Thermal expansion of NZP-family alkali-metal (Na, K) zirconium phosphates

The thermal expansion of the A _x Zr_2.25-0.25x(PO₄)₃ phosphates with A = Na(x = 0.5,1.0,2.0,3.0,4.0, 5.0) and K(x = 1.0, 3.0, 5.0), crystallizing in structures of the NaZr₂(PO₄)₃ type (sp. gr.R3c or C2/c), was studied by high-temperature x-ray powder diffraction in the range 20–700‡C. The lattice parametersa and c and thea- andc-axis thermal expansion coefficients (α_a and α_c) were determined. The thermal expansion of the phosphates was found to be highly anisotropic (α_a < 0, α_c > 0). The strongest anisotropy was found in NaZr₂(PO₄)₃ (α_a = -4.89 x 10^-6 K^-1, α_c = 22.02 x 10^-6 K^-1), KZr₂(PO₄)₃ (α_a =-5.30 x 10^-6 K^-1, α_c = 5.41 x 10^-6 K^-1), and Na₅Zr(PO₄)₃ (α_a = -5.82 x 10^-6 K^-1, α_c = 20.73 x 10^-6 K^-1). K₅Zr(PO₄)₃ exhibited the smallest thermal expansion and weak anisotropy (α_a = -2.14 x 10^-6 K^-1, α_c = 2.65 x 10^-6 K^-1). The effects of M(l) and M(2) site occupancies on α_a, α_c,a, and c were assessed. The relative magnitudes of crystal-chemical and thermal expansion in the Na and K compounds were analyzed.     

Chemical vapour deposition of Si3N4 from a gas mixture of Si2Cl6, NH3 and H2

Si₃N₄ layers were obtained on a quartz substrate from a gas mixture of Si₂Cl₆, NH₃ and H₂ under a reduced pressure in a temperature range of 800 to 1300‡ C. Amorphous Si₃N₄ layers that were dense and adherent to the substrate were obtained in a temperature range of 800 to 1100‡ C. On the other hand,α-Si₃N₄ layers were obtained at 1200‡ C and a source-gas ratio (N/Si) of 1.33 to 1.77. The lowest deposition temperature of amorphous Si₃N₄ was considered to be about 700‡ C. The microhardness of amorphous Si₃N₄ obtained in a temperature range of 800 to 1100‡ C was 2400 to 2600 kg mm⁻² (load: 50 g), and that ofα-Si₃N₄ obtained at 1200‡ C was 3400 kg mm⁻². Chlorine contents in the Si₃N₄ layer decreased with increasing deposition temperature and source-gas ratio (N/Si), and with decreasing total pressure.     

Low-temperature structural studies of Tl2Ba2CaCu2O x

Low-temperature x-ray diffraction studies of superconducting Tl-2212 ceramics revealed anomalies in thec cell parameter andc-axis thermal expansion coefficient near the superconducting transition. Air and vacuum anneals in the range 400–550‡C demonstrate that these anomalies correlate with the superconducting properties of the material. The anomalous temperature variation of thermal expansion suggests that the superconducting transition is accompanied by significant changes in the phonon spectrum.     

Thermal expansion of ternary chalcogenides of molybdenum containing infinite chains of (Mo6/2Se6/2)

The coefficients of thermal expansion of three ternary chalcogenides of molybdenum Tl₂Mo₆Se₆, Cs₂Mo₆Se₆, and Na₂Mo₆Se₆ have been determined in the temperature range 16 to 400° C using a Guinier-Lenne camera and a powder diffractometer. In all the three compounds, the coefficient of expansion along thec-axis (α_∥) is found to be much smaller than that in the perpendicular direction (α_⊥). This behaviour has been explained in terms of the strength of the interatomic bonds in the two directions.     

Electrical conductivity and oxygen nonstoichiometry of SrCo0.25Fe0.75O3-δ

The oxygen stoichiometry and defect structure of SrCo_{0. 25}Fe_0.75O_3-δ were studied by solid-electrolyte coulometry, thermogravimetry, and conductivity measurements at temperatures from 20 to 1000‡C and oxygen partial pressures from 1 to 10⁵ Pa. The results were used to construct thep-T-x andp-T-a phase diagrams and identify the sequence of phase transformations in the Co-substituted strontium ferrate (low-temperature perovskite → brownmillerite → high-temperature perovskite). SrCo_0.25Fe_0.75O_3-δ was shown to be p-type throughout thepo ₂ and temperature ranges studied. Cooling in flowing oxygen or air was found to give rise to sharp changes in the resistivity of SrCo_{0. 25}Fe_0.75O_3-δ.     

Reaction sintering and the α-Si3N4/β′-sialon transformation for compositions in the system Si-Al-O-N

The reaction sintering of three powder compositions corresponding to points near theβ′-phase line in the region of z=0.75 has been studied using apparatus which allowed the continuous monitoring of the densification kinetics. The powder compositions were prepared from mixtures ofα-Si₃N₄, Al₂O₃ and AIN, and the weight changes in the compacts could be kept to less than ∼ 1% over a two hour period. The densification rate is sensitive to small changes in powder composition and decreases markedly as theβ∼'-sialon phase is approached from the oxygen-side. Theα-Si₃N₄ toβ′-sialon conversion rate, on the other hand, is almost independent of the powder composition. The sintering and transformation kinetic data, combined with surface area measurements and observations on the microstructure of the sintered compacts, indicate that the sintering behaviour is controlled by at least two processes, namely the vapour phase transport of material and a solution-diffusion-reprecipitation process involving a grain boundary liquid phase. Both processes result in the conversion ofα-Si₃N₄ toβ′-sialon and result in microstructural coarsening but only the second process leads to overall densification in the powder compact.     

Framework flexibility of sodium zirconium phosphate: role of disorder, and polyhedral distortions from Monte Carlo investigation

A detailed Monte Carlo investigation of the structural changes of the framework of sodium zirconium phosphate, [Zr₂P₃O₁₂]⁻,—NASICON (acronym for Na-SuperIonic CONductor)—accommodating alkali ions of varying sizes (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) is carried out over a range of temperatures. Simulation results are critically compared with the structural models proposed earlier and available experimental results. Anisotropic changes of the rhombohedral cell parameters—a contracts while c expands with the size of the alkali ion substituted—is observed in good agreement with previous experimental results. The mechanism of anisotropic variation of lattice parameters involves dominantly, coupled rotations of the polyhedra as proposed by Alamo and co-workers. It is, however, observed that the distortions of the PO₄ tetrahedra and ZrO₆ octahedra are significant, and accounts for nearly one-third of the total change in a and c—parameters as the size of the alkali ion increases. This suggests that ‘rigid’ polyhedral models, permitting only angular distortions of the polyhedra, are of limited quantitative applicability in these solids. The same mechanism is found to be responsible for the low/anisotropic thermal expansion of these solids. Evidence that the polyhedral rotations are dynamic, opposed to a static-frozen-in disorder, is provided.     

Electrical,thermal and infrared studies of cadmium metavanadate

Cadmium(II) metavanadate has crystal structure related to brannerite (ThTi₂O₆) structure. The high temperatureβ-CdV₂O₆ phase isn-type semiconductor between 185 and 750°C. The electrical conduction in theβ-CdV₂O₆ occurs due to deviation from oxygen stoichiometric composition of the lattice. The seebeck coefficient (α) of the sample is negative and constant in the entire range of investigation. The mechanism of transport in cadmium metavanadate lattice is via thermally activated hopping of localized electrons on vanadium (V⁵⁺) sites of the lattice. The DTA result indicated that CdV₂O₆ undergoes phase transition at 185°C and not at 670°C as reported earlier. There is no DTA evidence to show the possibility ofβ →α phase reverse transition. The XRD powder patterns of the two modifications are nearly similar indicating brannerite related structures. The infrared absorption band of vanadium-oxygen stretching vibration modes of distorted VO₆ octahedra ofβ-CdV₂O₆ is exhibited at 855 cm⁻¹.     

Influence of Co Doping on the Zn Distribution in?Nd1.05Ba1.95Cu3?xZnxOz

The structural effects of Zn doping and Zn, Co co-doping in Nd_1.05Ba_1.95Cu₃O _z have been investigated by using neutron diffraction. The Zn atoms are found to occupy exclusively the planar sites in Nd_1.05Ba_1.95Cu_3−x Zn _x O _z samples with x≤0.4. An orthorhombic-to-tetragonal transition behavior was observed in the Nd_1.05Ba_1.95Cu_3−x Zn _x O _z (x=0–0.6) system, while a tetragonal-to-orthorhombic transition was observed in the Nd_1.05Ba_1.95Cu_2.8−x Zn _x Co_0.2O _z (x=0–0.6) system as the Zn doping content x increases. A comparative analysis of the structural behavior of Nd_1.05Ba_1.95Cu_3−x Zn _x O _z and Nd_1.05Ba_1.95Cu_2.8−x Zn _x Co_0.2O _z (x=0, 0.2, 0.4, 0.6) samples showed that the Co doping can induce an increase in Zn occupancy at the Cu(1) sites as the Zn doping concentration increases above x=0.4, implying that the distribution of Zn ions between the chain and planar sites can be tuned by the Co substitution for the chain Cu sites.     

Some electrical properties of the defectiveγ phases of spinel structure during their transformation to the rhombohedral α phases

During the transformation of “cubic” iron sesquioxides, γ-Fe₂O₃, substituted by divalent or trivalent ions, to rhombohedral α phases, the electrical conductivity yields discontinuities and a change in plots of logσ againstf(1/T). The average temperature of these discontinuities is influenced by the particle size, and extent of oxidation and substitution. For divalent substituted defective spinels, the activation energy for the haematite precipitation appears to depend on the extent of substitution and the size of the particle.     

Thermal expansion of Li2O-ZnO-Al2O3-SiO2 glasses and corresponding glass-ceramics

The thermal expansion behaviour of some glasses and glass-ceramics within the system Li₂ZnSiO₄-LiAlSi₂O₆ is described. The effect of TiO₂ and ZrO₂ additions is also evaluated. The expansion coefficient () of the glasses increases with an increase of the Li₂ZnSiO₄ component in the glass composition. TiO₂ and ZrO₂ were found to decrease the thermal expansion of the glasses investigated. The dilatometric transition and softening points of the glasses showed the reverse behaviour. The thermal expansion of the glass-ceramics exhibited a wide range, depending upon the type and relative proportions of the crystalline phases present. The values of the glasses ranged between 73.6 and 97.4×10^–7C^–1 in the temperature range 20–450C and those for the crystalline products ranged from 36.1 and 102.6 × 10^–7 in the temperature range 20–450C.     

Thermal expansion behaviour of barium and strontium zirconium phosphates

Ba_1.5-xSr_xZr₄P₅SiO₂₄ compounds withx = 0, 0.25, 0.5, 0.75, 1.0, 1.25 and 1.5, belonging to the low thermal expansion NZP family were synthesized by the solid state reaction method. The XRD pattern could be completely indexed with respect to space group indicating the ordering of vacancy at the divalent cation octahedral sites. The microstructure and bulk thermal expansion coefficient from room temperature to 800°C of the sintered samples have been studied. All the samples show very low coefficient of thermal expansion (CTE), withx = 0 samples showing negative expansion. A small substitution of strontium in the pure barium compound changes the sign of CTE. Similarly,x = 1.5 sample (pure strontium) shows a positive CTE and a small substitution of barium changes its sign.X = 1.0 and 1.25 samples have almost constant CTE over the entire temperature range. The low thermal expansion of these samples can be attributed to the ordering of the ions in the crystal structure of these materials     

High-temperature creep and structure investigation of nearly stoichiometric Fe3Si

X-ray diffraction measurements were carried out on powdered single crystals of nearly stoichiometric Fe₃Si. The experimental data obtained in the temperature range from room temperature up to 750‡ C in terms of long-range order, thermal expansion, phase transition and Debye temperature (together with values of the Curie temperature) support the existence of two modifications of the DO₃ structure for Fe-26 at% Si alloys and a phase transition in the DO₃ structure field at 595‡ C. The high-temperature modification has a smaller thermal expansion coefficient, a higher Curie temperature and a higher Debye temperature.     

ESR characterization of Fe(III) ions in polycrystalline cation-deficient FeAl2O4 spinel

Polycrystalline samples of cation-deficient FeAl₂O₄ spinel with Fe³⁺ ions and vacancies on both octahedral and tetrahedral sites have been investigated by electron spin resonance (ESR). For samples annealing at 1100° C and slightly oxidized (⩽0.16 wt % Fe³⁺), the ESR spectrum originates from Fe³⁺ ions located in trigonally distorted octahedral sites ofα-Al₂O₃ formed during the annealing and also from Fe³⁺ ions introduced by oxidation at low temperature in octahedral sites of spinel structure. The samples with concentration between 0.16 and 1.6 wt % Fe³⁺ show the majority of Fe³⁺ ions to be on tetrahedral sites and the rhombic symmetry around the Fe³⁺ ions is attributed to the presence of cation vacancies.     

Effect of combined doping (y<Superscript>3 +</Superscript> + fe<Superscript>3 +</Superscript>) on structural features of nanodispersed zirconium oxide

Fully stabilized zirconium dioxide is widely used. One of the basic requirements to this material is the thermal stability of the structure. The most effective stabilizer for zirconium oxide is yttrium oxide. However, the structure of Y-ZrO₂ degraded at low temperature. Partial substitution of Fe^{3 +} for Y³⁺ decreases both the crystallization and sintering temperature of zirconia ceramic. The aim of present work is the investigation of structural peculiarities of zirconium oxide stabilized by combined dopant depending on chemical composition, synthesis conditions and heat treatment. The polymorphic composition of a ZrO₂-based materials has been determined in series of samples that correspond to the formula [1−(x+y)]ZrO₂⋅xY₂O₃⋅yFe₂O₃ in the temperature range 620–1570 K. It has been found that at the same molar ratio ZrO₂ : doping oxides, the degree of ZrO₂ stabilization increases, and the low-temperature degradation process is retarded by the partial substitution of Fe^{3 +} for Y³⁺. Nonequivalent sites of Fe^{3 +} ions have been identified: two with octahedral coordination for CPH and three with octa-, penta- and tetrahedral coordination for SPH. The possibility of cluster distribution of Fe³⁺ ions and the dependence of the number of vacancies on synthesis conditions have been shown.     

Study of lithium–zinc borophosphate glasses

Mixed lithium–zinc borophosphate glasses were prepared and studied in three compositional series xLi₂O–(50−x)ZnO–50P₂O₅, xLi₂O–(50−x)ZnO–10B₂O₃–40P₂O₅ and xLi₂O–(50−x)ZnO–20B₂O₃–30P₂O₅ with x = 0, 10, 20, 30, 40 and 50 mol% Li₂O. The obtained glasses were characterized by the measurements of the density (ρ), molar volume (V _M), glass transition temperature (T _g) and thermal expansion coefficient (α). For the investigation of structural changes ¹¹B and ³¹P MAS NMR and Raman spectroscopy were applied. The replacement of zinc by lithium in borophosphate glasses slightly decreases V _M and T _g, while α increases. In Li–Zn metaphosphate glasses the compositional dependence of T _g reveals a minimum, while at the borophosphate series T _g decreases monotonously with increasing Li₂O content. Chemical stability of Li–Zn borophosphate glasses is very good for glasses with x = 0–30 mol% Li₂O. Spectral studies showed in the glass series with 10 mol% B₂O₃ only the presence of BO₄ sites. In the glasses with 20 mol% B₂O₃ the presence of BO₃ and two BO₄ sites was revealed in ZnO-rich glasses and only one BO₄ site in Li₂O-rich glasses; the number of BO₃ groups decreases with increasing Li₂O content which is ascribed to the formation of P–O–Zn covalent bonds in ZnO-rich glasses.     

Study of the reactivity and <Emphasis Type="Italic">in vitro</Emphasis> bioactivity of Sr-substituted α-TCP cements

In this study the effect of strontium substitution on the hydrolysis of α -tricalcium phosphate (α -TCP) toward the formation of calcium deficient hydroxyapatite (CDHA) was investigated. For that purpose substituted α -TCP powders with 1, 5 and 10 mol% Sr substitution for Ca were synthesized by reacting at 1500 ^∘C stoichiometric amounts of CaCO₃, SrCO₃, and Ca₂P₂O₇, followed by rapid quenching in air. XRD analysis of the powders revealed the presence of α-TCP (traces of β-TCP) with enlarged unit cell volume at increased Sr contents, indicating the incorporation of Sr in the crystal structure. Strontium was also incorporated in the apatite phase as revealed by XRD analysis of the set cements. The hydrolysis of milled α-SrTCP powders and a pure α-TCP (control) was monitored by isothermal calorimetry and the compressive strength of set cements was tested. The results showed a decrease in the reactivity with increasing Sr content and similar final mechanical strength within the Sr series, though lower than the control. The in vitro bioactivity of the set cements after soaking in simulated body fluid for 4 weeks was also tested. The formation of a bone-like apatite layer on the surface of the set cements indicated a potential in vivo bioactivity.     

The Effect of Zn Substitution of Ca in BiPbSrCaCuO Superconductors Sintered at 830 °C

The effect of partial substitution of Ca by Zn in Bi_1.7Pb_0.3Sr₂Ca_2−x Zn _x Cu₃O _y at x=0.00, 0.05, 0.10, 0.15 and 0.20 levels on the electrical and structural properties was investigated in this work. The characterization of the ceramics prepared by the conventional solid-state reaction method were done by resistance–temperature measurements, XRD, SEM and density analysis. Low levels of Zn substitution of Ca caused significant changes in the properties of the ceramics. The low-T _c superconducting properties were enhanced and the fraction of the low-T _c (2212) phase were found to increase at x=0.15 level of Zn substitution at 830 °C sintering temperature. Zero resistance was observed only in sample D with x=0.15 and the T _c was determined as 92 K. The SEM micrographs and the density analysis have shown that this was the densest packed ceramic.     

Preparation and studies of some thermal,mechanical and optical properties of<Emphasis Type="Italic">x</Emphasis>Al<Subscript>2</Subscript>O<Subscript>3</Subscript>(1 −<Emphasis Type="Italic">x</Emphasis>)NaPO<Subscript>3</Subscript> glass system

Sodium aluminophosphate glasses having compositions of xAl₂O₃(1-x)NaPO3 (x = 0.05-0.2) were prepared using conventional melt-quench technique. Density, glass transition temperature, microhardness (MH), thermal expansion coefficient (TEC) and transmission characteristics were measured as a function of alumina content for different samples. They were found to depend on O/P ratio with pronounced changes taking place for O/P ratio ≥3.5. Density, glass transition temperature and microhardness were found to increase up to 15 mol% of alumina and then they showed a decreasing trend. Thermal expansion coefficient decreased continuously with alumina content. Optical gaps for different glass samples as measured from transmission characteristics were found to be in the range 3.13–3.51 eV. It initially decreased with alumina content up to 15 mol% and then increased. The behaviour was explained on the basis of change in the average aluminum coordination number from six Al(6) to four Al(4) (i.e. Al(OP)₆/Al(OP)₄ ratio) along with the changes in polyhedra linkages in the glass network due to change in O/P ratio.     

Erbium zirconium phosphate Er<Subscript>0.33</Subscript>Zr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> of the NaZr<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> family. Structure contraction on heating

Erbium zirconium phosphate Er_0.33Zr₂(PO₄)₃, a member of the family of structural analogs of NaZr₂(PO₄)₃ (NZP), was prepared by the sol-gel process and studied by X-ray phase analysis, IR spectroscopy, and differential scanning calorimetry. The behavior of erbium zirconium phosphate on heating in the temperature interval from 25 to 625°C was studied by high-temperature X-ray diffraction. Expansion and contraction along different crystallographic directions and contraction of the structure as a whole were found. The overall contraction is due to higher contribution of the negative axial thermal expansion coefficients α _a and α _b to α_av and hence to the volume expansion of the phosphate. On heating to 900°C, the NZP structure is preserved.