journal J. Am. Ceram. Soc., 72 [lo] 1918-24 (1989) Dispersion of Silicon Carbide Powders in Nonaqueous Solvents
Masahiko Okuyama*,*Gary J. Gamey,* Terry A. Ring**+and John S. Haggerty* Materials Processing Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Thirty-two pure solvents were used to disperse laser-synthesized powder dispersibility. Good dispersibility gave ceramic green SIC powder, oxidized laser-synthesized SIC powder, and com- bodies with high green densities. [Key words: silicon carbide, mercially available Sic powder. Five-day sedimentation tests dispersion, powders, surface, solvents.] were used to screen the solvents. Relative turbidity of the supernatant after 1 month was used as a quantitative meas- I. Introduction ure of the degree of dispersion. Coagulation kinetics were measured by photon correlation spectroscopy to determine the STABLE powder dispersion is necessary to produce uniform, coagulation rate. Stabilized powders were centrifugally cast A dease ceramic green bodies. Two types of dispersion systems into ceramic green bodies and their green densities measured. may be used: pure solvent systems and solvent-dispersant sys- Experimental dispersion results were correlated with various tems. Solvent-dispersant systems are known to be more effective solvent properties including dielectric constant, hydrogen- for dispersing fine powders than are pure solvent systems; how- bond index, acid dissociation constant (pK.), and Lewis acid/ ever, dispersants can be difficult to remove from the green body base interaction energy. Microcalorimetry was used to meas- during binder bum-out , particularly with small-diameter powders. ure the heat of wetting of the powders in various aFidic and Because the dispersion characteristics of powders are strongly basic solvents. The heat of wetting was used to determine the dependent upon their surface characteristics, the extremely pure Lewis interaction energy parameters for the powder surfaces. laser-Sic powders are expected to behave differently from com- Oxidized Sic powder, either laser or commercial, was shown mercial Sic powders with their air- and water-exposed surfaces. to have an acidic surface and was stabilized by basic solvents. The dispersion characteristics of pure silicon powders made from Pure laser-synthesized Sic powder was shown to have a basic laser-heated SiH4 were previously studied.' The dispersion stabil- surface and was stabilized by acidic solvents. Solvents with ity of silicon powder was found to depend upon the dielectric high hydrogen-bond indices gave high packing densities. constant of solvents. Other solvent properties had a much smaller influence on This paper examines the dispersibility of both pure Sic pow- ders made by laser synthesis and oxidized Sic powders in pure solvents without dispersants. To develop a more fundamental under- S. C. Danforth-contributing editor standing of the solvent physical properties that are responsible for the dispersion of a particular type of powder, powder dispersion was correlated with the following solvent properties: dielectric Manuscript No. 199380. Received December 28, 1987; approved January 26, constant, hydrogen-bond index, acid dissociation constant (pK,), 1989. and Lewis acid/base interaction energy. Supported by the Office of Naval Research and Army Research Office under Contract No. N00014-82-KO350 and a group of industrial sponsors: ABEX Cop, Akin Seiki, Alcoa, Japan Steel Works, NGK Spark Plug Co., Ltd., Nippon Steel Corp., Sumimoto Electric Corp.. and Toa Nenryo Kog:yo. Terry A. Ring was 11. Experimental Procedure funded by National Science Foundation Contract No. 8617500. 'Member, American Ceramic Society. (I) Materials :Now with NGK Spark Plug Co., Ltd., Aichi-Ken, Japan. Now with Powder Technology Laboratory, Ecole Polytechnique Federal Three types of pure laser-synthesized Sic powders made under Lausanne , Lausanne , Switzerland . different conditions -one commercial Sic powder,' one com- $Betarandom Ultrafine, Ibiden Co., Ltd., Ogaki, Japan. mercial Si02powder,s and an oxidized laser powder-were used $Hi Sil 233, Cabot Corp., Reading, PA. in this study. The general procedures* and particle formation mechanisms3 used to make the SIC powders from laser-heated mixtures of SiHI and either CH4 or C2H4gases are discussed Table I. Laser-Powder Synthesis Conditions elsewhere. The specific powder synthesis conditions used for these powders are given in Table I; their physical properties are Laser power Reaction temp Run No. Carbon reactant (W ("C) summarized in Table 11. The two methane-derived powders (B-060 and L-014) contained approximately 4 wt% free carbon as deter- B-038 Ethylene I50 1650 mined by thermogravimetric analysis (TGA) and chemical analysis. B-060 Methane 150 1680 L.-014 Methane 325 1830 The ethylene-derived powder (B-038) is stoichiometric within the precision of the analyses.
Table 11. Characteristics of Powders Laser oowders B-038 B-060 L-0 14 Commercial Sic Oxidized L-014 Sic Commercial SiO, TEM particle size (nm) 31 52 90 130 90 Specific surface area (m2/g) 44.9 44.3 22.4 19.4 23.3 133 Phase P P P P P Size distribution Narrow Narrow Narrow Wide Narrow Wide Morphology Spherical Spherical Spherical Irregular Spherical Spherical Surface Sic Sic + C Sic + C SiO, SiOl SiO, 1918 October 1989 Dispersion of Silicon Carbide Powders in Nonaqueous Solvents 1919 Table 111. Solvents Used and Results of Screening Tests Hydrogen- Drago4 E and C values ((J/rnol)”’) bond PK, Dielectric‘ Solvent c* CB E* Es index” Acid Base constant Hydrocarbons (1) Hexane 2.2 1.89 (2) Toluene 123.5 5.6 3.0 37 2.37 Chlorides (3) Methylene chloride 1.29 219.9 2.7 12.6 (4) Chloroform 9.70 214.2 2.2 4.8 (5) Carbon tetrachloride 2.2 2.238 (6) 1,2-Dichloroethane 2.7 10.36 (7) Trichloroethylene 2.5 3.4 (8) Chlorobenzene 2.7 5.621 Cyanide (9) Acetonitrile 86.69 57.32 4.5 25 - 10.1 Ethers (10) Isopentyl ether 206.39 71.81 (6.0) 2.82 (1 1) Tetrahydrofuran 276.26 63.27 5.3 -2.2 (12) Dioxane 153.98 70.52 5.7 -2.9 2.2 Ketones (13) Acetone 150.74 63.85 5.7 20 -7.2 20.7 (14) 2-Butanone 153.98 70.52 5.0 (15) 2-Heptanone (5.5) 9.8 Esters (16) Ethyl formate 5.5 7.10 (17) Ethyl acetate 112.57 63.08 5.2 26 -6.5 6.02 Aldehyde (1 8) Benzaldehyde 5.2 14.9 -7.1 17.8 Alcohols (19) Methyl alcohol 72.46 63.08 8.9 16 -2.2 32.63 (20) Ethyl alcohol 29.18 25 1.03 8.9 16 24.3 (21) a-Propyl alcohol 8.9 20.1 (22) Isopropyl alcohol 8.9 18.3 (23) Furfuryl alcohol (8.9) -3.2 9.78 (24) Benzyl alcohol 28.85 269.79 8.9 13.1 (25) n-Octyl alcohol 8.9 10.3 (26) Ethylene glycol 9.6 15.1 37.7 Amine (27) n-Propylamine 394.66 90.58 9.0 10.7 5.5 Carboxylic acids (28) Propionic acid 9.5 4.87 3.30 (29) Caprylic acid 9.5 4.89 (30) Oleic acid 9.5 2.46
The oxidized laser-Sic powder was prepared by heating laser- (
Table IV. Results of Screening Tests and Centrifugal Casting Tests -Dispersibility * Relative turbidity Packing density (96) Solvent Pure Oxid. Pure Oxid. B-038 8-060 L-014 Comml. Hydrocarbons (1) Hexane P P 0 0 11.9 16.4 20.5 28.4 (2) Toluene P P 0 0 12.8 19.5 23.6 31.0 Chlorides (3) Methylene chloride G G 1.9 1.3 11.8 17.2 23.9 26.8 (4) Chloroform G G 3.6 0 14.0 17.0 26.8 29.0 (5) Carbon tetrachloride P P 0 0 11.3 16.7 25.9 26.9 (6) 1,2-Dichloroethane G G 3.6 0 14.0 19.8 25.6 29.4 (7) Trichloroethy lene P P 0 0 12.8 17.7 25.9 27.7 (8) Chlorobenzene P P 0 0 13.9 18.7 26.2 30.9 Cyanide (9) Acetonitrile G G 3.0 3.5 11.1 15.8 25.1 34.8 Ethers (10) Isopentyl ether G P 0 0 25.5 29.0 34.4 (1 1) Tetrahydrofuran G G 3.6 1.3 13.6 21.8 34.1 (12) Dioxane P P 0 0 16.1 25.8 28.7 36.7 Ketones (13) Acetone G G 3.6 3.7 14.7 18.3 26.8 38.0 (14) 2-Butanone G G 0 3.4 15.5 20.7 26.2 34.9 (15) 2-Heptanone P G 0 2.7 22.2 27.5 36.9 Esters (16) Ethyl formate P VG 0 3.5 12.2 22.3 33.0 (1 7) Ethyl acetate G G 3.6 3.5 15.0 22.3 26.4 34.6 Aldehyde (18) Benzaldehyde P VG 0 2.2 26.5 29.5 39.1 Alcohols (19) Methyl alcohol P VG 0 2.3 16.2 24.3 25.0 37.6 (20) Ethyl alcohol P VG 0 3.2 16.1 25.2 27.7 37.8 (21) n-Propyl alcohol P VG 0 2.9 16.2 25.1 27.0 37.4 (22) Isopropyl alcohol P VG 0 3.3 16.3 25.6 30.2 38.6 (23) Furfuryl alcohol P VG 28.9 38.8 (24) Benzyl alcohol P VG 0 3.3 25.1 31.2 41.5 (25) n-Octyl alcohol G VG 0 1.6 26.4 32.4 38.4 (26) Ethylene glycol G VG 3.3 3.5 15.4 22.8 39.0 Amine (27) n-Propylamine P VG 0 4.2 12.7 21.8 28.3 40.2 Carboxylic acids (28) Propionic acid G P 3.4 0 21 .o 27.4 35.9 (29) Caprylic acid G G 2.5 0 22.1 29.7 39.9
(30)~, Oleic acid VG 6 3.8 0 22.3 25.7 32.4 *P, poor dispersion; G, good dispersion; VG, very good dispersion; F, flotation.
be incorporated into the liquid, an F designation was used. Using volumes were determined by measuring their heights on the cali- these designations, the sedimentation test results are given in brated centrifuge tubes. After the sediments were dried in a nitro- Table IV. The relative turbidity of the supernatant, also shown gen atmosphere, sediment weights were measured to permit in Table IV, was measured after 1 month, using a photometer.” calculation of packing densities, which are given in Table IV. The (4) Coagulation Rates microstructures of these sediments were observed by scanning electron microscopy (SEM). The coagulation rates of pure and oxidized laser-Sic L-014 powders in selected solvents were measured by photon correla- (6) Colloidal Pressing tion spectroscopy (PCS). Three kinds of solvents with differ- For the colloidally ressed samples, 290 mg of L-014 powder ent functional groups -n-propyl alcohol, propionic acid, and was dispersed in 9 cmrl. of either hexane or octyl alcohol solvents. n-propylamine -were chosen for this measurement. The suspen- The suspensions were placed in a 2.54-cm-diameter press with sions prepared as above were diluted to optimize the powder con- a permeable membrane on the top die surface and pressed to centration for the PCS measurement. The powder concentration 69 MPa (10 000 psi). When the solvent was completely removed, used in this study was 7.0 X lo9 particles/cm3. After ultrasonic the pressure was released. The disk was dried, and its dimensions agitation, the mean particle sizes were measured periodically measured so that the green packing density could be calculated. with a submicrometer particle analyzer.” The coagulation rate constants were calculated from the size versus coagulation time data, using a technique described by Barringer et ~1.~The vis- 111. Results and Discussion cosities of propyl alcohol and propionic acid use,d in the photon- (1) Dispersion Tests correlation measurements were obtained from Weast,6 and that The results of sedimentation tests, relative turbidities of the for n-propylamine was estimated using Soudre’s meth~d.~ supernatants, and centrifugal casting densities given in Table IV (5) Centrifugal Packing show significant differences in dispersibility between pure laser- Suspensions consisting of 290 mg of SIC powder and 9 cm3 of SIC powder and oxidized laser-Sic powder, but not between solvent were centrifugally cast at 3000g for 2 h. The sediment specific pure powder types. In the case of pure laser-Sic powders, “very good” dispersion was observed only for oleic acid. “Good” dispersion was observed for the other carboxylic acids (such as “Universal light-scattering photometer, Series 2000, Phoenix Precision Instrument propionic acid and caprylic acid) and high-molecular-weight al- Co., Philadelphia, PA. cohols (such as n-octyl alcohol, ethylene glycol, etc.). Oxidized ‘+Model N4, submicrometer particle analyzer, Coulter Electronics, Inc., Hialeah, n. October 1989 Dispersion of Silicon Carbide Powders in Nonaqueous Solvents 1921
51 Table V. Measured Heats of Wetting t I -AN ( x lo-’ J/mol) Solvent SO,* Pure Sic L-014 Oxidized SIC L-014 TEA 92.0 95.4 56.9 Acetone 50.6 45.6 44.6 Cyclohexane 33.4 38.9 35.3 Chloroform 46.0 61.5 38.7 SbC15 463.8 130.6 126.4 *Hi Sil 233, Cabot Corp.
5 The dispersion phenomena observed are related to the acidity Relative Turbidity - Oxidized Sic and basicity of the solvents and powder surfaces. n-Propylamine ! and low-molecular-weight alcohols act as Lewis bases; they Fig. 1. Comparison of relative turbidities of pure and disperse oxidized laser-Sic powder well, but not pure laser-Sic oxidized Sic powder dispersions in various solvents. powder, in spite of high hydrogen-bond indexes. In contrast, oleic acid and propionic acid act as Lewis acids, dispersing pure laser-Sic powder well, but not oxidized laser-Sic powder. (2) Microcalorimetry Using the temperature rise due to the wetting of a powder sur- laser-Sic powders dispersed very well in many kinds of sol- face by a particular solvent, the heat of wetting was calculated vents such as all alcohols, n-propylamine, ethyl formate, and per mole of solvent molecules at the surface of the powder. The caprylic acid. heats of wetting for the various solvents are given in Table V. With the relative turbidity values in Table IV, higher turbidity The E and C values and other pertinent physical properties of corresponds to better dispersion. For pure laser-Sic powder, the these solvents used in this calculation are given in Table VI. Two solvents with carboxylic acid groups, such as oleic acid, caprylic solvents, triethylamine (TEA) and acetone, are bases; two sol- acid, and propionic acid, gave high relative turbidities corre- vents, chloroform and antimony pentachloride, are acids; and one sponding to stable suspensions. For oxidized laser-Sic powder, solvent, cyclohexane, is neutral. Using Fowkes’ meth~d,~~’~the n-propylamine, acetone, ethyl formate, and all the alcohols gave Lewis acidibase interaction energies (AHab)between solvents and high relative turbidity values. powder surfaces were determined. Lewis acidibase interaction The relative turbidities observed for each solvent with both energies for the pure and oxidized laser-Sic powders as well as a oxidized and pure laser-Sic powders are summarized in Fig. 1. commercial SiOz powder are given in Table VII. The heats of The data points are divided into the three encircled areas. Solvent wetting (AH) were corrected for the dispersional interaction en- groups are concentrated in specific areas. Carboxylic acids such ergy using the heat of wetting for cyclohexane, which has only as oleic acid, caprylic acid, and propionic acid are concentrated dispersional interactions but not acid/base interactions.’.’’ in the upper left, showing that these solvents dispersed pure pow- Based on these Lewis acid/base interaction energies, Drago E der well, but not oxidized powder. Amine, alcohol, and ketone and C values for the powder surfaces were determined, using solvents are concentrated in the lower right, showing that they Fowkes’ slope intercept plotting method.’ The E and C values for dispersed oxidized Sic powder well, but not pure Sic powder. the different powder surfaces show both the SiOz (EA = 300.8 Inert and amphoteric solvents were generally at the origin or on (J/mol)”’, CA = 42.7 (J/mol)”’) and the oxidized Sic power the diagonal, indicating that the powder surface did not influ- (E, = 93.8 (J/mol)”’, C, = 27.8 (J/mol)’/’) to be acidic, and ence turbidity. the pure laser-Sic powder (EB = 119.7 (J/mol)”’, C, = 95.1 Comparisons of the relative turbidity with the hydrogen bond (J/mol)”’) to be basic. The EA and CAvalues measured for SiOz index and pK,(acid) and pK,(base) values showed a great deal of are similar to those reported by Fowkes’’ (EA = 282.0 (J/mol)”*, scatter without any discernible trends. The poor fit of turbidity C, = 68.8 (J/mol)”’). with respect to pK,(acid) and pK,(base) is contrary to that ob- served by Bolger,* where the arithmetic difference of either the (3) Coagulation Rates isoelectric point (IEP) and the pK,(acid), or the IEP and the The mean particle sizes for pure laser-Sic powder were meas- pK,(base), gave a parameter that correlated with powder dispersion. ured as a function of coagulation time. The mean particle size for Comparison of relative turbidity with dielectric constant6 (E) of isopropyl alcohol and isopropylamine (e.g., bases) increased lin- the solvents was performed, to see whether high+ solvents fa- early with coagulation time, signifying coagulation, but propionic vored stable suspensions, and whether low+ solvents tended to acid showed almost constant size within the measuring interval of give rise to flocculation analogous to the work of Mitzuta et at. ‘ 180 min, indicating colloid stability. In contrast, the oxidized on silicon powder. No correlation was found for pure Sic pow- laser-Sic powder showed a different trend; the particles dispersed ders; e.g., propionic acid showed good dispersion characteristics in isopropyl alcohol or isopropylamine (e.g., bases) were stable in spite of its low E. The oxidized powder gave results generally in size, and the mean size of the particles dispersed in propionic similar to those observed with silicon powder:’ dispersion stabil- acid increased linearly with coagulation time, indicating coagula- ity generally improved with the increasing dielectric constant of tion. The coagulation rate constant, k, was calculated from the the solvent; however, a large amount of scatter was observed. slope of the size-versus-time curves. These values are given in
Table VI. Solvent Properties Drago E and C values ((J/mo1)“2) Density Mol wt Molar surface area Surface energy Solvent (g/cm3) (g/mol) (X lo5 m2/mol) (X 10’ J/m*) EB CB E.4 CA TEA 0.727 101.2 2.267 22.7 64.1 717.5 Acetone 0.790 58.1 1.482 23.7 63.8 150.7 Cyclohexane 0.779 84.2 1.927 25.5 Chloroform 1.483 119.4 1.575 27.1 214.1 9.7 SbClS 2.336 299.0 2.145 35.8 477.4 331.9 1922 Journal of the American Ceramic Society-Okuyama et al. Vol. 72, No. 10
Table VII. Lewis Acid/Base Interactional Energies Table VIII. Coagulation Rate Constant -AHeb (x lo-’ J/rnol) k (cm’/s) Solvent SOz* Pure Sic L-014 Oxidized Sic L-014 Solvent Pure Sic Oxidized Sic TEA 50.6 58.6 23.5 Isopropyl alcohol 1.23 X lo-” 1.37 X lo-’* Acetone 25.6 8.2 10.5 Propionic acid 7.41 x 10-1~ 1.37 X lo-’’ Cyclohexane -0.3 Isopropylamine 3.38 X lo-’* 5.15 x 10-l4 Chloroform 17.7 21.4 2.2 SbC15 424.0 84.5 84.6 *Hi Sil 233, Cabot Corp. morphology, or degree of aggregation. Type L-014 powder was approximately twice as large as the other two. The B-038 and B-060 powders consist of aggregated primary particles which can Table VIII. Smaller values of the coagulation rate constant indi- be dispersed but do not pack well. The relatively high packing cate more stable dispersions. density achieved with the commercial powder is attributable to its (4) Centrgugal Packing wide particle size distribution and spherical morphology. Packing densities of centrifugal sediments as a function of Packing densities of centrifugal sediments are given in Table IV hydrogen-bond indices of the solvents” are shown in Fig. 2. for various solvents. Packing densities correlate with the liquid Reasonably good correlations were observed between packing functional group, the powder type, and powder run number. Cor- densities of all powders and hydrogen-bond indices of the sol- relations between centrifugal packing densitie:s and the sedimen- vent; high hydrogen-bond indices provided high packing densi- tation results were observed for the commercial powder, which ties. Good correlations between the hydrogen-bond index and had an oxidized surface layer, while some exceptions were ob- sedimentation results were also observed for the oxidized powder. served for pure laser-Sic powders. For pure laser-Sic powders, However, sedimentation results for the pure powders and the dif- octyl alcohol showed the highest overall packing density; but the ferences in dispersibilities between pure and oxidized Sic powders packing density for oleic acid, the best solvent in the sedimenta- could not be interpreted solely with the hydrogen-bond index. tion test, was not very high. These exceptions may be attributed The E and C values for powder surfaces can be used to predict to the relatively low dispersibilities of these particular pure laser- the Lewis acid/base interaction energy for a powder surface with Sic powders. a solvent using Drago’s4 four-parameter theory Significant differences between the centrifugal sediment densi- ties were observed among the four kinds of powders: B-038, -AHab = EAEB + CACB (1) B-060, L-014, and the commercial powder. These differences are attributable to powder characteristics such as particle size distri- where the E and C values of the powder surface are used for bution, particle morphology, and degree of aggregation. Packing either the acid (subscript A) or the base (subscript B) and the density should be independent of particle si:ze when the parti- solvent for the other E and C values. (Note: an acidic surface cles are large enough to neglect electrostatic forces, surface can have only an acid/base interaction with a basic solvent and films, and boundary effects. An increased width of the particle vice versa.) size distribution increases the packing density. When oriented, Using the above equation, the Lewis acid/base interaction en- high-aspect-ratio particles pack to higher den:sities than spheres. ergy for each type of Sic powder was calculated for each solvent Agglomerates and aggregates generally (but not always) have with appropriate E and C values for the various powder surfaces. lower packing densities than individual particles. These Lewis acidlbase interactions were compared with the tur- The packing densities achieved with type LO14 powder were bidity data listed in Table IV, but no correlation was found. The consistently higher than those achieved with type B-038 and correlations between packing densities of centrifugal sediments B-060 powders; the commercial powder had a higher packing and Lewis acidlbase interaction energies are shown in Figs. 3 and density than the laser powders. Since all three pure powders ex- 4. For both oxidized Sic-laser powder (Fig. 3) and pure SiC- hibited nominally the same dispersibilities, differences in packing laser powder (Fig. 4), the packing densities increase weakly with densities must be a result of particle size distribution, particle the Lewis acidlbase interaction energies. Some degree of scatter in the data is observed. This correlation between packing density and Lewis acid/base interaction energy suggests that acid/base interactions are one but not the only factor in dispersion and packing of non-oxide ceramic powders.
- Commercial -s 40 * 50 I .-- I In L-014 1 nQ 30 IJ) C x.- 20 a 8-038
0 5 10 15 20 01 ” ’ I ” ’ d -Interactional Energy I1000 (joules I mole) 0 2 4 6 8 10 Hydrogen Bond Index of Solvent Fig. 3. Centrifugal packing density as a function of Lewis acidlbase interactional energy (-AH“b) for Fig. 2. Centrifugal packing densities of four types of Sic oxidized Sic powder with the investigated solvents. powders as a function of hydrogen-bond indexes of solvents. E, = 93.8 (J/rnol)”’, C, = 27.8 (J/rnol)”*. 1923 October 1989 Dispersion of Silicon Carbide Powders in Nonaqueous Solvents
(5) Characterization of Sediment and Colloidally Pressed Bodies Micrographs of the top surfaces of centrifugal sediments from suspensions of L-014 with hexane and octyl alcohol are shown in Fig. 5. These packing densities are 20.5% and 33.2%, respec- tively. The particles dispersed in octyl alcohol are packed much better, as shown by the smaller number of large voids and loosely packed agglomerates. Micrographs of a fracture surface and a side surface of a colloidal-pressed pellet using octyl alcohol are shown in Fig. 6.
No voids larger than the particle size are present, and several ”3 areas show ideal close packing of spheres. The packing density 0 200 400 600 of this pellet was 62%, approximately the maximum achievable -Interactional Energy / 1000 (joules / mole) level with uniform-diameter, spherical powders in either random or ordered arrays. Fig. 4. Centrifugal packing density as a function of Lewis acidlbase interaction energy (-AHob) for pure Sic powder with the investigated solvents. E, = 543.5 (J/mol)”’, C, = 1,255 (J/rnol)1’2.
Fig. 5. SEM micrographs of top surfaces of centrifugally packed sediments: (A) powder L-014 in hexane, and (B) powder L-014 in octyl alcohol.
Fig. 6. SEM micrographs of a colloidally pressed compact of powder L-014 in octyl alcohol: (A) fracture surface, and (B) side surface. 19% Journal of the American Ceramic Society-Ohtsuka et al. Vol. 72, No. 10
IV. Conclusions References ‘S. Mitauta, W. R. Cannon, A. Bleier, and J. S. Haggerty, “Dispersion and Cast- An evaluation of the effect of solvent properties on pure and ing of Silicon Powder without Deflocculants,” Am. Ceram. SOC. Bull., 61 [8] oxidized Sic powder dispersibility and packing density has been 872-75 (1982). performed. Dielectric constant, pK,, hydrogen-bond index, and ’K. Sawano, J. S. Haggerty, and H. K. Bowen, “Formation of Sic Powder from Laser Heated Vapor Phase Reactions,” Yogyo Kyokaishi, 95 [I] 64 (1987). Lewis acidibase interaction energy did not correlate well with ’J. H. Flint, and J. S. Haggerty, “Models for Synthesis of Ceramic Powders by powder dispersibility. Packing density was correlated with high Vapor Phase Reactions”; to be published in the Proceedings of the First International hydrogen-bond index and high Lewis acidlbase interaction energy; Conference on Ceramic Powder Processing Science. however, some scatter was observed with both correlations. “R. S. Drago, G.C. Vogel, and T. E. Needham, “A Four-Parameter Equation for Predicting Enthalpies of Adduct Formation,” J. Am. Chem. Soc., 93, 6014-20 Heat-of-wetting studies showed that oxidized Sic powders, (1971). either laser or commercial, have acidic surfaces and were dis- ’E. A. Barringer, B. E. Novich, and T. A. Ring, “Determination of Colloid persed best by basic solvents. Pure laser-synthesized Sic powder Stability using Photon Correlation Spectroscopy,” J. Colloid Interface Sci., 100, was shown to have a basic surface and was dispersed best by 584-86 (1984). ‘R. C. Weast, Handbook of Chemistry and Physics, 47th ed. Chemical Rubber acidic solvents. Oleic acid showed the best dispersibility for pure Co., Cleveland, OH, 1967. laser-Sic powder; however, it may not be desirable for ceramic 7M. Soudres, Ir., “Viscosity Prediction Equations,” J. Am. Chem. Soc., 60, processing because it has a high viscosity and a high boiling 154-56 (1938). point. Octyl alcohol is the most suitable pure solvent studied for ‘J. C. Bolger, “Acid-Base Interactions between Oxide Surfaces and Polar Or- ganic Compounds”; pp. 4-18 in Acid-Base Interactions. Edited by K.L. Mittal. both pure and oxidized Sic powder. Elsevier, New York, 1981. Using colloidal pressing with pure Sic powder dispersed in 9F. M. Fowkes and M. A. Mostafa, “Acid-Base Interactions in Polymer Adsorp- octyl alcohol permitted uniform, -70% densify green bodies to tion,” Ind. Eng. Chem. Prod. Res. Dev.,17, 3-7 (1978). be obtained. This result showed that maximum density green ‘OF.M. Fowkes, “Acid-Base Contributions to Polymer-Filler Interactions,” Rub- ber Chem. Technol., 57, 328-44 (1984). bodies can be made with the laser-synthesized powders if they ““Paint Technology Manual,” pp, 6-12. E.I. Dupont de Nemours and Co., are properly dispersed. Wilmington, DE, 1968. 0
journal J. Am. Cerum. Soc., 72 [lo] 1924-30 (1989) Fabrication of Metal-layer (Nickel] Silicate Microcomposite Particles by a Surface-Nucleated Precipitation Route Kunio Ohtsuka, Johji Koga, Mitsuru Suda, and Mikiya Ono* Research and Development Center, Ceramics. Mitsubishi Minina and Cement Co.. Ltd.. 2270 Yokoze, Yokoze-machi, Clhichibu-gun, Saitama-ken 368, Japan
Clay complexes with surface-grown nickel(I1) basic salts were properties on their external surfaces as well as on their internal produced from nickel(I1) nitrate solutions containing dis- surfaces. Little work has been carried out, however, to form clay persed layer silicate by homogeneous precipitation using hy- complexes using their external cation-exchange properties. drolysis of urea. These clay-nickel@) salt complexes were We attempted to prepare the clay complexes with surface- obtained with clay having cation-exchange properties; in con- grown metal precursors, which would be converted to metal-clay trast, using clay without these properties caused formation of composites, by depositing metal basic salts as precursors for a separate precipitate. Metal-layer (Ni) silicate microcom- metal on the clay surfaces, using their ion-exchange properties of posite powders, consisting of layer silicate uniformly covered the outer surfaces. We believe that the fundamental requirement with metal particles, were derived by reduction of these prod- for the complex formation lies both in a growth of nuclei on the ucts; the metal/clay weight ratio was varied from 0.76 to silicate surfaces and in a uniform growth of precipitates on these 8.01. At a low metal/clay ratio (=0.76), highly divided fine nuclei. When expandable layer silicate is used, nucleation at the metal (40 nm) was fixed on the clay surface. [Key words: cation-exchange sites and cniform growth of precipitates on these silicates, particles, processing, powders, clays. 1 nuclei should be generated by means of a homogeneous precipita- tion process. Homogeneous precipitation techniques have been I. Introduction applied to prepare monodispersed uniform powder^,^-^ which are indispensable to the research and development of high- XPANDABLE layer silicates are characterized by their large performance ceramics, as well as less-contaminated and easily Ecation-exchange capacity, and a variety of intercalation com- filtered precipitate^',^ in the field of analytical chemistry. pounds have been prepared utilizing these properties. Obvi- In this investigation, urea decomposition is employed for ously, these expandable layer silicates possess cation-exchange homogeneous precipitation: urea hydrolyzes slowly in aqueous solu- tions at elevated temperatures (80” to 100”C), causing a rise in the pH of the solution uniformly and leading to the formation of precipitates. The conventional precipitation method (precipitation K. E. Spear-contributing editor by addition of a base) cannot be employed for these purposes, because addition of precipitating agents to the solution would cause the supersaturation to be locally very high and result in Manuscript No. 198947. Received August 12, 1988; approved March 14, 1989. free precipitates, even if slow addition and vigorous stirring ‘Member, American Ceramic Society. were employed.