (Taki et al 1969).

Of particular note was the long TOA for the 26 amu (C2 H2 ) species, as compared with the Cn ions. A similar effect can be noted in the results of

Lincoln (1969). For comparison, both our data and Lincoln's show a time

difference between 26 and 36 amu of 150 to 200 ns . This time delay could

result from a number of factors, including the C2 H2 + arising from electron

impact fragmentation of a heavier polyacetylene precursor (the delay implies a species of 110 ± 15 amu), a reaction time such as diffusion in the substrate,

or a different degree of gas-dynamic cooling for C2 H2 as compared with Cn

. The 57 amu (C4 H9 +

) ion showed an even larger time-delay and the TOA profile

was exceptionally wide.

3.2 Vapor species abundance

The relative ion intensities listed in table 1 are believed to be a good

indication of the relative species partial pressures. That is, species

dependent electron impact fragmentation and mass spectral sensitivity factors

can be neglected to a first approximation, with an estimated uncertainty of

30% in the relative partial pressures. More precise partial pressure data

cannot be obtained as no ionization cross section (a) information exists for

any of the polyatomic Cn species. However, various estimates suggest a

11
gradual increase in a with n. Under our experimental conditions this trend is

compensated for by the known (calibrated) decrease in the quadrupole mass

filter transmission efficiency with increasing mass. A potentially more

serious source of error in relating ion intensities to partial pressures

arises from possible electron impact fragmentation which would enhance lower

molecular weight ions at the expense of heavier parent ions. At the 26 eV

electron energies used this effect should not be too large. Examination of

the TOA profiles indicated no significant (< 20%) ion intensity contributions

from fragmentation of heavier ions. Further discussion of the fragmentation

question is given in later sections.

Typical TOA profiles, from which integrated ion intensity data are

obtained, are given in figures 2-4 for data-set B. Similar TOA profiles

were obtained for the C x

- C5 species of data-set A but with lower

sensitivity. The TOA profiles at half -height (0.5 I max) were wider and the

peak- intensity times were systematically shorter for set A. Both of these TOA

features are indicative of a faster but cooler (narrower velocity

distribution) beam for set A compared with set B. This, in turn, indicates a higher sample pressure and surface temperature for data-set A. Future

development of a deconvolution model is planned to extract temperature

information from the TOA profiles which are somewhat delayed and broadened by

the electronic (RC) system time constant (~ 100 - 200 /is)

Ion intensities were computed from the TOA profiles (eg, figures 2-4)

by two methods. The first used the peak maximum at the peak centroid, and the

second used the integrated peak intensity. Both approaches gave consistent

relative intensity results which were almost independent of ionizing electron

energy at 26 and 96 eV.

12
Under carefully controlled conditions, the data were very reproducible.

However, as seen by the significant difference between data-sets A and B

(table 1), the data are sensitive to experimental conditions. With fixed

laser conditions, the following factors were found to affect the absolute and

relative intensities: sample-type (A and B were from different graphite supply

sources), surface morphology (flat or cratered), and surface -heating history.

At present, it is not possible to isolate and quantify these effects though

future systematic work is planned. For data-set A, the initial intensities

resembled those of set B and as a crater developed the intensity variations

stabilized to the level shown by set A. We believe that this set most

reasonably represents the equilibrium thermodynamic distribution of Cn species

partial pressures. Non- equilibrium effects, such as non-unity accommodation

coefficients and impurities, are less likely from a pre-heated crater source.

.The intensity- time profiles from Cn (e.g., see figures 2-4) are quite

similar to those observed in analogous measurements by Olstad and Olander

(1975) for Fen species. These authors provide a theoretical analysis of the

atom density pulses in terms of an equilibrium vaporization model with known

laser pulse energy-time profiles. A similar analysis should be possible for

the Cn system. However, in the present case the time -scales of the thermal

pulse and the TOA profiles are not expected to overlap.

The validity of the assignment of ions to neutral precursors was borne

out by the observation of insignificant changes in ion ratios from 26 to 96 eV

ionizing electron energy. However, this is only a necessary but not

sufficient proof of ion assignment. Additional, more compelling, evidence for

the lack of significant electron impact fragmentation is given by the TOA data

discussed below (Section 3.3.1).

13
Comparison, in table 1, of the present ion intensity data with the

JANAF (1971, 1985), Livermore (Leider et al, 1973) and Russian (Gurvich

et al 1979) literature data for relative partial pressures and the ion

intensity data of Zavitsanos (1968) and Berkowitz and Chupka (1964) indicates a much closer agreement with JANAF (1971, 1985) than with the other data

sources. The only significant discrepancy between the present results and

JANAF (1971, 1985) occurs for the C x species when formed under a non-cratered

condition (set B in table 1) , which is discussed in more detail below.

However, the present C : results (data-set B) are in satisfactory agreement

with those of Berkowitz and Chupka (1964) but C 5 is not in agreement. In

fact, no two data-sets show agreement for C5 for reasons yet to be determined.

In this connection, it is noteworthy that the Russian tables (Gurvich

et al 1979) recommend a much higher partial pressure for C5 than observed by

us, or others, with the exception of the Zavitsanos (1968) data. It is

probably significant that in a subsequent study Zavitsanos and Carlson (1973)

were unable to obtain data on C5 owing to the presence of a large background

peak at 60 amu. Based on the good agreement between the present results and those of

JANAF (1971, 1985), we can argue that the Cn (n = 2 - 5) plume species are

most likely in local thermodynamic equilibrium and that the graphite species

vaporization coefficients appear to be at or near unity. Other significant

causes of data- differences between this and earlier work may arise from our

efforts to minimize laser/plume interactions and our use of very short laser

pulses to reduce perturbation of the gas-dynamic process. Also, as the

present study was performed off the central plume axis, it is possible that if

non- equilibrium effects occurred only along the perpendicular axis they would

14
not have been observed in the present work. As the plume is highly forwardpeaked in the perpendicular direction this possibility needs further study.

3.3 Temperature determination

For these initial studies, several indirect methods, summarized in table

2 for data-set A, have been used to determine a temperature (T0 ) of

4100 ± 200 K for the laser heated spot. For data-set B, using similar

methods, the T0 value was determined to be 3800 ± 300 K. Similar methods were

used with very satisfactory results for the BN studies described elsewhere

(Hastie et al 1987) . Application of potentially more accurate optical

pyrometric methods have been deferred due to uncertain emissivity data and the

difficulty of accounting for plume contributions to the emission signals.

Also conventional pyrometric systems do not have the time response needed.

Under the laser pulse conditions used, it is likely that only the surface

layer (a few wavelengths, i.e., - 2000 to 5000 nm) is heated directly and the

total extent of vaporization is confined to an area nearly equal to that of

the laser spot (Hall 1984)

.

3.3.1 Beam velocity analysis

The TOA profiles of the beam species may be used to obtain the postexpansion plume temperature. The time position (usually at peak intensity)

may be taken as an indication of the group velocity and the profile -shape is

characteristic of the velocity distribution. The corresponding temperatures

do not necessarily have the same value for gas-dynamic beams. However,

regardless of the model used to describe the vaporization and beam- forming

process, if a collection of gas molecules is in thermal equilibrium their

15
velocity function is proportional to (T/M)^ , where M is the individual species

molecular weight and T is the post -expansion molecular beam vapor species

temperature (< T0

) . This relation follows from the equipartitioning of

translational energy, established under thermal equilibrium conditions, where

Mv2 /2 = 3kT/2

and v is the root-mean- square (average) velocity, which is proportional to the

most probable velocity, ie, the peak of the thermal velocity distribution, and

k is the Boltzmann constant. Identification of the peak location of the

observed TOA profile is affected by the noise level. Hence relative TOA

values were selected at the half-height position of the leading edge of the

TOA profile. Correlating this time (t^) with the most probable velocity

yields the time -of- arrival function

t

% a (d/3kT)* . M* ,

where d is the flight distance, 26 cm. Hence a linear plot of t

% versus M

3* is

indicative of thermal equilibrium. Figure 5 shows that the various Cn species

exhibit such behavior. The linearity of the curves in figure 5 is also

excellent evidence for the correct assignment of ions to molecular precursors

of the same mass. It should be noted that the zero-mass time intercept

(in figure 5) has no physical significance here, owing to the use of halfheight TOA's arvd the affect of the RC time constant on the experimental TOA's.

Further support for thermal equilibration among the various Cn species

can be derived from the widths (half-height At) of the TOA profiles, such as

16
those given in figures 2-4. For a common species temperature, it follows from

the above discussion that At (half -height) is proportional to M^ and this was

generally found to be the case.

In general, the TOA profile curve shapes appear to follow MaxwellBoltzmann distributions characteristic of a cooled beam. Further,

more-detailed, analysis of this preliminary observation will be pursued in

future work when the instrumental RC time -delay effects are quantitatively

analyzed. It is noteworthy that the TOA peaks do not exhibit the unusual

shoulders or splitting present in the and C3 profiles obtained by Milne

et al (1972) under Knudsen effusion conditions (~ 2700 K beam temperature)

.

These authors attributed these anomalous effects to ion source time -delay

effects, including trapping and back-reflection of neutrals into the source.

At 50 eV ionizing electron energy, their C3 TOA profile showed the expected

distribution, indicating an absence of fragmentation. However, the C x and C2 TOA profiles did show a broadening at 50 eV, as compared with 17 eV, which was

believed to arise from fragmentation of C3 + by separate reaction channels to

C 1 + and C2 + (< 40% contribution to C 1 + ). In the present study, with beam

temperatures of < 750 K (produced by gas-dynamic cooling), no broadening of

TOA profiles occurred when the electron energy was increased from 26 to 96 eV.

However, some broadening of the C 1 TOA profile was noted during the first few

laser shots at a fresh surface which could arise from impurity fragmentation.

The TOA profiles will be further examined for evidence of secondary effects

when the RC time -delay component is eliminated.

From the slope of the data-set A curve in figure 5, we obtain an apparent

beam temperature of 1100 ± 200 K. Correction for the time difference factor

(empirical value = 0.68) between t^ and the peak- intensity time gives a beam

17
group temperature of 750 ± 200 K. This value is an upper limit as it is

affected by the instrumental RC time-delay. However, it is consistent with

the cooling affect expected from the isentropic free- jet expansion process.

Comparison of this value with that obtained similarly for the BN system

(670 K) (Hastie et al 1987) , together with the BN surface temperature of

2900 K, indicates a graphite surface temperature of 3300 ± 600 K. We expect

to improve the accuracy of this determination when the instrumental

(RC-delay) contribution to the TOA profiles is established. Direct

comparisons of individual TOA curves provide more accurate temperatures

For instance, by comparing the relative C2 + arrival time with that for B x + at

T0 = 2900 K (Hastie et al 1987) we obtain a graphite surface temperature value

of 4000 ± 300 K, which is in good agreement with other temperature indicators

(see table 2)

The relative beam temperatures for data-sets A and B, obtained from the

curve slopes in figure 5, indicate about an order of magnitude lower source

pressure for data-set B. That is, for the conditions of data-set B, there is

less gasdynamic acceleration of the beam due to the lower source pressure.

This difference in source pressure is consistent with the difference in source

temperatures for the two data sets.

3.3.2 Temperature from partial pressure determination

Independent evidence for a pre- expans ion (and hence surface) temperature

in the region of 4100 K is provided by the absolute partial pressure, P i , data. The relationship between and the integrated ion intensities, I

+

, for

species i is given by

= kI+ T

18
where k is a constant determined by vaporization of a caiibrant material at

known pressure and temperature. A calibration was performed using the well

established NaCl system with the same mass spectrometer and molecular beampath conditions. The graphite gas-dynamic conditions were simulated by use of

transpiration mass spectrometry to produce a supersonic expansion of NaCl in

N2 (Bonnell and Hastie 1979) . The value of k obtained was adjusted for the

C 1 case using the known ionization cross section (a) differences between NaCl

(Bonnell and Hastie 1979) and C 1 (Mann 1969) at the 26 eV ionizing electron

energy used (ac / aNaCi = 1.6). With this approach, we find

Pc =0.1 atm

for data-set A of table 1. A similar result is obtained if one compares the

cr-adjusted ion intensities of C2 + and B^ (Hastie et al 1987) . For an

equilibrium condition with the JANAF (1971, 1985) thermodynamic functions,

this partial pressure corresponds to a temperature of 4100 + 200 K. The error

represents the temperature span over which Pc changes by a factor of 3 , which

we consider a conservative estimate of the probable uncertainty in the derived

Pc

. For the data in table 1 , the sum of the individual species partial

pressures is in the region of 1.0 atm, the sublimation pressure of graphite.

It also follows that the species detectability limit for the present

experimental conditions was - 10“ 4 atm. If one considers the source pressure

of 1 atm this sensitivity value is much lower than normal for high temperature

mass spectrometry. However, the free -jet expansion process produces a gas of

final expansion pressure in the region of ~ 10“ 5 atm. Hence the 10“ 4 atm

detectability limit effectively refers to this 10“ 5 atm source and the

sensitivity is then comparable with Knudsen mass spectrometry.

19
3.3.3 Temperature from species relative intensities

The TOA evidence for thermal equilibrium suggests that the "known" Cn species partial pressure distribution with temperature can be used to infer

the sample temperature (T0 ) from the measured species concentration ratios.

Also, the beam- formation process is quite similar to that for more usual

supersonic expansions where the beam composition represents a "frozen

equilibrium" which corresponds to the pre- expansion temperature/pressure

conditions (Bonnell and Hastie 1979) . The high pumping speed of the vacuum

system used for the present studies, and the pulsed nature of the gas

evolution process, further ensures high expansion ratios and "frozen

equilibrium" compositions in the gas-dynamic process.

Values of "known" Cn species concentration ratios were calculated as a function of temperature using JANAF (1971, 1985) and other thermochemical

tables (Leider et al 1973, Meyer and Lynch 1973; Gurvich et al 1979).

Figure 6 gives the JANAF (1971, 1985) -based data and it is apparent that the

C 5 /C3 and C4 /C3 ratios are the most sensitive temperature indicators. Table 3

gives temperatures calculated from the experimental Cn /C3 ratio data and it is

clear that the most realistic temperature data result from use of the

experimental intensity ratios together with the JANAF (1971, 1985)

thermochemical data. That is, the Livermore (Leider et al, 1973) and Meyer

and Lynch (1973) revisions and reassessments of the earlier JANAF (1971, 1985)

thermodynamic functions are not as consistent with the present results. The

more recent Russian tables (Gurvich et al 1979) appear to overestimate the

importance of C 5 as discussed earlier.

Anomalous / C3 behavior : the anomalous low temperature result

indicated (in table 3) by the C^C-j ratio for data-set B could be interpreted

20
as being due either to an apparent excess of C1 or to a deficiency in C3

. Arguments for and against additional sources of C x + include:

o An excess intensity level of Cx + could arise from electron impact

fragmentation of higher mass Cn species or of contaminants such as

CO or hydrocarbons. In this connection, it should be noted that the

literature data were usually obtained at lower electron energies

than for the present study. However, the present results are consistent with the lower eV data of Berkowitz and Chupka (1964) and

Drowart et al (1959).

o A compelling argument against significant fragmentation ' in the

present study is the Maxwellian form of the TOA profile (figure 5)

which indicates that C x + arises primarily from a 12 amu neutral

precursor, ie, C 1

.

o The fact that the Cx + TOA follows the t^ vs M^ correlation (figure

5) is also very good evidence for the absence of higher molecular

weight species contributing to C x + by electron impact fragmentation.

o Analysis of the Cn

+ ion- stabilities (bond dissociation energies)

indicates no loss of stability on ionization of Cn , hence no

major fragmentation effect is expected (eg. see Has tie and Margrave

1968, 1969).

21
o The constancy of ion-ratios over the range 26 to 96 eV is consistent

with an absence of fragmentation and suggests the existence of a single ion-precursor for all ions. However, this is not a definitive proof as the fragmentation efficiency may not change

between 26 and 96 eV.

o The C 1 + appearance potential curve was inconclusive with respect to

fragment- ion contributions owing to an unusually large tailing near

the thresh-hold energy which did, however, agree with that for

atomic carbon. This effect requires further study with improved

sensitivity.

o Anomalous (and unprecedented) variations in relative ionization

cross sections or vaporization coefficients for C x or C3 'could also

account for some of the observed data, though this is not considered

likely.

In summary, the above arguments indicate that the high C 1 + intensity most

likely arises from an anomalously high concentration. Supporting evidence

for this conclusion is provided by the observations of Wachi and Gilmartin

(1970) to the effect that the C 1 /Cn ratios decreased with time and also

depended on the sample preparation and surface morphology. The fact that

data-sets A and B (table 1), which show a significant difference in C x +

relative intensity, represent two different samples and surface morphologies

is also consistent with Wachi and Gilmartins' (1970) observations. Hence the

22
C., + data do not appear to have thermodynamic significance for C 1 concentration

measurement except for the cratered condition used to obtain data-set A.

Another, less -likely, argument as to why C^/Cg gives a lower T0 than

Cn /C3 (n > 1) is based on the higher sublimation enthalpies for Cn (n > 1),

than for C : (as may be seen from the curve slopes of figure 7) . This means

that C x becomes a relatively more important species at lower temperatures and;

as the hot spot cools, C 1 could become relatively more significant than

Cn (n > 1). However, such an effect should also be manifested in the

B 1 /B2 ratio data obtained for the BN system (Hastie et al 1987) , and this was

not the case.

For the case of the C2 /C3 ratio (data-set A, table 1) , if the relative

ionization cross-sections were 1/1.5, which is quite reasonable based on

empirical trends in other systems, the calculated temperature of 3400 K would

be increased to 4200 K which is in closer agreement with the values obtained

using the C 4 and C5 species. This cross-section ratio agrees favorably with

the estimate of 1/1.23 given by Meyer and Lynch (1973). The higher polymeric

Cn species (n > 3) should have cross-section ratios close to unity, which we

have assumed in the present analysis.

Finally, the possibility that C 3 rather than C x or C2 behaves anomalously

should be considered. At temperatures less than 2700 K it is known that C 3 has a low vaporization coefficient (a ) , but this is expected to increase with

increasing temperature. Baker and Covington (1982) provide indirect evidence

for the average , over all Cn species, to fall in the range of 0.5 - 1.0 at

temperatures in the region of 4000 K. In the present experiments, both C3 and

C2 would need to have a values below this range to account for the observed

variations of Cn /C3

. Future measurements of Cn /C3 as a function of surface

23
morphology are planned to test for possible non-unity behavior for crn . However, the present data do not support an values of much less than unity.

A similar conclusion was also made by Lundell and Dickey (1977), based on

CW- laser vaporization studies over the temperature range 4000 - 4500 K. The

observed mass -loss rates were also consistent with that predicted from the

JANAF (1971, 1985) data using a gas-dynamic model.

4 Summary and conclusions

Laser pulses at 532 nm wavelength, with pulse widths in the range 7-20 ns and power densities in the range of ~108 - 109 W/cm2

, impacting graphite

surfaces at 15-45° incidence angles, produce vapor plumes with total pressures

in the range of - 0.1 - 1 atm and pre-expansion temperatures between

3800 ± 300 and 4100 ± 200 K. The pre- expans ion vapor and surface temperatures

were determined by a number of independent, indirect methods. The vapor

plumes formed originated either from craters or from a relatively flat

surface, depending on the number of laser shots used to obtain data at a fixed

sample location. It is believed that the cratered condition more closely

approaches equilibrium. Partial pressures were determined for the principal

vapor species C^-Cg.

No significant mass spectral interferences due to electron impact

fragmentation were detected. We surmise that the observed isentropic cooling

(to < 700 K) , which results from plume free-jet expansion into the high vacuum

chamber, reduces the normal thermal excitation and the availability of

dissociation channels for electron impact fragmentation. In this connection,

it is interesting to note the tendency for Cn cluster (ie non- equilibrium)

24
species to photodissociate according to the reaction,

Cn + hi/ = Cn

_ 3 + + C3 (Geusic et al 1986) . This process was found to be particularly prominant for

n > 6 and the driving force can be attributed to the high stability of C 3

. It

is reasonable to expect similar behavior for electron impact conditions,

although the relative abundance of Cn species differs significantly under the

non-clustering conditions of the present work. In the present case, we could

expect to see the following fragment- ion to neutral precursor pairings:

C x + (C4 ), C2 + (C5 ) , C3 + (C6 ) , C 4 + (C7 ) , C 5 + (C8 ) , etc.

in decreasing order of significance. However, the TOA profiles do not show

the presence of any significant (> 20%) high mass shoulders or broadening.

For the relatively noisy TOA data corresponding to C 7 , C8 , and C9 it is

presently not possible to exclude contributions from higher masses although

this is very unlikely because of the reduced thermodynamic significance of

n > 9 precursor species.

Future use of TOA mass spectral intensity profiles for temperature

determination shows promise once instrumental time -delay effects are reduced,

or at least quantified, in the present apparatus. Similarly, once an absolute

time scale is established for the TOA profiles, their degree of conformity to

Maxwell -Boltzmann or other temperature distributions can be investigated. It

should also be possible to deconvolute these profiles and provide unique

mechanistic information on electron impact fragmentation pathways,

temperature -dependent ionization cross sections, cluster formation, and other

plume -dynamics phenomena. One of the early suggested explanations for data

25
inconsistencies in the C3 thermodynamic functions was, in fact, the

possibility of a temperature -dependent a which would affect both Second and

Third Law analyses of vaporization data (attributed to L. Brewer by Liskow

et al 1972).

Comparison of the Cn (n = 1-5) species partial pressures with values

calculated from the JANAF (1971, 1985) thermodynamic functions is very

favorable, particularly for vaporization from a crater. However, the C 5 and

C6 species are much less significant than predicted from the Livermore (Leider

et al 1973) thermodynamic functions but the C7 species partial pressure agrees

well with their prediction. Possible sources of error in previous studies

include, the presence of hydrocarbon impurities, and low gas-dynamic expansion

ratios with the kinetic formation of cluster species. We expect that with

further development of the LVMS technique, the uncertainties in partial

pressures and temperatures can be significantly reduced and a revision of the

thermodynamic functions can be made. However, the present results do not

support the arguments of Baker et al (1983) to the effect that the Livermore

(Leider et al 1973) functions are more reliable than those for JANAF (1971,

1985)

The Cx species appeared to be particularly sensitive to the experimental

surface conditions, which probably accounts for the lack of reproducibility

between various workers. Further systematic study is planned to better define

the reasons for the apparently anomalous behavior of C x , relative to C3. With

this exception, the data suggest the various Cn species to be formed at local

thermodynamic equilibrium, even for very short laser heating times.

26
From the pressure information determined by this approach, one can

calculate an approximate rate of mass transport for a given surface area.

For the hot spot sizes used, we calculate this rate to be in the region of

-4

-10 gm/s . Using the JANAF (1971) vaporization enthalpy data, it follows

that the heating and vaporization process removes only about 10 ^ Joule per

laser pulse from the system. As this energy is several orders of magnitude

less than that provided by the laser then it appears that the primary energy

loss mechanism is not by vaporization but by conduction through the sample.

Work is in progress on development of an energy loss model.

ACKNOWLEDGEMENTS

A.B. Sessoms and M. Wilke have provided valuable technical assistance in the

construction and maintenance of the Laser- Induced Vaporization Mass

Spectrometry System. Financial support for this work has been provided, in

part, by the Air Force Office of Scientific Research. Initial development of

the basic multistage vacuum system was supported by the Army Research Office

for flame analysis experiments.

27
REFERENCES

Afanasev Yu V, Krokhin 0 N, 1967 Soviet Phys . JETP 2.5, 639.

Arnold W, Kowalski J, Putlitz zu G, Stehlin T, and Trager F, 1986,

Z Phys. D - Atoms, Molecules and Clusters 2 , 329.

Baker R L, Covington M A, and Rosenblatt G M, 1983 "The Determination of

Carbon Thermochemical Properties by Laser Vaporization" , in High

Temperature Materials II, Eds. Z. A. Munir and D. Cubicciotti

(Electrochem. Soc., Pennington, NJ) p. 143.

Baker R L, Covington M A, 1982 The High Temperature Thermochemical

Properties of Carbon, Report SD-TR-82-19 (ADA112977, NTIS)

Berkowitz J, Chupka W J, 1964 J. Chem. Phys. 40, 2735.

Bonnell D W, Hastie J W, 1979 in Characterization of High

Temperature Vapors and Gases, J. W. Hastie, ed. , NBS SP 561, (U.S.

Govt Printing Office, Washington, D.C.) p. 357.

Covington M A, Liu G N, Lincoln K A, 1977 AIAA J. .15, 1174.

Drowart J, Bums R P, De Maria G, Inghram M G, 1959 J. Chem.

Phys. 31, 1131.

Geusic M E, Jarrold M F, Mcllrath T J, Bloomfield L A, Freeman R R, Brown W L,

1986, Z. Phys. D. - Atoms, Molecules and Clusters, 3., 309.

Gurvich L V, Viets I V, Medvedev V A, Khachkurozov G A, Yungman V S,

Bergman G A, et al. 1979, "Thermodinamicheskie Svoistva Individual ' nykh

Veshchestu" (Thermodynamic Properties of Individual Substances);

Glushko, V. P. , gen. ed. , Vol II parts 1 and 2.

Hall R, 1984 Private Communication.

Hastie J W, Margrave J L, 1968 Fluorine Chem. Rev., 2, 77.

Hastie J W, Margrave J L, 1969 J. Phys. Chem. 73., 1105.

Hastie J W, Bonnell D W, Schenck P K, 1987 "Laser- Induced

Vaporization Mass Spectrometry of Refractory Materials. Part I,

Apparatus and the BN System", High Temp. Sci. in press.

Hastie J W, Bonnell D W, Schenck P K, 1984 "Molecular Basis for

Laser- Induced Vaporization of Refractory Materials", NBSIR 84, 2983

Hoch M, Ramakrishnan D, Vernardakis T, 1974, Advances in Mass

Spectrometry, vol. 6, A. R. West, ed. (Applied Science, Pub. Essex, U.K.)

p. 571.

28
JANAF Thermochemical Tables, 2nd Ed., NSRDS-NBS 37 (Washington,

D.C. 1971) and supplements (e.g., J. Phys . Chem. Ref. Data, 3, 311-480

(1974); 4, 1-175 (1975); 7 793-940 (1978); U, 695-940 (1982); 14,

Supplement 1, 535-681 (1985).

Leider H R, Krikorian 0 H, Young D A, 1973 Carbon 11 . 555.

Lincoln K A, Covington M A, 1975 Int. J. Mass Spec. Ion Phys. 16 . 191.

Lincoln K A, 1969 in "High Temperature Technology," (Butterworths

and Co., London), p. 323.

Liskow D H, Bender C F, Schaefer III H F, 1972 J. Chem. Phys. 66 , 5075.

Lundell J H, Dickey R R, 1977 Progr. Astron. and Aeron, 56 . 405.

Lundell J H, 1982 Progr. Astron. and Aeron. 83., 472.

Mann J 3, 1969 "Ionization Cross Sections of the Elements," in Recent

Developments in Mass Spectroscopy, Ed. K. Ogata and T. Hayakawa, (Univ.

Park Press, Baltimore) also, private communication, for cross sections as a function of electron energy, p. 814.

Meyer R T, Lynch A W, Freese J M, 1973 J. Phys. Chem. 77 . 1083.

Meyer R T, Lynch A W, 1973 High Temp. Science 5, 192.

Milne T A, Beachey J E, Greene F T, 1972 In "Vaporization

Kinetics and Thermodynamics of Graphite Using the High Pressure Mass

Spectrometer," AFML- TR-72-227 (AD 753713).

Ohse R W, Babelot J F, Cercignani C, Kinsman P J, Long K A,

Magill J, Scotti A, 1979 in Characterization of High

Temperature Vapors and Gases, J. W. Hastie, ed. , NBS SP 561, U.S. Govt.

Printing Office, Washington, D.C.) p. 83.

Olstad R A, Olander D R, 1975 J. Appl. Phys. 46, 1509.

Rohlfing E A, Cox D M, Kaldor A, 1984 J. Chem. Phys. 8JL, 3322.

Stickley C M, May 15-16, 1985 "Countermeasures Development (LCMMD) Program

Review," (The BMD Corp .

)

Taki K, Kim P H, Namba S, 1969 Bull. Chem. Soc. Japan 42, 823.

Wachi F M, Gilmartin D E, 1970 Carbon 8., 141.

29
Zavitsanos P D, 1968 Carbon 6, 731.

Zavitsanos P D, Carlson G A, 1973 J. Chem. Phys . 59, 2

30
Table

1.

Hass

Spectral

Ion

Intensity

and

Relative

Partial

Pressure

Data

(Normalized

to C3)

for

Laser

Vaporized

Graphite >0 3 CM

cd <r CO vo CM CO

co r 140 0 0 CM rH •H rH rH P Pi

•iH O H rH O 0 0 0 0 0 > O

cd O N rH<f1

43N4J

•H PS CM rH

r»» rH CM 00 0 rH O 0 o 0 CO CO 0 O 0 0 0 O 0

Pi 0 • • • e • • • 54 0 O 0 rH O 0 0 0 O 0

0) CQ i

433

cd PS 00 CO CO Ov

CO 0 O H 0 O rH

CO 0 • • « • • 1 1 1 1 3 rH O 0 rH O 0

oi <f

43

a)54 Pi o£ 0 VO 00 rH CO

54 0 O 0 0 O rH 0 O

a) rH • • • • • • 1 1 > <f O 0 rH O 0 0 0 «H

43 Pi CM r>»

1*1 rH 140 rH CM < 0 rH rH 0 0 O

J25 0 1 • • • • 1 1 1 1 <3 rH 0 0 rH 0 O *“5 <r

PS O'- 00 rH co CM rH

CO CM 0 O 0 O O O 0 VO CM 0 0 O 0 O O O » 0 • • • • • • • • PS CO 0 O rH 0 O 0 O O O 54 CO O& CQ

CO /^S •H PS 42 140 CM rH CM H O rH rH 0 0 O O • • • • 0 O O 0

rH 0 0 rH 0 O <

CO0

•H H CV] CO

m CD rv CO o> 0 O 0 CJ 0 0 0 0 0 0a

CO

CO /N

CO 3 CM <r VO co 0 CM vO 00

cd £ rH CM co mT VO 00 av O E cd rH

5-1 o

4-1*0P

0)0cn

CM

co

a)u3

b0 •H

4430>

*HbO

a)

coo

42P

43O3

co

•t-4 44O

5-1 a<o

54O

4-1

140 CM

T333

cQ

T3 3 3 0 co 03 < P0 CO 42 P

Cl) co co

cd cd 3 4J bO cd i4 T)

CO 44

T3 O 0P CO

cd 3

54 O

bO •<4 0 P P •H 3 3

*H •H44 £ 0 O X5

54 f4-1 0

co

-a OS

31 e

text

for

literature

citations.

JANAF,

Livermore,

and

Russian

data

are

relative

partial

pressures

normalized

toC3,

where

the

absolute C3

partial

pressures

are

0.91

and

1.32

atm

for

the

JANAF

and

Russian

data,

respectively,

for

instance.
Table 2 . Surface Temperature Results in the Graphite System

(Data-Set A)

Method Temperature (K)

C2 + arrival time relative to B 1 + at 2900 K) 4000 ± 300

Pc from P = k ICC/) T, with JANAF (1971) 4100 ± 200

C 1 + /B1 + intensity ratio (for B at 2900 K) 4000 ± 300

C2 + /Bi

+ intensity ratio (for B at 2900 K) 3900 ± 300

C//C3 + intensity comparison with JANAF (1971) 4000 ± 300

C 5 + /C3 + intensity comparison with JANAF (1971) 3900 ± 300

Selected Value = 4100 ± 200

32
Table 3. Temperatures (K) Calculated from Partial Pressure Ratios, Relative

to C3 , (as Listed in Table 1 for Data-Sets A and B) for Various

Thermochemical Data Sources

Species JANAF Livermore

(1971) (Leider et al. 1973)

A B A B

Meyer and Lynch

(1973)

A

Gurvich et al.

(1979)

A B

Ci 3600 2300 -0 -0 >3500 2500 3200

c2 3400 4700 >5000 GO 3500 4000 4600

C3

- - - - - - -

C* 4000 3700 3300 3100 >5000 _ 3600 3300

C5 3900 3500 3200 3100 >5000 3100 2700

33
Captions

Figure 1

Figure 2

Figure. 3

Figure 4

Figure 5

Figure 6

Figure 7

Schematic of laser- induced vaporization mass spectrometric facility

(side-sectioned view), showing laser- sample -molecular beam

geometries

Mass spectral TOA profiles (scaled to unit intensity) for the C 1 +

, C2 +

, C3 + species produced by 26 eV ionizing electron energy.

Relative peak ion intensities are 2.4, 2.1, and 3.4 respectively.

The time- scale has been corrected for the 180 /is laser pulse

initiation time. Note that the curves connect data points taken

every 500 fi s

Mass spectral TOA profiles (scaled to unit intensity) for the C 4 +

, C5 +

, C6 + species produced by 26 eV ionizing electron energy.

Relative peak ion intensities are 0.9, 0.6, and 1.3 respectively.

Other conditions similar to figure 2 (see caption)

Mass spectral TOA profiles (scaled to unit intensity) for the C7 +

, Cg +

, C9 + species produced by 26 eV ionizing electron energy.

Relative peak ion intensities are 0.5, 0.5, and, 0.1 respectively.

Other conditions similar to figure 2 (see caption)

Dependence of the half -height TOA values for Cn (n = 1 to 5 or 9)

ions as a function of molecular weight (square root)

The circular

and square data points are for data-sets A and B, respectively,

obtained at 26 eV. The time -scales have been corrected for the nonzero time of the laser pulse initiation, ie 60 /is and 180 jus for

data-sets A and B, respectively.

Comparison of experimental (data set A, table 1) and JANAF (1971,

1985) predicted Cn species partial pressure ratios at 4100 K.

Comparison of experimental (data-set A, table 1 for C 1 -C 5 and B for

C6 -C9 ) and JANAF (1971, 1985) -predicted partial pressure versus

reciprocal temperature data for Cn (n = 1-9) species.

34
Figure 1
LO

(sqmn AjBjqiqje) Aq.jsu0q.u1 uo|

Relative

Time

of

Arrival

(msec)

Figure

2
LO

Relative

Time

of

Arrival

(msec)

Figure

3
N- OO CD

o o o

I I

in

c\i

Relative

Time

of

Arrival

(msec)
Square Root of Mass (amu)

Figure 5
Pressure

Ratio

TEMPERATURE K

Figure 6
PARTIAL

PRESSURE

(atm)

TEMPERATURE K

5000 4000 3000 2500

Figure 7
NBS«Y14A !Rt*v. 2 «ac> U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

1. PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. Publication Date

NBSIR 87-3561 MAY 1987

Thermochemistry of Materials by Laser Vaporization Mass Spectrometry, Part II Graphite

5. AUTHOR(S)

John W. Hastie, David W. Bonnell, Peter K. Schenck

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE S. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS ( Street. City . State, ZIP)

National Bureau of Standards

Gaithersburg, MD 20899

10. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si ghificant information. If document includes a significant

bibliography In an earlier study (Part I of this series) an experimental

approach which couples laser heating of refractory materials under vacuum

with mass spectrometric detection of the vapor plume, for thermochemical

determinations, of 2900 K was demonstrated using the BN refractory system

at 2900 K. This paper describes the results of a similar study on graphite

at temperatures around 4100 K and total species pressures in the vicinity

of an atmosphere (1 atm = 1.01325 x 105 Nm“ 2 ). A Nd/YAG laser system,

focused to power densities in the range of 109 - 10 10 W/cm2

, was found to

be a convenient energy source for producing controlled vapor plumes and

with negligible post-vaporization perturbation of the neutral species

identity and concentration. The principal vapor species were found to be

Cn (n = 1 - 9) and their relative intensities were found to be more

consistent with the JANAF Thermochemical Tables than more recent literature

assessments and results. No means were available to directly determine

surface temperatures under the experimental conditions of very short

heating times (7 - 20 ns) and luminous vapor plumes. Therefore various

indirect methods were used, including comparison of the total and partial

pressure data with literature results and_£ime- of -flight analysis of the plume species.

Surface temperatures were found to be in the region of 4100 + 300 K and the sum of

the Cn species partial pressures was determined to be in the vicinity of 1 atm.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

graphite; high temperature; laser, mass spectrometry,; refractory materials;

thermochemistry; vaporization.

13. AVAILABILITY 14. NO. OF

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