Production and properties of mullite foam ceramic

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In the first article [i] it was shown that the evolutionary development of refractory concretes based on high-alumina cements (VGTs) led to the creation of new concretes with low (4-8%) and superlow (i-2%) concentrations of such cements.Beside the important advantages of such concretes, their technology is connected with a range of features which complicate'their use.In particular, in the composition of dry concretes the significant quantities of ultrafine powders is connected with high energy outlay during their production and the undesirable ecological aspects during their use (especially in relation to silicosis-forming, amorphous~ SiO 2 [2]). Moreover, very high quality control is needed for batching, moisture content, and application technology, in connection with which even slight variations in these requirements at the user-factory lead, as indicated in [3], to a reduction in the resistance of the linings by a factor of 2-3.A further drawback of the new range of concretes based on VGTs is their variety of bonding systems (consisting, as a rule, of a complex system involving uitrafine particles, various inorganic and organic reagents -regulators of rheological and technological properties of the mixtures) and refractory filler.This may impair some of the properties of the concrete, in particular, a reduction in the refractoriness under load (RUL).Ceramic concretes:basic characteristics of technology and properties. From the information in [i] and the present article it is evident that the ideal composition for refractory concrete should be that in which both the filler and the bond are similar or equal in chemical composition (nature); this principle is precisely realized in the technology of ceramic concretes [4,5].It is relevant that the first reports on the development of ceramic concretes and also low cement concretes were made simultaneously in 1976. According to the chronological table for the development of work on VGTs and concretes based on them, laid out in [6], in line with the above proposals, the patent (7,622,344, France) was unknown to the authors of the present article; this patent covered the production of low cement-concrete with ultradispersed additions of SiO 2 and Cr=O 3. In 1976 reports were published on the technical principles for ceramic bonds and ceramic concretes, the content of which was published in [4,5]. Thus, simultaneously and independently, there appeared two very different concepts about the solution to one and the same problem -the production of qualitatively new refractory concretes.The concept of ceramic bonds (or VKVS) as the basis of ceramic concretes is based on the fact that most natural siliceous and aluminosilicate rock-forming minerals, in crystalline structure, are inorganic polymers [7]. Thus, the problem consisted in developing such technological processes which would enable us, in industrial conditions, to realize the capacity of the original mineral raw materials to form polymerization structures.The VKVS obtained with a special technology [8][9][10] consi...

Section: Ceramic Concretesmentioning

confidence: 91%

New population of refractory concretes: Cement free concretes (review)

Pivinskii

Trubitsyn

1990

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“…In accordance with this, with an increase in dma x for concrete with dense or porefree filler, there is a reduction in the porosity of the concretes. Other "ideal" grain-size distributions curves exist, and are analyzed in [5,18].Analysis of grain distributions of ceramic concretes, e.g., in [19,20], shows that they are close to the curves shown in Fig. i.…”

mentioning

confidence: 85%

New population of refractory concretes: Grain size composition and volume factors

Pivinskii

1992

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Previous articles in this series [1][2][3] showed that one of the outstanding characteristics of new refractory concretes -low-cement refractory concretes (NTsOB) or cast vibration thixotropic refractory bodies (NVTOM) -is their high density. Together with other factors the density is largely determined by the choice of the optimum grain-size composition.Selection Principles and Grain Distribution Curves. The choice of grain size in refractory concretes is governed by both production and service requirements. Bearing in mind the need for the minimum water requirement of the concrete mixture and the minimum porosity, the problem is one of obtaining the maximum packing density of the components. On the other hand, concrete mixtures should possess the necessary production properties (mobility and placement facility with the optimum vibration-compaction cycles, the absence of dilatancy preventing mixing and placement). Moreover, the choice of the optimum grain-size composition helps save cement -the most expensive component. In many applications of these concretes it is also true that excess cement impairs the properties of the concrete.The grain-size composition of refractory concretes to a marked degree determines their shrinkage during drying (setting) and in service, the thermal-shock resistance, and the relationship between temperature and strength. Uncompensated shrinkage [4,5] has a special influence on the properties.Questions of the choice and compilation of a grain-size composition for the ceramic concretes and low-cement refractory concretes have been examined in many papers [2,[4][5][6][7][8][9][10][11][12].The production of the new type of concretes usually involves a mixture of continuous composition which is more suitable technologically than discontinuous mixtures. The packing effectiveness can be improved by using equations to determine the best relationship between the contents of the various fractions and the maximum grain size in the mixture dma xIn particular, in several projects with NTsOB the grain composition was chosen taking account of the maximum packing density curve according to the Furnas equation [13][14][15][16][17]. As an example, Fig. i, using data in [13][14][15], presents the optimum curves for mixtures. It follows from curve 6 that for a concrete with dma x = 4.8 mm the content of particles <74 Dm should be 37.6%, and <0.5 ~m 4%. With an increase in dma x there is a proportional increase in the median diameter d m of the mixture, and also the polydispersion factor Kp [5]. Thus, when dma x equals 4.8, 1.2 and 0.3 mm, then Kp = Ks0/K20 is 160, 84, and 50 respectively. In accordance with this, with an increase in dma x for concrete with dense or porefree filler, there is a reduction in the porosity of the concretes. Other "ideal" grain-size distributions curves exist, and are analyzed in [5,18].Analysis of grain distributions of ceramic concretes, e.g., in [19,20], shows that they are close to the curves shown in Fig. i. Thus, ceramic concretes based on high-colloid (VKVS) mullite an...

“…Pivinskii and Bevz [5] studied the possibility of obtaining heat resistant mullite--corundum refractories having a granular structure.…”

mentioning

confidence: 99%

“…Molding of products according to the method of controlled solidification differs from the well known schemes [3][4][5] of solidification of melts in closed castings (molds). Prior to the beginning of shaping of the product, the melt is fed into the mold.…”

mentioning

confidence: 99%

Refractory products for the production units of amorphous metals and alloys

Aleksandrov¹,

Litovskaya²,

Aleksandrova³

et al. 1992

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In recent years, extensive studies have been carried out on the synthesis of amorphous metals and alloys having improved physicochemical properties. Centrifugal-casting units are used for maintaining the melt in its amorphous state during the course of its solidifcation. Their operation is based on rotating a bowl at a speed of approximately i0,000 rpm and continuously feeding a thin jet of the molten material having a temperature of 1500-1600~ onto its bottom portion.The melt comes into contact with the bottom of the bowl and forms a film having a thickness up to 200 ~m under the action of the centrifugal force. This film is driven upwards to the edge of the bowl and is separated from it to form granules having a size of approximately 200 zm. The bowl is cooled using liquid nitrogen (T = 173 K) so as to ensure retention of the amorphous state of the granules during their fall and to avoid ~heir crystallization.Until recently, graphite bowls or water-cooled copper bowls have been used for carryin g out atomization of the melts. However, the extent of their corrosion under the action of the melt is so significant that they can not be reused after the very first melting. Furthermore, the molten metal is contaminated with the material of the bowl. Bowls made from fused quartz are also not being used widely because of cristobalitization and cracking during the course of cooling.This paper deals with a study of the possibility of replacing such bowls and other products used under the conditions involving the action of aggressive chemical media and temperature gradients with ceramic components. When developing the components, it was borne in mind that they must possess the following set of properties simultaneously: a high thermal shock resistance (heating internally up to 1550-1600~ and cooling from the outside up to 100~ nonwettability with the melts; ability to retain mechanical strength under the conditions involving prolonged action of temperature gradients along (across) the wall of the bowl; and sufficient mechanical strength for withstanding the stresses develped in the edge of the bowl under the action of significant centrifugal loads and the shear stresses developed during the operation.The accumulated experimental and the theoretical data show that some of the properties (for example, refractoriness and the limits of temperature compatibility of the products) are controlled by the physicochemical nature of the compact ceramic materials. However, their mechanical strength and corrosion resistance can be improved using different technological routes and new heterophase materials having a specific structure [i].In view of the fact that the structure consists of an aggregate of pores and a solid region containing crystals, a glass, and an amorphous phase, one can develop materials having the required service characteristics by changing the quantitative and the qualitative ratios of the structural elements and establishing the structure--property relationships.It is known that the nature of the pore structure of ...