The problem of thermal stability of ceramic materials

The thermal stability of ceramic materials is considered from the standpoint of fracture mechanics as a property determined by the capacity of the structure to resist the appearance and propagation of cracks on critical defects under the effect of thermal stresses. A method is suggested for comparative evaluation of the thermal stability of ceramics according to which specimens with a notch simulating a structural defect responsible for fracture are subjected to thermal shock. The absence of induced defects in the region adjoining the tip of the notch is a necessary condition for providing reproducible results. The resistance to thermal shock is determined from the relative decrease in the crack resistance after the thermal shock, the “insensitivity” of the structure to defects, and the degree of their accumulation in the region of the tip of the notch as a result of the thermal shock. The first and second criteria for evaluation of thermal stability involve the coefficient of relaxation of thermal stresses and the ultimate bending strength of the notched specimen. A ceramics of ZrO₂ partially stabilized by 3 and 12% Y₂O₃ and a cermet with a composition of ZrO₂-3 vol.% Y₂O₃-50 vol.% Cr are chosen for studying thermal stability by the method developed. Calculated results on the thermal stability of the cermet are compared with results obtained directly by thermocycling with variation of the metal content from 10 to 50%. The maximum mechanical properties are shown to correspond to a metal content of 40% due to formation of double-skeleton structure in the cermet. The method can be helpful for evaluating the initial stage of fracture caused by a thermal shock in structural ceramics. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 3, pp. 5–10, March, 1998.     

Effect of an Iron Catalyst and Process Parameters on Si-Based Ceramic Materials Synthesized From Rice Husks

Silica and carbon are naturally occurring in rice husks (RH) and these were used for the production of ceramic materials made of Si, C, N and O as the main constituents. The various “silicon-based” ceramics were produced from the thermal decomposition of rice husks and posterior heat treatment at temperatures varying from 1200–1450 °C under a pure nitrogen atmosphere. FeSO₄ in various concentrations was introduced to the decomposed rice husk prior to the heat treatment. The formation of various Si/C/N/O ceramics in general and silicon nitride in particular, were studied with respect to the concentration of FeSO₄ (4–10%) as well as temperature (1200–1450 °C). The formation of different phases were confirmed by XRD and FT-IR analysis. Morphology and surface properties of the products have been studied using SEM. The maximization of whiskers of the ceramic products was also studied through microstructural analysis. Elemental analysis of the whiskers product was analyzed through EDX. The chemical analysis of raw rice husk was also carried out.     

Strength and Notch Sensitivity of Porous-Matrix Oxide Composites

The effects of matrix strength on the notched and unnotched tensile properties of a family of porous-matrix oxide composites are examined both experimentally and theoretically. Experiments are performed on three composites, distinguished from one another by the amount of binding alumina within the matrix. Increases in alumina concentration produce elevations in unnotched tensile and shear strengths, but the benefits are offset by an increase in notch sensitivity. The degree of notch sensitivity is rationalized on the basis of a model that accounts for interactions between notch tip tensile and shear bands. The model predictions are cast in terms of the ratio of the notch length to a characteristic bridging length scale. These results, in turn, form the basis for a simple analytical formula for notched strength, accounting for effects of elastic anisotropy and finite sample size. The utility of this formula in predicting notched strength is assessed. Issues associated with bridging law shapes and bridging length scales are addressed. The effect of alumina concentration on notch sensitivity is discussed in terms of its influence on the bridging length scale, dictated by the interplay between the unnotched tensile strength, the longitudinal Young's modulus, the degree of in-plane elastic anisotropy, and the fracture energy. The net result is a decreasing bridging length scale and hence increasing notch sensitivity as the matrix is strengthened with alumina.     

Formation of microstructure in bauxite ceramics

The effect of TiO₂ additives on the formation of the microstructure of bauxite ceramics is considered. It is shown that the thialite phase formed in the roasting process “cleans” the corundum phase and the glass phase from impurities. The thialite grains reinforce the corundum matrix, hamper the growth of corundum grains in roasting, and improve the resistance of the material to crack propagation when it fractures. Playing the role of bridges connecting the “banks” of a crack, thialite grains enhance the crack resistance of the material. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 3, pp. 2–6, March, 2000.     

Scientific and practical approaches to creating ceramic refractory materials and technology

A comprehensive methodological approach is proposed for resolving metallurgical problems within the scope of a multistage problem formula “composition – structure – properties” – “technology” – “thermal shock-corrosion-erosion-resistant material” followed by development of material technology and objects conforming to refractory conditions and operating regimes in heating units. Practical scientific approaches are considered for forming heat and corrosion resistant refractory materials. A general structural model is proposed for ceramic refractory technology making it possible by computer modelling to perform systematic analysis, prediction and calculation of specific technological parameters in the main production stages.     

Rheotechnological properties of mixed suspensions in the SiO2 - Al2O3 system and some properties of materials based on them. 1. Molten quartz – Alumina system

The rheotechnological properties of mixed suspensions in the SiO₂ - Al₂O₃ system obtained by the method of mixing of individual suspensions of molten quartz and alumina are described. Some properties of the materials after their heat treatment at 1000 – 1300°C are investigated. The ranges of compositions (30 – 40% Al₂O₃, 60 – 70% SiO₂) in which the parameters of thermal expansion of the materials are 2 – 3 times lower than those calculated under the condition of additivity are determined. The obtained materials possess an elevated mechanical strength. Translated from Ogneupory i Tekhnicheskaya Keramika, No.7, pp. 18 – 23, July, 2000.     

Preparation and thermal shock resistance of cordierite-spodumene composite ceramics for solar heat transmission pipeline

Cordierite-spodumene composite ceramics with 5, 10, 15 wt% spodumene used for solar heat transmission pipeline were in-situ prepared via pressureless sintering from kaolin, talc, γ-Al₂O₃ and spodumene. Effects of spodumene on densification, mechanical properties, thermal shock resistance, phase composition and microstructure of the composite ceramics were investigated. The results showed that spodumene used as flux material decreased the sintering temperature greatly by 40–80 °C, and improved densification and mechanical properties of the composite ceramics. Especially, sample A3 with 10 wt% spodumene additive sintered at 1380 °C exhibited the best bending strength and thermal shock resistance. The bending strengths of A3 before and after 30 thermal shock cycles (wind cooling from 1100 °C to room temperature) were 102.88 MPa and 96.29 MPa, respectively. XRD analysis indicated that the main phases of the samples before 30 thermal shock cycles were α-cordierite, α-quartz and MgAl₂O₄, and plenty of β-spodumene appeared after thermal shock. SEM micrographs illustrated that the submicron β-spodumene grains generated at the grain boundaries after thermal shock improved the thermal shock resistance. It is believed that the cordierite-spodumene composite ceramics can be a promising candidate material for heat transmission pipeline in the solar thermal power generation.     

Some methodological features of determination of crack resistance in ceramic materials

The known methods for forming stress concentrators (cracks and notches) in ceramic specimens in order to determine their crack resistance are described. A method for forming a notch with a tip curvature radius of at most 10 µm is suggested. The notch is first formed in the process of pressing the specimen in a specially designed mold and then the green specimen is cut additionally from the tip of the notch by a steel blade with a thickness of 0.1 mm and a grinding angle of 14°. After sintering, this specimen does not contain induced defects that are possible when sintered specimens are notched by a diamond disk by the conventional method. It is shown for a ZrO₂ ceramics partially stabilized by 12 mol.% CeO₂ and an Al₂O₃ ceramics with 0.5 wt.% MgO possessing a layered granular structure that an incorrect choice of the tip curvature radius can result in an erroneous evaluation of the optimality of the structure of the material and an incorrect choice of the technological parameters for its production. Notching by the suggested method made it possible to establish the discrete nature of fracture of the layered granular structure of the ceramics from strain diagrams and the mechanisms of crack propagation causing this kind of fracture.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 9, pp. 26 – 30, September, 1996.     

Development of technology for the production of microspherical alumina support for the alkane dehydrogenation catalyst: III. The effect of the phase composition of microspherical supports on their thermal stability

This publication continues a series of our reports on the optimization of preparation conditions for obtaining a thermally stable support for the alkane dehydrogenation catalyst. The phase composition effect on the stability, particle size distribution, structure, texture, and mechanical properties of supports heated to 1100°C is reported. Microspherical alumina supports obtained by successive thermal and hydrothermal treatments of gibbsite are compared to commercial supports obtained by the thermochemical activation (TCA) of gibbsite. The dimensions of the support granules decrease upon heating because of shrinkage, which is governed by the phase composition of the granules and by the packing of their constituent boehmite and alumina crystallites. Three temperature intervals can be distinguished in the shrinkage of the granules. In region I (<600°C), there is intensive shrinkage via the diffusion glide of crystallites, the mechanical strength of the granules remaining invariable. In region II (600–900°C), the polymorphic transformations of alumina accompanied by sintering via surface diffusion do not affect the dimensions and strength of the granules. In region III (>900–1000°C), shrinkage takes place via coalescent sintering. For commercial manufacturing of microspherical alkane dehydrogenation catalysts and for ensuring their stability at 550–900°C, it is recommended to use alumina supports containing the minimum possible amount of χ-Al₂O₃. As the single-phase boehmite support obtained by our technology is heated to 1100°C, its granules shrink by no more than 14.4% and show an attrition resistance of 89% or above. The support based on the gibbsite TCA products, which contains 14–23 wt % χ-Al₂O₃, is characterized by 3–5% greater granule shrinkage and 6–12% lower mechanical strength.     

Synthesis and Study of Composite Materials in the ZrO 2 (Y 2 O 3 )–MgAl 2 O 4 System

Powders–precursors of a tetragonal solid solution based on partially stabilized zirconium dioxide (t-ZrO2) and aluminum-magnesium spinel (MgAl2O4) are synthesized using the method of the cocrystallization of solutions of nitrate salts from which nanocrystalline (<100 nm) composite materials are fabricated at 1400°C in the ZrO2(Y2O3)–MgAl2O4 system with an open porosity of 3%. The structure, physical–mechanical properties, and thermal stability of the nanocomposites are investigated. It is established that the introduction of MgAl2O4 into the matrix of the solid t-ZrO2 solution increases the thermal resistance of the ceramics under the thermal cycling conditions (20–1000°С). The effect of thermal cycling on the phase composition, hardness, and bending strength of the ceramics in the ZrO2(Y2O3)–MgAl2O4 system is investigated.     

Investigating the effect of SPRM on mechanical strength and thermal conductivity of highly porous alumina ceramics

For recycling waste refractory materials in metallurgical industry, porous alumina ceramics were prepared via pore forming agent method from α-Al₂O₃ powder and slide plate renewable material. Effects of slide plate renewable material (SPRM) on densification, mechanical strength, thermal conductivity, phase composition and microstructure of the porous alumina ceramics were investigated. The results showed that SPRM effectively affected physical and thermal properties of the porous ceramics. With the increase of SPRM, apparent porosity of the ceramic materials firstly increased and then decreased, which brought an opposite change for the bulk density and thermal conductivity values, whereas the bending strength didn’t decrease obviously. The optimum sample A2 with 50 wt% SPRM introducing sintered at 1500 °C obtained the best properties. The water absorption, apparent porosity, bulk density, bending strength and thermal conductivity of the sample were 31.7%, 62.8%, 1.71 g/cm³, 47.1 ± 3.7 MPa and 1.73 W/m K, respectively. XRD analysis indicated that a small quantity of silicon carbide and graphite in SPRM have been oxidized to SiO₂ during the firing process, resulting in rising the porous microstructures. SEM micrographs illustrated that rod-like mullite grains combined with plate-like corundum grains to endow the samples with high bending strength. This study was intended to confirm the preparation of porous alumina ceramics with high porosity, good mechanical properties and low thermal conductivity by using SPRM as pore forming additive.     

Thermal characteristics of high-strength and thermostable aromatic fibres

The thermal characteristics of para-aramid, polyoxadiazole, and polyimide fibres were comparatively investigated by dynamic thermogravimetric analysis, differential scanning calorimetry, and thermomechanical analysis. It was shown that thermooxidative degradation of these types of fibres began at 400–450 °C and intensified at higher temperatures. The fibres investigated are characterized by size stability up to the initial temperature of thermooxidative processes (400–450 °C). With respect to thermal stability, these fibres are in the following order: polyimide > polyoxadiazole, and carbocyclic para-aramid fibres. The correlation of the “hydrogen index” I_H and “aromaticity index” I_Ar for thermostable fibres with their thermal stability was demonstrated. Translated from Khimicheskie Volokna, No. 3, pp. 72–74, May–June, 2008.     

Effect of hollow spheres on the properties of lightweight periclase-magnesium aluminate spinel refractories

This study presents new lightweight periclase-magnesium alumina spinel refractories for the working lining of cement rotary kilns in which magnesium alumina spinel hollow spheres are used to replace conventional dense fused magnesia-aluminate spinel aggregates. The effects of adding spinel hollow spheres on the physical properties, mechanical strength, thermal conductivity, and slag resistance of the samples were explored. The results showed that compared with the sample prepared with dense aggregates, the sample prepared with hollow spheres had a 10.3% higher cold compressive strength, 44.1% higher modulus of rupture (MOR), and lower bulk density. Additionally, with increasing hollow spheres content, the thermal conductivity decreased from 3.79 W/(m·K) to 2.53 W/(m·K), and the high-temperature MOR increased from 2.82 to 4.09 MPa. The highest residual strength ratio was 90.73% (15 wt.% hollow spheres), which is 1.17 times that of the sample prepared without hollow spheres. Moreover, microstructure and energy dispersive spectroscopy of crucible specimens after corrosion by cement clinker showed that specimens with 15 wt.% hollow sphere additions had a better slag resistance. Introducing hollow spheres reduced the thermal conductivity of the refractories, providing a new strategy for improving the heat insulation performance of kiln linings.     

Efficiency of the use of silicon nitride ceramics as an armor material

The efficiency of the use of materials based on Si₃N₄ for armor is estimated experimentally in comparison with conventional alumina ceramics. It is shown that the studied compositions of monolithic and composite ceramics based on Si₃N₄ are 12–30% more efficient in ballistic applications than alumina ceramics. Monolithic high-density OTM-917 ceramics based on Si₃N₄ (PCS) is shown to have the highest physicomechanical properties among the studied materials. The choice of the composition depends on the requirements on the mass, size, and cost of the armor. The experimental results have been confirmed by tests for bullet and splinter strength. Translated from Ogneupory i Tekhnicheskaya Keramika, No. 6, pp. 9–12, June, 1997.     

Enhancement of thermal shock and slag corrosion resistance of MgO–ZrO2 ceramics by doping CeO2

The purpose of this paper was to develop ceramics materials with high thermal shock resistance and corrosion resistance for preparing gas blowing components. In this paper, MgO-rich MgO–ZrO₂ ceramics were obtained by using MgO powder and ZrO₂ powder as starting materials and CeO₂ as an additive. Changes in the properties in terms of thermal shock resistance, mechanical properties, and slag corrosion-resistance with chemical compositions were examined correlated to microstructure and phase changes. Especially, the effect of doping CeO₂ on phase transition of zirconia in MgO-rich system was discussed. The results showed that doping amount of CeO₂ significantly improved properties of MgO–ZrO₂ ceramics. Especially when doping amount of CeO₂ was 2 wt%, residual strength ratio was enhanced over 100% after thermal shock testing. In samples doped with CeO₂, ZrO₂ was stable in cubic or tetragonal form due to complete solution of CeO₂, which was important reason for the improvement of various properties of MgO–ZrO₂ ceramics.     

Preparation of porous Al2O3 ceramics by starch consolidation casting method

Porous alumina ceramics were fabricated by starch consolidation casting using corn starch as a curing agent while their microstructure, mechanical properties, pore size distribution, and corrosion resistance were examined. Results showed that the porous alumina ceramics with the flexural strength of about 44.31MPa, apparent porosity of about 47.67% and pore size distribution in the range of 1‐4 μm could be obtained with 3wt% SiO₂ and 3wt% MgO additives. Corrosion resistance results showed mass losses: hot H₂SO₄ solution and NaOH solution for 10 hours were 0.77% and 2.19%, which showed that these porous alumina ceramics may offer better corrosion resistance in acidic conditions.     

Methods for determining hydration resistance of refractories

The “SPb SEC” Association has developed a standard of the organization (STO) “Refractories. Method for Determining Hydration Resistance.” The standard covers powders and articles based on magnesium oxide. The hydration resistance of powders is evaluated in terms of the change in the mass or in the grain composition of a sample after treating it with steam under pressure; the hydration resistance of articles is estimated from the change in their appearance. __________ Translated from Novye Ogneupory, No. 4, pp. 62–66, April, 2007.     

Enhanced mechanical properties of 3D printed alumina ceramics by using sintering aids

Stereolithography based 3D printing provides an efficient pathway to fabricate alumina ceramics, and the exploration on the mechanical properties of 3D printed alumina ceramics is crucial to the development of 3D printing ceramic technology. However, alumina ceramics are difficult to sinter due to their high melting point. In this work, alumina ceramics were prepared via stereolithography based 3D printing technology, and the improvement in the mechanical properties was investigated based on the content, the type and the particle size of sintering aids (TiO₂, CaCO₃, and MgO). The flexural strength of the sintered ceramics increased greatly (from 139.2 MPa to 216.7 MPa) with the increase in TiO₂ content (from 0.5 wt% to 1.5 wt%), while significant anisotropy in mechanical properties (216.7 MPa in X-Z plane and 121.0 MPa in X–Y plane) was observed for the ceramics with the addition of 1.5 wt TiO₂. The shrinkage and flexural strength of the ceramics decreased with the increase in CaCO₃ content due to the formation of elongated grains, which led to the formation of large-sized residual pores in the ceramics. The addition of MgO help decrease the anisotropic differences in shrinkage and flexural strength of the sintered ceramics due to the formation of regularly shaped grains. This work provides guidance on the adjustment in flexural strength, shrinkage, and anisotropic behavior of 3D printed alumina ceramics, and provides new methods for the fabrication of 3D printed alumina ceramics with superior mechanical properties.     

Improvement of the mechanical properties of SiC reticulated porous ceramics with optimized three-layered struts for porous media combustion

Silicon carbide reticulated porous ceramics (SiC RPCs) with three-layered struts were fabricated by polymer replica method, followed by infiltrating alumina slurries containing silicon (slurry-Si) and andalusite (slurry-An), respectively. The effects of composition of infiltration slurries on the strut structure, mechanical properties and thermal shock resistance of SiC RPCs were investigated. The results showed that the SiC RPCs infiltrated with slurry-Si and slurry-An exhibited better mechanical properties and thermal shock resistance in comparison with those of alumina slurry infiltration, even obtained the considerable strength at 1300 °C. In slurry-Si, silicon was oxidized into SiO₂ in the temperature range from 1300 °C to 1400 °C and it reacted with Al₂O₃ into mullite phase at 1450 °C. Meantime, the addition of silicon in slurry-Si could reduce SiC oxidation of SiC RPCs during firing process in contrast with alumina slurry. With regard to slurry-An, andalusite started to transform into mullite phase at 1300 °C and the secondary mullitization occurred at 1450 °C. The enhanced mechanical properties and thermal shock resistance of SiC RPCs infiltrated alumina slurries containing silicon and andalusite were attributed to the optimized microstructure and the triangular zone (inner layer of strut) with mullite bonded corundum via reaction sintering. In addition, the generation of residual compressive stress together with better interlocked needle-like mullite led to the crack-deflection in SiC skeleton, thus improving the thermal shock resistance of obtained SiC RPCs.     

High P–T Nano-Mechanics of Polycrystalline Nickel

We have conducted high P–T synchrotron X-ray and time-of-flight neutron diffraction experiments as well as indentation measurements to study equation of state, constitutive properties, and hardness of nanocrystalline and bulk nickel. Our lattice volume–pressure data present a clear evidence of elastic softening in nanocrystalline Ni as compared with the bulk nickel. We show that the enhanced overall compressibility of nanocrystalline Ni is a consequence of the higher compressibility of the surface shell of Ni nanocrystals, which supports the results of molecular dynamics simulation and a generalized model of a nanocrystal with expanded surface layer. The analytical methods we developed based on the peak-profile of diffraction data allow us to identify “micro/local” yield due to high stress concentration at the grain-to-grain contacts and “macro/bulk” yield due to deviatoric stress over the entire sample. The graphic approach of our strain/stress analyses can also reveal the corresponding yield strength, grain crushing/growth, work hardening/softening, and thermal relaxation under high P–T conditions, as well as the intrinsic residual/surface strains in the polycrystalline bulks. From micro-indentation measurements, we found that a low-temperature annealing (T < 0.4 T_m) hardens nanocrystalline Ni, leading to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of impurity segregation to the grain boundaries of the nanocrystalline Ni.