AFOSR TN 59-481 ASTIA AD 215 734 T HE UN IV ER SIT Y OF MICHIGAN COLLEGE OF ENGINEERING Department of Chemical and Metallurgical Engineering SINTERING OF VANADIUM PENTOXIDE G. Parravano V. J. Lee UMRI Project 2832 under contract with: AIR FORCE OFFICE OF SCIENTIFIC RESEARCH AIR RESEARCH AND DEVELOPMENT COMMAND CONTRACT NO. AF 49(638) -493 PROJECT NO. 9762, TASK NO. 37621 WASHINGTON, D. C. administered by: THE UNIVERSITY OF MICHIGAN RESEARCH INSTITUTE ANN ARBOR June 1959

ABSTRACT The sintering of vanadium pentoxide microspheres has been studied in the temperature range of from 560 to 6500C in air and with spheres of diameters varying from 0.2 to 0.9 mm. Rate data have been interpreted in terms of a sintering mechanism involving migration of vacancies along the surface and grain boundaries of the spheres. The role of whisker growth on sintering is discussed. The overall sintering process is shown to be the result of several steps; the initial stage of the kinetic sequence is the growth of oxide whiskers. iii

INTRODUCTION The role played by chemical processes on the sintering of zinc oxide has recently been investigated.1 These studies have shown that zinc oxide, whose initial composition differs from the composition corresponding to the thermodynamic redox conditions prevailing during sintering, undergoes sintering at relatively low temperatures. In this instance, the driving force for the process is supplied by the oxidation reaction of the stoichiometric excess of zinc. Conversely, oxide samples, which are "equilibrated" with the surrounding atmosphere prior to sintering, cannot be welded together even at high temperatures, and the oxide disintegrates without sintering appreciably. To compare this sintering process (chemical sintering) with a sintering operation not controlled by a chemical process (physical sintering), efforts have been made to find a metal oxide which would undergo sintering under conditions of compositional equilibrium with the surrounding atmosphere. Preliminary runs have shown that vanadium pentoxide fulfilled this requirement. We have, therefore, investigated in some detail the kinetics of the welding together of vanadium pentoxide microspheres at temperatures below their melting point. The results of these studies are reported here, as is information on the effect on sintering of sphere size and temperature. EXPERIMENTAL Vanadium pentoxide microspheres were produced by directing a blast of purified air against a falling mass of molten vanadium pentoxide (C.P. grade).2 The resulting spheres, sized by means of a series of -standard sieves, were subjected to an air annealing treatment to relieve stresses and amorphous regions. A string of spheres of similar size was placed in a platinum boat and heated at constant temperature in a well type of furnace under a slow stream of air. An indication of the extent of sintering was obtained by measuring the width of the neck formed between the spheres as a function of time. These measurements were carried out by means of a creep test microscope, directly placed above the platinum boat. Each value reported for the neck width represents the average of five to six contacts growing under similar conditions. No appreciable bonding was formed between sintering microspheres and the platinum container, which was slightly tilted horizontally to assure the initial contact between microspheres.

The results thus obtained were quite reproducible. Sintering characteristics of nonannealed spheres, however, were erratic and poorly reproducible. RESULTS Typical sintering data are summarized in Fig. 1, where the logarithm of the half-width of the neck connecting the spheres is plotted against the logarithm of time, for different temperatures and constant radius (0.73 mm). All runs were performed in air except for one run conducted in one atmosphere of steam. The average inverse slope, computed from the straight lines drawn through the experimental point of Fig. 1, is - 4. Data obtained at constant temperature on microspheres with different diameters, 2r (0.2 to 0.9 mm), have been used to investigate the effect of the size of the spheres on the rate of sintering. These experiments showed that the relationship between x, r, and time t is of the type x4/r!Ct. The effect of temperature on the rate of sintering has been obtained by noting the time required to obtain a constant value of x at different temperatures. From these data, a value of 32 + 5 kcal/mole was calculated for the activation energy of the sintering process. DISCUSSION Since the composition of the surrounding atmosphere in the course of the reported sintering experiments was similar to composition of the atmosphere prevailing during the preparation of the vanadium pentoxide microspheres, it can be assumed that the chemical composition of the oxide did not vary significantly in the course of sintering. In the present case, therefore, no major contribution to sintering from a chemical process can be expected. Sintering becomes a physical process, whose driving force is provided by the decrease in surface free energy of the system. Supporting evidence for this conclusion can be obtained from the fact that the relationship between x and time was found to be constant throughout the whole range of sintering investigated. This situation does not prevail when a chemical reaction controls the sintering operation.1 The reported relationship, (x4/r2)a time, can then be taken as an indication that sintering of vanadium pentoxide is controlled by a diffusional process, involving migration of vacancies along the surface and grain boundaries, which act as vacancy sink.3 In view of the polycrystalline nature of the vanadium pentoxide used, grain-boundary processes are likely to be kinetically pronounced. A difference in concentration of vacancies between the surface of the spheres (neck) and the bulk can result from several factors: frozen equilibria during preparation, shape, and stresses. The relatively low value of the activation energy supports the view of diffusion along grain boundaries. The presence of water vapor in the surrounding atmosphere during

sintering did not noticeably alter the rate of the process (Fig. 1). This implies that no large composition changes were produced by the presence of water vapor, possibly due to a large (r 54 kcal/mole) positive free-energy change for the reaction: H20 + V205 + 0adsorbed on V205 + H2 In the course of this work, growth of whiskers on the microsphere surface was observed.4 To study the role played by whisker on sintering, several runs were performed by placing the vanadium pentoxide microspheres at some distance apart ( 0.1 mm) in the platinum vessel and observing whisker growth in the space between the spheres at constant temperature. As soon as whiskers, growing at constant cross section, contacted the adjacent sphere, a rapid radial growth of the whisker ensued. Observations of this effect, recorded at 6490C, are schematically represented in Fig. 2. Morphological changes on the initial whisker were clearly discernible after contact was established. In particular, crystallization around the neck area was quite apparent, since the neck became surrounded by rod-like crystals after 150 min (Fig. 2). Possibly, they were oriented with their main direction parallel to the c-axis, since this is the direction of most rapid crystal growth in V205. Kinetically, different growth equations were found to apply to the steps shown in Fig. 2. Therefore we may conclude that sintering of vanadium pentoxide involves several stages, which can be quantitatively characterized by the following relationships in chronological sequence:4 i. Whisker growth at constant cross section up to time tc, when contact with an adjacent microsphere is attained: x = xo; 2. Radial growth of whisker connecting two adjacent spheres: x = xo exp kl(t-tc), k = constant; 3. Radial linear growth: x - const + k2(t-tc); 4. Growth of width of connecting bridge: x = const + k3(t-tc)l/n where n - 4. It is worth pointing out that, since whiskers grow generally by a dislocation process at the expense of the local vacancy concentration, it is plausible that a whisker-bridge mechanism of sintering should be controlled by grainboundary vacancy diffusion.

CONCLUSION The mechanism of sintering of vanadium pentoxide, deduced from the reported experiments, is the result of a physical process (physical sintering). This is in contrast with sintering controlled by a chemical reaction (chemical sintering).1 Therefore two general mechanisms for sintering seem to be available, depending on the chemical nature of the sintering materials and ekperimental conditions. Furthermore, it has been shown that in the case-of vanadium pentoxide several stages are followed by the sintering process, whisker growth providing the initial phase of the process. It is possible that a similar, stepwise mechanism is also followed by other metal oxides, REFERENCES 1. Lee, V. J., and Parravano, G., AFOSR Report No. 58-811. 2. Rollman, W. F., U. S. Patent 2,489,347. 3. Coble, R. L., Journ. Amer. Ceram. Soc., 41, 55 (1958). 4. Lee, V. J., and Parravano, G., AFOSR Report No. TN 59-184.

10 SINTERING OF V205 MICROSPHERES 9 SPHERE DIAMETER = 0.73 mm 8 7 6 5 xxIO mm 4 AIR; 56 C,. n 3,95 o 6260C. n=3.5O e 6260C. n=4,54 0 6490C. n=4.I0 STEAM; * 6260C. n=4.80 2 hr.3 45678910 Fig. 1. Sintering of V205 microspheres; relationship between width of bridge connecting spheres, x, and time of sintering.

9~'SOFA UT T:lAOJ-?fDGU jo SG2Sq-s PGAIGxsqo JO uoTqsquGsGJdGj DTT-~siGmS -E ~rTT.7 ~~~~~~~~'-7 ~T — ~ ~ r. Zx x, 0 ~r~~~~ x1.x.//, -,00, 00, /00~~,* 0 100, oo/~ ~ ~ i00 ~../. NIW~~~~~~~~~~~~~~~~ZI 00 ~~~~'~~ \z/' / oL~~~~~~~~~~~~~~~~~~' ~~~~~~~~~~~~E /~ N~dOL ~ ~7."75 NV /~~~ /.'. // Nmlq~O 7"".-'/'..~~~~~~~oo /./././ NIW 09~~~~~~~~~~~~~~40,0,o 00 tp v,-Z 0 /00 7 *,*'..'....7/ "'"' — 7,/ ~ f ~ ~ J9 7, 7-Nltj31NIS.7./ ~ I I/' 9 o ~o~,9.L S3J3 d *77.^ NI1NSNIIYQ I3N-09H~t 7. 00 0*0 77900, 9, 7*7~~~11,10,000 *-"7'~~~~~~~~~~~~~~~~~~~~~~~~~~~~~7 7~~~~~~~~~~~01 *71-,7' 777.7y7 y'*7 / 77" 100 7'' 7.7, 10' 7 SN~~~dILNIS~,00 ONI'fll>,3N dO HIM0J

DISTRIBUTION LIST No. of No. of Agency Copies Agency Copies Commander, Headquarters 5 Commander 10 AF Office of Scientific Armed Services Technical Research Information Agency Washington 25, D. C. Arlington Hall Station Attn: SRQB Arlington 12, Virginia Attn: TIPDR Commander 4 Wright Air Development Center Director 1 Wright-Patterson AF Base, Research and Development Ohio Hq., U. S. Air Force Attn: WCRRH Washington 25, D. C. Attn: WCRRL Attn: AFDRD7RE-3 Attn: WCRTL Attn: WCRTM-1 Department of the Navy 2 Office of Naval Research Commander 2 Washington 25, D. C. AF Cambridge Research Center Attn: Code 423 L. G. Hanscom Field Attn: Code 421 Bedford, Massachusetts Attn: Technical Library Officer in Charge 1 Attn: CRRF Office of Naval Research FPO No. 100 Commander 1 New York, New York Rome Air Development Center Griffiss Air Force Base Commanding Officer 1 Rome, New York Naval Radiological Defense Lab. Attn: Technical Library San Francisco Naval Shipyard San Francisco 24, California Director 1 Office for Advanced Studies Director 1 AF Office of Scientific Research and Development Division Research General Staff P. O. Box 2035 Department of the Army Pasadena 2, California Washington 25, D. C. Superintendent 1 Division of Research 1 Diplomatic Pouch Rooms U. S. Atomic Energy Commission Department of State Division Office Washington 25, D. C. Washington 25, D. C. For Transmittal to: Commander Oak Ridge National Laboratory 1 European Office, ARDC P. O. Box P c/o American Embassy Oak Ridge, Tennessee Brussels, Belgium Attn' Central Files

DISTRIBUTION LIST (Concluded) No. of No. of Agency Copies Agency Copies U. S. Atomic Energy Commission 1 Office of Technical Services 1 Library Branch Department of Commerce Technical Information Division Washington 25, D. C. ORE, P. O. Box E Document Custodian 1 Oak Ridge, Tennessee Document Custodian Los Alamos Scientific Laboratory P. O. Box 1663 Brookhaven National Laboratory 1 Los Alamos, New Mexico Upton, Long Island, New York Attn: Research Library Arnold Engineering Development Center 1 Argonne. National Laboratory 1 P. 0. Box 162 ~~P. 0. Box 299 ~Tullahoma, Tennessee.Lemont, Illinois Attn: Technical Library Lemont, Illinois Attn: Librarian Commanding Officer 1 Ordnance Materials Research Office Ames Laboratory 1 Watertown Arsenal Iowa State College Watertown 72, Massachusetts P. O. Box 14A, Station A Commanding Officer 1 Watertown Arsenal Watertown 72, Massachusetts Knolls Atomic Power Laboratory 1 Attn: Tech. Reports Section,WAL P. O. Box 1072 Schenectady, New York NACA 1 Attn: Document Librarian 1512 H Street, N.W. Washington 25, D. C. National Bureau of Standards 1 Room 203, Northwest Building Room 203, Northwest Building Air Technical Intelligence Center Washington 25, D. C. Wright-Patterson Air Force Base, Attn: Library Ohio Attn: Deputy for Documentation National Science Foundation 1 1901 Constitution Avenue, N.W. Commander, Headquarters 2 WashinD. C. AF Office of Scientific Research Washington 25 Washington 25, D. C. Attn: Technical Library, SREC Director 1 Office of Ordnance Research Commandant 1 Box CM, Duke Station AF Institute of Technology Durham, North Carolina Wright-Patterson Air Force Base, Ohio Cormrmander 1 Attn: Technical Library, MCLI Western Development Division Commander 1 P. O. Box 262 Wright Air Development Center Inglewood, California Wright-Patterson Air Force Base, Attn; WDSIT Ohio Attn: Mr. R. Besamson, WCLTY-3

UNIVERSITY OF MICHIGAN 3111 III 1111111111111111111511111 612111 3 9015 03022 6412