Role of cold isostatic pressing in the formation of the properties of ZrO2-base ceramics obtained from ultradisperse powders

The physicomechanical properties of ceramics obtained from plasmachemical and sol-gel powders of partially stabilized (3% Y₂O₃) zirconia (PSZ) and its compositions with 20% Al₂O₃ by cold isostatic pressing (CIP) at a pressure of at most 2 GPa and sintering at 1300–1650°C are investigated. It is established that plasmachemical PSZ exhibits its best properties (K _lc=7.8 MPa · m^1/2, a strength of 650 MPa) only after complete disintegration at a CIP of 0.1 GPa and a sintering temperature of 1650°C, when the material is sintered to a density of 5.5 g/cm³. After partial stabilization and CIP at 0.1 GPa the plasmachemical composition of PSZ+20% Al₂O₃ is sintered at 1650°C to a density of 4.7 g/cm³, but hasK _lc=8.5 MPa · m^1/2 and a strength of 700 MPa. The deagglomerated sol-gel powder exhibits properties at a level ofK _lc=12.4 MPa · m^1/2 and a strength of 950 MPa at a density above 6.0 g/cm³ after CIP at 0.3 GPa and sintering at 1450°C. The latter obviously has the best mechanical properties of all the investigated materials.Translated from Ogneupory, No. 2, pp. 12 – 19, February, 1995.     

Conditions for formation of corundum briquets from crude technical-grade alumina

Conclusions In order to obtain a compact corundum briquet, unfired alumina must be pulverized to monocrystals, which is achieved when the content of grains with a diam. of less than 10 is more than 95%. Introduction of even a small amount of unpulverized alumina greatly increases the porosity of the annealed material. The briquets should be pressed by the semidry method under a pressure of 300–600 kg/cm² and fired at 1750°C for 3–6 h.Addition of titanium dioxide to improve the sintering of the briquets is expedient only when the annealing temperature is low (1550°C) and 0.1% of additive is used.Translated from Ogneupory, No.4, pp.43–48, April, 1969.     

Obtaining corundum-zircon suspensions,their rheological and bonding properties

Conclusions It is shown that the combined grinding of corundum (80%) and zircon (20%) will produce suspensions with high concentrations and binding properties. A study was made of the effect of pH and slip concentration on their rheological properties, density, and the strength of the castings. With a pH of 2.5–3.5 and of 0.50–0.54 we obtained castings with an open porosity of up to 28–30% and _bend of up to 8–9 MPa.Substantial sintering of the material is accomplished even at reduced (1450°C) temperatures. The factors for _bend (on specimens measuring 7×7×70 mm) and _comp (on specimens measuring 10×10×10 mm) with an open porosity of 16–18% are up to 180 and 740 MPa respectively. At 1100 and 1200°C the value of _comp equals 300–340 and 250–300 MPa, respectively.Translated from Ogneupory, No. 6, pp. 22–25, June, 1984.     

Effect of some factors on the rheological and technical properties of alumina slips

Conclusions An increase in the concentration of the suspension of alumina from 70 to 75% improves the structural-mechanical characteristics and the casting-up rate.An increase in the temperature of pre-calcination of the alumina from 1450 to 1550° impairs the structural-mechanical characteristics of the suspension in connection with a reduction in the chemical activity of the alumina due to the lower hydration of the particles of alumina and the weakening of their interaction with hydrochloric acid. Under these conditions there is a reduction in the strength of the casts.The presence in the suspension of gaseous phases, reducing the forces of intermolecular attraction between the particles, impairs its structural-mechanical characteristics. Vacuum treatment of the suspension increases the density of the castings by 0.04–0.05 g/cm³ and reduces the casting-up rate.During storage of the slip up to 30 days there is an increase in the zeta-potential of the particles as a result of the reinforcement of the interaction of the hydrochloric acid with the alumina; under these conditions the properties of the slip and the castings are scarcely changed.An increase in temperature of the slip to 30°C reduces the viscosity of the structural breakdown, increases the rate of easting and density of the castings, and increases their strength.     

Preparation and properties of zirconium refractories stabilized by cerium dioxide

Conclusions We studied the reaction of zirconium dioxide with cerium dioxide in mixtures with CeO₂ contents of 6, 8, 10, 12, 15, 18, 20, 33, and 50%. The properties of the samples of these compositions were determined.On the addition of amounts of CeO₂ from 6–12% complete stabilization of ZrO₂ was not achieved by a single sintering at 1750°C. The samples with the composition 88% ZrO₂+12% CeO₂, sintered twice at 1750°C with an intermediate grinding, stabilized almost completely as a solid solution of tetragonal structure.The thermal stress resistance of dense, completely stabilized samples with CeO₂ contents of 15–18%, prepared from finely ground raw oxides, was 3–4 thermal cyclings. It improved when the CeO₂ content was decreased, or when more monoclinic ZrO₂ was added.It was found that the onset and the inversion temperature interval depend on the CeO₂ content, the granular composition of the original oxides, the temperature, and the gaseous sintering medium. We studied the properties of synthesized compositions and their dependence on the reducing or oxidizing conditions of sintering due to a change in the valency of cerium. In order to obtain zirconium-cerium refractories with definite properties it is necessary to have strict control of the gaseous medium during sintering.Translated from Ogneupory, No. 3, pp. 37–44, March, 1969.     

Effect of titanium dioxide on sintering and stabilization of ZrO2 in zirconium-alumina and spinel-zirconium mixtures

Conclusions The addition of titanium dioxide considerably reduces the sintering point of alumina and zirconia-alumina mixtures. Titanium dioxide does not have this effect on the sintering of the ternary equimolecular mixture ZrO₂MgO Al₂O₃. But the zero porosity was not even attained during firing up to 1700°. When the three-component mixture ZrO₂ + MgO + Al₂O₃ was sintered, spinel formed first, after which, at a higher temperature, there formed a solid solution ZrO₂ and MgO. In the presence of titanium dioxide some of the magnesium oxide apparently forms magnesium titanate with the TiO₂, and this impairs the stabilization of the zirconium dioxide, in view of which it is partially detected in monoclinic form in the fired mixtures. The addition of 2% TiO₂ reduces the temperature of polymorphous transitions of ZrO₂ by approximately 200°.Specimens of the composition 90% Al₂O₃ + 10% ZrO₂ and ZrO₂MgOAl₂O₃=111 show better spalling resistance than those made of alumina and those made of zirconium dioxide stabilized with magnesium or calcium oxide.Pure pre-synthesized spinel does not react with zirconium dioxide when fired up to 1600°, nor with titanium dioxide up to 1500°.The coefficient of thermal linear expansion of the equimolecular mixtures ZrO₂-MgO-Al₂O₃ and ZrO₂-CaO-Al₂O₃ is considerably lower than that of the corresponding mixtures without alumina.When the three component equimolecular mixtures ZrO₂-CaO-Al₂O₃ is sintered, calcium aluminate and the solid solution ZrO₂-CaO are formed.The two-component compositions Al₂O₃-ZrO₂ and three-component MgO-Al₂O₃-ZrO₂ have high refractoriness, satisfactory spalltng resistance, good stability-underload at high temperatures, and can be used as super duty refractories.     

Mechanical Properties of Ceramics Prepared from a Nanocrystalline ZrO2 – 3 mol.% Y2O3 Powder

Ceramics of ZrO₂ – 3 mol.% Y₂O₃ composition prepared from commercial raw materials by cold isostatic pressing technique at sintering temperatures from 1100 to 1500°C are studied. Best mechanical properties were found in material sintered at 1350°C: density, 5.95 g/cm³; bending strength, 1200 MPa, and crack resistance, about 18 MPa m^1/2. The mechanical properties of ceramic materials sintered at 1300 and 1500°C are inferior by roughly a factor of 1.5, despite virtually the same density and grain size. This difference is explained as due to the different properties of the intergranular material in ceramics sintered at different temperatures.     

Role of hydrostatic treatment in the formation of the structure and the properties of reaction-bonded ceramics based on mullite and zirconium dioxide

Conclusions Comparative studies were made on the densification of castings during hydrostatic pressing (GSO) and the hydrostatic pressing of powders of corundum-zirconia composition, and sintering of the greenware in relation to the pressure of the hydrostatic pressing treatment. Some properties of the resulting ceramic were analyzed.It was established that the sinterability of the castings obtained by water slip casting with subsequent GSO is much higher than for the greenware obtained by hydrostatic pressing of powders at the same pressure. This is connected with the presence in the castings of unpolymerized colloidal component.A reduction in the porosity of the castings to 35–38% and less after GSO for pressures above 500 N/mm² facilitates sintering of the ceramic to zero open porosity in a wide range of rises in temperature, and contributes to an increase in strength on account of the dissipation mechanism which is reinforced with increase in GSO pressure.Translated from Ogneupory, No. 4, pp. 24–27, April, 1989.     

Densification of RO2 type oxides during hot pressing

Conclusions The optimal conditions for obtaining high-density products by the hot pressing of pure oxides of the type RO₂ were established. In order to prepare high-density samples with simultaneous hot pressing and stabilization of the zirconium and hafnium dioxide the heating must be slowed down in the polymorphic interval (800–1200°C) to generate the necessary number of vacancies which ensure significant densification. The amount of stabilizer (calcium or magnesium oxide) should not be less than 7% (by weight). Prestabilized zirconium and hafnium dioxide can be sintered by hot pressing with heating without holding in the interval 800–1200°C to a relative density of 0.98–0.99 under a pressure of 160 kg/cm², and also with a holding time of 10 min and a temperature of 1700–1800°C (zirconium dioxide) and 2100–2200°C (hafnium dioxide).Fused quartz can be sintered by hot pressing to zero water absorption and a relative density of about 1.0 at 1400°C, a pressure of 200 kg/cm², and a holding time of 10 min.Deceased.Translated from Ogneupory, No. 2, pp. 36–40, February, 1969.     

The grinding method and additives as factors in the sintering of zircon

Conclusions It was shown that specimens with an open porosity below 3% can be produced from zircon wet-ground in a vibromill, treated with acid, and containing over 80% grains smaller than 10 including 60–70% grains smaller than 2; the density of the specimens is highest (porosity below 1%) when they are fired at 1500°C and contain a maximum of 0.8–1% TiO₂.Specimens prepared from dry-ground zircon can be sintered at 1500–1580°C to a porosity of 3–20%) depending on the composition and amount of additive and on the content of grindings of metallic iron.Translated from Ogneupory, No. 2, pp. 26–32, February, 1977.     

Role of sintering additives in the production of lime refractories

Conclusions A production technology is proposed for lime refractories covering the preparation of high-density lime clinker, followed by crushing, batching, adding waterproof bond, pressing the goods, and firing at 2020 K.The technology for obtaining high-density lime clinker resistant to atmospheric hydrolysis, includes milling CaCO₃ to a particle size of 100–150 m with simultaneous incorporation of sintering additives (2–6% TiO₂ or 2–4% titanium ferrites), and the use of water solutions of sulfite lye as bond; pelletizing and firing at 2020 K. The lime clinker has an apparent density of 2.75–3.03 g/cm³, and an open pelletizing and firing at 2020 K. The lime clinker has an apparent density of 2.75–3.03 g/cm³, and an open porosity of 3.4–9.8%.The lime products have an apparent density of 2.65–2.97 g/cm³, open porosity 4.4–16.4%, compressive strength 60–140 N/mm², and refractoriness under load 1610–1700°C and above.Translated from Ogneupory, No. 2, pp. 13–16, February, 1992.     

Investigation of the sintering of zirconia

Conclusions We investigated the sintering of zirconium dioxide as a function of the activity of the starting material. It was shown that preliminary firing or stabilization of zirconium dioxide reduces its capacity for densification at temperatures of up to 1500°C, while stabilizing it directly during sintering intensifies this capacity. The prestabilized zirconia has the lowest sintering rate.Introducing monoclinic unfired zirconia into the prestabilized material intensifies sintering; the optimal addition is 30%.It is established that the zirconia is sintered by volume diffusion of vacancies.We investigated the sintering of active zirconia obtained by decomposing zirconium nitrate. The greatest degree of densification is obtained with a preliminary short heat processing of the nitrate at 400°C. Activation is connected with the formation of metastable tetragonal and cubic modifications with defect crystal lattices. Increasing the heat-process temperature of the nitrate or prolonging the soak at 400°C, leading to the formation of stable monoclinic ZrO₂ with an ordered crystalline lattice, impairs sintering.Incorporating small additions of active ZrO₂ in the industrial material, and providing rapid firing in an oxidizing atmosphere, greatly increases the degree of sintering. Introducing stabilizing additives intensifies sintering. The maximum densification is obtained by the formation of 60–70% solid solution. Further increase in these additions reduces the shrinkage and densification of the specimens.Translated from Ogneupory, No. 6, pp. 33–40, June, 1968.     

Performance Evaluation of Doped Titanium Oxide for Low‐Voltage Applications

Varistors are the electronic devices which are used in various industries to protect the electrical and electronic systems from sudden surges. In this research, the electrical properties of titanium dioxide (TiO₂) doped with tantalum pentoxide (Ta₂O₅), tungsten trioxide (WO₃), cobalt oxide (Co₃O₄), and bismuth oxide (Bi₂O₃) and fired at different temperatures were investigated for low‐voltage applications. The adequate amount of dopants at suitable sintering temperature had beneficial effect in improving the properties of TiO₂. The relative density was found to be more than 97% of theoretical density when samples sintered between 1300°C and 1400°C for all composition compared to undoped samples. On the other hand, the addition of dopants enhanced hardness and compressive strength of varistor disks. The average grain size was also increased with the dopants system, making it suitable for low‐voltage application. Furthermore, the current–voltage characteristic of the TiO₂ revealed a significantly high value of nonlinearity of 19.6. A high dielectric constant of 10⁴ with minimum dissipation factor of 0.002852 at 1 kHz was also obtained, thereby making it suitable for low‐voltage application.     

Studies on the conditions of synthesis of picolinic acid by heterogeneous catalytic oxidation of 2-picoline

Heterogeneous oxidation of 2-picoline over binary P–Ti, Sb–Ti, P–Sb, and V–Ti oxide catalysts was studied over the temperature range of 200–300°C. The vanadium–titanium catalysts based on titanium dioxide (anatase) were found to be the most selective for picolinic acid. With binary catalysts containing 20–50% of vanadium pentoxide, the selectivity for picolinic acid was 19–22% at the 36–74% conversion of 2-picoline. A distinguishing feature of these catalysts is regular surface stacking of V₂O₅ and TiO₂ crystallites.     

Sintering behavior and thermal shock resistance of aluminum titanate (Al2TiO5)-toughened MgO-based ceramics

In order to improve the thermal shock resistance of MgO-based ceramics, aluminum titanate (Al₂TiO₅)-toughened MgO-based ceramics were successfully prepared by solid state sintering at 1450 °C and 1550 °C for 3 h starting from MgO and as-synthesized Al₂TiO₅ powders. The effects of various contents of Al₂TiO₅ second phase on the sintering behavior and thermal shock resistance of MgO-based ceramics were investigated. The sintering behavior of sintered samples was evaluated by comparing the relative density, apparent porosity, bending strength, phase composition as well as microstructure. The thermal shock resistance of sintered samples was characterized by using the residual bending strength after three thermal cycles and thermal expansion coefficient. The obtained samples with 10 wt% Al₂TiO₅, which were sintered at 1550 °C for 3 h, showed the highest relative density, lowest apparent porosity as well as optimum bending strength. In addition, the samples added 15 wt% Al₂TiO₅ at 1550 °C with a dwell time of 3 h were the highest residual bending strength and lowest thermal expansion coefficient. It revealed that the enhancement in thermal shock resistance was ascribed to the reduction of thermal expansion coefficient.     

Features of the production technology and the properties of the components made from Al2O3 and SiO2 of different particle-size distributions

Conclusions It is recommended that the mullite-silica products made from polydispersed mixtures of alumina and silica in the presence of oxide additives be fired at a temperature of 1380–1410°C according to the firing regime of the dinas refractories. We obtained products containing a mixture of iron and calcium oxides (maintaining their total content not exceeding 3% and a 21 ratio). Oxide additions exceeding 3.5% have a negative effect on the technological effectivenesss and the properties of the products.In the absence of additives, mullite-silica products must be fired at 1550–1600°C maintaining the heating and cooling rates below 30°C/h. The mullite-silica products based on the polydispersed mixtures of corundum and crystalline quartz are recommended for the linings of the units used in the production of steel and cast iron and for lining coke-ovens and other thermal units (furnaces).Translated from Ogneupory, No. 7, pp. 9–12, July, 1988.     

Casting of products from aqueous acid alumina slips in gypsum molds

Conclusions An investigation was carried out of the pH of slips of commercial alumina GA85 fired together with 0.1% MgO and 1.5% H₃BO₃ (on 100%) at 1300°C and ground to particles of 1–3 as a factor in the growth rate, density, and moisture content of the castings and in the process of water elimination to the gypsum mold.The added MgO and B₂O₃ narrow the pH range for casting to 2–3 from the 2.5–4.5 for all-alumina slips.The process of water elimination from the slip to the gypsum mold depends not only on the density of the casting but also on the form of bond of the water with the solid phase which depends in turn on the pH.Translated from Ogneupory, No. 3, pp. 50–52, March, 1978.     

Phase formation during the sintering process of a chromium ore-concentrate having a titanium-containing additive and during the firing process of the periclase-spinellide products based on it

Conclusions It was shown that the interaction occurring between the titanium-containing sintering additive and the chromium ore-concentrate during the firing process leads to the formation of the solid solutions of titanium dioxide in spinellide and the titanates of the trivalent metals [(Cr_1–x–yAl_xFe_y)₂TiO₅], that are localized mainly in the form of films and inclusions at the surface of the spinellide grains.The disappearance of the titanates during the firing process of the periclase-spinellide products obtained using the aforementioned chromium ore-concentrate and periclase can be attributed to the interaction of the free magnesium oxide with the titanates and the subsequent dissolution of the intermediate spinellide in the original spinellide.Thus, a complete and uniform appropriation (distribution) of the titanium dioxide additive in the form of a solid solution in the spinellide is ensured by introducing an excess quantity (with respect to the composition of the spinellide) of magnesium oxide into the charge.It is advisable to carry out a detailed study of the effect of the content of titanium dioxide in the spinellide on the physical and technological properties and the service characteristics of the refractories.Translated from Ogneupory, No. 7, pp. 2–5, July, 1993.     

Investigating sintering of magnesia obtained by chemical beneficiation

Conclusions A study was made of the sintering behavior of samples of magnesium oxide obtained by chemical purification. The magnesia obtained by the nitrate method is a low sinter-active material. The density of the material after firing at up to 2400°C equals about 3.0 g/cm³. Partial hydration of the material improves the sintering results. The magnesia obtained by the ammonium-sulfate technology is an active material which when fired yields a densely sintered product with a density of 3.3–3.4 g/cm³.Translated from 0gneupory, No. 5, pp. 25–33, May, 1991.     

Comparison of rapid and slow sintered pulverised fuel ash

This study has investigated the properties of sintered PFA using conventional (20 °C/min) and rapid heating rates. Rapid heating was achieved by directly placing pressed PFA samples into a furnace preset at the sintering temperature. Slow sintered PFA samples show an increase in shrinkage and fired density and a reduction in water absorption as the sintering temperature increases, with maximum density obtained at around 1250 °C. At higher temperatures samples show a bloating effect associated with the enlargement of closed porosity. This peak in density is not observed in rapidly sintered PFA. Rapid sintering at temperatures between 1150 °C and 1300 °C resulted in constant values of shrinkage (6.5%), fired density (1.4 g/cm³) and water absorption (15%). During rapid sintering the residual carbon in the PFA is present in the sample at the sintering temperature whereas conventional sintering removes the carbon at lower temperatures before sintering occurs. Rapid sintering also retains significant carbon in the core of the sintered sample. This carbon is believed to act as rigid inclusions that inhibit PFA sintering, limiting shrinkage and densification. Rapid sintering is associated with black coring which is also characteristic in lightweight aggregate manufactured from PFA sintered on a sinter strand. The rapid heating rate experienced by PFA pellets on a sinter strand and the associated inhibition of sintering by carbon is essential for producing sintered PFA products with properties appropriate for use as lightweight aggregate.