Obtaining mullite ceramics by slip casting

Bevz, V. A.; Pivinskii, Yu. E.

doi:10.1007/bf00702768

Cited by 5 publications

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“…The same significant effect of the influence of original porosity of the greenware is noted during the sintering of ceramics made of synthetic mullite3Al203.2SiO2 (74.5%A1203, 23% Si02, and 2.5% CaO, MgO, Fe=O~, and other impurities [12]). In this case the considerable difference in the original porosity of the greenware is due to the shaping process.…”

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confidence: 60%

“…The analysis was done as a means of processing previously published data on thesintering of quartz [5, ii] and mullite [12] ceramics, and also on the basis of the results of new research, for example, on zirconia ceramics. The analysis was done as a means of processing previously published data on thesintering of quartz [5, ii] and mullite [12] ceramics, and also on the basis of the results of new research, for example, on zirconia ceramics.…”

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confidence: 99%

“…This is mainly due to the fact that the low porosity values of greenware (i.e., those for which the effect being examined shows itself to a significant degree) [5,Ii,12,16] were not previously attained. This is mainly due to the fact that the low porosity values of greenware (i.e., those for which the effect being examined shows itself to a significant degree) [5,Ii,12,16] were not previously attained.…”

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Influence of original greenware porosity and solid-phase hydration on the sintering of ceramic materials

Pivinskii¹

1985

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Accelerating or activating the sintering of ceramics is an urgent problem since it provides savings of fuel and energy.The term activation of sintering is normally understood to mean speeding up the change in porosity (shrinkage), making interparticle contacts and affecting the physicomechanical properties of the material under the action of factors other than temperature and sintering time [i, 2]. In such conditions the methods of bringing about these processes are classified into two groups: chemical and physical.The activity of the powders during sintering is ensured either by structural defects or by defects generated during sintering. However, we should note that the preparation and use of active powders, as a rule, causes inadequate density to develop in the greenware, and localization of the shrinkage, leading to the formation of gaps between the various microsections of the material ("subnormal" sintering) [3].In contrast to the above methods the present article will examine two special cases of accelerated (activated) sintering in ceramic materials.The first of them is revealed during the sintering of ceramic greenware with a high density, and the second with a certain hydration of the particles of solid phase in the original green article.Influence of the Original Porosity of the Greenware.In most cases in ceramic technology it is important to attain the maximum volume filling of the system by solid phase C d [4] at the forming stage, and also the minimum changes in C d at the stage of forming the final structure, for example, during drying and firing.The latter occurs because considerable volume changes in systems located in the compact form (greenware) frequently cause cracking, buckling, or destruction of the shapes.With a marked reduction in the original porosity of the greenware it becomes possible not only to reduce the temperature or duration of the heat treatment of the material to obtain the prescribed density (strength), but in most cases to attain the maximum values [4,5]. Moreover, there is also a significant fall in the shrinkage, which is of great practical importance in the production ofhigh-precision articles (especially large ones).Starting from the Ivensen phenomenological theory of sintering it is usual to consider [3, 6-8] that during the sintering, in the same conditions, of articles with different original porosity, we observe the same coefficient of relative reduction in the volume of the pores K:where m m Vg--m m v~=~ a~; a-g ~' ,~2) d k is the true density of the solid phase; dg is the apparent density of the greenware; d c is the apparent density of the material after sintering; and m is the mass of the specimen.As follows from Eqs. (1) and (2),the magnitude Vc/Vg represents the ratio of the absolute values of the pore volumes after sintering V c and before sintering Vg. It was shown in [9, i0] that for sinterable bodies with different original porosities the ratio of the porosities before and after sintering, expressed in percentages of the volume of the body, also is a consta...

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Influence of original greenware porosity and solid-phase hydration on the sintering of ceramic materials

Pivinskii¹

1985

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“…As was indicated in our review [I], HCBS of other materials have also been obtained in alkali media. We note that there is a significant difference in the effectiveness (efficiency) of milling in an alkaline medium for the HCBS of acidic and acidic-amphoteric comopositionso For example, while the maximum achievable values of Peas for the HCBS of quartz sand that are obtained by milling in an acidic or an alkaline medium do not virtually differ and amount to 11-12% [21,23,24], the values of Pea, are significantly lower for the HCBS made from mullite [25,22] and zircon [I] by milling in an alkaline medium.…”

Section: Principle Of Optimumthinning Of Hcbs and D~ssolution Of Dispmentioning

confidence: 90%

Highly concentrated ceramic binder suspensions. Technological principles (Review)

Pivinskii¹

1987

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It was previously shown [I, 2] that single-stage wet milling in 'ceramic' ball mills is the best method of producing the most extensively studied highly concentrated ceramic binder suspension (HCBS) of acidic and acidic-amphoteric systems (according to the proposed classification [i]). In this case, optimum characteristics are achieved by following numerous principles among which the fundamental principle of attaining a high concentration directly during the process of wet milling has been examined in detail [2].Based on the generalizations made on the basis of the available literature and the previously published and new experimental results, this paper* formulates other basic technological principles of HCBS, viz., the principles of increased temperature, optimum dispersion and granulometric distribution, and optimum thinning (dilution) of HCBS and dissolution of the dispersed phase. Principle of Increased TemperatureA significant increase of the temperature of the charge is observed during wet milling of HCBS in ball mills. As was indicated by Avvakumov [3, p. 66], during mechanical activation (milling), heat is liberated due to the conversion of kinetic energy into thermal energy at the moment of impact of the grinding bodies and due to interparticle friction. No information regarding the influence of the temperature factor on the wet milling process of the suspensions was found in our literature survey. However, it was mentioned [3, p. 50] that the effectiveness of dispersion of solids decreases with increasing temperature. This was attributed to the fact that as the temperature increases, a significantly more lattice deformation is required so as to create the conditions necessary for forming potential cracks.The positive role of the increased temperature during wet milling was indicated first in our work [4] on the conditions of obtaining highly concentrated suspensions of quartz glass. The concept of the 'incubation' stage of the wet milling process was introduced in that paper; this stage is initiated by the increased temperature. In subsequent extensive studies, it has been shown that besides concentration, the principle of increased temperature is decisive in the technology of HCBS.Temperature largely determines the rheological properties of the system directly during the milling process and the properties of HCBS after milling. With increasing temperature, one observes a significant decrease of the total viscosity of the system (its flowahility increases) and the parameters of its dilatational properties (thickening). All these facts make it possible to carry out the process at high concentrations [5].At the same time, the rate of dissolution of the particles of the solid phase increases rapidly with increasing temperature. This leads to the formation of an increased amount of the colloidal component that decreases the anomaly of viscosity (dilatancy) due to a peculiar 'lubricating' effect. For example, according to the generalized data [6, p. 51], as the temperature increases from 25 up to I00~ the ...

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“…Using highly concentrated suspensions of various materials such as high-alumina firec]ay [5,12] or mullite [7,21], we obtained granular semifinished products in which the binder (matrix) contains 16-18% porosity.…”

Section: Comparative Evaluation Of the Shaping Methodsmentioning

confidence: 99%