R. P. Felten, Sprechsaal far Keramik, Glass, Email, Silikate, No. 24, 1088-1099 (1971). 6. A. E. Zhukovskaya, F. S. Kaplan, E. A. Sherman, et al,, Ogneupory, No.
corundum refractories, combining such frequently mutually exclusive properties as high density, thermal-shock resistance, abrasion resistance, and stable strength at high temperatures.
LITERATURE CITED i. 2. 3. 4. 5. 6. 7. 8 9 I0 Ii 12 13 14 15 16 17 18.
I. S. Kainarskii, E. V. Degtyareva, and I. G. Orlov, Corundum Refractories and Ceramics [in Russian], Metallurgiya, Moscow (1981), p. 161. Ya. A. Orlovskii, Use of Refractories Abroad [in Russian], (Review of Information), Refractories Production Series ll/Int. Chermetinformatsiya, No. I, 16 (1977). A. A. Mukhin, G. E. Karas', V. I. Entin, et al., Ogneupory, No. ii, 20-24 (1979). A. A. Kortel', Ogenupory, No. 5, 16-22 (1983). R. P. Felten, Sprechsaal far Keramik, Glass, Email, Silikate, No. 24, 1088-1099 (1971). A. E. Zhukovskaya, F. S. Kaplan, E. A. Sherman, et al,, Ogneupory, No. 7, 4-8 (1986). G. A. Gogotsi, Inelasticity of Ceramics and Refractories: Preprint [in Russian], IMP Akad. Nauk UkSSR, Kiev (1982), p. 68. A . G . Karaulov, T. E. Sudarkina, N. V. Gul'ko, et al., Ogneupory, No. ii, 48-53 (1976). N. Claussen and I. Steeb, J. Am. Ceram. Soc., 59, Nos. 9-10, 457-458 (1976). A . A . Dabizha and S. Yu. Pliner, Ogneupory, No. II, 23-29 (1986). N. Claussen, Z. Werkstofftechn., No. 13, 138-147, 185-196 (1982). A . E . Zhukovskaya, E. A. Sherman, et al., Ogneupory, No. 2, 46-50 (1981). T. Kosmac, I. S. Wallace, and N. Claussen, J. Am. Ceram. Soc., 65, No. 5, 66-67 (1982). M . N . Bluvshtein, Zavod. Lab., No. i, 113-116 (1956). G . A . Gogotsi, A. N. Negovskii, and A. E. Zhukovskaya, Ogneupory, No. 3, 16-19 (1984). R . G . Garvie, R. H. Hannik, and K. T. Pascoe, Nature, No. 258 (5537), 703-704 (1975). F . F . Lange, Fracture Mech. Ceram., ~, 799-819 (1978). W. Kreher and W. Pompe, J. Mater. Sci., 16, No. 3, 694-706 (1981).
PROPERTIES OF POROUS SILICON-NITRIDE MATERIALS
V. N. Antsiferov and V. G. Gilev
UDC 666.762.93-405.8.001.4
Various technological measures are used to increase the thermal-shock resistance of refractories; these enable us to obtain fragmented and other structures capable of undergoing substantial elastic and nonelastic deformations [1-8]. Another possibility is to use materials based on compounds with low temperature coefficients of expansion, amongst which most attention is being paid to silicon nitride [3, 7, 9]. Several methods are known for obtaining porous silicon-nitride materials* [9-11], but satisfactory resistance to oxidation is possessed only by the finely porous materials [II]. The present authors studied reaction-sintered, silicon-nitride materials obtained from the elements originally having the shape of curved, thin platelets 0.5-1.5 mm wide and 0.07 mm thick.# The elements are obtained mechanically from bodies based on silicon, plastics, and special additives. The articles are shaped at low pressures (from 0.5 x 10 .3 tO 7 • 10 .3 N/mm2), by heating to I00-150~ and comparatively simple devices are used. It is possible to make large articles in the form of plates, discs, bricks, etc. During the process of forming conglomeration and deformation (in the main flattening out) of the elements occur. By changing the shaping conditions it is possible to regulate the degree of deformation of the elements during shaping, and accordingly the density of the material in a wide range (from 0.5 to 1.8 g/cm3). The density of the blanks is reproduced *Patent No. 2105316, Great Britain. ?It is possible to vary the thickness from 0.25 to 0.005 mm. Perm Polytechnical Institute.
410
Translated from Ogneupory, No. 7, pp. 20-23, July, 1988.
0034-3102/88/0708-0410512.50
9 1989 Plenum Publishing Corporation
dV
g/cm 2
~m
1,0
i, W/(m.k) Ocom~N/mm2f,W~. N/ml2
n--- N lz
~y,~
b
~
I
60 80
,~lle
0,5
J
0,0ol 0,01
0 p'-J 0,5 1,0 I I
0,!
1,01", ~tm
Fig. i
I
I 0 0 =='="~----'-~"---'~ 0 1.59, g/cm3 1,0 f,5 9, glcm 3 l
80 70 60
I
I
p,%
I
I
l
I
I
O0 fO 60 p ,o/,
Fig. 2
Fig. i. Differential porogram for specimen of silicon nitride material made of thin-walled elements with a porosity of about 50%. Fig. 2. Relationship between the properties of silicon nitride material made of thin-walled elements and the density p (porosity P): a) thermal conductivity A at IO0~ along the pressing axis (i), and the compressive strength aco~ (2) at 20~ b) elasticity modulus E (3) and elastic deformation ~y (4) under compression at 20~
with a high degree of accuracy; in five series of experiments carried out in different conditions of shaping, the mean square deviation of the density did not exceed 3% from the mean. The structure of the materials consists of a combination of a framework made from interbonded curved platelike elements, filled with fine pores, and larger pores filled with fine, fibrous, silicon nitride formed in the process of reaction sintering [12]. This research involved using a batch based on silicon KrO (GOST 2169-69). As a result of sintering in technically pure nitrogen, in a channelless furnace, with a graphite heater, the increase in the mass equals 60-63% of the mass of the silicon in the batch. The residual silicon at the final stage of sintering is forced out of the elements of the framework and is located in the structure in the form of microscopic spheres arranged on the woollike product~ The specific surface of the materials Sspe=, determined by the BET method varies in the range 1.7-5.8 m2/g in relation to the porosity and composition of the batch. For material having a porosity of 50% (Sspec m v 3 mZ/g, Sspec = 5 m2/cm 3) the assessment of the effective radius of the pores, reff, according to the equation reff = P/Sspec, gives a value of 100 nm. This material, according to mercury porometry* may be characterized as finely porous with a wide range of pores from 8 to 3500 rum with a maximum in the region of 50 nm. The differential curve for the distribution of the pore volumes in relation to size is close to the logarithmic normal (Fig. I). =
The nature of the change in the properties with increase in the density varies (Fig. 2)~ The thermal conductivity A increases practically linearly; the curve for the rise in strength can be described as a second degree polynomial; and the elasticity modulus increases sharply with a relative density of more than 50%. The materials are anisotropic and the mechanical properties as a rule are higher in value in the direction perpendicular to the pressing axis. Compared with known porous, reaction-sintered silicon nitride materials, the materials obtained by combustible additives techniques in which polyvinyl alcohol powder is used as the additive [ii], the new materials described here have a low thermal conductivity. This is especially marked in the porosity range 40-50% where the materials being studied also have the advantage in strength (see Table
*Carried out by G. V. Kusov (Inst. Heat Physics UrO Akad. Nauk SSSR, in the town of Sverdlovsk on the PM-3A mercury porometer.
411
TABLE I. Properties of Porous, Silicon-Nitride Materials* Therm. cond., ICon~ress. str., Porosity,% w/(m'K), lO0~ IN/mm~ , 20~ 55
49
l,l/3,0 1,3/5,0
40
1,5/6,0
I
| |
40/37 70/51
!
85/58
[
*The numerator indicates the material made of elements, the denominator - w i t h combustible additives [ii].
i), and possess a uniquely high deformability (see Fig. 2b), which reaches 2-3% of the elastic deformation. The lower thermal conductivity in the materials studied is possibly connected with the more finely porous structure and the formation of a large number of cracklike pores during deformation (flattening) of the thin-walled elements. As is known, [i, 13] the greatest reduction in thermal conductivity arises from pores, and especially cracks measuring i #m and less. In this regard we should note the usefulness of creating heat-insulating materials based on refractory compounds with covalent bonds (Si3N4, SiC) for which, as a result of the difficulty of diffusion, the fine-pored structures are thermodynamically stable. The high deformation capacity of the materials is of special interest. The elastic deformation under compression of materials with a porosity of 70% equals 0.7-0.9%, and it increases with increase in the density (see Fig. 2b). In the porosity range 40-50% it reaches 2-3%, and for some specimens 4-5%. On the deformation diagrams for highly porous (about 70%) materials we note significant sections of nonelastic deformation. In terms of the deformability the specimens greatly exceed that attained for materials made of sintered microspheres of zirconium dioxide [4]. The high level of deformationis explained by the features of the microstructure consisting of a multiplicity of thin-walled, platelike elements which, due to the presence of hollow interlayers, are capable of significant bending deformations. The impact strength of the specimens with a density of 1.76 g/cm 3 (porosity 45%) equals 0.9-1.1 kJ/m 2. The features of the microstructure and mechanical properties of the materials mean we can carry out machining on ordinary lathes and other cutting machines, with ordinary tools. In this case, local damage takes place, i.e. chipping of the thin elements close to the cutting edge, and with the formation of a practicaly smooth surface; the material removed has the form of fine dust. The unusual frequency structure and the high deformability of the materials lead us to assume that they will have a high thermal-shock resistance. For the experimental assessment of this property we tested samples of 14 materials with densities from 1.30 to 1.83 g/cm 3 (porosity 60-43%) in the form of specimens measuring 14 x 14 (16-19) mm. Tests included 50 heating cycles in a muffle furnace at 800~ with a soak of I0 min, and cooling in air. The temperature of 800~ was selected in order to reduce to a minimum the influence of oxidation. After the heat cycling we observed an increase in the strength, on average by 29%, and a rise in the maximum deformation of 15%. The mass increase as a result of oxidation was 0.5-1.0%, and on average 0.76%. More rigid tests were made using a heating schedule of upto 900~ and cooling in water, on specimens* in the form of discs 62 mm in diameter and 14-17 mm high. We tested five specimens of various materials with a density of 1.17-1.56 g/cm 3 (porosity 51-63%). The first cracks appeared after 10-20 cycles. Two specimens were completely destroyed after 47 and 69 cycles; three specimens withstood i00 cycles without total destruction. The increase in the mass as a result of oxidation during heat cycling after 50 cycles equaled 1.5-2.0%. After 30-40 cycles we observed bending on the oxidation curve (accelera*Specimens were obtained from elements 0.01 mm thick.
412
& '[/ 0 # I
0
t
~
t
N o t. c y c l e s ,
5
1
t
i0
I
n
~[_
I
~
Time, h Fig. 3. Integral (I) and differential (2) curves for change in mass m during the oxidation of specimens (porosity 51%) during heat cycling: heating at 8000C (i0 min) - cooling in water. Am/m,
%
a c
0,71 8 o, a7
~m//n, %
0,07 1,33
c
2
0,87 1o
I
fO
b
I "
1,33 IJ 1,45 J
0
1,72
# 2
0
20
,',J5
0,87 1 1
40
Time, h
I
8
1,72
4
2 0
lO
20
30 Time, h
Fig. 4. Oxidation kinetics of porous silicon nitride material made of thin-walled elements in air at 850~ (a) 950 (b), and IIO0~ (c). Notation on the curves - density of material g/cm 3 .
tion of oxidation) due to the formation of cracks, tion surface (Fig. 3).
i.e. because of the increase in the oxida-
Oxidation was studied on specimens measuring 14 x 14 (16-19) mm by determining the mass on an analytical balance after holding in the muffle furnace. The generally accepted measure of oxidation assessed with respect to the increase in mass Am, related to unit surface of specimen S, in the case of highly porous materials in unsatisfactory, since this magnitude (Am/S) does not have a consistent relationship with density. Thus, after 50 h soaking at 850~ the maximum value of oxidation (i0 mg/cm z) was exhibited by specimens with a density of 1.33 g/cm3; for materials having a density of 0.87 and 1.72 g/cm 3 it was 8 and 7 mg/cm z, respectively. It is more satisfactory to evaluate the oxidation resistance of highly porous materials with respect to the increase in mass, whose maximum value for silicon nitride equals 28.5%.* *I. Yao Guzman, "Investigation into the field of reaction sintering of ceramics based on silicon compounds in the system Si-C-O-N," Author's Abstract of Dissertation for Dr. Tech. Sci., Moscow (1979), p. 36. 413
0
I
I
I
I
I
I
0 1o fO JO ~0 50 80 70
Time, h F i g . 5. A s y m p t o t i c i n t e r p r e t a tion of the oxidation kinetics of the specimens with a density of 1.72 g/cm 3 at 850~ (i) and 950~
(2). Oxidation at 850~ is accompanied by a rapid increase in mass for the first 5-10 h; further oxidation occurs very slowly (Fig. 4a). The final mass effect of oxidation gradually increases with increase in porosity. Obviously, the oxidation process is discontinued after the formation of a layer of oxide over the entire internal surface of the material, and the process is stopped because of the low diffusion mobility of the atoms in the oxide layer. At 950~ and II00~ we also observe a retardation of oxidation after i0 h soaking, but the rate of oxidation remains noticeable, and the final mass effects of oxidation are significant. The oxidation kinetics at 850 and 950~ are best described with the aid of asymptotic interpretation (Fig. 5) [14]. The marked incline in the curve i in Fig. 5 signifies more rapid attainment of saturation during oxidation at 850~ than at 950~ (curve 2). Judging from the combination of properties for these material it is possible to recommend them for prolonged working at 700-800~ for example, in muffle furnaces and baths of molten aluminum normally having a temperature of about 700~ in which silicon nitride is resistant and is not wetted [9]; and also for operation in conditions of short-term and periodic heating to high temperatures, for example, as heat insulation in the MGD channels of impulse nonstationary MGD generators. The materials being studied were successfully used by the authors as the reinforcing phase for the production of composition materials for various purposes with matrices of alloy metals (alloys) and polymers. CONCLUSIONS Newly developed porous silicon-nitride materials from elements have a thin walled porous structure, which in comparison with known materials for the same kind and similar porosity have a lower thermal conductivity, high strength, deformability, and thermal-shock resistance as well as satisfactory resistance to oxidation at temperatures of about 800~
LITERATURE CITED i. 2. 3. 4. 5. 6. 7. 8. 9. I0. ii.
414
K. K. Strelov, Theoretical Principles of the Technology of Refractory Materials [in Russian], Metallurgiya, Moscow (1986), p. 480. A. S. Vlasov, D. A. Ivanov, and G. A. Fomina, Ogneupory, No. 8, 10-13 (1985). I. Ya. Guzman, E. V. Kosokina, and E. I. Tumakova, Ogneupory, No. 6, 44-47 (1974). Yu. L. Krasulin, V. A. Timofeev, C. N. Barinov, et al., Porous Design Ceramics [in Russian], Metallurgiya, Moscow (1980), p. I00. G. V. Kukolev, I. I. Nemets, and G. B. Dobrovol'skii, Ogneupory, No. 3, 14-21 (1967). G. A. Gogotsi, Ogneupory, No. 5, 45-50 (1977). I. I. Nemets, Probl. Prochn. No. II, 48-51 (1981). A. I. Zabotka and A. Ya. Peras, Probl. Prochn. No. I, 40-43 (1984). R. A. Andrievskii and I. I. Spivak, Silicon Nitride and Materials Based on It [in Russian], Metallurgiya, Moscow (1984), p. 136. I. Ya. Guzman, Highly Refractory Porous Ceramics [in Russian], Metallurgiya, Moscow (1971), p. 208. P. A. Kornienko, V. Ya. Naumenko, V. A. Chekhovich, et al., Poroshk. Metall., No. II, 67-70 (1984).
12. 13. 14.
G. V. Samsonov, O. P. Kulik, and V. So Polishuk, Production and Methods of Analyzing Nitrides [in Russian], Naukova Dumka, Kiev, (1978), p. 320. E. Ya. Litovskii, F. S. Kaplan, and S. L. Bondarenko, Ogneupory, No. II, 42-46 (1986). F. Porz and F. Th~mmler, J. Mater. Sci., 19, No. 4, 1283-1295 (1984).
INFLUENCE OF HIGH-TEMPERATURE HEAT TREATMENT IN VACUUM ON THE ELECTRICAL RESISTANCE OF SILICON-CARBIDE HEATING ELEMENTS
V. i. Polyak, G. S. Rossikhina, V. L. Kalikhman, and M. A. Solov'ev
UDC 666.762.852.017.001.5
Silicon-carbide electric heating elements (KEN) are widely used in various regions of industry for carrying out electrical heating processes at temperatures up to 1500~ The use of electric furnaces fitted with such heaters ensures strict observation of the heating schedules for processing materials and a high quality in the finished product. As a result of the increasing specialization of electric furnaces with these heaters the required range of electrical resistance in the heaters has been extended [I]. However, the methods of regulating the electrical resistance of such heaters have not been developed satisfactorily. Meanwhile it is known that the high-temperature thermal treatment in vacuum of silicon carbide leads to a change in its phase composition, and as a result, in the electrical resistance [2]. We studied the possibilities of increasing the resistance of KEN heaters by changing their phase composition in the process of additional heat treatment in vacuum. The electrical conductivity of silicon carbide in the composition of KEN is governed on the one hand, by the ratio of the cubic (~) and hexagonal (~) modifications, and on the other hand, by the nonstoichiometric composition and the content of impurities, mainly in the form of free silicon. As is known, silicon carbide is a strictly stoichiometric compound. According to data in [3] the ratio Si:C is 1.049 for E-SiC and 1.032 for o-SiC. In other words the crystals of silicon carbide contain upto 59* nonstoichiometric excess of silicon. Moreover, the composition of the KEN heaters normally contains upto 19 free silicon as impurity [4]. The resistivity of silicon is 10 .3 ~'cm, which is two orders higher than that of silicon carbide, Therefore, we cannot exclude the fact that thin films of free silicon in the composition of KEN heaters, the total quantity of which may reach 19, makes such a contribution to the resistivity as to equal all the other parts of the material. It can be assumed that additional heat treatment of the KEN material leads to removal of the silicon as a result of volatilization, the result of which will increase the resistivity of the heating element~ As regards the role of the ratio of the ~ and ~ phases of the silicon carbide in the KEN composition, then a significant difference in the resistivity of these phases leads to a substantial difference in the electrical resistance of the KEN with different ratios of the ~- and o-SiC. Thus, according to data in [5] an increase in the content of E-SiC in the KEN material from 35.5 to 809 reduces the resistivity of the heaters from 0.032 to 0.006 ~'cm. Thus, it becomes possible to increase the electrical resistance of KEN by altering the phase composition of the fired heaters, using high temperature schedules. The electrical resistance of the heater can be increased if we reduce the mass proportion of E-SiC right down to complete exclusion. This condition is quite possibly realized in the supplementary high-temperature processing. Such treatment, through volatilization, may remove not only the impurity of free silicon from the heater material, but also its nonstoichiometric excess from the crystals of silicon carbide. With a certain limitation of the heating temperature, as *Here and subsequently mass parts are indicated. Moscow Institute of Chemical Technology. Branch of the All-Union Scientific-Research Institute of Electromechanics. Translated from Ogneupory, No. 7, pp. 24-25, July, 1988.
0034-3102/88/0708-0415512.50
9 1989 Plenum Publishing Corporation
415
