Sintering


    The first problem that I studied at Corning Glass Works, immediately after graduating from MIT, was sintering of optical fiber preforms. These were vapor-deposited bodies consisting of tiny glass particles (tens of nm in diameter) lightly sintered into a body with a porosity of ~85%. At that time, the preform was in the form of a cylinder with an axial hole about 6 mm in diameter and an outer diameter of 40-60 mm. (Now they are much larger and do not have a central hole.) Preforms have such low density that the standard viscous sintering models were not adequate, so I developed a “cell model” consisting of cylinders of glass (representing strings of particles) forming the edges of a cube. 

    The most important novelty in the sintering work was the introduction of the concept of constitutive models for sintering materials, which made it possible to take account of stresses resulting from constraints of various kinds. This could include applied stresses or rigid inclusions, but the original motivation was to account quantitatively for the gradients in composition and density in optical fiber preforms. The dopants used to raise the refractive index of the inner part (called the core) of a fiber also changes the viscosity and other properties. During sintering, the core region of the preform densifies first, owing to its lower viscosity. Since the outer part of the core is bound to the silica cladding, it cannot contract, so densification requires that the central hole becomes larger. As the temperature rises, the cladding finally begins to contract, so it squeezes the relatively fluid core and the central hole begins to contract again. By controlling the composition and density during deposition, one can cause the hole to close, so that the final product of sintering is a solid rod of glass - which makes it easier to draw fiber. The cell model for sintering was able to predict this phenomenon quantitatively, and was used for process control in the plant fabricating the preforms.

    I later learned that constitutive models for sintering had been independently developed by Skorokhod in the USSR in the 1960s, but his work was all published in Russian and was unknown in the west (so far as I know) until the 1980s. The concept is now widely used for problems such as shrinkage mismatch during firing of multilayer circuits.


Relevant papers:


Sintering Models

"Sintering of Low Density Glasses: I. Theory," G. W. Scherer, J. Am. Ceram. Soc. 60 [5-6] (1977) 236-239


"Sintering of Low Density Glasses: II. Experimental Study," G. W. Scherer, D. L. Bachman, J. Am. Ceram. Soc. 60 [5-6] (1977) 239-243


"Sintering of Low Density Glasses: III. Effect of a Distribution of Pore Sizes", G. W. Scherer, J. Am. Ceram. Soc. 60 [5-6] (1977) 243-246


“Cell Models for Viscous Sintering”, G.W. Scherer, J. Am. Ceram. Soc., 74 [7] (1991) 1523-1531


Constitutive Models

This was the first paper to propose a constitutive model for sintering:

"Sintering Inhomogeneous Glasses: Application to Optical Waveguides", G. W. Scherer, J. Non-Cryst. Solids 34 (1979) 239-256


"On Constrained Sintering: I. Constitutive Model for a Sintering Body", R.K. Bordia and G.W. Scherer, Acta Metallurgica 36 [9] (1988) 2393-2397. 


"On Constrained Sintering: II. Comparison of Constitutive Models", R.K. Bordia and G.W. Scherer, Acta Metallurgica 36 [9] (1988) 2399-2409.


"On Constrained Sintering: III. Rigid Inclusions", R.K. Bordia and G.W. Scherer,  Acta Metallurgica 36 [9] (1988) 2411-2416


“Constitutive Models for Viscous Sintering”, G.W. Scherer, pp. 1-18 in Mechanics of Granular Materials and Powder Systems, ed. M.M. Mehrabadi (ASME, New York, 1992)


This paper illustrates the difficulty of using constitutive models when grain growth occurs:

“Constitutive Behavior of Sintering Materials”, K.R. Mikeska, G.W. Scherer, and R.K. Bordia, Ceram. Trans., 7 (1990) 200-214


Constrained Sintering

"Sintering Inhomogeneous Glasses: Application to Optical Waveguides", G. W. Scherer, J. Non-Cryst. Solids 34 (1979) 239-256


"Viscous Sintering of a Bimodal Pore Size Distribution", G. W. Scherer, J. Am. Ceram. Soc. 67 [11] (1984) 709-715


"Viscous Sintering on a Rigid Substrate", G. W. Scherer and T. Garino, J. Am. Ceram. Soc. 68 [4] (1985) 216-220


"Viscous Sintering under a Uniaxial Load", G.W. Scherer, J. Am. Ceram. Soc. 69 [9] (1986)  C206-C207


"Sintering with Rigid Inclusions", G.W. Scherer, J. Am. Ceram. Soc. 70 [10] (1987) 719-725


"Creep and Densification During Sintering of Glass Powder Compacts", M.N. Rahaman, L.C. DeJonghe, G.W. Scherer, R.J. Brook, J. Am. Ceram. Soc. 70 [10] (1987) 766-774


"On Constrained Sintering: III. Rigid Inclusions", R.K. Bordia and G.W. Scherer,  Acta Metallurgica 36 [9] (1988) 2411-2416


"Viscous Sintering with a Pore Size Distribution and Rigid Inclusions", G.W. Scherer,  J. Am. Ceram. Soc. 71 [10] (1988) C447-C448


"Sintering of Composites: Critique of the Available Analyses", R.K. Bordia and G.W. Scherer, pp. 872-876 in Ceramic Powder Science B, Ceramic Transactions, Vol. 1, eds. G.L. Messing, E.R. Fuller, Jr., H. Hausner (Am. Ceram. Soc., Westerville, OH, 1988)


This paper analyzes the constraint from crystallization during sintering of glass:

“Effect of Inclusions on Shrinkage”, G.W. Scherer, pp. 503-514 in Better Ceramics Through Chemistry IV, eds. B.J.J. Zelinski, C.J. Brinker, D.E. Clark, and D.R. Ulrich  (Mat. Res. Soc., Pittsburgh, PA, 1990)


“Constitutive Behavior of Sintering Materials”, K.R. Mikeska, G.W. Scherer, and R.K. Bordia, Ceram. Trans., 7 (1990) 200-214


“Viscous Sintering of Particle-Filled Composites”, G.W. Scherer, Ceramic Bull., 70 [6] (1991) 1059-1063


“Effect of Inclusions on Viscous Sintering”, G.W. Scherer and A. Jagota, pp. 99-109 in Advanced Composite Materials, Ceramic Transactions Vol. 19 (Am. Ceram. Soc., Columbus, OH, 1991)


“Viscosities and Sintering Rates of a Two-Dimensional Granular Composite”, A. Jagota and G.W. Scherer, J. Am. Ceram. Soc. 76 [12] (1993) 3123-3135


“Viscosities and sintering rates of composite packings of spheres”, A. Jagota and G.W. Scherer, J. Am. Ceram. Soc. 78 [3] (1995) 521-528


“Sintering of sol-gel films”, G.W. Scherer, J. Sol-Gel Sci. Tech. 8 (1997) 353-363


“Coarsening in a viscous matrix”, G.W. Scherer, J. Am. Ceram. Soc. 81 [1] (1998) 49-54


Sintering Gels

"Sol–Gel–Glass: III. Viscous Sintering", G. W. Scherer, C. J. Brinker, and E. P. Roth, J. Non-Cryst. Solids 72  (1985) 369-389  


"Structural Evolution during the Gel-to-Glass Conversion", C. J. Brinker, E. P. Roth, G. W. Scherer, and D. R. Tallant, J. Non-Cryst. Solids 71 (1985) 171-185


“Sintering of Gels”, G.W. Scherer, pp. 221-256 in Sol-Gel Science and Technology, eds. M.A. Aegerter, M. Jafelicci Jr., D.F. Souza, and E.D. Zanotto (World Scientific, New Jersey, 1990)


“The Sintering of Silica Aerogels Studied by Thermoporometry”, T. Woignier, J.F. Quinson, M. Pauthe, M. Repellin Lacroix, J. Phalippou, and G.W. Scherer, J. Sol-Gel Sci. Tech. 2 [1/2/3] (1994) 277-282


“Densification kinetics and structural evolution during sintering of silica aerogel”, G.W. Scherer, S. Calas, and R. Sempéré, J. Non-Cryst. Solids 240 (1998) 118-130


“Sintering of aerogels”, G.W. Scherer, S. Calas, and R. Sempéré, J. Sol-Gel Sci. Tech. 13 (1998) 937-943


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