Strong magnetoelastic effect in Pr1−xCaxMnO3

Manganese oxides with distorted perovskite structure have attracted much attention during the last decade due to their colossal magnetoresistance (CMR), and the strong correlations among the various degrees of freedom involved. In particular, Pr_1−xCa_xMnO₃ compounds present in a wide Ca-doping range a charge ordering phenomenon, consisting of real space ordering of Mn³⁺ and Mn⁴⁺ in alternate lattice sites below a certain temperature T_CO. This ordering brings about a lattice distortion and a large hardening of the sound velocity below T_CO. Tomioka et al. have observed that an applied magnetic field can melt this charge ordered state and induce a transition from an insulating to a metallic state. In order to study the effects of this charge order melting, ultrasonic longitudinal sound velocity measurements were performed on polycrystalline Pr_1−xCa_xMnO₃ (x=0.35 and 0.5) as a function of magnetic field and temperature. Interesting anomalies were found related to the melting of the charge ordered phase into a metal-like state even at low temperatures.     

Synthesis and humidity sensitive properties of nanocrystalline Ba1−xSrxTiO3 thick films

Nanocrystalline Ba_1−xSr_xTiO₃ (x=0, 0.2, 0.4, 0.6, 0.8 and 1.0) precursors were synthesized using the stearic acid gel method. After the precursors had been calcined at 600–950°C for 0.5–1 h, nanocrystalline powders with the cubic perovskite structure were obtained and these were made into thick films. The powder samples were characterized by differential thermal analysis, X-ray diffraction and transmission electron microscopy, and the thick film samples were characterized by scanning electron microscopy and X-ray diffraction. The humidity-sensitive properties of the nanocrystalline Ba_1−xSr_xTiO₃ thick films were investigated. The results show that these nanocrystalline thick films possess higher humidity sensitivity and lower resistance than those of conventional materials.     

Magnetoimpedance effect at various temperatures for manganite La0.7Ca0.3MnO3−δ

In the present work, the AC magnetoimpedance effect in La_0.7Ca_0.3MnO_3−δ at various temperatures are investigated. The peak of the metal–insulator transition occurs in the temperature dependence of impedance. Negative magnetoimpedance effect in the La_0.7Ca_0.3MnO_3−δ is obtained at frequencies f≤10 MHz. In the magnetoimpedance effect of manganites, the magnetic field not only decreases the permeability μ_t, but also reduces the resistivity ρ by aligning the local spins and varying the transfer integral t_ij. The AC magnetoimpedance participated by the DC colossal magnetoresistance (CMR) in manganites, should be connected with the combined effects of double exchange interaction, electron–phonon coupling and skin effect.     

Dielectric and piezoelectric properties of sol–gel derived Ca doped PbTiO3

Synthesis of Ca doped PbTiO₃ powder by a chemically derived sol–gel process is described. Crystallization characteristics of different compositions Pb_1−xCa_xTiO₃ (PCT) with varying calcium (Ca) content in the range x = 0–0.45 has been investigated by DTA/TGA, X-ray diffraction and scanning electron microscopy. The crystallization temperature is found to decrease with increasing calcium content. X-ray diffraction reveals a tetragonal structure for PCT compositions with x ≤ 0.35, and a cubic structure for x = 0.45. Dielectric properties on sintered ceramics prepared with fine sol–gel derived powders have been measured. The dielectric constant is found to increase with increasing Ca content, and the dielectric loss decreases continuously. Sol–gel derived Pb_1−xCa_xTiO₃ ceramics with x = 0.45 after poling exhibit infinite electromechanical anisotropy (k_t/k_p) with a high d₃₃ = 80 pC/N, ′ = 298 and low dielectric loss (tan δ = 0.0041).     

Electrical transport and magnetic properties in La0.67Ca0.33MnO3 and CuFe2O4 composites

The (1 − x) La_0.67Ca_0.33MnO₃/xCuFe₂O₄ (x = 0, 0.04, 0.07, 0.10 and 0.15) composites have been prepared and investigated systematically for their microstructure, electrical and magnetic properties. The X-ray diffraction and scanning electronic microscopy analysis reveal that both La_0.67Ca_0.33MnO₃ (LCMO) and CuFe₂O₄ phases are distributed in the composites. Compared with pure LCMO, an obvious enhancement of magnetoresistance is observed over a wide temperature range for the composites. Under 3 T field, the magnetoresistance rises from a base value 44.5% for pure LCMO, to a maximum value of 66.7% for x = 0.1 composite. Meanwhile, a weak temperature-dependent MR platform is observed in the temperature range from 210 K to 240 K. The MR enhancement is discussed in terms of spin-polarized intergrain tunneling due to enhanced magnetic disorder and magnetic coupling near boundaries between LCMO and CuFe₂O₄ grains.     

Effect of B-doping on the structural, magnetotransport and magnetocaloric properties of La0.67Ca0.33MnO3 compounds

In this study, the effect of Mn site substitution of B on the structural, electrical and magnetocaloric properties of manganites was investigated. Polycrystalline manganites with the chemical composition La_0.67Ca_0.33Mn_1−xB_xO₃ (x = 0, 0.1, 0.2 and 0.3) were prepared by the standard solid-state process. It was found that the magnetisation, the Curie temperature and the maximum value of the magnetic entropy change |ΔS_m| decrease with increasing concentration of B.     

La(1−x)SrxCoO3 for thin film thermocouple applications

In this study, the LSCO (lanthanum strontium cobalt oxide) family has been investigated for thin film thermocouple applications. Thin films of La_(1−x)Sr_xCoO₃ (x=0.3,0.5,0.7) were prepared on sapphire substrates by pulsed laser deposition. The films were annealed at different temperatures in air and characterized for phase, composition and microstructure to determine their thermal stability. From the phase and composition analyses, it is clear that as the Sr content in LSCO increases, the thermal stability decreases. Among the three compositions studied, x=0.3 had the best phase and chemical stability, and microstructural properties. It was observed that La_0.7Sr_0.3CoO₃ possesses excellent phase, composition and microstructural stability up to 1273 K. Above 1273 K, however, LSCO decomposes resulting in the loss of cobalt and formation of individual oxide phases. Electrical resistivity and Seebeck coefficients were measured in situ as a function of temperature in air up to 1023 K. The electrical and Seebeck coefficient properties were found to be stable for all the three compositions up to 1023 K and studies indicated that electrical conduction occurs through a small polaron hopping mechanism. In conclusion, LSCO possessed good thermal stability in air up to 1273 K and exhibits excellent potential in thin film thermocouple applications.     

Chemical vapour deposition of molybdenum carbides: aspects of phase stability

Thin films of different molybdenum carbides (δ-MoC_1−x, γ′-MoC_1−x and Mo₂C) have been deposited from a gas mixture of MoCl₅/H₂/C₂H₄ at 800°C by CVD. The H₂ content in the vapour has a strong influence on the phase composition and microstructure. Typically, high H₂ contents lead to the formation of nanocrystalline δ-MoC_1−x films while coarse-grained γ′-MoC_1−x is formed with an H₂-free gas mixture. This phase has previously only been synthesized by carburization of Mo in a CO atmosphere and it has therefore been considered as an oxycarbide phase stabilized by the presence of oxygen in the lattice. Our results, however, show that γ′-MoC_1−x films containing only trace amounts of oxygen can be deposited by CVD. Stability calculations using a FP-LMTO method confirmed that the γ′-MoC_1−x phase is stabilized by oxygen but that the difference in energy between e.g. δ-MoC_0.75 and oxygen-free γ′-MoC_0.75 is small enough to allow the synthesis of the latter phase in the absence of kinetic constraints. Annealing experiments of metastable δ-MoC_1−x and γ′-MoC_1−x films showed two different reaction products suggesting that kinetic effects play an important role in the decomposition of these phases.     

Sintering behavior and microwave dielectric properties of Bi3(Nb1−xTax)O7 solid solutions

Solid solutions of Bi₃(Nb_1−xTa_x)O₇ (x = 0.0, 0.3, 0.7, 1) were synthesized using solid state reaction method and their microwave dielectric properties were first reported. Pure phase of fluorite-type could be obtained after calcined at 700 °C (2 h)⁻¹ between 0 ≤ x ≤ 1 and Bi₃(Nb_1−xTa_x)O₇ ceramics could be well densified below 990 °C. As x increased from 0.0 to 1.0, saturated density of Bi₃(Nb_1−xTa_x)O₇ ceramics increased from 8.2 to 9.1 g cm⁻³, microwave permittivity decreased from 95 to 65 while Q_f values increasing from 230 to 560 GHz. Substitution of Ta for Nb modified temperature coefficient of resonant frequency τ_f from −113 ppm °C⁻¹ of Bi₃NbO₇ to −70 ppm °C⁻¹ of Bi₃TaO₇. Microwave permittivity, Q_f values and τ_f values were found to correlate strongly with the structure parameters of fluorite solid solutions and the correlation between them was discussed in detail. Considering the low densified temperature and good microwave dielectric proprieties, solid solutions of Bi₃(Nb_1−xTa_x)O₇ ceramics could be a good candidate for low temperature co-fired ceramics application.     

Electronic structure of the ladder-chain compound Sr14−xCaxCu24O41

The electronic structure of the ladder-chain compound Sr_14−xCa_xCu₂₄O₄₁ is studied by ab initio calculations within the local density approximation. The effects of Ca substitution and structure modulation on electronic structure are discussed. It is found that 0.05 holes per copper atom are on the ladder layers for fully substituted compound, Ca₁₄Cu₂₄O₄₁.     

Highly conductive microcrystalline silicon carbide films deposited by the hot wire cell method and its application to amorphous silicon solar cells

Microcrystalline silicon carbide (μc-Si_1−xC_x) films were successfully deposited by the hot wire cell method using a gas mixture of SiH₄, H₂ and C₂H₂. It was confirmed by Fourier transform infrared and X-ray diffraction analyses that the films consisted of μc-Si grains embedded in a-Si_1−xC_x tissue. The p-type μc-Si_1−xC_x films were deposited using B₂H₆ as a doping gas. A dark conductivity of 0.2 S/cm and an activation energy of 0.067 eV were obtained. The p-type μc-Si_1−xC_x was used as a window layer of a-Si solar cells, in which the intrinsic layer was deposited by photo-chemical vapor deposition, and an initial conversion efficiency of 10.2% was obtained.     

Physical properties of (La,Sr)Ti(O,N)3 thin films grown by pulsed laser deposition

Thin films of La_xSr_1−xTiO_3+x/2 (x = 0, 0.25, 0.5, 0.75, 1) were grown by laser ablation on two different kinds of substrates (SrTiO₃ (STO) and MgO) and were subsequently ammonolysed to yield the corresponding oxynitrides La_xSr_1−xTi(O,N)₃. For both substrates all films were found to grow epitaxially to the (1 0 0) direction of the cubic perovskite structure, except for x = 0.5 that grew parallel to the (1 1 0) direction. For some of the films TiN was detected as impurity phase. Scanning electron microscopy revealed that the films are dense and homogeneous with thicknesses around 350 nm. Atomic force microscopy showed that the surface roughness of the films varied between 4.2 and 14.1 nm. The employed substrate had a strong influence on the electrical properties. Films grown on STO exhibited a metallic behaviour, in contrast to the films grown on MgO, which were insulating.     

Characterization and microstructure of highly preferred oriented lead barium titanate thin films on MgO (100) by sol-gel process

Highly preferred oriented lead barium titanate (Pb_1−x,Ba_x)TiO₃ thin film, with particular emphasis on (Pb_0.5,Ba_0.5)TiO₃, can be obtained by spin-coating on MgO (100) substrate by using the precursor sol, which was synthesized from acetylacetone chelating with titanium isopropoxide and ethylene glycol as a solvent, in the sol-gel process. Film thickness, pyrolysis temperature and heating rate were studied systemically to investigate their influences on the formation of preferred oriented thin films. The highly preferred (001)/(100) oriented thin film could be obtained by the pyrolysis of wet film at 500 °C and annealing at 600 °C at a slow heating rate of 5 °C/min. It is confirmed that the tetragonal perovskite structure of the titanate ceramic decreases with an increase of Ba content in (Pb_1−x,Ba_x)TiO₃. The (001)/(100) oriented films were synthesized from all compositions between x = 0.2 and x = 0.8, at a crystallization temperature of 600 °C. In particular, for the Ba content in the range of x = 0.50.6, highly preferred (001)/(100) planes were observed.     

Growth of lithium doped silver niobate single crystals and their piezoelectric properties

Lithium doped silver niobate (Ag_1−xLi_xNbO₃, 0 < x < 0.1) is one of the candidate materials for lead-free piezoelectric materials. In this study, Ag_1−xLi_xNbO₃ single crystals were successfully grown by a slow cooling method. Crystal structure was assigned to perovskite-type orthorhombic (monoclinic) phase. Dielectric properties were measured as a function of temperature. As a result, with increasing lithium contents, the phase transition at around 60 °C was shifted to lower temperature while the phase transition at around 400 °C was shifted to higher temperature. On the basis of these peak shifts, the lithium contents in Ag_1−xLi_xNbO₃ single crystals were determined. Moreover, P–E hysteresis measurement revealed that pure silver niobate crystal was weak ferroelectrics with P_r of 0.095 μC/cm² while Ag_0.9Li_0.1NbO₃ (ALN10) crystal was normal ferroelectrics with P_r of 10.68 μC/cm². About this ALN10 crystal, polling treatment was performed and finally piezoelectric properties were measured. As a result, high electromechanical coupling coefficient k₃₁ over 70% was observed.     

Pyroelectric properties of BST/PLT composite thick films with addition of xPbO–(1 − x)B2O3 glass

The Ba_xSr_1−xTiO₃ (BST)/Pb_1−xLa_xTiO₃ (PLT) composite thick films (20 μm) with 12 mol% amount of xPbO–(1 − x)B₂O₃ glass additives (x = 0.2, 0.35, 0.5, 0.65 and 0.8) have been prepared by screen-printing the paste onto the alumina substrates with silver bottom electrode. X-ray diffraction (XRD), scanning electron microscope (SEM) and an impedance analyzer and an electrometer were used to analyze the phase structures, morphologies and dielectric and pyroelectric properties of the composite thick films, respectively. The wetting and infiltration of the liquid phase on the particles results in the densification of the composite thick films sintered at 750 °C. Nice porous structure formed in the composite thick films with xPbO–(1 − x)B₂O₃ glass as the PbO content (x) is 0.5 ≥ x ≥ 0.35, while dense structure formed in these thick films as the PbO content (x) is 0.8 ≥ x ≥ 0.65. The volatilization of the PbO in PLT and the interdiffusion between the PLT and the glass lead to the reduction of the c-axis of the PLT phase. The operating temperature range of our composite thick films is 0–200 °C. At room temperature (20 °C), the BST/PLT composite thick films with 0.35PbO–0.65B₂O₃ glass additives provided low heat capacity and good pyroelectric figure-of-merit because of their porous structure. The pyroelectric coefficient and figure-of-merit F_D are 364 μC/(m² K) and 14.3 μPa^−1/2, respectively. These good pyroelectric properties as well as being able to produce low-cost devices make this kind of thick films a promising candidate for high-performance pyroelectric applications.     

Preparation of B-doped a-Si1−xCx:H films and heterojunction p–i–n solar cells by the Cat-CVD method

B-doped a-Si_1−xC_x:H films for a window layer of Si thin film solar cells have been prepared by the Cat-CVD method. It is found that C is effectively incorporated into the films by using C₂H₂ as a C source gas, where an only little C incorporation is observed from CH₄ and C₂H₆ under similar deposition conditions. Using a-Si_1−xC_x:H films grown from C₂H₂, heterojunction p–i–n solar cells have been prepared by the Cat-CVD method. The cell structure is (SnO₂ Asahi-U)/ZnO/a-Si_1−xC_x:H(p)/a-Si:H(i)/μc-Si:H(n)/Al. The obtained conversion efficiency was 5.4%.     

Co掺杂钙钛矿锰氧化物La0.8Sr0.2MnO3电磁特性研究

采用传统的高温固相反应法制备了La_0.8Sr_0.2Mn_1-xCo_xO₃(x = 0, 0.1, 0.3)多晶样品。系统研究了Co掺杂量对La_0.8Sr_0.2MnO₃(LSMO)多晶样品的类Griffiths相、磁熵变、临界行为和电输运性质的影响。研究结果表明: 制备的多晶样品均具有菱形对称结构; 三样品在低温磁转变温度(T_C2)以上均存在类Griffiths相; La_0.8Sr_0.2Mn_1-xCo_xO₃(x = 0, 0.1, 0.3)样品外加磁场为7 T的最大磁熵变ΔS_max分别为-2.28、-2.05和-2.75 J/(kg·K), Co元素的掺杂使得ΔS_max先减小后增大; 母相的临界行为与平均场模型拟合得最好, 掺杂后样品的临界行为和3D海森伯模型拟合最好; 母相为半导体材料, Co元素掺杂量达到0.1时在低温磁转变温度(T_C2)附近出现金属绝缘体转变; 高温区三样品的导电方式均满足小极化子模型。     

Optical and electrical properties of p-type AgInSnxS2−x (x = 0–0.04) thin films prepared by spray pyrolysis

AgInSn_xS_2−x (x = 0–0.2) polycrystalline thin films were prepared by the spray pyrolysis technique. The samples were deposited on glass substrates at temperatures of 375 and 400 °C from alcoholic solutions comprising silver acetate, indium chloride, thiourea and tin chloride. All deposited films crystallized in the chalcopyrite structure of AgInS₂. A p-type conductivity was detected in the Sn-doped samples deposited at 375 °C, otherwise they are n-type. The optical properties of AgInSn_xS_2−x (x < 0.2) resemble those of chalcopyrite AgInS₂. Low-temperature PL measurements revealed that Sn occupying an S-site could be the responsible defect for the p-type conductivity observed in AgInSn_xS_2−x (x < 2) thin films.     

Tungsten–carbon films prepared by reactive sputtering from argon–benzene discharges

Tungsten–carbon thin films have been deposited by reactive (Ar+C₆H₆) DC magnetron sputtering onto various substrates. Deposition onto glass, monocrystalline silicon, tantalum and stainless steel at room temperature yielded W–C films, having XRD patterns corresponding to the structure of heavily disordered W₂C or WC_1−x carbides. The samples deposited upon the Au or Cu foils were nanocrystalline cubic WC_1−x with the grain size of 2.9 nm. Disordered tungsten–carbon films were stable up to 1200°C. Microhardness of the films with disordered W₂C phase was about 5–6 GPa while that of the films with disordered WC_1−x phase was about 17 GPa. The characteristics of films can be understood considering the effects of the incorporation of free carbon and/or carbon–hydrogen fragments into the tungsten carbide layer.     

Supported La1--xCexMnO3 perovskite catalysts: preparation,characterization and catalytic performance in methane combustion

A serials of La_1--xCe_xMnO₃ (x = 0, 0.2, 0.4, 0.6 and 0.8) catalysts have been prepared via a sol--gel process using citric acid as a chelating agent. The phase composition and morphology of the catalysts are characterized by XRD, SEM and TEM. La_1--xCe_xMnO₃/γ-Al₂O₃/cordierite catalyst has also been prepared by immersion process using cordierite honeycomb ceramic as carrier and γ-Al₂O₃ as support agents. The effect of Ce-doping on the structure of perovskite and catalytic property of methane combustion is explored. The results show that with the increase of Ce substitution, the crystal structure of perovskite changes from the tetragonal system to the orthorhombic system and when the x is equal to 0.2, the catalytic property in methane combustion is the best.