Electrical conductivity of polycrystalline Fe2(MoO4)3

Measurements of the electrical conductivity of polycrystalline Fe₂ (MoO₄)₃ in the temperature range 370 to 900 K and in the oxygen partial pressure region 10^?4 to 1 atm are presented. Fe₂(MoO₄)₃ is found to be a semiconductor. Differing conduction mechanisms operate, depending on the crystallographic form of Fe₂(MoO₄)₃ and the oxygen partial pressure, and their nature is discussed.     

Preparation of the rod-like β-Si3N4 particles favoring the self-reinforcing Si3N4 ceramics

The β-Si₃N₄ particles were prepared by heating original α-Si₃N₄ powder with rare earth oxide Nd₂O₃ or Yb₂O₃ additives at 1600-1700 °C for 1.5 h. The transformation ratio of α-Si₃N₄ was also investigated by XRD. The results showed that Yb₂O₃ could accelerate the transformation of Si₃N₄ more effectively than Nd₂O₃ and the powder heated at 1700 °C with over 4 wt.% Yb₂O₃ has a high transformation ratio of over 98%. The morphologies of the heated powders were observed by scanning electron microscopy. The results showed that the powder heated at 1700 °C with 4 wt.% Yb₂O₃ had ideal β-Si₃N₄ rod-like morphology particles. This heated powder was used as a seed by adding it to the original α-Si₃N₄ powder to prepare self-reinforced Si₃N₄ ceramic by hot-pressed sintering. The fracture toughness of the seeded Si₃N₄ ceramics increased to 9.1 MPa m^1/2 from 7.6 MPa m^1/2 of the unseeded Si₃N₄ ceramics, while the high value of strength was still kept at 1200 °C.     

Substitutions in the Ca2Na6Al6Si6O24(SO4)2 haüyne structure

Four new haüynes having the chemical formula Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, X = Se, Te, Mo and W, were synthesized by solid-state reaction. Various substitutions were then attempted for Ca, Na, Al and Si. The replacement of Ca²⁺ by Sr²⁺ or Cd²⁺ was successful in Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, when X = S, Cr, Mo or W, except for Cd, when X = Cr. The synthesis of Mn₂Na₆Al₆Si₆O₂₄(XO₄)₂ could be made when X was Mo or W, and, among the Pb substitutions tried, Pb₂Na₆Al₆Si₆O₂₄(SO₄)₂ was successful. The solubility of Li, K and Ag was partial and was different in different haüynes. Maximum solubility of Li for Na was three atoms in Ca₂Na₆Al₆Si₆O₂₄(SO₄)₂ and the minimum was half an atom in Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, X = Mo or W. Maximum replacement of K or Ag for Na was two atoms in Ca₂Na₆Al₆Si₆O₂₄(XO₄)₂, X = Mo or W and the minimum was 0.5 in Ca₂Na₆Al₆Si₆O₂₄(SO₄)₂. The solubility of Li, K and Ag was 1.5 atoms in Ca₂Na₆Al₆Si₆O₂₄(CrO₄)₂. The solubility of Ga³⁺ or Fe³⁺ for Al³⁺ was partial. A maximum of 0.5, 3 and 4 atoms of Ga³⁺ can be substituted for Al³⁺ in sulfate, chromate and molybdate or tungstate haüynes, respectively. The solubility of Fe³⁺ was one atom in all haüynes. The complete replacement of Si⁴⁺ by Ge⁴⁺ was possible in M²⁺₂Na₆Al₆Si₆O₂₄(XO₄)₂, when M = Ca, Cd or Sr, and X = Mo or W.     

Analyses of the role of grain boundaries in mesoscale dynamic fracture resistance of SiC-Si3N4 intergranular nanocomposites

Silicon carbide (SiC)-silicon nitride (Si₃N₄) nanocomposites with SiC dispersions as well as Si₃N₄ matrix of mesoscale dimensions (∼1 μm) are considered to have exceptional strength attributed to interactions of SiC dispersions with Si₃N₄ grain boundaries (GBs). However, an account of GBs on the strength of these nanocomposites is not available. In order to analyze this issue, cohesive finite element method (CFEM) based mesoscale dynamic fracture analyses of SiC-Si₃N₄ nanocomposites with an explicit account of length scales associated with Si₃N₄ GBs, SiC particles, and Si₃N₄ grains are performed. Analyses indicate that primary mechanism of fracture in the nanocomposite microstructures is intergranular Si₃N₄ matrix cracking. GBs are responsible for crack deflection and accordingly damage is limited to a smaller geometric region in microstructures with GBs. On an average, a microstructure with GBs present is stronger than the corresponding microstructure with GBs removed. However, in cases where the second phase SiC particles are in the wake of microcracks the microstructure without GB becomes stronger against fracture in comparison to the corresponding one with GBs owing to the crack bridging effect caused by the second phase SiC particles.     

Effect of Si3N4-particles addition in Ag-Cu-Ti filler alloy on Si3N4/TiAl brazed joint

A novel composite filler alloy was developed by introducing Si₃N_4p (p = particles) into Ag-Cu-Ti filler alloy. The brazing of Si₃N₄ ceramics and TiAl intermetallics was carried out using this composite filler alloy. The typical interfacial microstructure of brazed joints was: TiAl/AlCu₂Ti reaction layer/Ag_(s,s) + Al₄Cu₉ + Ti₅Si_3p + TiN_p/TiN + Ti₅Si₃ reaction layer/Si₃N₄. Effects of Si₃N_4p content in composite filler alloy on the interfacial microstructure and joining properties were investigated. The distribution of Ti₅Si_3p and TiN_p compounds in Ag-based solid solution led to the decrease of the mismatch of the coefficient of thermal expansion (CTE) and the Young's modulus between Si₃N₄ and TiAl substrate. The maximum shear strength of 115 MPa was obtained when 3 wt.% Si₃N_4p was added in the composite filler alloy. The fracture analysis showed that the addition of Si₃N_4p could improve the mechanical properties of the joint.     

Syntheses and structures of Ta2(WO2)0.87H0.26(PO4)4 and Ta2(MoO2)(PO4)4

Tantalum hydrogen phosphate, β-TaH(PO₄)₂, has a three-dimensional structure that is stable to remarkably high temperature (∼600 °C) presumably due to the presence of strong hydrogen bonds. Impedance measurements indicate a low conductivity, 2.0 × 10⁻⁶ S/cm at 200 °C in 5% H₂. In further studies aimed at enhancing the conductivity by aliovalent doping, we have investigated systematically the synthesis of compounds in the TaH(PO₄)₂-W₂P₂O₁₁ system at 380 °C. As a result, a new phase, Ta₂(WO₂)_0.87H_0.26(PO₄)₄, was identified and subsequently the molybdenum analog Ta₂(MoO₂)(PO₄)₄ was also prepared. The structures were determined by single crystal X-ray diffraction techniques. The structures of Ta₂(WO₂)_0.87H_0.26(PO₄)₄ and Ta₂(MoO₂)(PO₄)₄ can be formally derived from the structure of β-TaH(PO₄)₂ by the replacement of two P-OH protons with an MO₂²⁺ (M = Mo and W) group together with a change in the orientation of some phosphate tetrahedra.     

Long wavelength Ce emission in Y6Si3O9N4 phosphors for white-emitting diodes

Y₆Si₃O₉N₄:Ce³⁺ phosphor was prepared by a solid-state reaction in reductive atmosphere. X-ray powder diffraction (XRD) analysis confirmed the formation of Y₆Si₃O₉N₄:Ce³⁺. Scanning electron microscopy (SEM) observation indicated that the microstructure of the phosphor consisted of irregular fine grains with an average size of about 5 μm. Photoluminescence (PL) measurements showed that the phosphor can be efficiently excited by near ultraviolet (UV) or blue light excitation, and exhibited bright green emission peaked at about 525 nm. Compared with Ce³⁺-doped Y₄Si₂O₇N₂ phosphors, Ce³⁺-doped Y₆Si₃O₉N₄ phosphors showed longer wavelengths of both excitation and emission. The Y₆Si₃O₉N₄:Ce³⁺ is a potential green-emitting phosphor for white LEDs.     

Synthesis of Si3N4 whiskers in porous SiC bodies

Si₃N₄ whiskers were synthesized by the carbothermal reduction process in porous SiC bodies. The SiC bodies had a sponge microstructure with pore sizes of approximately 600 μm. The raw materials for the Si₃N₄ whiskers were powder mixtures of Si₃N₄, SiO₂ and Si for silicon and phenolic resin for carbon. Cobalt was used as a metal catalyst. The carbothermal reaction was performed at 1400 °C or 1500 °C for 1 or 2 h. The α-Si₃N₄ whiskers grew inside the SiC pores by the VLS process, and their diameters ranged from 0.1 to 1.0 μm. The length of the grown Si₃N₄ whiskers was over 100 μm and their growth direction was [100].     

Assessment of the optimum degree of Sr3Fe2MoO9 electron-doping through oxygen removal: An X-ray powder diffraction and Fe Mössbauer spectroscopy study

We describe the preparation and structural characterization by X-ray powder diffraction (XRPD) and Mössbauer spectroscopy of three electron-doped perovskites Sr₃Fe₂MoO_9−δ with Fe/Mo = 2 obtained from Sr₃Fe₂MoO₉. The compounds were synthesized by topotactic reduction with H₂/N₂ (5/95) at 600, 700 and 800 °C. Above 800 °C the Fe/Mo ratio changes from Fe/Mo = 2-1 < Fe/Mo < 2. The structural refinements of the XRPD data for the reduced perovskites were carried out by the Rietveld profile analysis method. The crystal structure of these phases is cubic, space group , with cationic disorder at the two different B sites that can be populated in variable proportions by the Fe atoms. The Mössbauer spectra allowed determining the evolution of the different species formed after the treatments at different temperatures and confirm that Fe ions in the samples reduced at 600, 700 and 800 °C are only in the high-spin Fe³⁺ electronic state.     

Large-scale synthesis of ear-like Si3N4 dendrites from SiO2/Fe composites and Si powders

Large-scale ear-like Si₃N₄ dendrites were prepared by the reaction of SiO₂/Fe composites and Si powders in N₂ atmosphere. The product was characterized by field emission scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The results reveal that the product mainly consists of ear-like Si₃N₄ dendrites with crystal structures, which have a length of several microns and a diameter of 100-200 nm. Nanosized ladder-like Si₃N₄ was also obtained when changing the Fe content in the SiO₂/Fe composites. The Si₃N₄ nanoladders have a length of hundreds nanometers to several microns and a width of 100-300 nm. The ear-like Si₃N₄ dendrites are formed from a two-step growth process, the formation of inner stem structures followed by the epitaxial growth of secondary branches.     

Microwave sintering of Si3N4 with LiYO2 and ZrO2 as sintering additives

Phase transformation, microstructure development and mechanical properties of 2.45 GHz microwave-sintered silicon nitride (Si₃N₄) with lithium yttrium oxide (LiYO₂) and zirconia (ZrO₂) sintering additives were investigated. It was found that α to β phase transformation completed at a lower temperature of 1500 °C. Scanning electron microscopy (SEM) micrographs revealed a bimodal microstructure with a large number of elongated β-Si₃N₄ grains in addition to smaller grains. Surface residual porosity was observed in all sintered samples due to selective localized over heating of grain-boundary glassy phase. The high aspect-ratio of β-Si₃N₄ grains exhibited significant crack deflection, debonding and pull-out. It was observed that Vickers hardness and indentation fracture toughness increased with increasing sintering temperature.     

Interfacial microstructure of Si3N4/Si3N4 brazing joint with Cu–Zn–Ti filler alloy

In this study, Si₃N₄ ceramic was jointed by a brazing technique with a Cu–Zn–Ti filler alloy. The interfacial microstructure between Si₃N₄ ceramic and filler alloy in the Si₃N₄/Si₃N₄ joint was observed and analyzed by using electron-probe microanalysis, X-ray diffraction and transmission electron microscopy. The results indicate that there are two reaction layers at the ceramic/filler interface in the joint, which was obtained by brazing at a temperature and holding time of 1223 K and 15 min, respectively. The layer nearby the Si₃N₄ ceramic is a TiN layer with an average grain size of 100 nm, and the layer nearby the filler alloy is a Ti₅Si₃N_x layer with an average grain size of 1–2 μm. Thickness of the TiN and Ti₅Si₃N_x layers is about 1 μm and 10 μm, respectively. The formation mechanism of the reaction layers was discussed. A model showing the microstructure from Si₃N₄ ceramic to filler alloy in the Si₃N₄/Si₃N₄ joint was provided as: Si₃N₄ ceramic/TiN reaction layer/Ti₅Si₃N_x reaction layer/Cu–Zn solution.     

Fe3O4 magnetic nanoparticles synthesis from tailings by ultrasonic chemical co-precipitation

The aim of this study is to develop a new method for the preparation of high-value, environmentally friendly products from tailings. Magnetic Fe₃O₄ nano-powder was synthesized by ultrasonic-assisted chemical co-precipitation utilizing high purity iron separated from iron ore tailings by acidic leaching method. Magnetite particles with 15 nm average diameter were characterized by X-ray diffraction, field-emission scanning electron microscopy and vibrating sample magnetometer. Surfactant influence on particles shape and size was investigated. Fe₃O₄ nanoparticles coated with C₁₂H₂₅OSO₃Na exhibit better dispersion and uniform size. The product consisted of ferrous ferrite (Fe3O4) nanosized cubic particles with a high level of crystallinity and exhibit super-paramagnetism based on magnetization curves lacking hysteresis.     

Pretreatment and sintering of Si3N4 powder synthesized by the high-temperature self-propagation method

Self-propagation high-temperature synthesis (SHS) was applied for the synthesis of low-cost Si₃N₄ powder. The powder was purified and ground until its particle size reached submicron levels and its purity reached 98%. Using this pretreated powder, with α/β = 60/40 content, fully dense Si₃N₄ ceramics, having improved mechanical properties, were obtained by liquid-phase sintering in the presence of (Y, La)₂O₃-AlN. The mechanical properties achieved finally were as follows: strength, 784 MPa; hardness, 15.1 GPa; and fracture toughness, 5.2 MPa m^0.5. The behaviors of the SHS-Si₃N₄ powders before and after the pretreatment were compared. The relation between microstructure and mechanical properties of the sintered specimens and the effect of different β content in the powder on the sintering process of Si₃N₄ were also studied.     

Phase equilibrium diagram for the system Gd2O3-MoO3

Phase equilibria have been established in the binary system Gd₂O₃MoO₃ including Gd₂(MoO₄)₃ with ferroelectricity and ferroelasticity below 159°C, by means of differential thermal analysis and X-ray powder diffraction techniques. It is shown that Gd₂(MoO₄)₃ has no solubility of other compounds in this system and melts congruently on its stoichiometric composition. Three distinct intermediate compounds were found. Gd₂O₃.6MoO₃ and Gd₂O₃.4MoO₃ are formed by a peritectic reaction at 730°C and 825°C, respectively. The remaining compound Gd₂O₃.MoO₃ with the structure closely related to Eu₂O₃.MoO₃ does not decompose below 1400°C.     

Thermal chemistry of a high temperature solid lubricant, cesium oxythiomolybdate Part II Thermo-oxidative stability of Cs2MoOS3/Si3N4mixtures

Cesium oxythiomolybdate (Cs₂MoOS₃) may be an excellent high temperature lubricant, providing a friction coefficient below 0.2 at 650°C. However, oxidation products provide the lubrication above 400°C. Lubricant effectiveness depends strongly on the composition of the substrate materials in contact, such as Si₃N₄, suggesting that tribochemical and/or thermal reactions at the interface produce new compounds. The thermo-oxidative stability of Cs₂MoOS₃/Si₃N₄and Cs₂MoOS₃/SiO₂mixtures have been evaluated between room temperature and 1000°C in air. The transition temperatures and oxidation products were identified. The thermal chemistry of Cs₂MoOS₃/Si₃N₄mixtures was significantly different than that of Cs₂MoOS₃alone, largely due to the oxidation of Si₃N₄to glassy SiO₂. Cesium oxythiomolybdate formed cesium oxides, which melted below 600°C. As SiO₂is formed, the cesium oxides diffused into it, creating a cesium silicate glass. Also, Cs₂MoO₄was preferentially formed over complex cesium molybdates and molybdenum oxides. In a tribological application, Cs₂MoO₄, oxides, and cesium silicate glass may be formed at contacting interfaces from Cs₂MoOS₃films deposited on Si₃N₄substrates. Lubrication would be provided as the shear strength of these compounds decreases with increasing temperature.     

The synthesis and selected properties of new compounds: Mg3Fe4(VO4)6 and Zn3Fe4(VO4)6

New compounds: Mg₃Fe₄(VO₄)₆ and Zn₃Fe₄(VO₄)₆ were obtained from a solid state reaction. The temperatures of melting of Mg₃Fe₄(VO₄)₆ and Zn₃Fe₄(VO₄)₆ amount to 950±5 and 850±5°C, respectively. The indexing results and the calculated unit cell parameters for both compounds are given and suggest that both phases are isotypic with Mn₃Fe₄(VO₄)₆. The IR spectra of the above-mentioned compounds are presented.     

Reinvestigation of the binary diagram Na3PO4-FePO4 and crystal structure of a new iron phosphate Na3Fe3(PO4)4

A new iron(III) phosphate Na₃Fe₃(PO₄)₄ has been synthesized and characterized. It decomposes before melting at 860°C into FePO₄ and Na₃Fe₂(PO₄)₃. The structure of the compound was determined by single-crystal X-ray diffraction. The unit cell is monoclinic with the following parameters: a=19.601(8) Å, b=6.387(1) Å, c=10.575(6) Å and β=91.81(4)°; Z=4; space group: C2/c. Na₃Fe₃(PO₄)₄ exhibits a layered structure involving corner-linkage between FeO₆ octahedra, and corner- and edge-sharing between FeO₆ octahedra and PO₄ tetrahedra. The Na⁺ cations occupying the interlayer space are six- and seven-fold coordinated by oxygen atoms. The relationship between the structure of Na₃Fe₃(PO₄)₄ and the previous reported hydrate K₃Fe₃(PO₄)₄·H₂O will be discussed.     

Crystal Structure of Tl2[(UO2)2(MoO4)3] and Crystal Chemistry of the Compounds M2[(UO2)2(MoO4)3] (M = Tl, Rb, Cs)

A new compound, Tl₂[(UO₂)₂(MoO₄)₃], was prepared by a solid-phase reaction. The compound crystallizes in a rhombic system, space group Pna2₁, a = 20.1296(9), b = 8.2811(4), c = 9.7045(4), V = 1617.69(13) ų, Z = 4. The crystal structrue was solved by the direct method and refined to R ₁ = 0. 04 for 4884 unique reflections. The structural motif is a framework consisting of UO₇ pentagonal bipyramids and MoO₄ tetrahedra. The Tl coordination polyhedra are irregular, with seven and eight vertices. Large channels of the size 6 × 10.8 Å, occupied by Tl⁺ cations, are arranged parallel to the [001] direction. The compound is isostructural to the previously described α-Cs₂(UO₂)₂(MoO₄)₃ and Rb₂(UO₂)₂ (MoO₄)₃. __________ Translated from Radiokhimiya, Vol. 47, No. 5, 2005, pp. 408–411. Original Russian Text Copyright ? 2005 by Nazarchuk, Krivovichev, Burns.     

Study on particle-stabilized Si3N4 ceramic foams

The stability of bubbles and the microstructures of sintered Si₃N₄ ceramic foams produced by direct foaming method were investigated. The bubbles produced by short-chain amphiphiles (propyl gallate) have higher stability as compared with that produced by long-chain surfactants (TritonX-114). Si₃N₄ ceramic foams using short-chain amphiphile are particle-stabilized one, the pore cells are spherical and closed, and cell surfaces are smooth and dense. The pore cells of sintered Si₃N₄ ceramic foams using TritonX-114 foaming are coarse and large, and pore cells are polyhedral. High gas-pressure sintering is conducive to the development of the whisker-like microstructures in Si₃N₄ ceramic foams. The sintered Si₃N₄ ceramic foams with the whisker-like microstructure are quite promising for improving the mechanical strength of the ceramics by a simple and safe way.