Sialons have recently been obtained abroad from compounds in the system Si--AI--O-N [1-5]; they combine the main outstanding physicoehemical properties of oxides [6][7][8][9] and nonoxygen compounds* [10][11][12][13][14].Sialons are very resistant to oxidation and to the action of the metals Fe, AI, Cu, Zn, steels and also to sulfur, sulfuric acids, and alkalis.According to [15] the use of sialon for storing and transporting molten metals, including steel, is a very important application area.Abroad, sialons are synthesized mainly from AI=O~, Si3N~ and AIN [1][2][3][4], and in some cases nitrided mixtures of SiO2 and AI are used [5].Since sialons are essentially silicates or aluminosilicates in which the oxygen is partialiy replaced by nitrogen, the present authors synthesized sialon from various natural aluminosilicate materials whose compositions are shown in Table i. The carbon constituent consisted of graphite grade GL-3 (GOST 5279-74).Aluminosilicate material was mixed in a ball millwith the appropriate quantity of graphite calculated to obtain sialon with the composition 8i6_xAlxOxN6_ x , where x = 3~ The batch was used to make specimens (briquettes) at pressures of 30 and i00 MPa in the form of cylinders, 20 mm in length and diameter.The specimens and also the nonbriquetted batch (filling) were fired in an industrial muffle furnace at the Semiluksk refractories factory at 1450~ in a nitrogen atmosphere, purified to remove moisture and oxygen, at a pressure of 196 Fa. Firing was done in an open setting and also in graphite crucibles.The degree of synthesis in the sialon was evaluated on the basis of x-ray and petrographic investigations.Measurements were made of the intensity of the most typical lines with interplane distances of 0.270 and 0.331 nm (with an index of 200) for B'-sialon, and 0.362 nm for l-sialon.It was established that the sialon is synthesized from all the aluminosilicate materials studied.Judging from the intensity of the lines on the x-ray patterns corresponding to 8t and l-sialons, it is formed particularly actively in mixtures of Novoselitsk kaolin and carbon (Table 2), and furthermore with a reduction in the ratio of A1203:Si02 in the aluminosilicate materials we observe a reduction in the height of the lines for sialon (Fig. i). Firing in graphite crucibles speeds up the synthesis of sialon.The concentration of sialon expressed by the height of the line on the x-ray patterns is proportional to the amount of bonded nitrogen in the batch (Fig. 2).According to petrographic and x-ray data, in addition to sialon, all the specimens contained silicon oxynitride, residues of unreacted graphite, and glass. The sialon and silicon oxynitride have a fine-grained structure; their crystal sizes are not more than 3-8 ~m. The average refractive index of silicon oxynitride is 1.760• for the sialon it varies from 1.855 to 1.897, which indicates the variable composition of the compound.It is also established that during the nitriding of briquetted mixtures the amount of synthesized sialon is reduced, an...
The best hardening effects in the above-pressed specimens were obtained with sodium polyphosphate and periclase powder; and the worst --dunite powder; chromite powder with sodium polyphosphate hardly sets at all. The material is not weakened or embrittled at 800~ and is well sintered at 1600~ Thus, the Oeomp value after firing at 1600~ increases by approximately one order.Casting, compared with pressing, impairs the thermomechanical properties of the specimens, but A~ and ~ remain practically constant.In order to obtain denser specimens from magnesite concretes it is desirable to use combined grinding of microgranular mixtures of periclase powder with additions of 15-25% dunite, and for volume-constant concretss --with additions of 30-60% chromite. When water suspensions are cast it is necessary to know the rheological and technological properties of the sialon suspensions, but the literature contains absolutely no data on these properties. RHEOLOGICAL AND TECHNOLOGICAL PROPERTIES OF SUSPENSIONSThe present authors therefore studied the influence of pH on the rheological and technological properties of sialon suspensions.We used sialon prepared from a mixture of Novose, litsk kaolin and graphite of optimum composition, fired in nitrogen at 1450~ for 4 h [3]. The sialon was ground in a tungsten--carbide mill to a grain size of less than 4 ~m. According to x-ray data the main phase of the original material was B'-sialon, and the impurities --silicon oxynitride and mullite.The electrolytes used for altering the pH in preparing the supensions consisted of HCI and NaOH.The relationship between the velocity gradient and the effective viscosity and the shear stress for sialon suspensions has a thixotropic character, and is similar to the corresponding relationships for silicon carbide suspensions [4].An increase in the pH of sialon suspensions sharply reduces the shear stress necessary for destroying the limiting coagulation structure of the suspension, especially with a change in pH from 2 to 4-5 (Fig. i). Thus, with a pH of 2 the shear stress varies in the range 22-23 Pa, and when the pH is 4-5 it is 3-4 Pa.In an acid medium an increase in pH reduces the viscosity of the structure's breakdown threshold by a factor of 3 or more --from 0.3 Pa'sec for pH = 2, to 0.05-0.07 Pa.sec for a pH of 3-5 (Fig. 2). With a change to an alkaline medium the viscosity of the breakdown threshold in the structure again rises right up to 0.15 Pa'sec for pH = ii.The relationship between the minimumplastic viscosity of sialon suspensions has a similar character, which is apparently connected with the increase in the hydration of the particles of sialon and the accompanying impurities of mullite and silicon oxynitride with a significant increase in pH. Hydration, increasing the effective radius of the particles due to the intermicelle water, reduces the amount of dispersion medium.The zeta-potential of sialon suspensions has a negative value for all pH values (Fig.
The study of the phase formation in the system Si-AI-O-N is interesting in connection with the production of new ceramic materials (sialons) which are products of the reaction between nitrides and oxides.Sialons combine to a certain degree the advantages of nitrides and oxides, and are promising for use in various regions of technology.The sialon phases (while preserving the normal valence of the elements constituting the quaternary system Si-AI-O-N) are situated on the plane Si3N4-SiOz-AI203-AIN, formed by the binary compounds in the system. This plane may be divided into two equilateral triangles, consisting of the subsystems Si3N4-AI203-SiO 2 and Si3N4-AI203-AIN [i].In a study of the partial system Si3N4-AI203-AIN [1-3], the formation of solid solutions was detected on the basis of ~-Si3N 4. The crystallochemical prerequisites for the formation of such phases consist in the fact that the structure of ~-Si3N 4 and AIN is built up respectively from tetrahedra SiN 4 and AIN4, and the aluminum may exist in the tetrahedral surroundings of the ions of oxygen; and furthermore, the tetrahedra SiN 4, AIN 4, and AIO 4 are close in size. This creates the possibility of substitution of the silicon by aluminum and of nitrogen by oxygen in the structure ~-Si3N 4 and the formation of solid solutions of substitution on the basis of ~-Si3N4, consisting of tetrahedra (Si, AI) (O, N)4 , named ~'-sialons.In the present work we studied the formation and structure of sialon, obtained by the method of hot pressing, in the system Si3N4-AI203-AIN.To establish the region of extension of the monophase ~'-sialon solid solution in the stated system, we prepared a mixture having the composition (l-x) Si3N 4 + x (AI203"AIN). The starting components consisted of pure finely dispersed powdered ~-Si3N4, AIN, and AI203.These compositions (in the range 0 ~ x ~ 0.9) were carefully mixed and subjected to hot pressing at 1750~ soaking for 30 min, and using a pressure of 1.5 N/mm z.The phase composition of the specimens was determined by x-ray and crystal-optical methods. For this we used recordings on the DRON-I.5 diffractometer in CuK~ radiation (Ni filter) directly from the plane of the cross sections of the fired specimens. The parameters of the cell for the ~'-sialon were determined by the scanning method using lines (i01) and (210) on the hexagonal structure for the calculations. The determination errors for the parameters were • nm. The refractive indices were located with the MIN-7 microscope in immersion preparations.The x-ray studies of the fired specimens in the system Si3N4-AI203-AIN (compositions of which were selected along the lines with a ratio of cation/anion = 3/4) for establishing the extent of the region for the ~'-sialon showed that over the section (I -x) Si3N 4 + x (AI203"AIN) right up to x = 0.8 there forms a monophase region of hexagonal solid solutions on the basis of ~-Si3N 4 (~'-sialon). On the diffractograms of these samples we observed a shift in the reflexes typical of the structure of ~-Si3N 4 toward the smaller angle...
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