Optical and photoelectric properties of Cd<Subscript>1 - <Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>x</Subscript>As<Subscript>2</Subscript> single crystals

The optical absorption, photoconductivity, and short-circuit photocurrent spectra of structurally perfect Cd_{1 - x} Zn_xAs₂ (x = 0.03, 0.05, 0.06) single crystals are studied for the first time near the intrinsic edge in the range 80–300 K. The results demonstrate that the intrinsic edge in the solid solutions is due to indirect transitions involving the formation of excitons for both the E c and E c polarizations. The indirect gaps _g ⁱ of the solid solutions are determined. In the range 80–300 K, the data for x = 0–0.06 and both polarizations are well fitted by _g ⁱ (x) = _g ⁱ (0) + 0.0866x + 2.35x ². The introduction of 6 mol % ZnAs₂ into CdAs₂ increases its _g ⁱ by 14 meV.Translated from Neorganicheskie Materialy, Vol. 41, No. 3, 2005, pp. 268–272.Original Russian Text Copyright © 2005 by Morozova, Marenkin, Mikhailov, Koshelev.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Crystal structure of Gd<Subscript>14</Subscript>Cu<Subscript>48</Subscript>Ga<Subscript>3</Subscript> and Tb<Subscript>14</Subscript>Cu<Subscript>48</Subscript>Ga<Subscript>3</Subscript>

The compounds Gd₁₄Cu₄₈Ga₃ and Tb₁₄Cu₄₈Ga₃ have been synthesized, and their structures have been determined by powder x-ray diffraction (Gd₁₄Ag₅₁ type).     

Multivalent substitution in a quasi-binary Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>(II-Mn-IV)<Subscript>x</Subscript>As solid solution

A process of the multivalent substitution of Mn atoms for cations in Ga_1−x Mn_xAs (a base material) is proposed. According to this approach, the two-cation ternary compound with a chalcopyrite structure is used as a component forming a solid solution with GaAs. Based on the successful prediction and growth of high-temperature chalcopyrite ferromagnets, it is expected that the content of Mn in Ga_1−x Mn_xAs can be increased. Quasi-binary (GaAs)_1−x (ZnGeAs₂)_x:Mn and (GaAs_1−x (CdSiAs₂)_x:Mn solid solutions are considered as candidate materials possessing higher Curie temperatures as compared to that of Ga_1−x MnxAs.     

Electromagnetic wave absorption properties in the centimetre-band of Ti<Subscript>3</Subscript>C<Subscript>2</Subscript>T<Subscript>x</Subscript> MXenes with diverse etching time

In this work, Ti₃C₂T_x MXenes were fabricated by using HF solution for diverse etching time and then investigated as electromagnetic (EM) wave absorbing materials in the centimetre-band (2–18 GHz). Besides, the impact of etching time on the structure, morphology, surface termination and dielectric properties was explored. With growing etching time, the crystal structure and surface morphology of Ti₃C₂T_x MXenes were gradually destroyed and more C was exposed. Among the etching samples, Ti₃C₂T_x-24 exhibited optimal RL_min of ??42.5 dB and broad tunable absorption bandwidth of 13.8 GHz. The extraordinary absorption could be attributed to multiple reflections and scatterings and interfacial polarizations. More importantly, comprehensive comparisons were made among two-dimensional Ti₃C₂T_x MXenes, typical two-dimensional carbon-related and MoS₂ materials. The thorough explorations supplement and improve the study of dielectric properties and EM wave absorption performance of Ti₃C₂T_x MXenes in centimetre-band, thus broadband Ti₃C₂T_x based hybrids absorbers can be constructed reasonably and effectively.     

Synthesis,microstructure and microwave dielectric properties of Ca<Subscript>4-x</Subscript>Mg<Subscript>x</Subscript>La<Subscript>2</Subscript>Ti<Subscript>5</Subscript>O<Subscript>17</Subscript> ceramics

Ca_4-xMg_xLa₂Ti₅O₁₇ ceramics were prepared by a solid state ceramic route for x = 0, 0.5, 1, 2, 3 and 4. The structure and microstructure of the ceramics were investigated using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. X-ray diffraction results show that the Ca_4-x Mg _x La₂Ti₅O₁₇ adopts an orthorhombic crystal structure with no secondary phase observed for x from 0 to 0.5. Secondary phase, MgTiO₃ occurs with further increasing doping level (1 ≤ x ≤ 3). When x = 4, mixture phases La_0.66TiO_2.993, MgTiO₃ and a trace of unknown phase coexist. Ca₄La₂Ti₅O₁₇ ceramic exhibits a relative permittivity (ε_r) ~ 65, quality factor (Q × f) ~13,338 GHz (at ~4.75 GHz), and temperature coefficient of resonant frequency (τ _f ) ~ 165 ppm/°C. The sintering temperature was distinctly reduced from 1,580 °C for x = 0 to 1,350 °C for x = 4. With increasing Mg content, ε_r and τ_f obviously decrease, while Q × f value initially decreases and then increases. The ceramic for x = 2 shows ε_r ~ 50, Q × f ~ 9,451 and τ _f  ~ 62.5 ppm/°C. By the complete replacement of Ca with Mg, Mg₄La₂Ti₅O₁₇ ceramic sintered at 1,350 °C for 4 h combines a high dielectric permittivity (ε _r  = 31), high quality factor (Q × f ~ 15,021) and near-zero temperature coefficient of resonant frequency (τ _f  ~ 4.0 ppm/°C). The materials are suitable for microwave applications.     

Kinetics of Ge<Subscript>20</Subscript>Se<Subscript>80-x</Subscript>As<Subscript>x</Subscript> (<Emphasis Type="Italic">x</Emphasis> = 0, 5, 10, 15 and 20) in glass transition region

The results of differential scanning calorimetric (DSC) measurements on Ge₂₀Se_{80- x}As_x (x = 0, 5, 10, 15 and 20) system with the specific aim of investigating the effect of heating rate and composition on glass transition temperature have been discussed. The results indicate that the glass transition temperature (T_g) is dependent both on the heating rate and composition. The glass transition activation energy (E_t) and heat absorbed in glass transition region (ΔH) are higher for Ge₂₀Se₆₅As₁₅ as compared to the values of other compositions of arsenic. An effort has also been made to develop an empirical model for the composition dependence of ΔH. A good agreement has been observed between the experimental values and the results of model calculation.     

Optically addressed As<Subscript>50</Subscript>Se<Subscript>50</Subscript>-liquid crystal structure highly sensitive to He-Ne laser radiation

An optically addressed structure of the As _x Se_1-x -liquid crystal (LC) type, with the photoconductor composition deviating from stoichiometry toward excess arsenic, has been tested using holographic techniques. It is established that the As₅₀Se₅₀-LC structure (with maximum possible arsenic content in the photoconductor) exhibits record high sensitivity (2.2 × 10^?7 W/cm²) at a He-Ne laser radiation wavelength. Based on this structure, it is possible to implement nonlinear optical data-processing algorithms employing the entire transmission characteristic including the inversion region. The maximum diffraction efficiency achieved with the As₅₀Se₅₀-LC structure amounts to 36.5%.     

High-performance flexible supercapacitors based on C/Na<Subscript>2</Subscript>Ti<Subscript>5</Subscript>O<Subscript>11</Subscript> nanocomposite electrode materials

A new solution method to synthesize Na₂Ti₅O₁₁ with titanium powder is presented, and the C/Na₂Ti₅O₁₁ nanocomposite with high specific surface area and tunnel structure as the electrode material has excellent electrochemical performance. The single electrode composed of the C/Na₂Ti₅O₁₁ nanocomposite based on carbon fiber fabric (CFF) has the highest area capacitance of 1066 mF cm^?2 at a current density of 2 mA cm^?2, which is superior to other titanates and Na-ion materials for supercapacitors (SCs). By scan-rate dependence cyclic voltammetry analysis, the capacity value shows both capacitive and faradaic intercalation processes, and the intercalation process contributed 81.7% of the total charge storage at the scan rate of 5 mV s^?1. The flexible symmetric solid-state SCs (C/Na₂Ti₅O₁₁/CFF//C/Na₂Ti₅O₁₁/CFF) based on different C/Na₂Ti₅O₁₁ mass were fabricated, and 7 mg SCs show the best supercapacitive characteristics with an area capacitance of 309 mF cm^?2 and a specific capacitance of 441 F g^?1, it has a maximum energy density of 22 Wh kg^?1 and power density of 1286 W kg^?1. As for practical application, three SCs in series can power 100 green light-emitting diodes (LEDs) to light up for 18 min, which is much longer than our previous work by Wang et al. lighting 100 LEDs for 8 min. Thus, the C/Na₂Ti₅O₁₁ nanocomposite has promising potential application in energy storage devices.     

Electrical and photoelectric properties of electrodeposited <Emphasis Type="Italic">n</Emphasis>-Si/<Emphasis Type="Italic">n</Emphasis>-Cd<Subscript>1 - <Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>x</Subscript>S heterojunctions

n-Si/n-Cd_{1 - x} Zn_xS heterojunctions are produced by electrodepositing Cd_{1 - x} Zn_xS (0 x 0.6) films on silicon substrates, and their electrical and photoelectric properties are studied. The results demonstrate that the spectral response of the heterojunctions depends strongly on the film composition and heat-treatment conditions. The highest photosensitivity is achieved at x = 0.6 by heat treatment at 350°C for 7 min: V _OC = 0.5 V and I _SC = 3.8 mA/cm² under illumination of 1500 lx at 300 K.Translated from Neorganicheskie Materialy, Vol. 41, No. 3, 2005, pp. 276–280.Original Russian Text Copyright © 2005 by Mamedov, Gasanov, Amirova.This revised version was published online in April 2005 with a corrected cover date.This revised version was published online in April 2005 with a corrected cover date.     

Annealing Dependence of Exchange Bias in MnO/Ga<Subscript>1−</Subscript><Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>As Heterostructures

We discuss the annealing-dependence of exchange bias in Ga_{1− x} Mn _x As epilayers that are overgrown with Mn. Although pure Mn is a known antiferromagnet, we find that the as-grown Mn overlayer does not produce any exchange coupling with Ga_{1− x} Mn _x As. Rather, our studies indicate that annealing in air is essential for creating the standard signatures of exchange bias (a clear shift in the magnetization hysteresis loop and an increased coercive field). We use X-ray photoelectron spectroscopy to characterize the compositional depth profile of both as-grown and rapid thermal annealed samples. These studies demonstrate that the cleanest exchange bias arises when the Mn overlayer is completely converted into MnO.     

High-temperature oxidation behaviour of Ti<Subscript>3</Subscript>Si<Subscript>(1−<Emphasis Type="Italic">x</Emphasis>)</Subscript>Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>C<Subscript>2</Subscript> in air

The oxidation behaviours of bulk Ti₃Si_(1−x)Al _x C₂ prepared by hot pressing were investigated. The results showed that the isothermal oxidation behaviour of Ti₃SiC₂ obeyed a parabolic law between 900 and 1100°C, and followed a two-step parabolic rate law between 1200°C and 1300°C. The cyclic oxidation behaviour of material is assumed to obey a three-step parabolic rate law at 1100°C and 1200°C. The calculated activation energy of isothermal oxidation is 101·43 kJ·mol⁻¹. The oxide layers consisted of a mass of α-Al₂O₃ and little TiO₂ and SiO₂ are observed on Ti₃SiC₂ as a dense and adhesive protect scale. The oxidation mechanism varies with the additive aluminum that greatly improves the oxidation resistance of Ti₃SiC₂.     

Ab-initio Calculation of Electronic and Magnetic Properties of Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te

The electronic and magnetic properties of Mn_1−x Cr _x Te of NiAs-type and zinc-blend type structures are theoretically investigated using the first-principles KKR-CPA method. It is concluded that at low concentration region of Cr, an antiferromagnetic state is the ground-state, while a ferrimagnetic state is more stable at higher concentration region of Cr in the both types of structures. In particular, a new type of half-metallic ferrimagnets is found in zinc-blend type Mn_1−x Cr _x Te. On the other hand, a nearly half-metallic behavior is observed in NiAs-type Mn_1−x Cr _x Te, where the Fermi level locates slightly above the minority spin band gap. The features of the half-metallicity of Mn_1−x Cr _x Te in zinc-blend structure are also seen in the results of the conductivity calculations using the Kubo–Greenwood formula.     

Superhard pcBN tool materials with Ti<Subscript>3</Subscript>SiC<Subscript>2</Subscript> MAX-phase binder: Structure,properties, application

Superhard cutting tool materials were sintered in cBN–(Ti₃SiC₂–TiC) system via high pressure–high temperature method. Sintering was performed under the pressure 8 GPa in the 1400–2400°C temperature range. The initial mixtures of three compositions were chosen with 90, 80 and 60 vol % cBN. The mixtures were prepared by mixing cBN (1–3 μm) and Ti₃SiC₂–TiC (< 2 μm). It was found, that upon sintering, the compositions of the obtained samples differed from the initial mixtures in all cases as a result of chemical reactions. Microstructure observations, phase composition estimation, and mechanical properties of the obtained tool materials were carried out. The results indicate that both the varying cBN content and the applied sintering conditions have a direct effect on the structure, properties, and kinetics of reactions.     

Electric properties of Bi<Subscript>4</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript>(BIT)–CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> (CCTO) composite substrates for high dielectric constant devices

A study of the effect of the presence of BIT (Bi₄Ti₃O₁₂) in the dielectric and optical properties of the CaCu₃Ti₄O₁₂ (CCTO) is presented. The samples were prepared by the solid state procedure. Mechanical alloying followed by the solid state procedure has been used successfully to produce powders of CaCu₃Ti₄O₁₂ (CCTO) and BIT (Bi₄Ti₃O₁₂) to be used in the composites. We also look at the effect of the grain size of the BIT and CCTO in the final properties of the composite. The samples were studied using X-Ray diffraction, scanning electron microscopy (SEM), Raman and infrared spectroscopy. We also did a study of the dielectric function K and dielectric loss of the samples. The role played by the grain size of CCTO and BIT in the dielectric constant and structural properties of the substrates are discussed. For frequencies below 10 MHz the K value presented by the CCTO100 sample is always higher than the K value presented by the BIT100 sample. At 100 Hz the value of K 1900 for the CCTO100 sample and 288 for the BIT100 sample. However for the composite sample one has an unexpected result. The dielectric constant is higher for all the frequencies under study. At 100 Hz the value of the K is around 10.000 for the BIT10 sample. Which is more than one order bigger compared to the CCTO100 value for the same frequency. Therefore, these measurements confirm the potential use of such materials for small high dielectric planar devices. These composites are also attractive for capacitor applications and certainly for microelectronics, microwave devices (cell mobile phones for example), where the miniaturization of the devices is crucial.     

Crystal structure,microstructure and microwave dielectric properties of novel MgAl<Subscript>2</Subscript>Ti<Subscript>3</Subscript>O<Subscript>10</Subscript> ceramic

In the present work, a novel MgAl₂Ti₃O₁₀ ceramic was obtained using a traditional solid-state reaction method. X-ray diffraction and energy dispersive spectrometer showed that the main MgAl₂Ti₃O₁₀ phase was formed after sintered at 1300–1450 °C. With rising the sintering temperature from 1300 to 1450 °C, the bulk density (ρ), relative permittivity (ε _r ) and Q?×?f value firstly increased, reached the maximum values (3.61 g/cm³, 14.9, and 26,450 GHz) and then decreased. The temperature coefficient of resonator frequency (τ _f ) showed a slight change at a negative range of ??94.6 to ??83.7 ppm/°C. When the sintering temperature was 1400 °C, MgAl₂Ti₃O₁₀ ceramics exhibited the best microwave dielectric properties with Q?×?f?=?26,450 GHz, ε _r ?=?14.9 and τ _f ?=???83.7 ppm/°C.     

Electroluminescence of (Pb<Subscript>0.91</Subscript>La<Subscript>0.09</Subscript>)(Zr<Subscript>0.65</Subscript>Ti<Subscript>0.35</Subscript>)O<Subscript>3</Subscript> ceramics with nanopolar structure

Lanthanum-doped lead zirconate titanate (PLZT) relaxor ceramics with (Pb_0.91La_0.09)(Zr_0.65Ti_0.35)O₃ composition exhibits a repolarization-induced electroluminescence (EL) with a pronounced discrete character of emission. It is established that this behavior is related to the reorientation of nanodimensional polar regions in a strong pulsed electric field in the vicinity of a smeared phase transition. The temporal and temperature dependences of the EL intensity have been studied.     

Co-precipitation method for the preparation of ferroelectric CaBi<Subscript>4</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript>

A simple co-precipitation technique has been successfully applied for the preparation of pure single phase CaBi₄Ti₄O₁₅ (CBT) powders. Ammonium oxalate and ammonium hydroxide were used to precipitate Ca²⁺, Bi³⁺ and Ti⁴⁺ cations simultaneously. No pyrochlore phase was found while heating powder at 600 C and pure CBT phase was found to be formed by X-ray diffraction. Particle size and morphology was studied by transmission electron microscopy (TEM). The room temperature dielectric constant at 1 kHz is 400. The ferroelectric hysteresis loop parameters of these samples were also studied.     

Effect of high pressures and temperatures on the structure and properties of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript>

We have prepared ceramic CaCu₃Ti₄O₁₂ samples by solid-state reaction and investigated the effect of high-pressure/high-temperature processing (p = 8.0 GPa, t = 1100°C) on the structure and electrical properties of CaCu₃Ti₄O₁₂.