Hot-Pressed ceramics of ultradisperse composite powders

The regularities of variation of the microstructure, phase composition, degree of crystallinity, and properties of hot-pressed ceramics prepared from ultradisperse composite powders of the Si₃N₄ - Y₂O₃ system depending on the / phase ratio in Si₃N₄ and the concentration of Y₂O₃ are discussed. An increase in the concentration of Y₂O₃ in composite powders up to 15 – 18% and a decrease in the / ratio of Si₃N₄ lead to a change of the composition of the secondary crystalline phases with an increase in their refractoriness (from silicon oxynitride and yttrium silicates to complex yttrium — silicon oxynitrides), a lower concentration of the vitreous phase, a higher degree of crystallinity, and a higher content of stretched grains in the structure of the ceramics. The presence of the high-melting phase of yttrium — silicon oxynitrides improves the phase and granular homogeneity of the structure that forms by the type of solid solutions, with an essentially similar composition of the grains and the intergranular phase. The formation of a high-melting fine-grained self-toughening structure with a coefficient of grain elongation up to 8 – 9 ensures a high and practically stable level of the mechanical properties of the synthesized materials at temperatures up to 1400 – 1500°C.Translated from Ogneupory, No. 2, pp. 13 – 20, February, 1994.     

Properties of oxide structural ceramics made from mechanically milled powders

Conclusions A series of investigations established certain rules characterizing the development of oxide structural ceramics made from mechanically milled powders.The grain-size composition of the powders exerts a significant effect on the sintering processes. Using fine powders (average grain size 0.5–0.8m) reduces the sintering temperature of oxide ceramics to 1450–1500°C.The action of high pressures with cold isostatic pressing during shaping, and the use of hot isostatic pressing for the final densification of the ceramics provides conditions for developing pore-free ceramics with an average grain size of up to 1.0m.Ceramics were obtained based on alumina and zirconia with an average bending strength of 700–750 N/mm²- at room temperature and about 500 N/mm² at 1000°C.Translated from Ogneupory, No. 2, pp. 20–23, February, 1993.     

Infrared Spectra and Structure of Si3N4, Si2ON2, and Sialons

Sialon powders are prepared from Kyshtymskoe kaolin with 12 – 28 wt.% carbon mixtures by the carbothermal reduction-nitridation method. The infrared spectra of -, O-, C- and 15R-phases in the Si – Al – O – N system are recorded and compared with the IR spectra of Si₃N₄, Si₂ON₂, and kaolin. The IR data are shown to correlate with the chemical composition of sialons and provide information supplementary to x-ray phase analysis data. The spectra of sialon powders differ from each other and from the spectrum of silicon nitride.     

Sintering high purity,magnesia with additions of hafnium dioxide

Conclusions The introduction of hafnium dioxide into high-purity active magnesia accelerates its sintering and with 0.1–0.3 mol % HfO₂ a ceramic with an apparent density of 99–100% of the theoretical (3.56–3.63 g/cm³) can be obtained after firing at 1300–1400°C. The optimum quantity of HfO₂ additive is close to 0.25 mol %.The calcination temperature of the mixture of magnesium hydroxide and additive obtained by precipitation from a solution of MgCl₂ and Hf(SO₄)₂ with ammonia, and the fabrication pressure do not greatly affect the final density of the ceramics.Sintering of spectrally pure MgO containing 0.25 mol. % HfO₂ begins at 950°C, then the apparent density grows rapidly with rise in firing temperature, approaching 3.40 g/cm² at 1100°C. Selective recrystallization at these low temperatures is slow and sintering is not accompanied by a substantial grain growth. At 1300°C and higher firing temperatures densification of the ceramics approaches the limit in several minutes.The mechanism of the transfer of substance during the sintering of these specimens is volume self-diffusion from the grain boundaries to the surfaces of the bridges formed between them. The energy of activation of this process [3.9 eV (6.2 × 10^–16 J) in the 1000–1300°C range] and the coefficient of self-diffusion for MgO calculated in accordance with this (6 × 10^–14 cm²/sec at 1100°C) correspond to existing data on the diffusion of magnesium into MgO.With the incorporation of 0.25 mol. % HfO₂ in less pure magnesia obtained from chemically pure MgCl₂, sintering is little different from the sintering of spectrally pure MgO, but the limiting apparent density of the ceramics in this case is somewhat lower-of the order of 99% of the theoretical.     

High-strength corundum ceramics with a lowered sintering temperature

Factors affecting the sintering, formation of a fine crystalline structure, and strength of aluminum oxide ceramics are considered. A ceramics with a strength of up to 600 MPa fabricated at 1450°C in an air medium has been presented. It is shown that the sintering temperature of ceramics based on Al₂O₃ (including -Al₂O₃) can be decreased to 1200°C.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 10, pp. 2 – 5, October, 1996.     

Lowering of sintering temperature and microwave dielectric properties of BaTi4O9 ceramics prepared by the polymeric precursor method

Low temperature sintering and microwave dielectric properties of barium polytitanate (BaO–4TiO₂) ceramics prepared by means of polymeric precursor route based on the Pechini process were investigated. Pure and fine BaTi₄O₉ powders with particle sizes of 100–200 nm were derived by thermal decomposition of amorphous gel precursor (above 750 °C). They formed single orthorhombic BaTi₄O₉ phase and showed fine and well-dispersed by XRD and SEM observation. The high sintering ability of the prepared powders enabled the fabrication of dielectric ceramics at low sintering temperatures (1200–1300 °C). The well-sintered BaTi₄O₉ ceramics with high relative densities (95%) were found to show excellent microwave dielectric properties compared to those prepared by conventional method at the same sintering temperature.     

Preparation of silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction and liquid phase sintering

Dense silicon carbide ceramics using chemical treated powder by DCC via dispersant reaction method and liquid phase sintering was reported. Ammonium peroxydisulfate ((NH₄)₂S₂O₈) and ammonium carbonate ((NH₄)₂CO₃) were used as acid and base solutions to treat the silicon carbide powder, respectively. Influence of silicon carbide powder with chemical treatment on the preparation of silicon carbide suspension was studied. It was indicated that 50 vol% and 52 vol% silicon carbide suspensions with viscosities of 0.71 Pa s and 0.80 Pa s could be prepared using acid and base treated powders. Influence of silicon carbide powder with chemical treatment on the coagulation process and properties of green bodies and sintered ceramics were studied. It was indicated that silicon carbide green bodies with compressive strength of 1.13 MPa could be prepared using base treated powder. Dense silicon carbide ceramics with relative density above 99.3% and flexural strength of 697 ± 30 MPa had been prepared by DCC via dispersant reaction and liquid phase sintering using Al₂O₃ and Y₂O₃ as additives at 1950 °C for 2 h.     

KNbTeO6 transparent ceramics prepared by the combination of pressure-less sintering and pseudo hot isostatic pressing

KNbTeO₆ transparent ceramics were prepared by combining pressure-less sintering and pseudo-hot isostatic pressing (PHIP) of the synthesized submicron single-phase powder. The PHIP was conducted by wrapping coarse magnesium aluminate powders around the pre-sintered body in the spark plasma sintering (SPS) furnace. With an average grain size of 412 ± 23 nm, the in-line transmittance of transparent KNbTeO₆ ceramics reaches 80.25% at 2677 nm. By contrast, the density of the samples prepared by conventional SPS with the same sintering procedure is only 98.73%, and the highest in-line transmittance 64.25% occurs at 4976 nm. In particular, by investigating the sintering mechanism of PHIP, the improvement of microstructure and optical transmittance could be attributed to the plastic deformation caused by shear stress. The obtained ceramics exhibited excellent mechanical and dielectric properties, which was benefited from the novel sintering technology.     

Effect of oxygen on the strength of reaction-bonded silicon nitride

Conclusions When obtaining reaction-bonded Si₃N₄, drying of the silicon powder or the charge at 400°C leads to an increased oxygen content in it and can cause deterioration of the mechanical properties of the ceramics after sintering.Addition of small quantities of SiO₂ to the reaction-bonded Si₃N₄ improves the high-temperature strength to some extent although it affects the room temperature strength adversely.Translated from Ogneupory, No. 5, pp. 13–14, May, 1991.     

Good temperature stability of K0.5Na0.5NbO3 based lead-free ceramics and their applications in buzzers

(K_0.5−xLi_x)Na_0.5(Nb_1−ySb_y)O₃ (KLNNSx–y, x = 0–4 mol% and y = 0–8 mol%) lead-free piezoelectric ceramics were prepared by the conventional mixed oxide method. The denser microstructure and better electrical properties of the ceramics were obtained as compared to the pure K_0.5Na_0.5NbO₃ ceramic. The temperature stability of the electrical properties of the ceramics was also investigated. The experimental results show that the KLNNS2.5–5 ceramic exhibits good electrical properties (k_p  49%, k₃₁  30% and , tan δ  0.019), and possesses good temperature stability in the temperature range of −40 to 85 °C. The related mechanisms for improved electrical properties and temperature stability were also discussed. Moreover, buzzers based on the KLNNS2.5–5 ceramic have been fabricated and their characterization is presented. These results show that the KLNNS2.5–5 ceramic is a promising lead-free material for practical application in buzzers.     

Modern high-density oxide ceramics with a controlled microstructure. Part V. Dense chemically stable ceramics based on yttrium,scandium, and aluminum oxides

Data on the technology of dense Y₂O₃ ceramics are presented. Special features of the structure of powders and their effect on shaping of specimens and their sintering are described. Methods for leveling the characteristics of Y₂O₃ powders with different properties and decreasing the sintering temperature are developed.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 12, pp. 2–6, December, 1996.For the beginning of the article see No. 1 of 1996, for the continuation see Nos. 2, 4–7, and 9.     

Corundum Ceramics with a Eutectic Zinc-Bearing Additive

The sintering specifics of ceramics based on aluminum oxide with a eutectic zinc-containing additive are investigated. It is established that introduction of 1% sintering additive in the GLMK alumina makes it possible at 1450°C to obtain ceramics with zero open porosity, a mean density of 3.84 g/cm³, and a bending strength of 310 ± 40 MPa. This ceramics has a sealed porosity of around 1% and an average size of corundum crystals equal to 5 – 8 m.     

Microstructure and Strength of Ceramics Prepared from Powders Synthesized Via Simultaneous and Sequential Deposition Routes

Sol-gel techniques for simultaneous and sequential deposition of precursor powders for preparation of ceramics in the ZrO₂ – Al₂O₃ – CeO₂ system are described. The phase composition of ceramics varies with the method of powder deposition. The intracrystallite arrangement pattern of the secondary phase in the system obtained via simultaneous deposition route provides the base for a ceramic that is superior in strength characteristics to ceramic prepared from ZrO₂ – Al₂O₃ powders deposited via sequential route.     

Corundum ceramics with a low sintering temperature

A corundum ceramics with a high ultimate bending strength up to 550 MPa and an average crystal size of 3 µm is obtained. The initial material was highly active -Al₂O₃ powder with a particle size of less than 1 µm prepared from commercial aluminum hydroxide. The introduction of an addition based on Al₂O₃-ZrO₂ lowered the sintering temperature to 1500°C and gave a virtually poreless uniform fine-crystal structure.Translated from Ogneupory, No. 10, pp. 7–9, October, 1994.     

Effects of Initial Powder Size on the Densification of Barium Titanate Ceramics Prepared by Microwave‐Assisted Sintering

The effects of initial powder size on microwave‐assisted sintering (MWS) were investigated. BaTiO₃ powders with an average particle size of 50, 100, and 500 nm were prepared and sintered with MWS and conventional heating‐based sintering (CS). Samples of the 50 ‐ and 100‐nm‐sized BaTiO₃ powders were mechanically milled to study the effects of powder crystallinity on microwave absorption during the MWS process. The MWS of the 50‐nm‐sized BaTiO₃ powder resulted in a relative mass density of more than 90% when sintered at 1050°C, whereas the same density was achieved at 1200°C with CS. This difference between the optimal sintering temperatures, which is caused by the absorption of microwaves, was not observed when the 500‐nm‐sized BaTiO₃ powder was used. The sinterability of the BaTiO₃ ceramics prepared through the MWS of mechanically milled, 50‐nm‐sized powders decreased with increasing milling time. However, the sinterability was much higher than that of the BaTiO₃ ceramics prepared through the MWS of the 100‐ and 500‐nm‐sized unmilled powders. In conclusion, microwave absorption has significant effects on the sintering behavior of ~50‐nm‐sized powders, but is negligible for 500‐nm‐sized powders.     

Intergranular film thickness of self-reinforced silicon carbide ceramics

The intergranular film of self-reinforced SiC ceramics prepared by hot pressing and further annealing with SiO₂–Y₂O₃ and SiO₂–Al₂O₃ as sintering additives was observed by high-resolution transmission electron microscopy. The film thickness of SiC ceramics with SiO₂–Y₂O₃ was 1.2 nm whereas that of ceramics with SiO₂–Al₂O₃ was 0.8 nm. Based on the refined continuum model, an explanation on the variation of thickness with sintering additives is given. It seems that the behavior of intergranular glassy film of SiC ceramics is akin to that of Si₃N₄ ceramics.     

Rheological and technological properties of titanium dioxide slips and the properties of the castings

Conclusions An investigation of the rheological and technological properties of titanium dioxide slips and slip castings demonstrated the presence of two castability regions. The properties of the slip and castings are optimal in the alkaline castability region (pH 8).A study of the influence of the firing temperature of the titanium dioxide starting material on the properties of the slip and castings showed that the properties are optimal when the TiO₂ had been fired at 1550° C.An analysis of the sintering of titanium dioxide slip castings showed that the properties of the sintered castings are optimal when the castings were produced from slip with pH 8 because in this case the density of the castings is at maximum.The strength of TiO₂ castings in the sintering process depends on their porosity and that of the sintered castings on the size of the crystals.Titanium dioxide fired at 1550°C yielded castings with open porosity 1.8–2.0%, apparent density 3.90–3.92 g/cm³, _b 1020–1050 kgf/cm², acid resistance 97.5%, specific inductive capacitance 90–100, and tangent of the dielectric loss angle at 20°C and 1 MHz 8·10^–4–9·10^–4 Translated from Ogneupory, No. 3, pp. 28–33, March, 1978.     

Study on robust additive of CaCO3 for fast fabrication of highly transparent AlON ceramics by pressureless sintering

Fabrication of transparent AlON ceramics is extra sensitive to both particle size of starting powder and sintering additive due to shuttling transformation between AlON and Al₂O₃ + AlN during heating. One possible solution is to select robust additive to suppress the shuttling transformation. In this work, three AlON powders with different median particle sizes of 0.6, 0.9, and 1.1 μm were prepared. After studying the effect of CaCO₃ on densification process, AlON ceramics with the maximum transmittance of ≥81.1% were successfully fast prepared by pressureless sintering (PS) at 1880°C for only 2.5 h by using three AlON powders doped with different CaCO₃ amount. Specifically, AlON ceramics prepared from 1.1 μm with 0.5–0.8 wt.% CaCO₃ doping consistently showed the maximum transmittance of ≥85.3%, which indicates that CaCO₃ can serve as a robust additive to enable fast fabrication of highly transparent AlON ceramics even by PS.     

Chemical Synthesis,Sintering and Piezoelectric Properties of Ba0.85Ca0.15 Zr0.1Ti0.9O3 Lead‐Free Ceramics

The preparation of Ba_0.85Ca_0.15 Zr_0.1Ti_0.9O₃ (BCZT) powders by wet chemical methods has been investigated, and the powders used to explore relationships between the microstructure and piezoelectric properties (d₃₃ coefficient) of sintered BCZT ceramics. Sol–gel synthesis has been shown to be a successful method for the preparation of BCZT nanopowders with a pure tetragonal perovskite phase structure, specific surface area up to 21.8 m²/g and a mean particle size of 48 nm. These powders were suitable for the fabrication of dense BCZT ceramics with fine‐grain microstructures. The ceramics with the highest density of 95% theoretical density (TD) and grain size of 1.3 μm were prepared by uniaxial pressing followed by a two‐step sintering approach which contributed to the refinement of the BCTZ microstructure. A decrease in the grain size to 0.8–0.9 μm was achieved when samples were prepared using cold isostatic pressing. Using various sintering schedules, BCZT ceramics with broad range of grain sizes (0.8–60.5 μm) were prepared. The highest d₃₃ = 410.8 ± 13.2 pC/N was exhibited by ceramics prepared from sol–gel powder sintered at 1425°C, with the relative density of 89.6%TD and grain size of 36 μm.     

Hot-pressed ceramics based on SHS silicon nitride

The creation of new effective technologies and materials with an optimum ratio of quality and price is a significant achievement. The high cost of ceramics made of silicon nitride is a result of the expensiveness of the initial powder. The present study of the applicability of -Si₃N₄ powder prepared by the method of self-developing high-temperature synthesis (SHS) in hot-pressing technology has shown that although materials based on SHS -Si₃N₄ powders are inferior to materials based on plasmachemical compositions, their high strength, crack resistance, and hardness at a moderate density in combination with the lower cost of the initial raw material make them suitable for operation in aggressive media under moderate temperatures and loads. The materials developed have also shown high armoring properties.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 9, pp. 20 – 24, September, 1996.