Average Thermal Expansion Coefficient Research Articles

High-temperature crystallographic and thermodynamic investigations on synthetic zirconolite (CaZrTi2O7)

In this paper, we report the crystal structure and thermodynamic parameters of zirconolite (CaZrTi2O7) which is a potential ceramic matrix for incorporation of nuclear high-level waste. The room temperature structural studies revealed a two layer monoclinic (C2/c) structure for the compound. Thermal expansion behavior of zirconolite was investigated by the high-temperature XRD. The average thermal expansion coefficient of zirconolite, measured in the temperature range 298–1273 K, is found to be: a-axis (A) = 12.410(2) + 1.06(8) × 10−4 [T] + 3.1(5) × 10−8 [T]2/K, b-axis (A) = 7.252(1) + 6.2(4) × 10−5 [T] + 4.9(2.5) × 10−9 [T]2/K and c-axis (A) = 11.351(2) + 7.6(7) × 10−5 [T] + 3.2(4) × 10−8 [T]2/K. The standard molar enthalpy of formation of zirconolite (CaZrTi2O7, s) at 298.15 K determined employing high-temperature solution calorimetry is found to be − 3716.5 ± 3.7 kJ mol−1. The isobaric heat capacity of the CaZrTi2O7(s) has been derived from the enthalpy increment (H T–H 298.15) data. Based on the experimental data for the compound and the available thermodynamic data for the component oxides, thermodynamic functions like: $${\text{Cp}}_{\text{m}}^{\circ } ,S_{\text{m}}^{\circ } , H_{{{\text{m}},{\text{T}}}}^{0} , { }G_{\text{m,T}}^{\circ } , \left( {H_{\text{T}}^{\circ } - H_{298.15}^{\circ } } \right), - \left( {\frac{{G_{\text{T}}^{\circ } - H_{298.15}^{\circ } }}{T}} \right), \Delta_{\text{f}} H_{{{\text{m}},298.15}}^{\circ } \,\, {\text{and}}\,\,\Delta_{\text{f}} G_{{{\text{m}},{\text{T}}}}^{\circ }$$ for zirconolite (CaZrTi2O7, s) have been generated.

Specific heat measurement of Ge7.4Se92.6 glass

The Ge7.4Se92.6 glass was prepared by melt-quenched method. The characteristic temperatures (the strain point, annealing point, glass transition point, yielding point and soften point) were determined by thermal analysis. The result shows that the glass transition point is 52 °C; the average thermal expansion coefficient is ΔL/L 0 = 3 × 10−7 * T 2 + 8 × 10−6 * T − 0.0005. The specific heat of Ge7.4Se92.6 glass was measured by stepwise method. For undercooled liquid, the relation is C p l = 0.5001 + 4.037 × 10−4 T + 305.1T −2 J K−1 g−1, while for crystal, the relation is C p c = 0.3729 + 4.503 × 10−4 T + 15.71T −2 J K−1 g−1. With the result of specific heat, the Gibbs energy and the entropy curves were calculated, and the Kauzmann temperature was then determined as T k = 250 K.

A novel family of Nb-doped Bi0.5Sr0.5FeO3-δ perovskite as cathode material for intermediate-temperature solid oxide fuel cells

Cobalt-free provskite oxides Bi0.5Sr0.5Fe1−xNbxO3−δ (BSFNx, x = 0.05, 0.10 and 0.15) were prepared and evaluated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). In particular, the effects of Nb substitution on phase evolution, thermal expansion behavior and electrochemical performance were systematically investigated. The average thermal expansion coefficient (TEC) of BSFNx decreases from 13.3 × 10−6 K−1 at x = 0.05 to 12.6 × 10−6 K−1 at x = 0.15 within a temperature range of 50–800 °C. Among the BSFNx materials, Bi0.5Sr0.5Fe0.9Nb0.1O3−δ (BSFN0.10) oxide shows the best electrochemical performance. The polarization resistances (Rp) of BSFN0.10 cathode on CGO electrolyte are 0.038, 0.075 and 0.156 Ω cm2 at 700, 650 and 600 °C, respectively. Meanwhile the maximum power densities of the anode-supported single cells are 1.28, 1.54 and 1.34 W cm−2 at 700 °C for BSFNx cathodes with x = 0.05, 0.10, and 0.15, respectively. Furthermore, the relationship study of oxygen partial pressure dependence on Rp indicates that the oxygen reduction reaction (ORR) rate-limiting step is the oxygen adsorption-dissociation on the electrode surface. The desirable electrochemical performance demonstrates that BSFNx oxides are potential cathode materials for IT-SOFCs.

Thermal expansion of phosphate–sulfates of eulytite structure

New phosphate–sulfates of the next chemical formulae ASr2Eu(PO4)2SO4 (A = K, Rb, Cs), NaBa6Zr(PO4)5SO4, Pb2Mg2(PO4)2SO4, and BaxSr4−x(PO4)2SO4 (0 ≤ x ≤ 4) have been designed and synthesized with the aim of investigating their thermal expansion properties in the low-temperature region. Obtained samples have been characterized with X-ray, IR, DTA, and microprobe electron analyses. Crystal structure and unit cell parameters were derived from the least-squares refinement of powder X-ray diffraction data (eulytite-type, sp. gr. $$I 4 {\bar{\text{3}}}d$$ ). The magnitudes of average linear thermal expansion coefficients vary from 1.1 × 10−5 to 1.7 × 10−5 K−1, which is conducive for application in optics and electronics.

A-site Ba-deficiency layered perovskite EuBa1−xCo2O6−δ cathodes for intermediate-temperature solid oxide fuel cells: Electrochemical properties and oxygen reduction reaction kinetics

A-site Ba-deficiency layered perovskite oxides, EuBa1−xCo2O6−δ (EB1−xCO, x = 0.02 and 0.04), have been synthesized by a citric acid-ethylene diamine tetraacetic acid complexation sol-gel method, and evaluated as potential cathode materials for intermediate-temperature solid oxide fuel cells (IT-SOFCs). Room temperature powder X-ray diffraction patterns indicate that the EB1−xCO oxides crystallize in an orthorhombic symmetry with space group Pmmm. Among all of components, EB0.98CO exhibits a good chemical compatibility with Ce0.9Gd0.1O1.95 (CGO) electrolyte, as evidenced by phase analysis of mixed EB0.98CO-CGO after calcining at 950 °C for 12 h in air. Thermal expansion analysis gives an average thermal expansion coefficient of 16.7 × 10−6 K−1 for EB0.98CO. Thermogravimetric measurement confirms the high oxygen nonstoichiometric characteristic of EB0.98CO at elevated temperatures. The electrical conductivity values of EB0.98CO exceed 300 S cm−1 in the temperature range of 100–750 °C. When tested as cathode in IT-SOFCs, the polarization resistance of 0.107 Ω cm2 and the overpotential of 10 mV at current density of 77 mA cm−2 are achieved in the EB0.98CO cathode at 700 °C in air. The EB0.98CO cathode-based anode-supported single cell delivers the maximum power density of 505 mW cm−2 at 700 °C. Finally the rate-limiting steps for oxygen reduction reaction at the EB0.98CO cathode interface are determined to be the charge transfer reaction and gas-phase diffusion process.

Temperature-dependent structural behaviour of samarium cobalt oxide

The crystal structure and thermal expansion of the perovskite samarium cobalt oxide (SmCoO3) have been determined over the temperature range 295–1245 K by Rietveld analysis of X-ray powder diffraction data. Polycrystalline samples were prepared by a sol–gel synthesis route followed by high-temperature calcination in air. SmCoO3 is orthorhombic (Pnma) at all temperatures and is isostructural with GdFeO3. The structure was refined as a distortion mode of a parent $ Pm{\bar 3}m $ structure. The thermal expansion was found to be non-linear and anisotropic, with maximum average linear thermal expansion coefficients of 34.0(3) × 10−6, 24.05(17) × 10−6, and 24.10(18) × 10−6 K−1 along the a-, b-, and c-axes, respectively, between 814 and 875 K.

Structural, thermal and electrochemical properties of SrCo0.8Fe0.1Ga0.1O3-δ cathode material for intermediate-temperature solid oxide fuel cells

From the viewpoint of intermediate-temperature solid oxide fuel cells (IT-SOFCs) application, except for excellent electrochemical performance, a satisfactory cathode must also meet excellent thermal expansion compatibility with other cell components. Therefore, in order to reduce the thermal expansion coefficient (TEC) value and improve catalytic activity of Sr(Co0.9Fe0.1)O3−δ oxide, we report our effort on perovskite-type Ga3+ doped SrCo0.8Fe0.1Ga0.1O3-δ (SCFG) oxide as cathode material for IT-SOFCs. In this work, SCFG cathode was prepared using an improved sol–gel method and its properties were characterized by powder X-ray diffraction (XRD), X-ray Photoelectron Spectra (XPS), thermal expansion coefficient (TEC), electrical conductivity, and electrochemical impedance spectroscopy (EIS). The SCFG powder showed the expected orthorhombic crystal structure. XPS analysis disclosed multi-valence states of the transition-metal cations in SCFG, i.e., [Sr2+][Co3+/Co4+]0.8[Fe3+/Fe4+]0.1[Ga3+]0.1O3−δ. The maximum conductivity of SCFG was 122 S cm−1 at 800 °C. The average TEC value of the SCFG sample was 12.6 × 10−6 K−1 between room temperature and 1023 K, which is lower than the TEC of Sr(Co0.9Fe0.1)O3−δ cathode and is very similar to the value of the electrolyte. The Rp values of SCFG cathode on Ce0.8Sm0.2O1.9 (SDC) and La0.9Sr0.1Ga0.8Mg0.2O3-δ (LSGM) electrolytes were 0.047 and 0.053 Ω cm2 at 800°C, respectively. The maximum power densities of a single-cell with SCFG cathode on 300 μm thick SDC electrolyte were 504, 335 and 193 mW cm−2 at 800, 750 and 700 °C, respectively. These results indicate that SCFG oxide is a promising cathode material for IT-SOFCs duo to its good thermal expansion match with electrolyte and adequate electrochemical performance.

Iron phosphate glasses: Bulk properties and atomic scale structure

Bulk properties such as glass transition temperature, density and thermal expansion of iron phosphate glass compositions, with replacement of Cs by Ba, are investigated as a surrogate for the transmutation of 137Cs to 137Ba, relevant to the immobilisation of Cs in glass. These studies are required to establish the appropriate incorporation rate of 137Cs in iron phosphate glass. Density and glass transition temperature increases with the addition of BaO indicating the shrinkage and reticulation of the iron phosphate glass network. The average thermal expansion coefficient reduces from 19.8 × 10−6 K−1 to 13.4 × 10−6 K−1, when 25 wt. % of Cs2O was replaced by 25 wt. % of BaO in caesium loaded iron phosphate glass. In addition to the above bulk properties, the role of Ba as a network modifier in the structure of iron phosphate glass is examined using various spectroscopic techniques. The FeII content and average coordination number of iron in the glass network was estimated using Mössbauer spectroscopy. The FeII content in the un-doped iron phosphate glass and barium doped iron phosphate glasses was 20, 21 and 22 ± 1% respectively and the average Fe coordination varied from 5.3 ± 0.2 to 5.7 ± 0.2 with increasing Ba content. The atomic scale structure was further probed by Fe K-edge X-ray absorption spectroscopy. The average coordination number provided by extended X-ray absorption fine structure spectroscopy and X-ray absorption near edge structure was in good agreement with that given by the Mössbauer data.

Effects of Na+ substitution Cs+ on the microstructure and thermal expansion behavior of ceramic derived from geopolymer

AbstractAs part of a series of studies, effects of Na+ substitution on the thermal evolution of cesium‐based geopolymers on heating were studied. A series of sodium‐substituted cesium‐based geopolymers, Cs(1−x)NaxGPs (where x=0, 0.1, 0.2, 0.3, and 0.4), were prepared and treated at 1300°C for 2 hours to obtain the corresponding ceramic products. The thermal evolution process was disclosed by virtue of a variety of technical, including TG‐DTA, thermal shrinkage, XRD analysis, SEM, and TEM investigation. The results indicated that unheated Cs(1−x)NaxGPs was not completely amorphous after the substitution of Na+ and the crystallinity of Cs(1−x)NaxGPs gradually increased with the rise of sodium content. Meanwhile, the average particle sizes of Cs(1−x)NaxGPs also increased evidently with increases in sodium substitution. The final product after heat treatment mainly consisted of pollucite (CsAlSi2O6) and amorphous glass phase. The particle size of pollucite grain gradually decreased as more Cs+ were replaced maybe owing to the role of Na+ in the nucleation process of pollucite. Two forms of Na+ present in the final products: A small portion was present in the pollucite grains due to Na+ partial occupied the crystallographic sites of Cs+; and the rest were present in the amorphous glass phase among the pollucite grains. The average coefficient of thermal expansion (CTE) of resulting Cs(1−x)NaxGPs ceramics increased from 4.80×10‐6 K−1 (x=0) to 7.26×10−6 K−1 (x=0.4) with increases in sodium substitution, which could be due to the amorphous glass phase had a relatively higher CTE than that of pollucite.

Influence of Different Curing Modes on Polymerization Behavior and Mechanical Properties of Dual-Cured Provisional Resins.

This study determined the influence of curing mode on polymerization behavior and mechanical properties of dual-cured provisional resins. Three dual-cured bisacryl-based provisional resins were used: Tempsmart (TS; GC Corp), Luxatemp Automix Solar (LX; DMG Chemisch Pharmazeutishe Fabrik GmbH), and Integrity Multi·Cure (IG; Dentsply Caulk). A self-cured bisacryl-based provisional resin, Protemp Plus (PP; 3M ESPE) and a conventional poly(methyl methacrylate) (PMMA) provisional resin, Unifast III (UF; GC Corp) were used as controls. The inorganic filler content and coefficients of linear thermal expansion of the test materials were measured. Six specimens of each material were used to determine the flexural strength, elastic modulus, and resilience. The changes in ultrasound velocity during polymerization were measured. The average inorganic filler contents of the provisional resins, apart from UF, ranged from 24.4 to 39.3 wt%. The highest inorganic filler content was determined for LX, whereas TS showed the lowest value among the tested materials. The average coefficients of thermal expansion of the tested provisional resins ranged from 77.3 to 107.7 (×10-6/°C). TS and IG showed significantly lower thermal expansions than the other tested provisional resins. The mean flexural strengths of the provisional resins ranged from 70.4 to 122.6 MPa, the mean elastic moduli ranged from 1.8 to 3.7 GPa, and the mean resilience of the provisional resins ranged from 1.1 to 2.3 MJ/mm3, respectively. Dual-cured provisional resins showed significantly higher flexural strengths than the PMMA resin. However, in all cases, the light-curing mode showed significantly higher flexural strengths than the self-curing mode. In the initial polymerization phase, dual-cured resins in the light-curing mode showed a rapid increase in the speed of sound (V) during light irradiation, followed by a slower increase. Conversely, the dual-cured resins in the self-curing mode showed a slower initial increase, followed by a rapid increase. Although no significant difference in V was observed between 10 and 15 minutes in the light-curing mode of all tested dual-cured resins, a significantly higher V value was obtained at 15 minutes than at 10 minutes in the self-curing modes for LX and IG. Regardless of the curing mode, tested dual-cured provisional resins showed superior mechanical properties than the conventional PMMA provisional resin. However, dual-cured provisional resin flexural properties and polymerization behavior were affected by the curing mode. This study indicated that the light-curing mode might be recommended for all dual-cured provisional resins because of the enhancement of their mechanical properties and reduction of chair time.

Effect of fly ash on the overall performance of particulate‐filled polymer composite for precision machine tools

Particulate‐filled polymer composite (PFPC) consisting of epoxy resin, granite aggregate and fly ash (FA), is a new polymer composite that is prepared at room temperature. Exhibiting excellent vibration damping, PFPC has attracted wide attention in the field of precision machine tools. However, the major drawbacks that have limited the use of PFPC in manufacture of precision machine tools include their low mechanical properties, high coefficient of thermal expansion and high water absorption. In this article, the effect of FA content on the mechanical properties, damping ratio, thermal expansion coefficient, and water absorption of PFPC were systematically investigated. The experimental results showed that the compressive strength and elastic modulus of the PFPC initially increased and then decreased with the increase in the FA content. Also, it was found that the flexure strength, average thermal expansion coefficient (ATEC) and the maximum water absorption of the PFPC decreased as the FA content increased. The results also showed that the damping ratio first decreased and then increased with the increasing FA content, where the damping ratio of the PFPC exhibited a slight reduction when the FA content exceeded 5%. The maximum compressive strength, maximum elastic modulus, and the best damping ratio were obtained when the FA content was 5%. Simultaneously, the PFPC with a FA content of 5% exhibited high flexural strength, low thermal expansion, and low water absorption. Thus, the optimum FA content of PFPC was found to be 5% for use in precision machine tool fabrication. POLYM. COMPOS., 39:3986–3993, 2018. © 2017 Society of Plastics Engineers

The influence of Gd doping on thermophysical properties, elasticity modulus and phase stability of garnet-type (Y1-xGdx)3Al5O12 ceramics

In this work, Gd3+ was selected to partially substitute the Y3+ in yttrium aluminum garnet (YAG) in order to improve the thermophysical properties of YAG. A series of (Y1-xGdx)3Al5O12 (x=0, 0.1, 0.2, 0.3, 0.4) ceramics were synthesized through chemical co-precipitation route. The microstructure, thermophysical properties and elasticity modulus of (Y1-xGdx)3Al5O12 were investigated. The (Y1-xGdx)3Al5O12 ceramics was comprised of single garnet-type Y3Al5O12 phase. The thermal conductivities of (Y1-xGdx)3Al5O12 bulk samples decreased with increasing doping concentration to 0.2, but increased with furthering increasing the concentration to 0.4. The thermal conductivity of (Y0.8Gd0.2)3Al5O12 was 1.51Wm−1K−1 at 1200°C. The average thermal expansion coefficient of (Y0.8Gd0.2)3Al5O12 was slightly larger than that of Y3Al5O12. (Y0.8Gd0.2)3Al5O12 bulk sample exhibited the lowest elasticity modulus among the investigated (Y1-xGdx)3Al5O12. In addition, (Y0.8Gd0.2)3Al5O12 ceramic remained good phase stability from room temperature to 1600°C.

Phase equilibria, crystal structure, oxygen nonstoichiometry and thermal expansion of complex oxides in the Nd2O3 – SrO – Fe2O3 system

The phase equilibria in the ½ Nd2O3 – SrO – ½ Fe2O3 system were systematically studied at 1373K in air. The homogeneity ranges and crystal structure of the solid solutions Nd1-xSrxFeO3-δ with 0.0≤x≤0.6 (sp. gr. Pbnm) and with 0.7≤x≤0.9 (sp. gr. Pm3m), Sr2-yNdyFeO4-δ with 0.7≤y≤0.9 (sp. gr. I4/mmm), Sr3-zNdzFe2O7-δ with 0.0≤z≤0.4 and 1.8≤z≤1.9 (sp. gr. I4/mmm) and Sr4-uNduFe3O10-δ with 0.7≤u≤0.9 (sp. gr. I4/mmm) were determined by X-ray diffraction analysis of quenched samples. The structural parameters of single-phase oxides were refined by the Rietveld profile method. The changes of oxygen content in the solid solutions Nd1-xSrxFeO3-δ (0.2≤x≤0.9), Sr3-zNdzFe2O7-δ (0.0≤z≤0.4 and z=1.9) and Sr4-uNduFe3O10-δ (0.7≤u≤0.9) versus temperature in air were determined by thermogravimetric analysis. Gradual substitution of neodymium by strontium leads to the decrease of the oxygen content. The average thermal expansion coefficients for the Nd1-xSrxFeO3-δ (0.6≤x≤0.8) samples were calculated within the temperature range 298–1373K in air. The project of isothermal-isobaric phase diagram for the Nd-Sr-Fe-O system to the compositional triangle of metallic components was presented.

Phonon spectrum, IR and Raman modes, thermal expansion tensor and thermal physical properties of M2TiAlC2 (M = Cr, Mo, W)

The phonon spectra, phonon optical activities, thermal expansion and thermodynamic properties of ordered quaternary carbides M2TiAlC2 (M=Cr, Mo, W) and their ternary counterparts Ti3AlC2 and Cr3AlC2 are studied by first-principles phonon calculations. Phonon eigenvectors of all Raman and infrared modes are analyzed. We find that vibrational frequencies of some Raman active modes (A1g and E2g) can be correlated to the strength and reduced mass of covalent M (4f)C or metallic M (4f)Al bond in all studied structures. Phonon mode-Grüneisen parameter and macroscopic Grüneisen constants are evaluated from phonon spectra. The thermal expansion coefficients, specific heats and isothermal bulk moduli at finite temperatures are obtained under the quasiharmonic approximation (QHA). The average linear thermal expansion coefficient (LTEC) values are found to be αL=11.30×10−6K−1, 7.15×10−6K−1, 8.67×10−6K−1, 10.29×10−6K−1 and 15.70×10−6K−1 for Cr2TiAlC2, Mo2TiAlC2, W2Ti2AlC2, Ti3AlC2 and Cr3AlC2 in a range from 300K to 1400K, respectively. The hypothetical Cr3AlC2 phase shows strong anharmonic effects in the calculated thermal expansion profile above 600K. Using Slack-model, the thermal conductivities are also evaluated, and the values are κ=36.1W/mK (34.4W/mK, 27.2W/mK) for Cr2TiAlC2 (Mo2TiAlC2, W2Ti2AlC2) at room temperature.

Effects of B addition on glass forming ability and thermal behavior of FePC-based bulk metallic glasses

The FePC-based bulk metallic glasses (BMGs) have been demonstrated to possess high plasticity and good soft magnetic properties. However, the relatively poor glass forming ability (GFA) and thermal stabilities limited their application in industries. The effects of microalloying with B in FePC-based BMGs on the GFA and thermal behaviors were systematically investigated. It was found that a small amount of B addition can dramatically enhance the GFA of FePC-based BMGs, which in turn leads to the critical maximum diameter up to 2 mm for full glass formation even using low cost raw materials. The underlying mechanism of the enhancement of GFA from the competing crystalline phase with amorphous phase, the average thermal expansion coefficient and dynamic viscosity were discussed in detail.

Negative thermal expansion properties in tetragonal NbPO5 from the first principles studies

By using the first-principles calculations based on density functional theory combined with quasi-harmonic approximation, we have studied the geometric structural, thermal properties, and the negative thermal expansion (NTE) properties of tetrahedral NbPO5. The variations of cell parameter and cell volume of tetrahedral NbPO5 with temperature show that it displays NTE behavior in the range of 473-800 K along a-axis and the corresponding average coefficient of thermal expansion (CTE) is approximately -0.766 ×10−6 K−1, while the c cell parameter and the cell volume display positive thermal expansion behaviors. These results are in consistent well with the experiment observations. Further vibrational modes analysis, together with Grüneisen parameters calculations, revealed that the transverse vibration of O corner atoms accompanying the rocking motions of corner-shared NbO6 octahedron and PO4 tetrahedron dominate the negative thermal properties of tetrahedral NbPO5. Our findings will provide an understanding for the underlying mechanisms of the NTE in oxides materials.

ZrO2-NdO1.5 system: Investigations of phase relation and thermophysical properties

Exploring the structural and thermophysical behaviour of Nd-Substituted ZrO2 is vital in context of matrices for minor actinide transmutation. Hence, phase relations have been studied in this system using high temperature sintered samples prepared by gel combustion route. Phase characterization of Zr1−xNdxO2−x/2 (0.0≤x≤1.0) series was carried out using X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). With increasing NdO1.5 content, the system evolved in the sequence of (a) monoclinic zirconia (b) mixture of monoclinic+cubic stabilized zirconia, (c) cubic stabilized zirconia (d) mixture of stabilized zirconia+pyrochlore (e) pyrochlore (f) mixture of pyrochlore+hexagonal Nd2O3 and (g) pure hexagonal Nd2O3. Lattice and bulk thermal expansion of few selected compositions were characterized by high temperature XRD (298–1273K) and dilatometry (323–1673K). Average coefficient of thermal expansion decreased with increasing NdO1.5 fraction in Zr1−xNdxO2−x/2 series (x=0.2, 0.3, 0.4, 0.5) solid solutions, which has been attributed to increasing order in cation sub-lattice resulting in overall increase in lattice stability. Thermal conductivity has been reported for a representative composition Zr0.8Nd0.2O1.90.

Magnetocaloric effect and negative thermal expansion in hexagonal Fe doped MnNiGe compounds with a magnetoelastic AFM-FM-like transition

We report a detailed study of two successive first-order transitions, including a martensitic transition (MT) and an antiferromagnetic (AFM)-ferromagnetic (FM)-like transition, in Mn1-xFexNiGe (x = 0, 0.06, 0.11) alloys by X-ray diffraction, differential scanning calorimetry, magnetization and linear thermal expansion measurements. Such an AFM-FM-like transition occurring in the martensitic state has seldom been observed in the M(T) curves. The results of Arrott plot and linear relationship of the critical temperature with M2 provide explicit evidence of its first-order magnetoelastic nature. On the other hand, their performances as magnetocaloric and negative thermal expansion materials were characterized. The isothermal entropy change for a field change of 30 kOe reaches an impressive value of −25.8 J/kg K at 203 K for x = 0.11 compared to the other two samples. It demonstrates that the magneto-responsive ability has been significantly promoted since an appropriate amount of Fe doping can break the local Ni-6Mn AFM configuration. Moreover, the Fe-doped samples reveal both the giant negative thermal expansion and near-zero thermal expansion for different temperature ranges. For instance, the average thermal expansion coefficient ā of x = 0.06 reaches −60.7 × 10−6/K over T = 231–338 K and 0.6 × 10−6/K over T = 175–231 K during cooling.

Electrode properties of Cu-doped Bi0.5Sr0.5FeO3−δ cobalt-free perovskite as cathode for intermediate-temperature solid oxide fuel cells

A cobalt-free perovskite oxide Bi0.5Sr0.5Fe0.8Cu0.2O3−δ (BSFC) has been investigated as novel cathode for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The BSFC cathode exhibits good thermal stability and chemical compatibility with Ce0.9Gd0.1O1.95 (CGO) electrolyte below 950 °C. The average thermal expansion coefficient (TEC) of BSFC is 13.1 × 10−6 K−1 within a temperature range of 50–800 °C in air, which is close to that of CGO electrolyte. Meanwhile, the polarization resistant values (RP) of BSFC cathode on CGO electrolyte are 0.13, 0.32 and 0.76 Ω cm2 at 700, 650 and 600 °C, respectively; the lowest cathode overpotential is 37 mV at a current density of 125 mA cm−2 at 700 °C in air. The CGO electrolyte-supported single cell with BSFC cathode exhibits maximum power density of 290 mW cm−2 at 700 °C using dry H2 as the fuel. The rate-limiting step for oxygen reduction reaction (ORR) on BSFC cathode is oxygen surface adsorption-dissociation process. The attractive electrochemical performance demonstrates that BSFC oxide is a promising cathode material for IT-SOFCs.

Temperature dependent structural, vibrational and magnetic properties of K3Gd5(PO4)6.

Herein we report the evolution of the crystal structure of K3Gd5(PO4)6 in the temperature range from 20 K to 1073 K, as observed from variable temperature X-ray diffraction and Raman spectroscopic studies. K3Gd5(PO4)6 has an open tunnel containing a three dimensional structure built by [Gd5(PO4)6]3- ions which in turn are formed of PO4 tetrahedra and GdOn (n = 8 and 9) polyhedra. The empty tunnels in the structure are occupied by K+ ions and maintain charge neutrality in the lattice. Evolution of unit cell parameters with temperature shows a systematic increase with temperature. The average axial thermal expansion coefficients between 20 K and 1073 K are: αa = 10.6 × 10-6 K-1, αb = 5.5 × 10-6 K-1 and αc = 16.4 × 10-6 K-1. The evolution of distortion indices of the various coordination polyhedra with temperature indicates a gradual decrease with increasing temperature, while those of Gd2O9 and K2O8 polyhedra show opposite trends. The overall anisotropy of the lattice thermal expansion is found to be controlled largely by the effect of temperature on GdOn polyhedra and their linkages. Temperature dependent Raman spectroscopic studies indicated that the intensities and wavenumbers of most of the Raman modes decrease continuously with increasing temperature. Anharmonic analyses of Raman modes indicated that the lattice, rigid translation and librational modes have larger contributions towards thermal expansion of K3Gd5(PO4)6 compared to high frequency internal modes. The temperature and field dependent magnetic measurements indicated no long range ordering down to 2 K and the observed effective magnetic moment per Gd3+ ion and the Weiss constant are 7.91 μB and 0.38 K, respectively.