Na-polyelectrolytes (organic and inorganic), there is a large supply of Na+ ions. SiO2 is present in much lower concentrations. No major variations were observed in the organic polyelectrolyte (NH4- vs Na-) densification process. Some localized abnormal grain growth, in the form of elongated grains, can be explained by the higher Na+ levels in the Na-polyelectrolytes. Sodium silicate and sodium hexa-metaphosphate displayed inhibited sintering, therefore a liquid phase mechanism can be ruled out.
The issue of the organic portion of the additives must also be addressed. It has been shown that tape cast alumina will undergo a significantly larger amount of dilation (shrinkage and expansion), if it contains high concentrations of organic additives.25 Expansion in the later stages of binder burnout was seen by Gheo and Palmour.25 The lower temperature expansions were linked to the inhibition of sintering at intermediate temperatures (1200°C-1400°C). The organic polyelectrolytes of the present study displayed similar densification schemes. Those pellets with 1.25 mg/m2 dispersant had lower bulk densities at 1200°C and 1400°C than the pellets cast from suspensions at dispersant levels corresponding to a minimum in viscosity. At higher sintering temperatures, the high and low organic concentrations display common features. This type of additive-induced sintering inhibition can be attributed to a deterioration of particle-particle contacts during processing and binder burnout.
The inorganic dispersants resulted in a much different microstructure than the organic polyelectrolytes or the acid/base dispersants (Figure 10C). The mechanism by which the sodium silicate and sodium hexa-metaphosphate microstructures developed were similar--grain growth inhibition through second phase formation at the grain boundaries. The addition of sodium silicate to a suspension should provide the necessary
conditions for liquid phase development and subsequent abnormal grain growth, as outlined by Song and Coble. The formation of beta-alumina is a possible mechanism for sodium silicate sintering inhibition.
In the case of the sodium hexa-metaphospate dispersed samples, the lack of sintering can be attributed to the phosphate. Numerous studies in the refractories field have investigated the formation of aluminum phosphate phases.26-30 In all of the cases a hydrated aluminum phosphate transformed upon heating into aluminum orthophosphate.
Aluminum orthophosphate is polymorphic with thermal transitions resembling those of the SiO2 system.31 The three crystalline phases are berlinite (B-AlPO4), tridymite (T- AlPO4), and cristobalite (C-AlPO4). In several instances an amorphous phase was formed at low temperatures, followed by crystallization at higher temperature.25-28 The presence of phosphates in the system should initially accelerate sintering and densification with the formation of a liquid phase. At higher temperatures, crystallization of an alumino-phosphate would inhibit grain growth.25 A decrease in “green” density with increasing dispersant concentration may be the result of the formation of a glassy phase. The lack of densification and grain growth at higher temperatures is indicative of a crystalline phase forming at the grain boundaries, thus inhibiting sintering.
4.0 Conclusions
The dispersant used in the processing of a ceramic system, in particular alumina, will have an affect on the sintering and microstructure development. Small amounts of impurities introduced to the system, such as sodium and silica, have been shown to
influence grain growth. This most likely occurs through the formation of a liquid phase at the grain boundaries, which in turn results in abnormal grain growth.
The concentration of organic materials can affect the early stages of sintering, in the form of reducing the number of particle-particle contacts. Higher temperatures and/or longer sintering times are then required to obtain the same density as systems dispersed with an optimum concentration of dispersant.
Neither of the inorganic dispersants investigated resulted in dense microstructures. The inhibition of sintering was proposed as second phase formation at the grain boundaries. In the case of sodium silicate, the second phase is most likely a beta-aluminate, where as in the sodium hexa-metaphosphate system, the second phase is an alumino-phosphate.
Homogeneous, fine-grained, equiaxed microstructures were obtainable in systems containing no dispersant. Sulfates did not affect sintering.
5.0 References
1. M.N. Rahaman, Ceramic Processing and Sintering. Marcel Dekker, New York, 1995.
2. R.M. German, Sintering Theory and Practice. John Wiley and Sons, New York, 1996.
3. R.E. Mistler and R.L. Coble, "Microstructural Variation Due to Fabrication,"
Journal of the American Ceramic Society, 51 [4] 237 (1968).
4. P.A. Morris, "Impurities in Ceramics: Processing and Effects on Properties," pp.
50-85 in Ceramic Transactions, Vol. 7, Sintering of Advanced Ceramics. Edited by C. A. Handwerker, J. E. Blendell, and W. A. Kaysser. American Ceramic Society, Westerville, OH, 1990.
5. J. Zhao and M.P. Harmer, "Sintering of Ultra-High-Purity Alumina Doped Simultaneously with MgO and FeO," Journal of the American Ceramic Society, 70 [12] 860-866 (1987).
6. S.I. Bae and S. Baik, "Sintering and Grain Growth of Ultrapure Alumina,"
Journal of Materials Science, 28 [15] 4197-4204 (1993).
7. A. Kebbede, G.L. Messing, and A.H. Carim, "Grain Boundaries in Titania-Doped α-Alumina with Anisotropic Microstructure," Journal of the American Ceramic Society, 80 [11] 2814-2820 (1997).
8. W.C. Johnson and D.F. Stein, "Additive and Impurity Distributions at Grain Boundaries in Sintered Alumina," Journal of the American Ceramic Society, 58 [11-12] 485-488 (1975).
9. W.A. Kaysser, M. Aprissler, C.A. Handwerker, and J.E. Blendell, "Effect of Liquid Phase on the Morphology of Grain Growth in Alumina," Journal of the American Ceramic Society, 70 [5] 339-343 (1987).
10. H. Song and R.L. Coble, "Origin and Growth Kinetics of Platelike Abnormal Grains in Liquid-Phase-Sintered Alumina," Journal of the American Ceramic Society, 73 [7] 2077-2085 (1990).
11. C.A. Bateman, S.J. Bennison, and M.P. Harmer, "Mechanism for the Role of Magnesia in the Sintering of Alumina Containing Small Amounts of a Liquid Phase," Journal of the American Ceramic Society, 72 [7] 1241-1244 (1989).
12. S. Baik, "Segregation of Mg to the (0001) Surface of Single-Crystal Alumina:
Quantification of AES Results," Journal of the American Ceramic Society, 69 [5]
C--101-103 (1986).
13. R.I. Taylor, J.P. Coad, and R.J. Brook, "Grain Boundary Segregation in Al2O3,"
Journal of the American Ceramic Society, 57 [12] 539-540 (1974).
14. C.W. Li and W.D. Kingery, "Solute Segregation at Grain Boundaries in Polycrystalline Al2O3," pp. 368-378 in Advances in Ceramics, Vol. 10, Structure and Properties of MgO and Al2O3 Ceramics. Edited by W. D. Kingery. American Ceramic Society, Westerville, OH, 1984.
15. S.C. Hansen and D.S. Phillips, "Grain Boundary Microstructures in a Liquid- Phase Sintered Alumina (α-Al2O3)," Philosophical Magazine A, 47 [2] 209-234 (1983).
16. J. Rodel and A.M. Glaeser, "Anisotropy of Grain Growth in Alumina," Journal of the American Ceramic Society, 73 [11] 3292-3301 (1990).
17. A. Mamoun, T. Epicier, H. Gros, and G. Fantozzi, "Microstructural Study of a MgO-Doped Alumina-Based Ceramic," Materials Chemistry and Physics, 32 [2]
169-176 (1992).
18. "Standard Test Methods for Apparent Porosity, Water Absorption, Apparent Specific Gravity, and Bulk Density of Burned Refractory Brick and Shapes by Boiling Water," ASTM Standard C20-83. 1987 Annual book of ASTM Standards, Vol. 15.01. American Society for Testing and Materials, Philadelphia, PA, 1987.
19. R.T. DeHoff, "Problem Solving Using Quantitative Stereology," pp. 89-98 in Applied Metallography. Edited by G. F. Vander Voort. Van Nostrand Reinhold, New York, 1986.
20. S.J. Bennison and M.P. Harmer, "A History of the Role of MgO in the Sintering of α-Al2O3," pp. 13-49 in Ceramic Transactions, Vol. 7, Sintering of Advanced Ceramics. Edited by C. A. Handwerker, J. E. Blendell, and W. A. Kaysser.
American Ceramic Society, Westerville, OH, 1990.
21. G. Rossi and J.E. Burke, "Influence of Additives on the Microstructure of Sintered Al2O3," Journal of the American Ceramic Society, 56 [12] 654-659 (1973).
22. R.D. Bagley and D.L. Johnson, "Effect of Magnesia on Grain Growth in Alumina," pp. 666-678 in Advances in Ceramics, Vol. 10, Structure and Properties of MgO and Al2O3 Ceramics. Edited by W. D. Kingery. American Ceramic Society, Westerville, OH, 1984.
23. M.P. Harmer, "Use of Solid Solution Additives in Ceramic Processing," pp. 679- 696 in Advances in Ceramics, Vol. 10, Structure and Properties of MgO and
Al2O3 Ceramics. Edited by W. D. Kingery. American Ceramic Society, Westerville, OH, 1984.
24. H. Song and R.L. Coble, "Morphology of Platelike Abnormal Grains in Liquid- Phase-Sintered Alumina," Journal of the American Ceramic Society, 73 [7] 2086- 2090 (1990).
25. M. Gheo and H. Palmour III, "Sources of Sintering Inhibition in Tape-Cast Aluminas," Ceramic Engineering and Science Proceedings, 14 [11-12] 97-129 (1993).
26. S.J. Lukasiewicz and J.S. Reed, "Phase Development on Reacting Phosphoric Acid with Various Bayer-Process Aluminas," American Ceramic Society Bulletin, 66 [7] 1134-1138 (1987).
27. W.D. Kingery, "Fundamental Study of Phosphate Bonding in Refractories: I, Literature Review," Journal of the American Ceramic Society, 33 [8] 239-241 (1950).
28. V.A. Kipeikin, A.I. Kudryashova, L.N. Kuz'Minskaya, I.L. Rashkovan, and I.V.
Tananaev, "Formation of an Amorphous Phase in the Cementation of Materials Based on an Aluminophosphate Binder," Inorganic Materials (Translation of Neorganicheskie Materialy), 3 [4] 657-659 (1967).
29. F.J. Gonzalez and J.W. Halloran, "Reaction of Orthophosphoric Acid with Several Forms of Aluminum Oxide," American Ceramic Society Bulletin, 59 [7]
727-731 (1980).
30. Y.V. Klyucharov and L.I. Skoblo, "Compositions of the Products Formed by Hardening of Aluminum Phosphate Binder in Refractory Corundum Compositions," Journal of Applied Chemistry of the USSR (English Translation), 38 [3] 530-535 (1965).
31. W.R. Beck, "Crystallographic Inversions of the Aluminum Orthophosphate Polymorphs and Their Relation to Those of Silica," Journal of the American Ceramic Society, 32 [4] 147-151 (1949).
VIII. P
ROCESSINGA
DDITIVEI
NTERACTIONS1.0 Introduction
Polymeric additives are used extensively to aid in the processing of ceramic bodies.
In most instances, several different types of polymers are used in the same body for a variety of purposes. Typical uses include dispersants, temporary binders, plasticizers, and anti-foaming agents.1 Usually interactions among these polymeric systems are considered negligible. However, in many cases interactions between polymers occur which can affect product performance.
Interactions between polymers can occur in several ways. Two of the most common types of polymer-polymer interactions are complex formation and phase separation. Complex formation occurs between polymers that contain specific chemical groups that can interact through associative methods, such as hydrogen bonding. In these cases, the complex formed is much like a new polymer, with its own properties. When interactions are non-associative or repulsive, albeit in a liquid or solid phase, phase separation can occur.2 These types of interactions can be either beneficial or problematic, depending on the process conditions and desired performance. In particular, these interactions can result in varying product and process quality and consistency.1, 3 Therefore, it is necessary to better understand these types of interactions in order to maximize the effectiveness of the polymeric additives and product quality.
There were three types of polymeric interactions considered in this study: ideal, positive, and negative. The first situation is one in which the polymers behave separately,
as if there were no other materials in the system. However, this is rarely, if ever, the case.
Most polymers are influenced in some way by the presence of other materials in the solution. In ceramic systems, these materials can include salts, inorganic particles, and other polymers. When interactions occur between polymers they can be “positive” in the sense that the polymers can complex together to form a new material with properties much different from the separate, individual polymer chains. These complexes are often the result of strong associative bonding, such as hydrogen bonding. Interactions between polymers can also be “negative.” Negative interactions between polymers are the result of strong repulsive forces occurring due to differences in chemical structure. This is very similar to when oil and water are mixed. These negative interactions then result in a separation of the polymer systems. The phase separation of different polymer systems is the most common type of interaction. Although only interactions between the individual polymers and water are considered in this paper, it must be realized that ceramic system formulations can contain a number of different components, any of which may influence the interactions between polymers.2, 4, 5
Using the definitions just presented for the various polymeric interactions in solution, ternary phase diagrams can be constructed (i.e., polymer-polymer-solvent).
Flory and Huggins developed a comprehensive model for phase separation in polymer solutions.2 Problems with the model occur if pH fluctuations are to be incorporated, the salt/ionic concentration changes, different MW polymers are used, or the process is taking place at elevated temperatures. The main reason behind the inadequacy of the model revolves around the effect each of these changes has on the conformation of the polymer structure in solution. Although calculations of the phase diagrams are possible, their
accuracy is questionable in a manufacturing situation where many different and often uncontrollable variables exist. For this reason, a semi-quantitative means of monitoring the degree of polymeric interactions was preferred.
This investigation focuses on several commercial dispersants, binders, and plasticizers, used in spray-drying/dry-pressing applications. Interactions between these materials were evaluated using visual techniques and infrared analysis. These initial experiments are aimed at developing a basis to effectively use existing polymers and to set parameters for the development of new polymers, for enhanced performance in the manufacture of ceramic products.
2.0 Procedures