Influence of thermal treatments on the elastic parameters in iron rich metallic glasses

The change of magnetoelastic properties after thermal treatments has been investigated for two groups of metallic glasses. (Fe₇₉Co₂₁)_75+xSi_15−1.4xB_10+0.4x (x (at.%)=0, 2, 4, 6, 8, 10) has been studied both in the as-prepared state and after thermal annealing in an applied magnetic field, to achieve a particular domain structure, at temperatures well below the crystallization temperatures. Changes in the ΔE effect, magnetomechanical coupling (k) and internal friction coefficient (Q⁻¹) are reported, reaching values of about 60% of the saturation value E_S. Fe₆₄Ni₁₀Nb₃Cu₁Si₁₃B₉ alloys annealed in vacuum for 1 h in the temperature range 350–550 °C showed maximum values of the ΔE effect and k of 61% and 0.85, respectively, accompanied by a minimum value of Q of around 2 for the sample annealed at 460 °C. These variations are related to the progress of nanocrystalization. The properties achieved are among the best reported for magnetomechanical applications.     

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.     

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.     

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.     

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.     

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.     

Optical properties of a-(Se70Te30)100−x(Se98 Bi2)x thin films

Measurements of optical constants (absorption coefficient, refractive index, extinction coefficient, real and imaginary part of the dielectric constant) have been made on a-(Se₇₀Te₃₀)_100−x (Se₉₈Bi₂)_x thin films (where x=0, 5, 10, 15 and 20) of thickness 2000 Å in the wavelength range 450–1000 nm. It is found that the optical bandgap decreases with the increase of Se₉₈Bi₂ concentration in the a-(Se₇₀Te₃₀)_100−x(Se₉₈Bi₂)_x system. The value of refractive index (n) decreases, while the extinction coefficient (k) increases with increasing photon energy. The results are interpreted in terms of concentration of localized states varying effective Fermi level.     

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).     

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.     

Optical transitions near the band edge in bulk CuInxGa1−xSe2 from ellipsometric measurements

From the analysis of the variation of optical absorption coefficient with incident photon energy between 0.8 and 2.6 eV, obtained from ellipsometric data, the energy E_G of the fundamental absorption edge and E_G′ of the forbidden direct transition for CuIn_xGa_1−xSe₂ alloys are estimated. The change in E_G and the spin-orbit splitting Δ_SO=E_G′−E_G with the composition x can be represented by parabolic expression of the form E_G(x)=E_G(0)+ax+bx² and Δ_SO(x)=Δ_SO(0)+a′x+b′x², respectively. b and b′ are called “bowing parameters”. Theoretical fit gives a=0.875 eV, b=0.198 eV, a′=0.341 eV and b′=−0.431 eV. The positive sign of b and negative sign of b′ are in agreement with the theoretical prediction of Wei and Zunger [Phys. Rev. B 39 (1989) 6279].     

In situ co-precipitation synthesis and photoluminescence of YxGd1−xVO4:Tm microcrystalline phosphors by hybrid precursors

Y_xGd_1−xVO₄:Tm³⁺ (5 mol%) phosphors were prepared by in situ co-precipitation technology with the different content ratio of Y/Gd (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, respectively). During the process, rare earth coordination polymers with o-hydroxylbenzoate were used as precursors, composing with polyethylene glycol (PEG) as dispersing media. After heat-treatment of the resulting multicomponent hybrid precursors at 900 °C, the samples were obtained. SEM indicated the particles present good crystalline state, whose crystalline grain sizes were about 0.2–2 μm. Under the excitation of 257 nm, all the materials show the characteristic emission of Tm³⁺ which is the strong blue emission centered at 475 nm originating from ¹G₄ → ³H₆ of Tm³⁺. Besides this, concentration quenching appears in the system of YVO₄:Tm³⁺ and GdVO₄:Tm³⁺. And when x reaches 0.5, the system of Y_xGd_1−xVO₄:Tm³⁺ shows the strongest blue emission.     

Spectroscopic ellipsometry analysis of GaAs1 − xNx layers grown by molecular beam epitaxy

In this work, we present the effect of nitrogen incorporation on the dielectric function of GaAsN samples, grown by molecular beam epitaxy (MBE) followed by a rapid thermal annealing (for 90 s at 680 °C). The GaAs_1 − xN_x samples with N content up to 1.5% (x = 0.0%, 0.1%, 0.5%, 1.5%), are investigated using room temperature spectroscopic ellipsometry (SE). The optical transitions in the spectral region around 3 eV are analyzed by fitting analytical critical point line shapes to the second derivative of the dielectric function. It was found that the features associated with E₁ and E₁ + Δ₁ transitions are blue-shifted and become less sharp with increasing nitrogen incorporation, in contrast to the case of E₀ transition energy in GaAs_1 − xN_x. An increase of the split-off Δ₁ energy with nitrogen content was also obtained, in agreement to results found with MOVPE GaAs_1 − xN_x grown samples.     

Mechanical characterization of Ni1−xZnxFe2O4 prepared by non-conventional methods

The mechanical properties like hardness, H_v and compressive strength, σ of Ni_1−xZn_xFe₂O₄ (x = 0.2, 0.3, 0.4 and 0.5) prepared by the non-conventional flash combustion and citrate-gel decomposition techniques are studied and reported. It is observed that there is an increase in hardness with zinc content as well as sintering temperature. The hardness in the order of 2.0–3.63 GPa and compressive strength in the order of 150–240 MPa are obtained for Ni–Zn ferrites prepared by these non-conventional techniques. The influence of density, porosity and microstructure on hardness and compressive strength of Ni–Zn ferrites with respect to sintering temperature was studied.     

Magnetic properties of Fe---SiO films

The results of an experimental investigation of the magnetic properties of Fe_100−x(SiO)_x cermet thin films (where x is the volume percentage of SiO in the film) in the concentration range 10 vol.% x 93 vol.% are given. The films were synthesized using the vacuum deposition method. Films with a sufficiently large SiO content (x 25–40 vol.%) are amorphous and display a number of new properties. A T−x phase diagram of the ferromagnet-superparamagnet transition was obtained. A transition to the cluster glass state was found at low temperatures and fields. The correlation radius of the exchange parameter fluctuations was determined from measurements of the spin wave resonance at a number of concentrations.     

Ag glasses based on Zn-phosphate

The partial substitution of Zn²⁺ for Ag⁺ in Ag₄P₂O₇ leads to the formation of a wide glassy domain of composition [Ag₄P₂O₇] _(1−y) [Zn₂P₂O₇] _(y) with 0.20y0.87. The introduction of AgI in these materials results in a new series of glasses of formula [(Ag₄P₂O₇)_(1−y) (Zn₂P₂O₇)_(y)] _(1−X) [AgI] _(x), which domain for the composition y = 0.25 corresponds to 0x 0.64. The structure as well as the thermal and electrical properties of these materials are compared with those of the [AgPO₃] _(1−X) [AgI] _(x) and [Ag₄P₂O₇] _(1−x) [AgI] _(x) glasses.     

Quantum chemical modelling of point defects in KNbO3 perovskite crystals

We present results of semi-empirical quantum chemical calculations for several perovskite KNb_xTa_1−xO₃ (KTN) solid solutions, as well as point intrinsic defects – F centers and hole polarons bound to K vacancy – in KNbO₃. Method of the intermediate neglect of the differential overlap (INDO) was combined with typically 320-atom supercells and atomic geometry optimization. Analysis of the optimized atomic and electronic structure has clearly demonstrated that several nearest Nb atoms substituting for Ta in KTaO₃ – unlike Ta impurities in KNbO₃ – reveal the self-ordering effect, which probably triggers the ferroelectricity observed in KTN. We predict co-existence of one-site (atomic) and two-site (molecular) polarons with close absorption energies (≈1 eV). When available, the INDO results are compared with ab initio calculations. The relevant experimental data are discussed.     

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.     

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.     

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%.     

Determination of the E1 and E1+Δ1 energy bands by photoreflectance in AlxGa1−xAs in the region 0<x<0.55

The E₁ and E₁+Δ₁ energy bands of metal–organic chemical vapor deposition grown Al_xGa_1−xAs, with x in the range 0–0.55, have been determined using photoreflectance technique. The aluminum composition for each sample was determined using the energy of the room-temperature photoluminescence compensated peak value and a suitable fundamental band gap formula. The positions of the E₁ and E₁+Δ₁ peaks were determined from curve-fitting an appropriate theoretical model to our experimental data by a modified downhill simplex method. Using the results, we propose new E₁ and E₁+Δ₁ cubic expressions as functions of the aluminum composition, x, and compare them with the available reported expressions.