Sintering Of Glass Powders Research Articles

Finite Element Simulation of Sintering Process. 2nd Report. Constitutive Equation for Sintering and Prediction of Shape of Sintered Products.

A constitutive equation for sintering is proposed. The sintering is described as the deformation process of viscous porous materials under the action of sintering stress, the virtual hydrostatic stress produced by the surface tension. The parameters for the constitutive equation are determined from the experimental results on the sintering of glass powder. The proposed constitutive equation is applied to the prediction of the deformation behavior of glass powder compacts during sintering using the viscoplastic finite element method. The effect of powder particle size on the shape of the sintered body is examined. The sintering rate of the glass powder compact composed of small particles is high and the shape of the sintered body is not significantly affected by its own weight. The predicted shapes of the sintered bodies show good agreement with those obtained by experiment.

Creep and densification during anisotropic sintering of glass powders

The isothermal sintering behaviour of a barium magnesium aluminosilicate glass powder at 930°C was investigated using a heating microscope. The cylindrical samples exhibited a variable shrinkage anisotropy during sintering. The shrinkage anisotropy ratio, defined as the ratio of the relative change of height and diameter, varied linearly between ∼0.3 and ∼0.98 with the relative volume shrinkage during densification. Shrinkage anisotropy caused creep deformation of the samples. The creep rate varied exponentially with the densification rate and the ratio of creep to densification rates, \({{\dot \varepsilon _{\text{c}} } \mathord{\left/ {\vphantom {{\dot \varepsilon _{\text{c}} } {\dot \varepsilon _\rho }}} \right. \kern-\nulldelimiterspace} {\dot \varepsilon _\rho }}\), decreased as densification proceeded. This is in disagreement with most previous studies, which show a constant value of \({{\dot \varepsilon _{\text{c}} } \mathord{\left/ {\vphantom {{\dot \varepsilon _{\text{c}} } {\dot \varepsilon _\rho }}} \right. \kern-\nulldelimiterspace} {\dot \varepsilon _\rho }}\) during the densification. Overall, the study points out the relevance of variable shrinkage anisotropy and how it affects the densification behaviour of glass powders.

Effect of TiO2 addition on the nucleation of apatite in an MgO-CaO-SiO2-P2O5 glass

Glass-ceramic materials, which are applied to dental and bone implantation, are increasingly important in surgery due to the improvement on controlled crystallization and the development of glass-ceramic materials. Apatite-containing glass-ceramics are potentially important for surgical implantation due to the close crystallographic and chemical similarity of apatite to bone tissue [1]. The crystalline phases occurring in these glass-ceramics include essentially, first, apatite, the major crystalline phase which provides the biocompatibility of the glass-ceramics, and, secondly, secondary crystalline phase(s), crystallized at higher temperature and providing the interesting mechanical properties such as machinability [2], high strength [3-5], etc. The apatite phase exhibits a prominent surface and a little lower bulk nucleation. The secondary crystalline phases often show only surface crystallization [3-6] with a surface layer grown perpendicularly to the free surface of the bulk glass, so large cracks are seen due to a large amount of directional volume change [5]. This limits the application of these glass-ceramics, and hence the method of glass powder sintering was used to eliminate the surface effects [3-5]. Nevertheless, the casting method for the fabrication of a bulk glass article is simpler than the glass powder sintering method. One way in which it is possible to resolve the problem of crack formation in the casting method is the addition of nucleating agent into glass-ceramics to elevate the internal nucleation of apatite, so that the growth of the surface layer would be retarded in its initial stage. In the work described in this letter, TiO2 was used as a nucleating agent for an MgO-CaO-SiO2-P205 glass, which possesses a composition locating at the eutectic point of the 3CaO.PzOs-CaO-MgO.2SiO2-SiO 2 system. The nominal compositions of the glasses are given in Table I. Well-mixed dried powders containing appropriate amounts of reagent-grade chemicals were melted in a platinum crucible for 2 h at 1450 °C and quenched by pouring on to a copper plate. The glasses were annealed at 685 °C for 2 h, then furnace-cooled to room temperature. A two-stage heat-treatment method was used to study the effect of heat-treatment temperature on

Processes for silica glass powder sintering

Processes for silica glass powder sintering have been investigated in order to obtain glass-ceramics materials with definite physico-chemical properties. The influence of additions (BN and BPO 4) on the mechanism of sintering and structure of silica glass ceramics has been studied using X-ray diffraction, IR-spectroscopy, optical and electron microscopy. A method has been developed for quantitative IR-spectroscopic analysis with the help of which the concentration of cristobalite in silica glass ceramics can be determined with high accuracy. On the basis of the experimental results a mechanism for silica glass powder sintering is suggested.

Kinetic Processes Involved in the Sintering and Crystallization of Glass Powders

Viscometry, crystal growth kinetics, and glass powder sintering were used to determine the activation energies of viscous flow, crystal growth, and sintering behavior of a standard NBS 710 glass and two crystallizable glass powders in the calciaaluminosilicate system. In a system which sintered successfully to zero porosity, the activation energy for sintering was comparable to the activation energy of viscous flow. When the apparent activation energy for the sintering process was lower than that of viscous flow, sintering was found to proceed at a slower rate because the precipitated crystalline phase retarded viscous flow and the shrinkage of the pores. Initial crystallization appeared to occur on the surfaces of coalesced particles. A chemical treatment was partially successful in suppressing the onset of surface nucleation of the precursor powder.

Initial Sintering of Glass

The shrinkage isotherms of the powder compacts of spherical soda-lime glasses were measured in the temperature range of 590° to 660°C in air. An analytical equation for the initial sintering of glass powders was developed from the viewpoint of non-Newtonian flow. The shrinkage data measured with a dilatometer consisting of a linear variable transformer were best correlated with the equation. The results indicated that the length of unit flow, δ, was 3-10Å and that the viscosity coefficients were in fairly good agreement with those obtained by other authors.

Sintering of Glass Powders During Constant Rates of Heating

Analytical expressions for the initial sintering of glass powders were developed to describe shrinkage of powder compacts at a constant rate of heating. Data describing sintering of soda-lime glass spheres as measured by shrinkage of powder compacts at rates of heating from 0.46 to 2.91°C/min are consistent with the model analytical expressions. Compacts of glass spheres decreased in size exponentially with increasing temperature as predicted by the model.

Sintering and Crystallization Processes of the Glass Powder Having Cordierite Composition

Sintering and crystallization processes of the glass powder of various particle sizes with cordierite composition have been investigated by means of thermal shrinkage measurement, D. T. A., X-ray diffraction and polarized microscopic observation.The results obtained are summarized as follows:1) Viscous flow mechanism was applicable for the sintering of the glass powder that initiated at 820°-920°C, whereas plastic flow mechanism became to be available in the final stage of sintering where μ-cordierite began to crystallize from the glass phase at 900°-980°C. Sintering measured by thermal shrinkage of the compact body ceased at 950°-1000°C owing to the almost complete crystallization at the periphery of individual particles. Thereafter, μ-cordierite transformed to α-form at 970°-1070°C, and the microstructure of sintered bodies was unchanged up to 1300°C.2) As shown above, there existed ranges for the temperatures where sintering, crystallization and phase transformation started, since the initiation of the phenomena was affected by the particle size and the heating rate. The smaller particle size and the faster heating rate resulted in the larger thermal shrinkage and the denser sintered body. Accordingly, physical properties of the sintered body can be controlled to a considerable wide range.Activation energies for the sintering of glass powder by viscous flow, the crystallization of μ-cordierite and the transformation to α-form obtained by the Arrhenius plot were 36-115 (920°-855°C), 86, and 60kcal/mole respectively, and enthalpy changes estimated for the crystallization of μ-cordierite and the transformation were found to be 16 and 18kcal/mole respectively.