NBSIR 74-628

Mass Transport and Physical Properties of

Large Crystals of Calcium Apatites:

Studies of Ca(0Hj2 Crystals For Use in

Electrolytic Conversion of Calcium Fluorapatite

Crystals to Calcium Hydroxyapatite

A, D. Franklin and K. F. Young

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

September 1, 1973 -August 31, 1974

Annual Report II

Prepared for

Department of Health, Education, and Welfare

National Institutes of Health

National Institute of Dental Research

Bethesda, Md. 20014

Interagency Agreement No. Y01-DE-30010

NBSIR 74-628

MASS TRANSPORT AND PHYSICAL PROPERTIES OF

LARGE CRYSTALS OF CALCIUM APATITES:

STUDIES OF Ca(0H)2 CRYSTALS FOR USE IN ELECTROLYTIC CONVERSION OF CALCIUM FLUORAPATITE

CRYSTALS TO CALCIUM HYDROXYAPATITE

A. D. Franklin and K. F. Young

Inorganic Materials Division

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 20234

September 1, 1973 -August 31, 1974

Annual Report II

Prepared for Department of Health, Education, and Welfare

National Institutes of Health

National Institute of Dental Research

Bethesda,Md. 20014

Interagency Agreement No. Y01-DE-30010

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts. Director

ABSTRACT

In order to convert single crystals of calcium fluorapatite to

calciuip. hydroxyapatite , an electrolytic cell technique will be explored.

To utilize such a technique, the cathode compartment must consist of

a source of hydroxyl ions and a barrier to the flow of all others.

CaCOH)^ has been selected for the cathode material, backed

by a Pt electrode in an atmosphere containing H^O and 0^ . Ca(OE)

crystals have been grown and Ag as well as Pt electrodes applied to

them. Their ac admittance as a function of temperature has been

measured and analyzed. A Warburg contribution to the admittance

was observed to depend upon the presence of 0^ . Equivalent circuits

have been generated from the data allowing us to tentatively characterize

the electrical properties of the electroded crystal system.

1 . INTRODUCTION

We propose to electrolytically convert available large crystals

of either calciiom fluorapatite (Ca, „ (PO J ^F^ , or FA ) or calcium

10 4 6 2 p

chlorapatite (Ca^^ (PO^) ^Cl^ / or ClAp) to calcium hydroxyapatite

(Ca^Q (PO^) ^ (OH) 2 or OHAp) . In their closely related crystal structures"*",

the X~ ions in XAp are known to be relatively mobile.^ Possibly, then,

large crystals of OHAp can be prepared by replacing F in FAp or CI

in CIAp crystals by OH . 3 - ~ Young and Elliot have replaced all but 6% of the Cl with OH

ions in CIAp by using the diffusion controlled process of annealing

CIAp in steam at 1000°C. This would take an almost infinite amount of

time to replace the last traces of Cl ions with OH ions, whereas an electrolytic technique would drive out the X ions and drive in the

OH ions at a steady rate determined by the magnitude of the impressed

current.

It is, therefore, the first objective of this project to test the possibility of the electrochemical transformation of large single

crystals of FAp to OHAp , This is to be acomplished by constructing a solid electrolytic

cell of Pt:CaF :FAp:Ca(OH) :Pt. For the anode compartments, it

4

has been shown that the transport number of F ions in CaF^ is unity and

the conductivity of oxygen-doped CaF^ is sufficient for the desired

current to flow. For the cathode compartment, we have chosen

CaCOH)^ as the most likely candidate for the OH ion conductor.

We have diffusion-grown crystals suitable for characterizing OH

ionic conduction, but work is continuing on an electrolytic growth

technique which should produce larger crystals needed ultimately

for making sizeable OHAp crystals using the electrolytic conversion

cell

.

Silver paint electrodes were used on some specimens and made possible

electrical measurements that seem to hold out promise for characterization CaCOH)^. These data shows a Warburg dependence of the electrodespecimen interface which we interpret as a diffusion-controlled blocking

process. The next step, then, is to experiment with the electrodes which

1
eliminate this blocking to allow measurement of the bulk conduction of

Ca(0H)2 and which ultimately we can use in the electrolytic conversion

cell.

2. CRYSTAL GROWTH

Crystals of Ca(0H)2 suitable for most of the studies planned

have been grown by the diffusion technique described in the last

Annual Report. Ultimately, however, it will be desirable to have

larger crystals. The diffusion technique is limited in the apparent

crystal size produced, at least in small vessels. Once the original

charge of CaCl^ and NaOK have reacted, there appears to be no way

to add new reactants without nucleating new crystals. What is needed

is a way to draw continuously and slowly from large reservoirs of

reactant as growth proceeds.

Some experiments have been performed on a possible new technique

to do this, utilizing an electric current through an ionic solution

as a way to move ions from one compartment ot another. This technique

is illustrated by the sketch in Fig. 1 of one experimental apparatus.

A U-tube contains agar-agar made with 1 M KI solution so that liquid

cannot pass freely between the arm.s of the tube. A solution of KI

(1 M) , and CaCl^ (0.1 M) (slightly acidified to minimize CO^) is

placed in the cathode and anode compartments . The cathode compartment

also contains a stirring device and a probe to monitor the resistance

of the solution.

A small current, of the order of 1 mA, is then passed continuously

through the cell. At the cathode H^O is decomposed, forming gas find

OH ions. At the anode, I ions discharge, most reacting with the OH

ions to release 0^ gas and form H^O. The overall reaction is the for-

_ 2 2 mation of OH ions in the cathode, and their disappearance in the anode,

compartments. Because of the large concentration of "inert" ions

(k"*" and I ) there is probably little or no electric field in the bulk

of the cell. Thus, the concentration of OH ions builds up in the

cathode compartment and decreases in the anode compartment until a diffusion flux is created transporting OH ions from cathode to anode.

At the same time depletion of the OH ions concentration in the

anode compartment will cause the dissolution of some of the Ca(0H)2

in contact with the electrolyte in that compartment. As a result,
2+

the Ca ion concentration will be increased in the anode compartment

2+

and a diffusion flux will carry Ca ions toward the cathode

compartment. At steady state, the total current will be carried by

2+

the sum of the OH and Ca fluxes. By controlling the current,

the magnitude of these fluxes can be controlled, and hence, the value

of the concentrations of each ion in the two compartments. If the

current be properly adjusted, the solubility of Ca(0H)2 can be

exceeded in the cathode compartment and CalOH)^ made to crystallize

there. The result is a continuous transfer of Ca(0H)2 from the anode to the

the cathode compartments, at a rate determined by the easily-controllable

current.

In principle the transport equations governing the flow of mass

and charge in this system can be set-up and solved and the time

dependence of concentrations deteinnined in the various compartments,

at least for simplified geometries. This analysis has been started

but not yet completed. However, in a qualitative way the steady-state

behavior expected can be understood in terms of Fig. 2, drawn for the

2+ situation in which the initial concentration of Ca greatly exceeds

that of OH . At zero time both would be represented by the horizontal

dotted line at unit relative concentration.

If the Ca(0H)2 layer were not present, or if the dissolution of

the CaCOH)^ were very slow compared to the discharge of ions at the

electrodes , then the OH ion concentration would rise in the cathode

compartment and fall in the anode compartment until a concentration

gradient was established such that the charge transported by the

diffusion flux of the OH ions just equaled the current in the external

circuit. At the same time Ca(OH) would precipitate in the cathode

2+

compartment until the Ca ion concentration in the entire cell

became uniform at the value dictated by the solubility product and the

OH ion concentration in the cathode compartment. No more precipitation of Ca(OH) would take place, and no more charge transport by

2+

Ca ion. The entire current in the cell would be carried by

OH ions

.

3
With Ca(0H)2 dissolving from the reservoir at a finite rate, some of the OH ions reaching the anode would be provided by this

process and fewer by diffusion in the agar-agar layer. The negative

concentration gradient of OH ions in this layer would be correspond2+

xngly lower. The Ca ion concentration in the anode compartment

and in the Ca(OH) layer would be increased, giving rise

2+

to a negative concentration gradient of Ca ions in the agar-agar

layer and a flux of these ions into the cathode compartment where

they would react with some of the OH ions produced at the cathode

and precipitate as Ca(OH) . Hence some of the current in the bulk

2+

of the cell would be carried by the Ca ions flowing counter

to the OH ions. The proportion carried this way will depend upon

the rates of dissolution and precipitation of Ca(OH) and of diffusion

2+ - of Ca and OH ions. In the limit of very fast dissolution and

2+ 2+

Ca transport, all of the current would be carried by Ca ions, and

the net result would be the quantitative transport of CaCOH)^

from the reservoir to the cathode compartment. The actual situation

probably lies somewhere between the two extremes of transport

2+ purely by OH or Ca ions.

A number of experiments have been performed to test these

general ideas. In one set of experiments a simplified apparatus

consisting of two concentric tubes with a hole in the inner joining

the two was used. No reservoir of CaCOH)^ was provided, and the solution

contained IM.KCI plus 0.1 M CaCl^. Crystal plates of Ca(OE) ^ several mm across were grown on Cu and brass cathodes , demonstrating the

feasibility of the technique. The lack of the CaCOH)^ reservoir,

however, precluded extended growth.

In a second set of experiments, the configuration of Fig. 1 was used, with CaCl^ added to the cathode compartment only. Crystal

growth on the cathode has again been observed, as well as general

precipitation in the cathode compartment. The transport of CaCOH)^

from the reservoir into the cathode compartment has been demonstrated

but more experiments are needed to find the conditions of concentration

and current required to give smooth crystal growth without excessive

nucleation.
3. ELECTRODE APPLICATION

The electrodes in these experiments need to act both as catalysts

and as electron transfer agents to ensure an adequate supply of

hydroxyl ions. For this purpose the electrodes must be porous enough

for rapid diffusion of atmospheric H^O and 0^ to the sites where OH

ions can be formed and transferred to the crystal. Without sufficiently

porous electrodes capable of sufficiently rapid adsorption and catalytic

formation of OH ions, the obtainable current may be limited by the

diffusion and catalytic processes rather than by mass transport in

the crystal. This kind of situation has been studied recently for

stabilized zirconia solid electrolytes with Pt electrodes in various

atmospheres

Evaporation was tried as the first technique for putting down

Pt electrodes. The initial experiments, as reported in our last

Annual Report, were encouraging. Additional experiments during this

past year showed that the crystals could be handled in air with

reasonable precautions without measurable weight gain, and that

carbonate layers that did form, from long exposure to air at room

temperature appeared to do so only on the surface, even when the

crystal turned quite milky and opaque. A 2% HMO^ solution etched

this layer rapidly, leaving a clear crystal that dissolved only very

slowly.

Under the heating from the filament in the vacuum the crystals

sometimes were damaged, probably by loss of H^O. The resulting crystal

was milky rather than clear and cleaved into numerous milky slices

along the basal planes. To avoid the overheating a jig was built as shown in Fig. 3. Two removable radiation shields protected the

sample and holder from all but the part of the filament bearing the

evaporating Pt. The jig is made of brass and soft-soldered to a copper

plate to which copper tiibing is soldered for water-cooling. A

heating element in the base of the jig allows the temperature to be

raised in a controlled fashion for out-gassing , and a thermocouple

reaching into the center of the jig just below the specimen holder

monitors the jig temperature.
In order to ensure as smooth and controlled evaporation as possible, the tungsten supports with the Pt are cleaned with hot

concentrated HaOH followed by distilled K^O, and not touched except

with tweezers subsequently. Spiral tungsten baskets V7ith 10-30 mg

of Pt were found to be preferable to tungsten loops or "hair-pins".

With the latter the Pt tended to form discrete drops, undoubtedly as

a result of effects arising from the temperature dependence of the

surface tension. When this happened the filament was prone to burn

out in the thin regions between drops before much of the Pt evaporated.

With these arrangements it has been possible to deposit good

mirrors on a glass tube set beside the specimen jig in the vacuum,

but not to produce entirely satisfactory films on the specimens.

The films that were deposited tended to flake off readily and to be

quite coarse in comparison to the mirrors on the glass.

Possible problems are: 1) the size of the apertures in the

radiation shields, which may be too small; 2) the use of benzene

to protect the crystal, since a surface film may be left behind;

31 slow decomposition of the crystal itself, producing enough H^O

vapor to interfere with the deposition of Pt atoms on the crystal

surface. Experiments with the evaporation will continue to try to

determine the cause of poor films. At the same time, arrangements

have been made to sputter Pt films using equipment in another laboratory

to see if this rather different technique, involving essentially no

heat, can produce better films.

Initially, the Pt electrodes were evaporated onto as-grown crystals

2 which were about (10) mm in area and about (1) mm thick. This small area to thickness ratio of the two electroded surfaces yielded low

specimen capacitance. However, attempts to cleave the crystals were

very successful and thickness of the specimen could be reduced by

approximately a factor of 10. For measuring the capacitance of these

latest specimens v;e used silver paint electrodes. These electrodes

were not expected to provide satisfactory catalysts for exchange with

the atmosphere but were intended only as an expedient means in our

6
efforts to improve the signal-to-noise ratio of our electrical

measurements. However, the data indicated the expected dependence upon the presence of oxygen in the specimen environment.

This data is analyzed in the next section (electrical measurements)

althouth these silver paint electrodes are not well characterized.

We must have electrodes which will catalyze the reaction

2H^0 + 0^ + 4e~ 4=?40H~.

They must also be porous enough so that the reactants may diffuse

through to the double interface of solid-solid-gas [Pt-Ca(0H)2-

Reactant gas.] A thin {"^SOOK) film of platin\am, either evaporated

or sputtered, should fulfill these requirements best. The porosity,

thickness, and impurities of the silver paint electrodes which were

used for developing the measurement techniques are unknown and are probably not ideal. However, increases in the equivalent circuit

parameters generated from the data correlate well with the increases

of the partial pressure of oxygen, suggesting some catalytic activity.

4. ELECTRICAL MEASUREMENTS

The apparatus which was described in earlier reports had considerable

shunt capacitance relative to the very small capacitance of the specimen.

Measurements were made for specimens of CaF^ with known low capacitance

and also for "blank runs", i. e., no specimen in place. The shunt

capacitance was reduced from 0.23 pF to <0.08 pF by placing electrically

shielding stainless steel tubing and sheet between the lead and probes

within the environmental chamber. Hence, this capacitance to ground

is not included in the measurement and greatly improves our "signalto-noise" ratio.

The capacitance of a disc-shaped specimen varies as the inverse of

the thickness of the disc. Therefore, thinner specimens have a larger

capacitance and improve our measuring ability by increasing the ratio

of sample capacitance to background (or shunt) capacitance. Our cleaved

specimens are thinner than as-grown crystals by approximately a factor

of 5. This increased saimple capacitance along with the decreased shunt

capacitance yield the following overall improvement. With the shielded

specimen holder and thicker uncleaved specimens, we measured 0.23 pF

shunt capacitance and estimated the sample capacitance to be 0.15 pF.

7
With the improved shielding and thinner specimens, we measure 0.85 pF for

the sample and estimate shunt values to be <0.08 pF

When mass transfer impedance is governed by diffusional processes,

-1/2 there will be a contribution to the impedance that has an co dependence, where co is the circular frequency. This is called a

* 7 "Warburg impedance", Z , (see Sluyters-Rehback and Sluyters ). Typically,

in measurements on KHF Bruinink and Broers show this by simultaneously

1/2 plotting measured values of conductance versus O) and capacitance

-1/2 versus CO . If Warburg behavior exists, then there should be linear

regions with the same slope for each curve. This is so because,

Z* = (l-j)a co"-^/^ w

and ,2 Y* = (1+j) r-r w ^ = G + 3tOC. w 20

Equating real and imaginary parts

,

1 1/2

G = — CO

^ 20

r. 1 -1/2

We expected our porous electroded specimens to exhibit this behavior

and, indeed, as shown in Fig. 4 the linear central regions of the G

and C curves have equivalent slopes for the corresponding partial

pressures of 0^.

2 It appears that we can also qualitatively account for the deviations

from the Warburg behavior. For frequencies under 1 k Hz both the capacitance and conductance deviates somewhat from Warburg linearity; above

20 k Hz the conductance deviates significantly whereas the capacitance

does so only slightly.

For low frequencies, the time period taken for one oscillation of

the applied electric field becomes comparable to the time taken for the

ionic species to diffuse a length equal the electrode thickness. If

this is true, then as the frequency decreases there must be a transition

to a Debye relaxation region which would have an equivalent circuit of

a parallel resistance and capacitance. We call these resistance and

capacitance parameters R and C , respectively. This situation is w w

similar to that commonly observed when majority carriers are blocked

at an electrode but minority carriers can be discharged. There is

then a transition from high-frequency Warburg Behavior to lowfrequency effects dominated by a parallel conductance-capacitance

9 8

combination
At high frequencies our measurement errors become too large for us

to be certain of a Warburg dependence. However, the conductance cannot

increase indefinitely and, indeed, it tends in our data to level off

somewhat, as it must. Further, the capacitance at very high frequencies

should extrapolate to something like the geometric capacitance of the

specimen and our data show this behvior.

We can generate equivalent circuits for our specimen from these

data, keeping in mind the change in the nature of the diffusional

impedance as the frequency varies from high to low. The high frequency

extrapolation of a C versus O)"''"^^ curve gives us a shunt capacitance which

we denote by C^- This is most simply explained as the geometric capacitance of the plane parallel disc-shaped electrodes separated by the

CaCOH)^ specimen, although this may not be distinguishable from a double-layer capacitance at the electrodes in these data. Since all

of our data behave this way, the shunt element, C , must appear in

y

both equivalent circuits, shown in Fig. 5a and b.

Fig. 5a. Low frequency Fig. 5b. Mid-to-high frequency

As the frequency decreases below the Warburg , region , the tendency

toward larger G and a lower C is consistent with the transition of the

diffusional impedance to a parallel network of R and C shunted by w w

Cg. This is shown in Fig. 5a. In the mid-to-high-frequencies range

we see a Warburg element shunted by the capacitance as shown in

Fig. 5b. At high frequencies the apparent bending over of the

conductance curve is consistent with some bulk resistance, R^, in

series with the Warburg element. However, we cannot tell if is

shunting just or the Warburg in series with R^ as shown, because

Rg is apparently very small and our measuring precision at high

frequencies is not good enough to distinguish between these possibilities.

9
There is another way to show that the VJarburg dependence is obeyed

at mid-frequencies and the data are at least consistent with Fig. 5b

at high frequencies. That way is to plot the complex admittance, Y* , \ w

of the presumed Warburg element, where according to our model,

where

[Y*] = measured complex admittance = G+jcoC. m

= high frequency extrapolated geometric capacitance for

/ that particular trial or measurement.

Y* = Y'+jY" is shown in the complex plane in Fig. 6. The plot exhibits

W WW

the slope of unity for both the mid and high frequency ranges.

These electrical measurements are dependent upon the specimen's

environment when the proposed diffusional processes do occur. Figures

4 and 6 show four consecutive trials all measured at 365 °C where the atmosphere surrounding the specimen was varied between pure N and a 4:1 mixture

-1 -1

of N^:0^. The values obtained for the parameters (2a) , R , C and C and

2 2 w w g are listed in Table I. The slopes of the plots of G and C against frequency

(Fig. 4) yield values for (2a) which show a reproducible increase

when 0„ is introduced. The low frequency conductance, R , also shows

2 w

approximately the same increase whereas C and C show little or w g no effect. The expected effect of the presence of 0^ on the admittance

parameters is encouraging. It suggests that OH ions are conducting

and are exchanging with the atmosphere. Further, the apparently very

small value for the bulk resistance, if real, means that the transport

of OH ions may be as high as desired. Future experiments must measure

, however

5. DISCUSSION

The analysis described here seems to fit the data taken so far, and

indicates the direction to proceed with the studies. The strong

frequency dependences observed arise from limiting reactions occuring

at the electrode-electrolyte interfaces. The OH ions may "pile up" at

10
the CaCOH)^ side of the interface so that the low frequency conductivity

the bulk is screened, in which case is probably a space-charge

capacitance. In addition, the Warburg impedance implies that diffusion

of reactants through the electrodes is very slow. Our ultimate aim is

to remove this limitation on OH transport imposed by these processes

,

so that the DC conductance coincides with R^. That is, the experimental

problem involves electroding processes and the observational criterion

is the raising of the low-frequency portion of the conductance curve.

The limiting factor is the catalyzed exchange with the atmosphere,

so we need further work on the porous Pt electrodes, or possibly on other catalytically active materials.

11
REFERENCES

1. R. A. Young, Trans. N. Y. Acad. Sci., Ser. II, 29, 949 (1967).

2. C. Tse, D. 0. Welch, and B, S. H. Royce, Bull. Am. Phys . Soc, » Ser. II, 17, 265 (1972)

3. R. A. Young, and J. C. Elliott, Arch. Oral Biol., 11_, 699 (1966).

4. J. W. Hinze and J. W. Patterson, J. Electrochem. Soc, 120 , 96 (1973).

5. R. J. Brook, W. L. Pelzmann, and F. A. Kroger, J. Electrochem.

Soc. , 118 , 185 (1971)

6. T. H. Etsell and S. N. Flengas, J. Electrochem. Soc. 118 , 1890 (1971).

7. M. Sluyters-Rehbach and J. H. Sluyters, Electroanalytical Chemistry

ed. A. J. Bard (Marcel Dekker, Inc., New York, 1979), Vol. 4, p. 16.

8'. J. Bruinink and G. J. H. Broers , J. Phys. Chem. Solids, 32, 1713 (1972) .

9. J. R. MacDonald, J. Chem. Phys., 5£ 2026 (1971).

12
TABLE I

Environmental

Atmosphere (2a)

-1 -1w w g

xlO xlO

"'"'^ Siemens sec , Siemens

or Farads sec

pF PF

only

^2=°2'

only

1.90

1.76

1.88

1.64

6.5

5.7

7.6

5.7

.6+.1 .853

.855

.859

.861

13
FIGURE CAPTIONS

1. Schematic representation of electrolytic cell for crystal growth

of CaCOH)^. "Reservoir" indicates layer of finely-divided solid

Ca(OH) . - 2+

2. Schematic concentration profiles for OH and Ca ions during

2+

. electrolytic crystal growth for transport by OH only, Ca only,

2+

and both OH and Ca

3. Jig for radiation shielding of specimen during evaporation of Pt

electrodes. Materials is brass. Attachment to cooling coils not

shown.

1/2 -1/2

4. Conductance, G, versus w ; and capacitance, C, versus OJ , for

4 sequential runs. The more steeply sloped curves were measured

in an ^2'''^2 ^'^''^^ mixture, whereas the others were measured in

only. The symbol 03 denotes the circular frequency.

5. (In text) Equivalent circuits drawn from frequency dependences

of G and C; Fig. 5a. Low frequency; Fig. 5b. Mid-to-high frequency.

6. Complex admittance modified to show the Warburg-like behavior region.

Y' = G and Y" = aj(C-C ) , where the value of C were obtained from w w g g

high frequency extrapolations of the corresponding chapacitance

curves shown in Fig. 4.

USCOMM-NfiS -DC

LU

-J

5

Ca(0H)2

'Reservoir

AIN/UUh 1 AbAK - AoAK C/A 1 rfUUt

Transport by OH" Ions- S/ow

Disso/ufion of Co(OH)2

OH

Co2 +

o

UJ

>

<t

-J

UJ

O

<

q:

LUO

o

o

Transport by Ca^* Ions- Rapid Dissolution

of Co{OH)2 and Rapid Ca^-^ Diffusion

I -

Transport by Both OH" and Ca^"*"- Rapid

Dissolution of Ca(0H)2

OH"
Removable Cup

With Specimen

In Masking

Holder

Radfofion

Shields

Thermocouple

Well
8 .01

X {6D-0)n^ = *A
(8-01x5)9

ro e\j — o o> 00 ^-. <o — — — dddd
r»DS-l I4A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PliHI.K A HON OR RKPORT NO.

NBSIR 74-628

2. Ciov 't Acccs.s ion No.

3. Recipient's Accession No.

4. TITI.F AND SUBTITI.K

Mass Transport and Physical Properties of Large Crystals of

Calcium Apatites: Studies of Ca(OH) Crystals for Use in

Electrolytic Conversion of Calcium Fluorapatite Crystals

to Calcium Hydroxyapatite

5. Publication Date March 1975

6. Performing Organization Code

7. AUTHOR(S)

A. D. Franklin and K. F. Young

8. Performing Organ. Report No. NBSIR 74-^ 28

9. PERFORMING 0R( . AN l/ATION NAMF AND AOHRI-SS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Proiect/Task/Work Unit No. 3130457

11. C ontract/Grant No. YOl-DE-30010

12. Sponsoring Organ i/ at i on Name .ind ( onipli-K- Address (Street, City, Slate, ZIP)

National Institute of Dental Research

Bethesda, Maryland 20014

13. Type of Report & Period Covered

Ahnvial. 9/73 - 8/74

14. Sponsoring Agency Code

15. SUPPLKMFNTARY NOTFS

16. AHSTRA( r (A 200-word or less factual summary ol most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

In order to convert single crystals of calciiam fluorapatite to calcium hydroxyapatite,

an electrolytic cell technique will be explored. To utilize such a technique, the

cathode compartment must consist of a source of hydroxyl ions and a barrier to the

flow of all others. Ca(OH) has been selected for the cathode material, backed by

a Ft electrode in an atmosphere containing HO and 0^. CaCOH)^ crystals have been

grown and Ag as well as Pt electrodes applied to them. Their ac admittance as a function of temperature has been measured and analyzed. A Warburg contribution to

the admittance was observed to depend upon the presence of O^. Equivalent circuits

have been generated from the data allowing us to tentatively characterize the

electrical properties of the electroded crystal system.

17. KFY WORDS f.six to twelve entries; alphabetical order; capitalize only the first tetter of the first key word unless a proper name; separated by semicolons

)

Ac impedance; calcium apatites; calcium hydroxide; crystal growth; electrolysis;

interfacial polarization; ionic conduction; mass transport.

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