Microstructure and high-temperature strength of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Er<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>/ZrO<Subscript>2</Subscript> ternary melt growth composite

A new Al₂O₃/Er₃Al₅O₁₂(EAG)/ZrO₂ ternary MGC (Melt Growth Composite) with a novel microstructure has been fabricated by unidirectional solidification. This ternary MGC has a microstructure consisting of continuous networks of single-crystal Al₂O₃, single-crystal EAG and fine cubic-ZrO₂ phases without grain boundaries. The ternary MGC has also characteristic dimensions of the microstructure of around 2–4 m for EAG phases, around 2–4 m for Al₂O₃ phases reinforced with around 0.4–0.8 m cubic-ZrO₂ phases. No amorphous phases are formed at interfaces between phases in the ternary MGC. The ternary MGCs flexural strength at 1873 K is approximately 700 MPa, more than twice the 330 MPa of the Al₂O₃/EAG binary MGC. The fracture manner of the Al₂O₃/EAG/ZrO₂ ternary MGC at 1873 K shows the same intergranular fracture as the Al₂O₃/EAG binary MGC, but is significantly different from the transgranular fracture of the sintered ceramic.     

Sintering behavior and thermal property of Mg<Subscript>2</Subscript>SnO<Subscript>4</Subscript>

The sintering behavior of Magnesium Orthostannate (Mg₂SnO₄) by Magnesium oxide (MgO) and tin oxide (SnO₂) was investigated. Mg₂SnO₄ compound was formed by traditional solid state reaction (SSR) at elevated temperatures over a range of 600 C–1300 C. X-ray studies have revealed that the resulting, as-fired, compound is polycrystalline composed of an inverse spinel-structure grains separated partially by porosity. However it failed to detect any phase change as a result of annealing. Scanning electron microscope (SEM) was employed to follow-up any change in the morphology throughout the sintering and reduction processes. Thermal property Differential Scanning Calorimetry (DSC) revealed that reduction takes place at a temperature between 400 and 600 C, depending on the concentration of H₂ in the atmosphere in accordance with the X-ray studies. The technique employed has also demonstrated the stability of reduced species in typical atmospheres and working conditions.     

Phase transitions and high-temperature crystal chemistry of polymorphous modifications of Cs<Subscript>2</Subscript>(UO<Subscript>2</Subscript>)<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)

Phase transitions and thermal deformations of - and -Cs₂(UO₂)₂(MoO₄)₃ were studied by high-temperature X-ray diffraction analysis. In heating of -Cs₂(UO₂)₂(MoO₄)₃ to 625 ± 25°C, the reconstructive phase transition proceeds. -Cs₂(UO₂)₂(MoO₄)₃ is stable up to 700 ±25°C. The thermal expansion of both phases is sharply anisotropic: ₁₁ = 10 × 10^–6, ₂₂ = 33 × 10^–6, ₃₃ = 10 × 10^–6, _V = 53 × 10^–6 deg^–1 for -Cs(UO₂)₂(MoO₄)₃ and ₁₁ = 13 × 10^–6, ₃₃ = 3 × 10^–6, _V = 31 × 10^–6 deg^–1 for -Cs₂ (UO₂)₂ (MoO₄)₃. The anisotropy of thermal expansion is explained by features of the crystal structure of the compounds.Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 405–407.Original Russian Text Copyright © 2004 by Nazarchuk, Krivovichev, Filatov.     

Hydroxylapatite-zirconia composites: Thermal stability of phases and sinterability as related to the CaO-ZrO<Subscript>2</Subscript>phase diagram

Composites of hydroxylapatite (HA) with pure zirconia, and 3 and 8% Y₂O₃ in zirconia, were pressure-less sintered at temperatures from 900 to 1300C, and hot-pressed at 1200C in argon gas atmosphere for 1 h. The reactions and transformations of phases were monitored with X-ray diffraction and thermal analysis. At sintering temperatures higher than 1000,C, calcium from HA diffused into the zirconia phase, and the HA phase decomposed to tri-calcium phosphate (TCP). Above about 1200,C, CaZrO₃ was formed. These reactions and transformations were interpreted in terms of the ZrO₂-CaO phase diagram. On the other hand, zirconia and hydroxylapatite phases in hot pressed composite remained mainly stable suggesting that air in the sintering environment increased the reactivity between the phases. The highest densification was found in a composite initially containing 10% monoclinic ZrO₂ sintered at 1300,C. The densification of the composites decreased at lower sintering temperatures and higher zirconia contents upon air-sintering.     

Toughening mechanisms in duplex alumina-zirconia ceramics

A series of Al₂O₃-ZrO₂ ceramics has been fabricated using both conventional sintering and a hot-pressing route, which results in various microstructures including (i) Al₂O₃ with well-dispersed ZrO₂ single crystals; (ii) Al₂O₃ With TZP (tetragonal zirconia polycrystals) agglomerates (20 to 50m); and (iii) Al₂O₃-ZrO₂ duplex structures, in which both well-dispersed ZrO₂ single crystals and TZP agglomerates are dispersed. The fracture strength of the composites has been measured by means of three-point bending and the fracture toughness by means of the micro-indentation technique. The microstructural characterization was carried out using scanning and transmission electron microscopy, and phase analysis of the zirconia by means of X-ray diffraction. The high toughness values of 12 M Pa m^1/2 measured for the duplex structure have been correlated with the toughening mechanisms operative and the fracture strength with the matrix grain size and with larger defects present in the structure. A combined toughening process is proposed to account for the improved properties, including transformation toughening, microcrack toughening and crack deflection, which are discussed in context with the property measurements and the microstructural observations.     

Effects of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> on the microstructure and electrical properties of Pr-ZnO varistors

Effects of Y₂O₃ contents on the microstructure and electrical properties of Pr₆O₁₁-based ZnO varistor have been studied. The varistor voltage (V_1mA) and the nonlinear exponent () increased with increasing Y₂O₃ contents, whereas the dielectric constant decreased. Y-rich phase segregated at nodal points in grain boundaries. The average grain size decreased with increasing Y₂O₃ content. The specimen with 4.0 mol% Y₂O₃ sintered at 1285 C exhibited the highest nonlinear exponent ( = 77) with the dielectric constant, ~352 at 1 kHz.     

Homogeneity Range of ScMnO<Subscript>3</Subscript> in Air

Sc_cMn_{2 – c} O₃ (0.9 c 1.1) samples were prepared by reacting Sc₂O₃ and Mn₂O₃ in air between 800 and 1350°C and were characterized by x-ray diffraction. The results were used to determine the homogeneity range of ScMnO₃. The Mn- and Sc-rich phase boundaries of ScMnO₃ are at c 0.975 (800–1250°C) and c 1.025 (800–1200°C), respectively. At higher temperatures, the homogeneity range narrows down. At 1305 ± 5°C, stoichiometric ScMnO₃ dissociates to form Sc₂O₃- and Mn₃O₄-based solid solutions.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 5, 2005, pp. 608–610.Original Russian Text Copyright © 2005 by Golikov, Balakirev, Titova, Fedorova, Antonov.     

Fabrication and mechanical behaviour of Al2O3/Mo nanocomposites

Two types of Al₂O₃/Mo composites were fabricated by hot-pressing a mixture of - or -Al₂O₃ powder and a fine molybdenum powder. For Al₂O₃/5 vol% Mo composite using -Al₂O₃ as a starting powder, the elongated molybdenum layers were observed to surround a part of the Al₂O₃ grains, which resulted in an apparent high value of fracture toughness (7.1 Mpa m^1/2). In the system using -Al₂O₃ as a starting powder, nanometre sized molybdenum particles were dispersed within the Al₂O₃ grains and at the grain boundaries. Thus, it was confirmed that ceramic/metal nanocomposite was successfully fabricated in the Al₂O₃/Mo composite system. With increasing molybdenum content, the elongated molybdenum particles were formed at Al₂O₃ grain boundaries. Considerable improvements of mechanical properties were observed, such as hardness of 19.2 GPa, fracture strength of 884 MPa and toughness of 7.6 MPa m^1/2 in the composites containing 5, 7.5, 20 vol% Mo, respectively; however, they were not enhanced simultaneously. The relationships between microstructure and mechanical properties are also discussed.     

Kinetics of mechanically activated P<Subscript>2</Subscript>O<Subscript>5</Subscript> incorporation into Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The incorporation of phosphoric anhydride into - and -aluminas during mechanical activation is investigated. An empirical equation is proposed for the kinetics of P₂O₅ incorporation. The effect of the specific power of the mill on the amount of bound P₂O₅ is analyzed.Translated from Neorganicheskie Materialy, Vol. 41, No. 3, 2005, pp. 321–323.Original Russian Text Copyright © 2005 by Kosenko, Smirnova, Denisova.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Preparation of Y-TZP/AI2O3 whisker preform by an in situ method

In situ growth of AI₂O₃ whiskers into the matrix of Y-TZP (yttria-doped tetragonal zirconia polycrystals) was examined in order to prepare Y-TZP/AI₂O₃ whisker preform for the composites. Various shapes of AI₂O₃ particles were grown by the reaction of AI₂O₃ and AIF₃ powders with moist nitrogen or oxygen gases at high temperature. They showed a trend to change the particle shapes from massive rhombohedron whisker platelet as the processing temperature was increased. These particles, however, grew only on the surface and not inside the pellets It was found necessary to introduce the carrier gas inside the pellets for particle growth to occur internally. AI₂O₃ whiskers can be synthesized inside the pellets by mixing with an organic space-forming agent having a relatively large particle size.     

Sintering mechanisms and microstructural development of coprecipitated mullite

Coprecipitated mullite precursor powders of the bulk compositions 78 wt% Al₂O₃+22 wt% SiO₂ (high-Al₂O₃ material) and 72 wt% Al₂O₃+28 wt% SiO₂ (low-Al₂O₃ material) have been used as starting materials. The precursor powders were calcined at 600, 950, 1000, 1250, and 1650 C, and test sintering runs were performed at 1550, 1600, 1650, 1700, and 1750 C. Homogeneous and dense ceramics were obtained from cold isostatically pressed (CIPed) powders sintered in air at 1700 C. Therefore, all further sintering experiments were carried out at 1700 C. After pressureless sintering, sample specimens were hot isostatically pressed (HIPed) at 1600 C and 200 bar argon gas pressure. Sintering densifications of low Al₂O₃ materials ranged between 94% and 95.5%. There was no clear dependency between densification and calcination temperature of the starting powders. High-Al₂O₃ compositions displayed sintering densities which increased from 97% at 600 C calcination temperature to 99% at 950 C calcination temperature. Higher calcination temperatures first caused slight lowering of the sintering density to 95.5% (calcination temperature 1250 C) but later the density strongly decreased to a value of 85% (calcination temperature 1650 C). HIPing of pressureless sintered specimens prepared from powders calcined between 600 and 1100 C yielded 100% density. At the given sintering temperature of 1700 C, the microstructure of sample specimens was influenced by Al₂O₃/SiO₂ ratios and by calcination temperatures of the starting powders. Homogeneous and dense microstructures consisting of equiaxed mullite plus some minor amount of -Al₂O₃ were produced from high-Al₂O₃ powders calcined between 600 and 1100 C. Low-Al₂O₃ sample specimens sintered from precursor powders calcined between 600 and 1100 C were less dense than high-Al₂O₃ materials. Their microstructure consisted of relatively large and elongated mullite crystals which were embedded in a fine-grained matrix of mullite plus a coexisting glass phase. The different microstructural developments of high- and low-Al₂O₃ compositions may be explained by solid-state and liquid-phase sintering, respectively. The microstructure of HIPed samples was very similar to that of pressureless sintered materials, but without any pores occurring at grain boundaries.     

Effects of silica on the bioactivity of calcium phosphate composites <Emphasis Type="Italic">in vitro</Emphasis>

In the present study, silica-calcium phosphate composites (SiO₂-CaP composites) were developed by mixing the starting materials (SiO₂ and CaHPO₄) in different ratios with the addition of 0.1% w/v NaOH solution. The phase composition of the SiO₂-CaP composites was determined by XRD and FTIR. After thermal treatment at 350 ^C/1 h and at 1000 ^C/3.5 h; all SiO₂-CaP composites composed of -quartz, -cristobalite and -Ca2P2O7. The presence of calcium phosphate enhanced the transformation of -quartz into -cristobalite at 1000 ^C. SEM observation indicated favorable attachment and spreading of neonatal rat calvaria osteoblasts onto the surface of silica-rich SiO₂-CaP composites. After attachment, these cells produced significantly higher amount of protein and expressed higher AP activity than cells attached to silica-poor samples. Results of the study suggested that the silica-based composites are more bioactive than calcium phosphate-based composites. Silica promoted the expression of osteoblast phenotype by both solution-mediated effect and direct interaction with the surface of the substrate.     

Metal-nonmetal transition and the electronic transport behavior in disordered PbO<Subscript>2</Subscript>-Ag<Subscript>2</Subscript>O-xCsystem synthesized by ball milling

The electronic transport properties as a function of temperature and the graphite content have been investigated for disordered PbO₂-Ag₂O-xC (0 x 3) composite materials. The components of the system change from (1) lead-silver oxides of Ag₅Pb_2.5O₆ and/or Ag₂PbO₂ to (2) PbCO₃ + 2PbCO₃PbO + Ag and (3) PbCO₃ + Ag + Pb with increasing the graphite content. Ag₅Pb₂O₆ is synthesized directly by mechanical milling with a simple solid-state reaction of Ag₂O and PbO₂. The structure, thermal and electric properties of silver-lead oxides are characterized by means of X-ray diffraction, scanning electron microscope, a differential scanning calorimeter analysis and DC resistivity measurements. Strengthening the degree of disorder through mechanical milling and/or doping the lead oxide into Ag₅Pb₂O₆ causes the conductivity transition from metallic to semiconducting behavior, while decreasing the degree of disorder by annealing and the segregation of the lead oxide from the solid solution leads to a reverse transition.     

Synthesis and crystal structure of Cu<Subscript>2</Subscript>{(UO<Subscript>2</Subscript>)<Subscript>3</Subscript>[(S,Cr)O<Subscript>4</Subscript>]<Subscript>5</Subscript>}(H<Subscript>2</Subscript>O)<Subscript>17</Subscript>

Cu₂{(UO₂)₃[(S,Cr)O₄]₅}(H₂O)₁₇ crystals were prepared by evaporation of aqueous solutions. The crystal structure was solved by the direct method and refined to R ₁ = 0.064 (wR ₂ = 0.177) for 8120 reflections with ¦F _hkl¦ 4 ¦F _hkl¦. Rhombic system, space group Pbca, a = 18.0586(8), b = 19.9898(9), c = 20.5553(8) Å, V = 7420.2(6) ų. The structure is based on {(UO₂)₃[(S,Cr)O₄]₅}^4– anionic layers, formed by combination of UO₇ pentagonal bipyramids and TO₄ tetrahedra through common vertices. The { (UO₂)₃ [(S,Cr)O₄]₅}^4– layers are parallel to the (010) plane. The Cu²⁺ (H₂O)₆ octahedra and additional water molecules are located in the interplanar space and provide binding of the layers in the structure by hydrogen bonds. Based on the occupancy of tetrahedral positions, more accurate chemical formula of the compound should be written as Cu₂{(UO₂)₃[(S_0.804 Cr_0.196)O₄]₅} (H₂O)₁₇.Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 408–411.Original Russian Text Copyright © 2004 by Krivovichev, Burns.     

Fabrication and mechanical properties of silicon carbide–silicon nitride nanocomposites

A powder mixture of ultrafine –SiC–35 wt% –Si₃N₄ containing 6 wt% Al₂O₃ and 4 wt% Y₂O₃ as sintering additives were liquid–phase sintered at 1800°C for 30 min by hot–pressing. The hot–pressed composites were subsequently annealed at 1920°C under nitrogen–gas–pressure to enhance grain growth. The average grain–size of the sintered bodies were ranged from 96 to 251 nm for SiC and from 202 to 407 nm for Si₃N₄, which were much finer than those of ordinary sintered SiC–Si₃N₄ composites. Both strength and fracture toughness of fine–grained SiC–Si₃N₄ composites increased with increasing grain size. Such results suggested that a small amount of grain growth in the fine–grained region (250 nm for SiC and 400 nm for Si₃N₄) was beneficial for mechanical properties of the composites. The room–temperature flexural strength and fracture toughness of the 8–h annealed composites were 698 MPa and 4.7 MPa · m^1/2, respectively.     

Synthesis,crystal structure,and proton conductivity of KFe(H<Subscript>2</Subscript>P<Subscript>2</Subscript>O<Subscript>7</Subscript>)<Subscript>2</Subscript>

KFe H₂P₂O7)₂ is synthesized at 443 K in molten polyphosphoric acids containing K and Fe ions, and its crystal structure is determined: triclinic unit cell with a = 4.9974(6) Å, b = 7.4766(9) Å, c = 7.8185(9) Å, = 82.29(2)°, = 83.37(2)° , = 74.13(2)° ; Z = 1, sp. gr. P . The structure is made up of infinite ribbons formed by corner-shared PO₄ tetrahedra and FeO₆ octahedra, with the K atoms in between. Neighboring ribbons are linked by hydrogen bonds. The proton conductivity of potassium iron(III) dihydrogen diphosphate is rather low.Translated from Neorganicheskie Materialy, Vol. 41, No. 1, 2005, pp. 74–77. Original Russian Text Copyright © 2005 by Chudinova, Murashova, Ilyukhin, Tarnopolskii, Yaroslavtsev.     

Sintering and crystallization of mullite powder prepared by sol-gel processing

Mullite powder with the stoichiometric composition (3Al₂O₃.2SiO₂) was synthesized by a sol-gel process, followed by hypercritical drying with CO₂. Within the limits of detection by X-ray diffraction, the powder was amorphous. Crystallization of the powder commenced at 1200 °C and was completed after 1 h at 1350 °C. In situ X-ray analysis showed no intermediate crystalline phases prior to the onset of mullite crystallization and the pattern of the fully crystallized powder was almost identical to that of stoichiometric mullite. The synthesized powder was compacted and sintered to nearly theoretical density below 1250 °C. The microstructure of the sintered sample consisted of nearly equiaxial grains with an average size of 0.2 m. The effect of heating rate (1–15 °C min^–1) on the sintering of the compacted powder was investigated. The sintering rate increased with increasing heating rate, and the maximum in the sintering curve shifted to higher temperatures. The sintering kinetics below 1150 °C can be described by available models for viscous sintering.     

Dielectric anomalies in monoclinic TlInS<Subscript>2</Subscript> polytypes

Comparison of room-temperature x-ray diffraction data and temperature-dependent (190–220 K) permittivity data for different polytypes of monoclinic TlInS₂ demonstrates that the anomalies in (T) at the phase transition correlate with the c cell parameter of the polytypes. The (T) curve of single-phase crystals with c 15 Å shows a single, sharp peak at about 205 K. The peaks in the of the polytypes with c 30 and 60 Å occur at other temperatures, indicating that permittivity data near the phase transition can be used to identify the polytypes present in TlInS₂ crystals.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1423–1426.Original Russian Text Copyright © 2004 by Alekperov, Nadjafov     

Production and characterization of ZrO<Subscript>2</Subscript> ceramics and composites to be used for hip prosthesis

Tetragonal ZrO₂ polycrystalline (TZP) ceramics with varying yttria and ceria content (2–3 mol%) and distribution (coated or co-precipitated), and varying second phase content Al₂O₃ were prepared and investigated by means of microstructural analysis, mechanical properties, and hydrothermal stability, and ZrO₂-based composites with 35–60 vol% of electrical conductive TiN particles were developed. The effects of stabilizer content and means of addition, powder preparation, sintering conditions, and grain size have been systematically investigated. Fully dense Y-TZP ceramics, stabilized with 2–3 mol% Y₂O₃, 2 wt% Al₂O₃ can be achieved by hot pressing at 1,450 °C for 1 h. The hydrothermal stability increased with increasing overall yttria content. The jet-milled TiN powder was used to investigate the ZrO₂–TiN composites as function of the TiN content. The experimental work revealed that fully dense ZrO₂–TiN composites, stabilized with 1.75 mol% Y₂O₃, 0.75 wt% Al₂O₃, and a jet-milled TiN content ranging from 35 to 60 vol% could be achieved by hot pressing at 1,550 °C for 1 h. Transformation toughening was found as the primary toughening mechanism. The decreasing hardness and strength could be attributed to an increasing TiN grain size with increasing TiN content, whereas the decreasing toughness might be due to the decreasing contribution of transformation toughening from the tetragonal to monoclinic ZrO₂ phase transformation. The E modulus increases linearly with increasing TiN content, whereas the hydrothermal stability increases with addition of TiN content.     

Fabrication of carbon nanofiber(CNF)-dispersed Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composites by pulsed electric-current pressure sintering and their mechanical and electrical properties

Dense Al₂O₃-based composites (≥99.0% of theoretical) dispersed with carbon nanofibers (CNFs) were fabricated using the pulsed electric-current pressure sintering (PECPS) for 5 min at 1300°C and 30 MPa in a vacuum. The dispersion of CNFs into the matrix depended much on the particle size of the starting Al₂O₃ powders. Mechanical properties of the composites were evaluated in relation with their microstructures; high values of three-point bending strength σ_b (∼800 MPa) and fracture toughness K _IC (∼5 MPa·m^1/2) were attained at the composition of CNF/Al₂O₃ = 5:95 vol%, which σ_b and K _IC values were ∼25% and ∼5%, respectively, higher than those of monolithic Al₂O₃. This might be due to the small Al₂O₃ grains (1.6 μm) of dense sintered compacts compared with that (4.4 μm) for the pure Al₂O₃ ceramics, resulting from the suppression of grain growth during sintering induced by uniformly dispersed CNFs in the matrix. Electrical resistivity of CNF/Al₂O₃ composites decreased rapidly from >10¹⁵ to ∼2.1 × 10⁻² Ωm (5vol%CNF addition), suggesting the machinability of Al₂O₃-based composites by electrical discharge machining.

---