LIQUEFACTION OF HIGHLY LOADED PARTICULATE SUSPENSIONS

Abstract: Pourable suspensions having a high volume percent of solids are produced by mixing colloidal particles or particles having a narrow size distribution, or both (selected from the group consisting of oxide ceramics and metal particles having an oxide surface), liquid, and dispersant.

Description: LIQUEFACTION OF HIGHLY LOADED PARTICULATE SUSPENSIONS This application is a continuation-in-part of U.S. patent application serial number 856,806, filed April 25, 1986. DESCRIPTION Field of the Invention This invention relates to ceramic and related technologies, and in particular, to the production of pourable suspensions of small size particulates having greater than 55 volume percent of solids, and to pourable suspensions of narrow size distribution particulates having greater than 55 volume percent solids. Background Art Advances in ceramic processing have permitted the replacement of various components of electrical and mechanical equipment with sintered ceramic parts. Ceramics have found widespread use in electronics as a substrate for integrated circuit packages. Ceramic substrates are typically formed by sintering slips produced from suspensions of ceramic particles. However, ceramics have exhibited problems of cracking, large and inconsistent shrinkage upon drying and firing, and nonuniform formability, leading to nonuniform microstructure. Bowen, et al. (1980), "Basic Needs in High Temperature Ceramics," Journal of Material Science and Engineering, Vol. 44, p. 156ff; Rhoades (1981), "Agglomeration of Particle Size Affects Sintering of Y 2 0 3 -Stabilized Zr0 2 ," Journal of the American Ceramics Society, Vol. 64, pp. 19-22; and Lange, et al. (1983), "Processing Related Fracture Origins: I, II, III," Journal of the American Ceramics Society. Vol. 65, pp. 396-408; and Aksai, et al. (1983), "Uniformity of Alumina-Zirconia Composites by Colloidal Filtration," Journal of the American Ceramics Society. Vol. 66, #10, pp. C190 and C193, have demonstrated that processing related defects can greatly affect the performance of a high performance ceramic article. Further, these defects can, and are typically property-limiting. These investigators also demonstrated that by wet processing of colloidal ceramic particles, processing flaws can be virtually eliminated. This is carried out by controlling the packing of the particles on a particulate scale and not on a part-sized scale. Commercial colloidal processing has experienced several difficulties in the formation of ceramic parts due to one or more of the following factors: 1. Due to the inability to overcome interfacial and wetting forces of these fine particulates that serve to keep particles as discrete suspension units, processing has been typically limited to dilute suspension. 2. Typical dilute suspensions result in a low green particle packing density, which generally results in large drying, debinderizing, firing shrinkages and stresses, low green compact strength, and inhomogeneous miσrostructure factors that produce nonreproducable processing and properties. 3. Dilute suspensions tend to have very slow rate-limiting processing steps. Dewatering or debinderizing the carrier situated between fine particles is generally very slow due to the permeability, settling, sedementa ion, and diffusion limitations for particles and compacts of these fine particle and pore sizes. A critical step to overcoming these processing difficulties would be the ability to form concentrated (> 55 volume percent) pourable suspensions of narrow size range micron and submicron size particles. Typically in the prior art the forming process is deemed substantially complete when the particle packing density has reached a value of 55 volume percent. But, low particle density results in low green strength, and the problem is exacerbated by the requirement of removing the binder used in the casting process. A higher volume percent of solids would result in less shrinkage upon drying and sintering, superior green strength, and a reduction in the probability of a large defect or void in particle packing. In addition, faster, processing results from the reduced volume fraction of liquid. A better ceramic piece, i.e., one exhibiting good green * strength and a more controllable drying shrinkage could be achieved if one were able to cast or mold a slip containing a high volume percent solids of either colloidal particles (typically submicron to approximately 2 micrometers in size) and/or narrow size range particles (which may or may not be colloidal) . Prior attempts in making these fine particulate slurries to high solids loading (> 55 volume percent) have been unsuccessful, as the conventional slurry tends to be so viscous at this solids loading that it is not suitable to wet forming techniques including slip casting, tape casting, extrusion, and injection molding. This conventionally limiting rheology can be understood in terms of colloidal particle interaction. For example, colloidal alumina in deionized water usually has negatively charged surfaces. This charge extends away from the particle surface and influences the surrounding solution. This "sphere" of influence is called a diffuse double layer. When two double layers interact (i.e., when two alumina particles come close together) the particles are repulsed and stay dispersed. However, while the double layer keeps particles are apart, the double layer interactions prevent high solids loading due to spacial exclusion. Countering and balancing the double layer repulsion is an attractive force between the alumina particles called the Van der Waals attraction. This attactive force is inversely proportional to the distance between the two particles. This is another colloidal interaction that prevents high solids loading as particles are forced to be within the Van der Waals sphere of influence. While within this sphere, these attactive forces cause agglomeration leading to significant viscosity increases. Decreasing particle size tends to result in an increase in viscosity because of an increase in particle-particle interaction. For a given volume fraction of solids, the smaller the particle size, the closer the particles will be to each other and the greater the. chance for interaction. Even in a relatively dilute suspension of submicron sized particles, the Van der Waals forces present can become quite appreciable. As this particle-particle attraction increases, the viscosity of the slurry increases as it requires more and more force to move one particle past another. Aside from an increase in viscosity, another problem encountered in attempting to make a highly loaded slurry is that the particle-particle interactions may cause the particles to agglomerate. Thus, the suspension looses its homogenity. Further, at very high solids loading, the agglomerates begin to interact with each other, further increasing the viscosity. To counteract the particle-particle interactions, a dispersing agent is typically added. A dispersant is a chemical additive that adsorbs on the particle surface and reduces the double layer interaction while keeping the particles outside the Van der Waals sphere of influence. Several types of dispersant mechanisms are postulated with two primary types of dispersants: steric dispersants, which utilize the size and conformation of the dispersant molecule; and electrostatic dispersants, which use an electrical charge to cause dispersion. Steric dispersants are understood to operate by presenting (i) functional groups exhibiting strong surface interaction with the particle surface while being only marginally soluble in the solvent and (ii) stabilizing moieties that are highly soluble in the solvent. Electrostatic dispersants include acids and bases which operate to modify the pH of the suspending medium. Typically, dispersing agents have been used in aqueous and other solutions to create pourable suspensions of submicron particles having a maximum solids fraction of approximately 50 volume percent. Although this maximum solids fraction can be higher if the particles do not have a narrow size range, uniform size particles may be desirable to make for uniform ceramics having uniform properties, such as reproducible shrinkage for net shape forming, and for high performance applications. Thus, the key to creating a pourable suspension having a high solids loading of colloidal and/or uniform sized particles is providing an effective dispersant. The prior art does not provide such a dispersant. Accordingly, there exists a need for a system for dispersing and maintaining a maximum solids content of colloidal and/or narrow size range particles, thereby maximizing green strength and minimizing shrinkage, while still maintaining the components in a flowable or pourable state. Disclosure of Invention We have discovered a class of dispersants which interact with colloidal and/or narrow size range particles in such a way as to allow the creation of highly loaded, yet low viscosity slurries, suitable for numerous forming techniques. Surprisingly, these systems generally do not need the addition of an organic binder to impart green strength, and thus, the problems associated with burning out these organics are obviated. Th±s invention includes both aqueous, non-aqueous, and mixecE systems. Generally, a dispersant for aqueous " and mixed systems may include one or more of the polymeric polyelectrolytes, discussed in further detail below. A dispersant for non-aqueous systems may generally be an organo-functional-coupling agent or triethanolamine (TEOA) . Moreover, slips made from suspensions produced in accordance with the present invention demonstrate superior green strength. This superior green strength is due in part to the closer packing of narrow size range particles in a high solids content suspension. Importantly, such enhanced green strength also results from the fact that selected dispersants act as a binder in high solids content suspensions. Further details and embodiments of the „ invention are described below. As used throughout this specification and claims, the term- "narrow size range" refers to particles whose size range has a standard deviation not greater than 50% of the mean particle size. Such particles can be produced in a variety of ways, including by centrifugal classification techniques as taught in co-pending U.S. patent serial number 028,891, filed March 23, 1987, which is hereby incorporated herein by reference. - i - Description of Specific Embodiments We have found that conventional dispersing agents will not permit the production of high solid content suspensions of colloidal particles and/or particles having a narrow size distribution having low viscosity. The physical chemistry of these types of colloidal particles is markedly different from that of conventional colloidal particles, so that conventional dispersing agents that are satisfactory at low volume fractions of solids are unsatisfactory at higher volume fractions of solids. Despite these results, we have found that certain polymeric polyelectrolytes and other steric dispersants will work satisfactorily at volume fractions above 50 percent in both colloidal sized and narrow size range dispersions. The dispersant must be readily adsorbed on to the particles, and accordingly, the selection of dispersant is a function of both the particle and solvent choice. The first group of dispersants that we have found useful, especially in aqueous systems, -include the polymeric polyelectrolytes, and, in particular, those having a molecular weight less than about 10,000. Of the polymeric polyelectrolytes, we have found that the acrylic acid-based polymers and co-polymers to be particularly useful. Many of these are commercially available. Preferred ones, particularly with alumina in aqueous systems, include Narlex LD-45, a co-polymer of polyacrylic acid with propylene glyσol and ammonium salt appendages, Narlex LD-42, a similar co-polymer, but with sodium salt appendages, and DB-300, an ammonium salt of a co-polymer of acrylic acid and polypropylene glycol. (All are obtained from National Starch and Chemical Corporation, Bridgewater, NJ.) LD-45 is currently used as a dispersant for titania and other organic pigments in paint formulations. An added advantage of using LD-45 is that its low molecular weight facilitates its burnout. Other polymeric polyelectrolytes which are of interest in the present invention include, but. are πcεt limited to, Darvan C and Darvan 821A brand of ammonium polyacrylate co-polymer dispersants, available commercially from R.T. Vanderbilt Co., Norfolk, CT. The polymeric polyelectrolytes are able to disperse a high volume of solids in non-aqueous systems as well as aqueous. In one embodiment of this invention, suspensions of greater than 60 volume percent of alumina were made in a lower: carbon alcohol-based system using LD-45. &. second type of dispersant found to be helpful in forming; highly loaded, yet pourable dispersions of colloidal and/or narrow size range particles can be roughly classified as steric dispersants. Such a dispersant may be an ethoxy-coupling agent. Steric dispersants of interest include the organo-functional oxy-metal type dispersants and their derivatives generally referred to as organo-silane esters. One preferred class of organo-silane esters and their derivatives include the gamma- amihopropyl-triethoxysilanes and their derivatives, and specifically the commercially available one sold under the name A174 (obtained from Union Carbide Corp., Specialty Chemicals Division, Danbury, CT) . Other dispersants of this general type which are also useful include other organoethoxy-type esters and their derivatives, such as phosphate esters and their derivatives. We have found that RE-610, a commercially available organophosphate ester (obtained from GAF Corp., Wayne, NJ) is particularly preferred. TEOA may also be used, in one preferred embodiment, it dispersed 60 volume percent of submicron sized alumina in alcohol, yet the dispersion retained a viscosity of less than approximately 200 cP. Generally, a preferred embodiment of the invention involves the step of mixing colloidal particles, having micron or submicron size and/or narrow size range, in a slightly greater than critical amount of liquid, with dispersant and an appropriate grinding medium used for deagglomerating the particles, for a relatively brief time (less than approximately one hour) , preferably with powder being added to the liquid in stages in a high amplitude vibratory mill until the desired volume fraction of solids is attained. This mixing step, which should preferably involve mixing at high shear rates, is of critical importance, and can mean the difference between achieving an optimal dispersion at a high solids fraction or not. Alternatively, mixing could take place at a lower than desired volume fraction of solids, and liquid extracted until the desired volume fraction of solids is attained. Suspensions produced in accordance with the present invention subsequently enter a slip forming phase, in which the slips are deaired, using a defoaming agent such as octanol, and agitated to avoid the formation of ordered arrays prior to forming the part. Prior art suspensions having volume fraction solids below 50 percent exhibit substantial shrinkage on drying, and require controlled drying procedures. We have found that pourable suspensions of colloidal particles can be made with such a high volume fraction of solids that when only a small quantity of liquid is removed from the suspension, as for example by evaporation, the suspension sets very rapidly into a solid with a concomitantly small amount of shrinkage. It will be appreciated that the invention may be generally employed with colloidal particles of oxide ceramics and with metal powders. Examples of particles of particular interest in this invention include, but are by no means limited to: alumina, silicon nitride and its alloys, zirconia (including partially stabilized) , and any combination thereof. The following examples are illustrative of the inventio : Example 1 Suspensions of alumina in alcohol, having greater than 60 V% alumina and viscosity less than approximately 200 centipoise, were prepared in the following manner. Dried alumina particles of narrow size distribution having a mean size of approximately .5 micrometers and ranging in size from approximately .3 to .8 micrometers were mixed with isopropyl alcohol and 1/2 weight percent TEOA (such fraction based on weight of dried powder) , and mixed in a container with the appropriate amount of grinding media (for deagglomerating particles) . The mixtures were then milled in a high amplitude vibratory mill for up to approximately one hour, and were then ready for use. The slips exhibited high green strength and rapid set (approximately 5 minutes) at room temperature; less than l minute at 40° C) and low shrinkage (approximately 1%) upon drying. Example II Suspensions of alumina in water, having greater than 60 V% alumina and viscosity less than approximately 200 centipoise were prepared using LD-45 as a dispersant. The basic procedure was similar to that described above, using the same type of dried alumina particles as described above, with water as the liquid in lieu of alcohol, and LD-45 in lieu of TEOA. The concentration of LD-45 was 1.5 weight percent (based on weight of dried powder) . The slips exhibited high green strength, rapid set (approximately 1 hour at room temperature, 5 minutes at 40° C, and less than one minute at 100° C) , and low shrinkage (approximately 1%) . Example III A suspension was made in generaly accordance with the procedure of Example II, except that 20 ml methanol, 13 ml dimethylsulfoxide, 7 ml ethylene glycol, 23 ml LD-45, and 206 ml water were used with 1421 g AKP-20 alumina (from Sumitomo Chemical Co., Japan) The resulting suspension had a viscosity of less than approximately 455 cP at 99.8 sec " .

Claims: What is claimed is: 1. A process for forming a dispersion of particles comprising: (a) preparing a suspension of particles, dispersant and at least one other liquid; and (b) agitating the mixture until the particles are thoroughly dispersed, these steps being carried out to produce a pourable dispersion having greater than 55 volume percent solids. 2. A process according to claim 1, wherein step (a) includes preparing a suspension of particles having a narrow size range. 3. A process according to claim 1, wherein step (a) includes preparing a suspension of particles which are colloidal. 4. A process according to claim 1, wherein step (a) includes preparing a suspension of colloidal particles having a narrow size range. 5. A process according to claim 2, wherein step (a) includes preparing a suspension of particles which are ceramic particles. 6. A process according to claim 2, wherein step (a) includes preparing a suspension of particles which are metallic powders. 7. A process according to claim 5, wherein step (a) includes preparing a suspension of particles selected from the group consisting of: alumina, silicon carbide, silicon nitride and its alloys, zirconia, partially stabilized zirconia, cordierite, forsterite, mullite, titania, and any combination thereof. 8. A process according to claim 7, wherein step (a) includes preparing a suspension of the particles, dispersant, and water. 9. A process according to claim 8, wherein step (a) includes preparing a suspension including a polymeric polyelectrolyte as a dispersant. 10. A process according to claim 9, wherein step (a) includes preparing a suspension including at least one acrylic acid based polymer or co-polymer. 11. A process according to claim 10, wherein step (a) includes preparing a suspension of particles, water, and a dispersant selected from the group consisting of LD-42, LD-45, DB-300, Darvan C, and Darvan 821A. 12. A process according to claim 6, wherein step (a) includes preparing a suspension particles, dispersant, and at least one organic liquid. 13. A process according to claim 12, wherein step (a) includes preparing a suspension including at least one alcohol. 14. A process according to claim 13, wherein step (a) includes preparing a suspension including a ethoxy-coupling agent as a dispersant. 15. A process according to claim 14, wherein step (a) includes preparing a suspension including a dispersant selected from the group consisting of gamma aminopropyl triethoxysilane, A-174, organophosphate esters, RE-610 and their derivatives. 16. A process according to claim /3, wherein step (a) includes preparing a suspension including a dispersant including TEOA. 17. A process according to claim 3, wherein step (a) includes preparing a suspension of particles which are ceramic particles. 18. A process according to claim 3, wherein step (a) includes preparing a suspension of particles which are metallic powders. 19. A process according to claim 17, wherein step (a) includes preparing a suspension of particles selected from the group consisting of: alumina, silicon nitride and its alloys, zirconia, partially stabilized zirconia, and any combination thereof. 20. A process according to claim 19, wherein step (a) includes preparing a suspension of the particles, dispersant, and water. 21. A process according to claim 20, wherein step (a) includes preparing a suspension including a polymeric polyelectrolyte as a dispersant. 22. A process according to claim 21, wherein step (a) includes preparing a suspension including at least one acrylic acid based polymer or co-polymer. 23.. A process according to claim 21, wherein step (a) includes preparing a suspension of particles, water, and a dispersant selected from the group consisting of LD-42, LD-45, DB-300, Darvan C, and Darvan 821A. 24. A process according to claim 19, wherein step (a) includes preparing a suspension including particles, dispersant, and at least one organic liquid. 25. A process according to claim 24, wherein step (a) includes preparing a suspension including at least one alcohol. 26. A process according to claim 25, wherein step (a) includes preparing a suspension including a ethoxy-coupling agent as a dispersant. 27. A process according to claim 25, wherein step (a) includes preparing a suspension including a dispersant selected from the group consisting of gamma aminopropyl triethoxysilane, A-174, organophosphate esters, RE-610 and their derivatives. 28. A process according to claim & , wherein step (a) includes preparing a suspension including a dispersant including TEOA. 29. A process according to claim 4, wherein step (a) includes preparing a suspension of particles which are ceramic particles. 30. A process according to claim 4, wherein step (a) includes preparing a suspension of particles which are metallic powders. 31. A process according to claim 29, wherein step (a) includes preparing a suspension of particles selected from the group consisting of: alumina, silicon nitride and its alloys, zirconia, partially stabilized zirconia, and any combination thereof. 32. A process according to claim 31, wherein step (a) includes preparing a suspension of the particles, dispersant, and water. 33. A process according to claim 32, wherein step (a) includes preparing a suspension including a polymeric polyelectrolyte as a dispersant. 34. A process according to claim 33, wherein step (a) includes preparing a suspension including at least one acrylic acid based polymer or co-polymer. 35. A process according to claim 34, wherein step (a) includes preparing a suspension of particles, water, and a dispersant selected from the group consisting of LD-42, LD-45, DB-300, Darvan C, and Darvan 821A. 36. -A process according to claim 31, wherein step (a) includes preparing a suspension particles, dispersant, and at least one organic liquid. 37. A process according to claim 36, wherein step (a) includes preparing a suspension including at least one alcohol. 38. A process according to claim 37, wherein step (a) includes preparing a suspension including a ethoxy-coupling agent. 39. A process according to claim 38, wherein step (a) includes preparing a suspension including a dispersant selected from the group consisting of gamma a inopropyl triethoxysilane, A-174, organophosphate esters, RE-610 and their derivatives. 40. A process according to claim 37, wherein step (a) includes preparing a suspension including a dispersant including TEOA. 41. A suspension of particles comprising: (a) particles; (b) at least one liquid; and (c) a dispersant; the suspension comprising at least 55 volume percent particles that are thoroughly dispersed in the liquid and the suspension being pourable. 42. A suspension according to claim 41, wherein the particles have a narrow size range. 43. A suspension according to claim 41, wherein the particles are colloidal. 414;.. &. suspension according to claim 41, wherein the particli_ss have a narrow size range and are colloidal. 45. A suspension according to claim 42, wherein the particles are ceramic particles. 46. A suspension according to claim 42, wherein the particles are metallic particles. 47. A suspension according to claim 39, wherein the particles are selected from the group consisting of: alumina, silicon nitride and its alloys, zirconia, partially stabilized zirconia, and any combination thereof. 48. A suspension according to claim 47, wherein the liquid includes water. 49. A suspension according to claim 48, wherein the dispersant includes a polymeric polyelectrolyte. 50. A suspension according to claim 49, wherein the dispersant includes an acrylic acid based polymer or co-polymer. 51. A suspension according to claim 50, wherein the dispersant is selected from the group consisting of LD-42, LD-45, DB-300, Darvan C, and Darvan 821A. 52. A suspension according to claim 47, wherein the liquid includes at least one organic liquid. 53. A suspension according to claim 52, wherein the liquid includes at least one alcohol. 54. A suspension according to claim 53, wherein the dispersant is an ethoxy-coupling agent. 55. A suspension according to claim 54, wherein the dispersant is selected from the group consisting of gamma aminopropyl-triethoxysilane, A-174, organophosphate esters, RE-610, and their derivatives. 56. A suspension according to claim 52, wherein the dispersant includes TEOA. 57. A suspension according to claim 43, wherein the particles are ceramic particles. 58. A suspension according to claim 43, wherein the particles are metallic particles. 59. A suspension according to claim 57, wherein the particles are selected from the group consisting of: alumina, silicon nitride and its alloys, zirconia, partially stabilized zirconia, and any combination thereof. 60. A suspension according to claim 59, wherein the liquid includes water. 61. A suspension according to claim 60, wherein the dispersant includes a polymeric polyelectrolyte. 62. A suspension according to claim 61, wherein the dispersant includes an acrylic acid based polymer or co-polymer. 63. A suspension according to claim 62, wherein the dispersant is selected from the group consisting of LD-42, LD-45, DB-300, Darvan C, and Darvan 821A. 64. A suspension according to claim 59, wherein the liquid includes at least one organic liquid. 65. A suspension according to claim 64, wherein the liquid includes at least one alcohol. 66. A suspension according to claim 65, wherein the dispersant is an ethoxy-coupling agent. 67. A suspension according to claim 66, wherein the dispersant is selected from the group consisting of gamma aminopropyl-triethoxysilane, A-174, organophosphate esters, RE-610, and their derivatives. 68. A suspension according to claim 65, wherein the dispersant includes TEOA. 69. A suspension according to claim 44, wherein the particles are ceramic particles. 70. A suspension according to claim 44, wherein the particles are metallic particles. 71. A suspension according to claim 68, wherein the particles are selected from the group consisting of: alumina, silicon nitride and its alloys, zirconia, partially stabilized zirconia, and any combination thereof. 72. A suspension according to claim 70, wherein the liquid includes water. 73. A suspension according to claim 71, wherein the dispersant includes a polymeric polyelectrolyte. 74. A suspension according to claim 72, wherein the dispersant includes an acrylic acid based polymer or co-polymer. 75. A suspension according to claim 74, wherein the dispersant is selected from the group consisting of LD-42, LD-45, DB-300, Darvan C, and Darvan 821A. 76. A suspension according to claim 70, wherein the liquid includes at least one organic liquid. 77. A suspension according to claim 75, wherein the liquid includes at least one alcohol. 78. A suspension according to claim 77, wherein the „dispersant is an ethoxy-coupling agent. 79. A suspension according to claim 78, wherein the dispersant is selected from the group consisting of gamma aminopropyl-triethoxysilane, A-174, organophosphate esters, RE-610, and their derivatives. 80. A suspension according to claim 77, wherein the dispersant includes TEOA.

Inventors: NOVICH BRUCE D, OCCHIONERO MARK A, NOVICH, BRUCE, D., OCCHIONERO, MARK, A.
Applicants: CERAMICS PROCESS SYSTEMS
Application year: 1987
Main classification: B01F3
IPC codes: B01F3, B01J13, B22F3, B28C1, C04B35