Polycrystalline Compounds Research Articles

The analysis of the magnetic properties in the intermetallic YxGd1−xNi3 compounds

In the paper an influence of Gd/Y substitution on the magnetic properties and exchange interactions of the YxGd1−xNi3 (x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0) polycrystalline compounds have been studied. The partial replacement of Gd by Y atoms is reflected in decreasing of the Curie temperature (TC) as well as decreasing of effective the magnetic moment (μeff). It has been shown that such a behaviour strongly depends on the magnetic interactions. Exchange coupling parameters of R–R (ARR), T–T (ATT) and R–T (ART) have been evaluated from M(T) magnetization curves (2–300K, 2T) based on the mean field theory (MFT) calculation. The magnetocaloric effect (MCE) has been estimated from the family of magnetic isotherms. The magnetic entropy indicates relatively small change with the Gd/Y substitution. The value of ΔSm(T,H) is higher for Gd-rich compounds and, respectively, decreases with Gd/Y substitution.

Synthesis, EPR and optical spectroscopy of the Cr-doped tetraborate glasses

Electron paramagnetic resonance (EPR), optical absorption and photoluminescence spectra as well as luminescence kinetics of the Li2B4O7:Cr and KLiB4O7:Cr tetraborate glasses were investigated at T=300K. The Li2B4O7:Cr and KLiB4O7:Cr glasses containing 0.4 and 1.6mol.% Cr2O3 of high optical quality were obtained from polycrystalline compounds by fast cooling of the melts. The X-band EPR spectroscopy shows that the Cr impurity is incorporated in the tetraborate glass network as isolated Cr3+ centers and Cr3+–Cr3+ pairs coupled by magnetic dipolar and exchange interactions. The EPR spectral parameters (geff and ΔBpp) of both Cr3+ and Cr3+–Cr3+ centers in the Li2B4O7:Cr and KLiB4O7:Cr glasses were measured and analyzed. All transitions in optical absorption, luminescence excitation and emission spectra of these glasses are identified. Broad complex bands that peak near 615, 405, and 350nm in optical absorption and luminescence excitation spectra correspond to the 4A2g(F)→4T2g(F), 4A2g(F)→4T1g(F), and 4A2g(F)→4T1g(P) spin-allowed transitions of the Cr3+ centers in distorted octahedral sites of the tetraborate glass network. The octahedral (cubic) crystal field strength (10Dq) and Racach parameters (B and C) for Cr3+ centers in Li2B4O7:Cr and KLiB4O7:Cr glasses are estimated. Narrow and broad emission bands in red – NIR regions are assigned to the 2Eg(F)→4A2g(F) (R1 line) and 4T2g(F)→4A2g(F) (electron-vibration) transitions, which correspond to the Cr3+ centers in high-field and low-field sites, respectively. All observed emission bands are characterized by non-exponential decay. Measured average lifetimes and local structure of the Cr3+ centers in high-field and low-field sites of the Li2B4O7:Cr and KLiB4O7:Cr glass network have been discussed.

Electron transport properties of some new 4-tert-butylcalix[4]arene derivatives in thin films

Temperature dependences of electric conductivity and thermoelectric power of some recently synthesized organic compounds, 4-tert-butylcalix[4]arene derivatives, are studied. Thin-film samples (d = 0.10–0.40 μm) spin-coated from chloroform solutions onto glass substrates were used. Organic films with reproducible electron transport properties can be obtained if, after deposition, they are submitted to a heat treatment within temperature range of 295–575 K.The studied polycrystalline compounds show typical p-type semiconductor behavior. The activation energy of the electric conduction ranges between 0.82 and 1.12 eV, while the ratio of charge carrier mobilities was found in the range of 0.83–0.94.Some correlations between semiconducting parameters and molecular structure of the organic compounds have been discussed.In the higher temperature ranges (T > 420 K), the electron transport in examined compounds can be interpreted in terms of the band gap representation model, while in the lower temperature range, the Mott's variable-range hopping conduction model was found to be appropriate.The investigated compounds hold promise for thermistor applications.

Confined and Chemically Flexible Grain Boundaries in Polycrystalline Compound Semiconductors

AbstractGrain boundaries (GBs) in polycrystalline Cu(In,Ga)Se2 thin films exhibit only slightly enhanced recombination, as compared with the grain interiors, allowing for very high power‐conversion efficiencies of more than 20% in the corresponding solar‐cell devices. This work highlights the specific compositional and electrical properties of Cu(In,Ga)Se2 GBs by application of appropriate subnanometer characterisation techniques: inline electron holography, electron energy‐loss spectroscopy, and atom‐probe tomography. It is found that changes of composition at the GBs are confined to regions of only about 1 nm in width. Therefore, these compositional changes are not due to secondary phases but atomic or ionic redistribution within the atomic planes close to the GBs. For different GBs in the Cu(In,Ga)Se2 thin film investigated, different atomic or ionic redistributions are also found. This chemical flexibility makes polycrystalline Cu(In,Ga)Se2 thin films particularly suitable for photovoltaic applications.

Amorphous and nanocrystalline silicon thin film photovoltaic technology on flexible substrates

This paper reviews our thin film silicon-based photovoltaic (PV) technology, including material and device studies as well as roll-to-roll manufacturing on a flexible substrate. Our current thin film silicon PV products are made with hydrogenated amorphous silicon (a-Si:H) and amorphous silicon germanium (a-SiGe:H) alloys. The advantages of a-Si:H-based technology are low cost, capability of large scale manufacturing, abundance of raw materials, and no environmental concerns. One disadvantage of a-Si:H PV technology is lower energy conversion efficiency than solar panels made of crystalline and polycrystalline silicon and compound crystal thin film semiconductors. Significant efforts have been made to improve efficiency. First, a-Si:H and a-SiGe:H material quality has been improved by optimizing deposition conditions, especially using high hydrogen dilution to deposit the amorphous materials close to the amorphous/nanocrystalline transition. Second, cell efficiency has been improved by engineering the device structure, such as bandgap profiling. In order to use the solar spectrum effectively, multijunction structures that incorporate a-SiGe:H in the middle and bottom cells have been used. The authors achieved record high solar cell efficiency with an a-Si:H/a-SiGe:H/a-SiGe:H triple-junction structure. Using the same structure, we fabricated solar laminates on flexible stainless steel coils with roll-to-roll production systems. Our current product has a stable aperture area efficiency of 8.2%. In recent years, hydrogenated nanocrystalline silicon (nc-Si:H) has emerged as a potential replacement for the a-SiGe:H bottom cell in multijunction structures. The authors have conducted a great deal of research and development of a-Si:H- and nc-Si:H-based multijunction PV technology. The authors have significantly improved the efficiency of a-Si:H and nc-Si:H multijunction solar cells and modules by optimizing the nc-Si:H material quality and device structure. The authors achieved an initial active-area (∼0.25 cm2) solar cell efficiency of 16.3% using an a-Si:H/a-SiGe:H/nc-Si:H triple-junction structure and an initial aperture-area module (∼400 cm2) efficiency of 12.0% and a stable aperture-area module (∼800 cm2) efficiency of 11.3% using an a-Si:H/nc-Si:H/nc-Si:H triple-junction structure. The authors expect to launch a new a-Si:H/nc-Si:H/nc-Si:H triple-junction product in near future with much higher efficiency than the current product.

D.c. electric conduction mechanism of some newly synthesized indolizine derivatives in thin films

Temperature-dependent d.c. electric conductivity of some recently synthesized indolizine derivatives is studied. Thin-film samples (d=0.10–0.30μm) spin-coated from chloroform solutions onto glass substrates were used. Organic films with reproducible electric properties can be obtained by submitting as-prepared samples to a heat treatment within temperature range of 301–493K.The studied polycrystalline compounds show typical n-type semiconductor behavior. The activation energy of the electric conduction ranges between 0.32 and 1.07eV.Some correlations between semiconducting characteristics and molecular structure of the organic compounds have been discussed.In the higher temperature range (T>415K), the electron transport in present compounds can be interpreted in terms of the band gap representation model, while in the lower temperature range (T<339K), the Mott’s variable-range hopping conduction model was found to be suitable.

Pulsed laser deposition and characterization of La1−xSrxMnO3

La1−xSrxMnO3 manganite films with x=0.15, 0.33 and 0.4 were deposited onto silicon substrates by pulsed laser deposition in an 80/20 Ar/O2 atmosphere at room temperature. After being deposited, the films were annealed at 900°C in air in order to obtain the desired crystalline phase. Structural characterizations using X-Ray diffraction showed polycrystalline compounds having the predominant peaks corresponding to the perovskite structure. Morphological studies carried out by scanning electron microscopy revealed a very rough surface and led to deducing the nature of the growth process. Spectral ellipsometry was performed between 1.5 and 5eV range at room temperature, showing electronic transitions near to 2.2 and 4.7eV in a range of thicknesses between 94 and 107nm.

Effect of strontium substitution on the physical properties of Nd0.5Ca0.5−xSrxMnO3(0.0≤x≤0.5) manganites

The structural and magnetic properties of Nd0.5Ca0.5−xSrxMnO3 (0.0≤x≤0.5) were investigated experimentally. All our samples were synthesized using the standard ceramic elaborating method at high temperatures. X-ray diffraction was systematically carried out on Nd0.5Ca0.5−xSrxMnO3 polycrystalline compounds. All our samples crystallized in the orthorhombic system. The unit cell volume increased with increasing strontium amount. The substitution of Ca by Sr destroyed the charge order state observed in the parent compound and induced a ferromagnetic phase at low temperatures. The Curie temperature Tc increased from 150 K for x = 0.1 to 260 K for x = 0.5.

Electrical conductivity of Ca1−xSrxTi0.65Fe0.35O3−δ, x = 0, 0.5 and 1, polycrystalline compounds in the 300–500 K range

Bulk and grain boundary electrical conductivity of oxygen deficient CaxSr1−xTi0.65Fe0.35O3−δ, x=0, 0.5 and 1.0, polycrystalline specimens were evaluated by impedance spectroscopy measurements in the 5Hz–13MHz frequency range from 300 to 500K. The ceramic powders were synthesized by solid state reaction and by a chemical route, the polymeric precursor technique. The X-ray diffraction of the samples at room temperature shows the following perovskite crystalline structures: cubic for x=0 and orthorhombic for x=0.5 and 1.0. The impedance plots are composed of two semicircles ascribed to grains (bulk) and interfaces (grain boundaries) contributions. The impedance data show that sintered pellets using powders prepared by solid state synthesis present higher intergranular and intragranular resistivity values than pellets prepared by the chemical route. Observations of scanning probe microscopy topographic images of the surfaces of the sintered pellets show evident differences between the grain morphology of the pellets prepared with powders synthesized by the two routes.

The influence of doping with Ca and Mg in YBa2Cu3O7-δceramic

We have investigated the effect of partial substitution of Ca for Y and/or Mg for Cu on structural, compositional and magnetic properties in γBa2Cu3O7-δ polycrystalline compounds. All prepared samples were found to be single phase with small fraction of Ba-secondary phases. Substitution by more than 2% of magnesium causes an increase of spurious phases. Energy Dispersive Spectroscopy (EDS) revealed that the distribution of Ca in the sample is quite homogenous. DC susceptibility measurements show that superconducting transition temperature Tc is reduced much more by Ca than Mg. Hysteresis loops reveal that magnetic irreversibility is decreased by Ca and Mg content. The deduced critical current density Jc does not follow the same variation. Ca alone reduces Jc for x=0.1 and x=0.2. Together with Ca, Mg compensates the reduction of Jc and increasing its content near the solubility limit gives higher Jc than in the undoped sample.

Defects in Cu(In,Ga)Se2 Chalcopyrite Semiconductors: A Comparative Study of Material Properties, Defect States, and Photovoltaic Performance

AbstractUnderstanding defects in Cu(In,Ga)(Se,S)2 (CIGS), especially correlating changes in the film formation process with differences in material properties, photovoltaic (PV) device performance, and defect levels extracted from admittance spectroscopy, is a critical but challenging undertaking due to the complex nature of this polycrystalline compound semiconductor. Here we present a systematic comparative study wherein varying defect density levels in CIGS films were intentionally induced by growing CIGS grains using different selenium activity levels. Material characterization results by techniques including X‐ray diffraction, scanning electron microscopy, transmission electron microscopy, secondary ion mass spectrometry, X‐ray photoelectron spectroscopy, and medium energy ion scattering indicate that this process variation, although not significantly affecting CIGS grain structure, crystal orientation, or bulk composition, leads to enhanced formation of a defective chalcopyrite layer with high density of indium or gallium at copper antisite defects ((In, Ga)Cu) near the CIGS surface, for CIGS films grown with insufficient selenium supply. This defective layer or the film growth conditions associated with it is further linked with observed current‐voltage characteristics, including rollover and crossover behavior, and a defect state at around 110 meV (generally denoted as the N1 defect) commonly observed in admittance spectroscopy. The impact of the (In, Ga)Cu defects on device PV performance is also established.

Mössbauer and calorimetric studies of spin reorientation processes in Er2−xPrxFe14C

The Er2−xPrxFe14C (x = 0.25, 0.5) poly crystalline compounds have been synthesized and investigated by means of X-ray diffraction, 57Fe Mössbauer spectroscopy and Differential Scanning Calorimetry (DSC). The influence of carbon on the spin reorientation phenomena with increasing praseodymium content was of special interest in these compounds. Investigations of spin reorientation phenomena were performed by means of 57Fe Mössbauer spectroscopy with a narrow step temperature scanning in the neighborhood of the spin reorientation temperature. Obtained Mössbauer spectra were analyzed using a procedure of simultaneous fitting and the transmission integral approach. Consistent description of Mossbauer spectra were obtained. Temperature dependencies of hyperfine interaction parameters and subspectra contributions were derived from fits and the transition temperatures were determined for all the compounds studied. DSC studies revealed weak thermal effect (heat absorption) during the transition process. Heat absorption was detected only for the compound with the lowest content of Pr (x = 0.25). The results obtained with different methods were analyzed and data were compared with those for Er2−xPrxFe14B series. The spin arrangement diagram was constructed.

Passive Amplification Effect in Thin Pyroelectric Films

Here we continue the description of the previously found passive amplification effect in thin pyroelectric films placed on a massive heat-conducting substrate. We show that in the definite modulation frequency range the amplification effect occurs i.e., a current generated by a thin pyroelectric film placed on substrate exceeds that in freely suspended film. The frequency dependence a pyroelectric current is illustrated by set of examples including achiral ferroelectric liquid crystals, ferroelectric polymers and polycrystalline compounds.

Fabrication of dye sensitized solar cell using Cr doped CuZnSe type chalcopyrite thin film

AbstractChalcopyrites are a versatile class of semiconductors known for their potential in photovoltaic applications. Considering the well established CuInSe2 as a prototype system, a new compound of the chalcopyrite type, Cu1–xZn1–ySe2–δ, by replacing In with Zn, has been prepared (both undoped and 2% Cr doped) by the metallurgical method. Thin films have been deposited by the thermal evaporation technique using the stabilized polycrystalline compounds as charge. Structural, compositional, morphological, and optical properties of the films are analyzed and reported. Use of these films as electrodes in dye sensitized solar cell (DSSC) is demonstrated.

μSR Investigation of magnetic phases in R 1− xSr xCoO 3 oxides (R=Pr, Nd)

We report results of our muon spin relaxation measurements in the series of polycrystalline compounds Pr 1− x Sr x CoO 3 ( x=0.3, 0.4 and 0.5) and Nd 1− x Sr x CoO 3 ( x=0.3 and 0.5). For the Pr-based samples our data clearly indicate the existence of two magnetic transitions, as also inferred from macroscopic measurements. While the high temperature transition is typical for cobaltites (∼200 K), the low temperature one is unusual. In our experiments it occurs below about 120 K and it manifests itself as a change in the slope of the temperature variation of the muon spin depolarization rate λ( T). For the Nd-based samples we found an increase of the muon spin depolarization rate below 45 K, temperature at which the sample is ferrimagnetic, when the Nd sublattice coupled antiparallel to the Co lattice. No phase separation could be evidenced in our samples by μSR experiments.

Synthesis and crystal structure characterisation of sodium neptunate compounds

The present work reports studies of the chemical reactions between neptunium dioxide and sodium oxide either in the presence of oxygen or inert gas (Ar), leading to compounds with hexavalent, heptavalent or pentavalent/tetravalent neptunium, respectively. Solid state synthesis with different NpO 2/Na 2O ratios led to the following polycrystalline compounds: Na 2Np 2O 7 monoclinic (P12 11), α-Na 2NpO 4 orthorhombic (Pbam), β-Na 2NpO 4 orthorhombic (Pbca), β-Na 4NpO 5 tetragonal (I4/mmm), Na 5NpO 6 monoclinic (C2/m) and a cubic compound (Fm-3m) that could either be Na 3NpO 4 or Na 4NpO 4. The crystal structures of the α-Na 2NpO 4 and Na 2Np 2O 7 compounds were refined by Rietveld analysis. Evolution of the cell parameters of α-Na 2NpO 4 was also followed as a function of temperature up to 1273 K by X-ray diffraction. The corresponding linear thermal expansion coefficients along the different axis were determined: α a = 41.3 × 10 −6 K −1, α b = 35.0 × 10 −6 K −1, α c ∼ 0 K −1. From the high temperature X-ray diffraction experiment it was also possible to evidence formation of diverse phases at different temperatures and to review parts of the Na–Np–O system.

Determination of the magnetic ground state of a polycrystalline compound based on susceptibility measurements

The diruthenium compound [Ru2(O2CMe)4]3[Cr(CN)6] contains two interpenetrating sublattices that behave like giant antiferromagnetically-coupled moments with strong anisotropy. The preferred orientations of the total moment of each sublattice are determined from susceptibility measurements on a polycrystalline sample. In agreement with previous mean-field calculations for the magnetic ground state, the fits to the experimental magnetization imply that the sublattice moments are restricted to cubic diagonals rather than the cubic axis or the edge diagonals. The parameterization of the sublattice susceptibility indicates that the sublattice spin states are more distorted when they are aligned antiparallel.

Large magnetic entropy change near room temperature in La0.7(Ca0.27Ag0.03)MnO3 perovskite

In this paper, the magnetic properties and magnetocaloric effect (MCE) of La0.7(Ca1−xAgx)0.3MnO3 (x=0, 0.1, 0.2, 0.7, and 1) powder samples are reported. Our polycrystalline compounds were synthesized using the solid state reaction method at high temperature. Magnetization measurements versus temperature showed that all our samples exhibited a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature, TC, has been found to increase from ∼250K for x=0–270K for x=1. Ag doping weakens the first order phase transition, and at higher Ag doping, the phase transition is of second order. For the La0.7(Ca0.27Ag0.03)MnO3 composition, the maxima of the magnetic entropy changes from the applied magnetic field (ΔSM) at 2 and 5T are about 4.5 and 7.75J/kgK, respectively, at the Curie temperature of ∼263K. The relative cooling power (RCP) values without hysteresis loss are about 102 and 271J/kg for the applied fields of 2 and 5T, respectively. Due to the large ΔSM, large RCP, and high Curie temperature, La0.7(Ca0.27Ag0.03)MnO3 is promising for application in potential magnetic refrigeration near room temperature.

Establishment of Structure-Conductivity Relationship for Tris(2,2′-bipyridine) Ruthenium Ionic C60 Salts

Three ionic C(60) salts with [Ru(bpy)(3)](m+) (bpy = 2,2'-bipyridine) as cations were synthesized. The UV-vis-NIR spectra, XPS spectra, and elemental analysis have demonstrated their compositions: [Ru(bpy)(3)](2)(C(60)) (1), [Ru(bpy)(3)](C(60)) (2), and [Ru(bpy)(3)](C(60))(2) (3). Single crystals of polycrystalline compounds 1 and 2 were obtained as solvates. At room temperature, all three salts are semiconducting with the highest four-probe conductivity observed for compound 1 at ∼10 S m(-1). The electronic conduction mechanisms can be described appropriately by an electron hopping model in this immobilized polyvalent redox system.

Properties of clinopyroxene LiFeGe2O6

The polycrystalline compound LiFeGe2O6 has been synthesized by the solid-phase reaction. The X-ray diffraction, Mossbauer, calorimetric, and magnetic investigations have been carried out. The Mossbauer spectrum at 300 K represents a single quadrupole doublet. The isomer shift with respect to the metal iron α-Fe is 0.40 mm/s, which is characteristic of the Fe3+ high-spin ion in the octahedral coordination. The quadrupole splitting of 0.42 mm/s indicates a distortion of the oxygen octahedron around the iron cation. The results of the measurement of the temperature dependence of the heat capacity in the range 2–300 K have shown the presence of the only anomaly with a maximum at T m ∼ 20.5 K, which indicates the occurrence of a magnetic phase transition in this point. The data of the measurement of the temperature dependence of the magnetization have confirmed that the magnetic order with the dominant antiferromagnetic interaction of magnetically active ions exists in LiFeGe2O6 at a temperature below 20.5 K. The investigation of the temperature dependence of the heat capacity in the magnetic field H up to 9 T has demonstrated that the external factor insignificantly changes the order-disorder transition point (at H = 9 T, there occurs a shift of ∼0.5 K toward the low-temperature range).