Electrical and optical characteristics of Au/PbS/n-6H-SiC structures prepared by electrodeposition of PbS thin film on n-type 6H-SiC substrate

To realize Schottky barrier height (SBH) modification in the Au/n-6H-SiC Schottky diodes, lead sulfide (PbS) thin films were grown on n-6H-SiC by electrodeposition method. At first, XRD experiments were performed to investigate the crystal structure of the PbS film electrodeposited on n-6H-SiC. It has been deduced from the diffraction profile that the PbS thin film has a crystal structure more strongly oriented along the [2 0 0] direction. An optical energy band gap value of 1.42 eV for the PbS film was obtained from its optical absorption spectra. Then, we have prepared Au/PbS/n-6H-SiC Schottky barrier diodes (SBDs) with interface layer and reference Au/n-6H-SiC/Ni SBDs. The SBH enhancement has been succeeded by the PbS interlayer, influencing the space charge region of the SiC. The SBH values of 1.03 and 0.97 eV for the samples with and without the interfacial PbS layer were obtained from the forward bias current-voltage (I-V) characteristics. The SBH increase in the Au/PbS/n-6H-SiC SBD with the interfacial PbS layer has been attributed to the fact that the interface states contain a net negative interface charge in metal/n-type semiconductor contact due to the presence of the interfacial PbS layer.     

Current–voltage and capacitance–voltage characteristics of a Sn/Methylene Blue/p-Si Schottky diode

Electrical and interfacial properties of Sn/Methylene Blue (MB)/p-Si Schottky diode have been determined by using current–voltage (I–V) and capacitance–voltage (C–V) measurements of the device at room temperature. Cheung functions and modified Norde functions have been used to obtain the electrical characteristics such as barrier height and series resistance of the diode. It has been seen that the MB layer modifies the effective barrier height of the structure because the layer creates the physical barrier between the metal and the semiconductor. Electrical properties of the device obtained from C–V characteristics have been compared with the ones obtained from its I–V characteristics. It has been seen that at sufficiently high frequencies, the charge at the interface cannot follow an ac signal. The interface state density of the diode has been also calculated.     

Multiple-barrier distribution behavior of Mo/p-GaTe fabricated with sputtering

A Molybdenum Schottky diode on unintentially doped p-GaTe was fabricated using DC sputtering. I-V characteristics of the fabricated diode were measured as a function of temperature at the range of 50-300 K. The barrier parameters of Mo/p-GaTe are interpreted using thermionic emission theory and inhomogeneities observed in the barrier are characterized with Gaussian distribution approach on the basis of parallel conduction model. The barrier height and the ideality factor values at 300 K and at 80 K of Mo/p-GaTe were calculated to be 0.581 eV, 1.097 and 0.472 eV, 1.349, respectively. The barrier parameters changed resolutely at 140-300 K temperature range and a strong temperature dependence was observed below 130 K. The weighting coefficients, standard deviations and mean barrier heights were calculated for sub distributions. Richardson plot was interpreted with a new approach and Richardson constant was found to be 117.96 AK⁻² cm⁻² for p-GaTe.     

Electrical characterization of Au/n-GaN metal-semiconductor and Au/SiO2/n-GaN metal-insulator-semiconductor structures

In the present work, we have investigated the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of Au/SiO₂/n-GaN metal-insulator-semiconductor (MIS) Schottky diode and compared with Au/n-GaN metal-semiconductor (MS) Schottky diode. Calculations showed that the Schottky barrier height and ideality factor of the MS Schottky diode is 0.79 eV (I-V), 0.87 eV (C-V) and 1.45, respectively. It is observed that the Schottky barrier height increases to 0.86 eV (I-V), 0.99 eV (C-V) and ideality factor deceases to 1.3 for MIS diode. For the MS diode, the calculated doping concentration is 4.17 × 10¹⁷ cm⁻³. However, in the case of the MIS Schottky diode, the decrease in doping concentration is observed and the respective value is 2.08 × 10¹⁷ cm⁻³. The obtained carrier concentration of the MIS diode is reduced about 50% when compared to the MS diode. The interface state density as determined by Terman's method is found to be 3.79 × 10¹² and 3.41 × 10¹⁰ cm⁻² eV⁻¹ for the MS and MIS Schottky diodes, respectively. The calculated interface densities are 2.47 × 10¹¹ cm⁻² eV⁻¹, 3.35 × 10¹¹ cm⁻² eV⁻¹ and 3.5 × 10¹¹ cm⁻² eV⁻¹ for the sweep rates of 300, 450 and 600 mV/s from MOS C-V measurements for the MIS Schottky diode. The interface state density calculated from Terman's method is found to be increased with sweep rate. From the C-V measurement, it is noted that the decrease in the carrier concentration in MIS diodes as compared to MS diode may be due to the presence of oxide interfacial layer. DLTS measurements have also been performed on MIS Schottky diode and discussed.     

Effect of chemical composition and heat treatment on mechanical characteristics of microlayer composite materials of the Cu-Zr-Y-Mo System in a wide temperature range

The structure, hardness, and characteristics of short-term strength and ductility in the temperature range 20–800°C are determined for microlayer composite materials of the Cu-Zr-Y-Mo system obtained by the method of electron beam evaporation and layer-after-later condensation from a vapor phase depending on the chemical composition and heat treatment. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 18–22, March, 2006.     

A study of methane adsorption and accumulation on microporous carbon adsorbent in a wide temperature range

This work studies the adsorption properties of microporous activated carbon AU-2 to determine the efficiency of methane accumulation in a wide temperature range, particularly in the low temperature range. Absolute adsorption isotherms of methane are measured in the pressure range of 20 Pa to 25 MPa and temperature range of 178–260 K. It is shown that the adsorbent accumulates up to 130 m³(ntp, CH₄)/m³ at 7 MPa and 298 K. A decrease in the temperature by 55° allows reaching the value of 180 m³(ntp, CH₄)/m³. The experimental data are used to plot methane adsorption isosteres that are well approximated by straight lines in the coordinates of lnp = f(1/T)_a. The values of differential and integral adsorption heats of methane on the adsorbents are calculated on the basis of the experimental isotherms and are used to calculate an increase in the adsorber temperature as a result of adsorption.     

Electrical characteristics of light-emitting diode based on poly(p-phenylenevinylene) derivatives: CzEH-PPV and OxdEH-PPV

In the light-emitting devices (LEDs) based on the π-conjugated polymers, the relationship between the quantum efficiency and the balance of hole (μ_h) and electron (μ_e) mobility has been investigated. In order to measure the μ_h and μ_e of the LEDs based on π-conjugated polymers, we fabricated the hole transport device (HTD) and the electron transport device (ETD) by using various metal electrodes with different work functions. For the materials of light emitting layer, we synthesized poly[2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (CzEH-PPV) and poly[2-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazolyl]-phenyl}-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (OxdEH-PPV) with electron-rich groups. The poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), which is well known material for the polymer-based LED, was synthesized for the reference. We measured the current density vs. applied field (J–E) characteristics of the HTD and ETD with various thickness at different temperatures. The results of the J–E curves were analyzed by using the space charge limited conduction (SCLC) model. Based upon the SCLC model, μ_h and μ_e of MEH-PPV sample was measured to be 10⁻⁶ cm²/V s and 10⁻⁸ cm²/V s, respectively. For CzEH-PPV and OxdEH-PPV samples with electron-rich groups, μ_h was similar to μ_e with 10⁻¹⁰–10⁻¹¹ cm²/V s. The μ_h and μ_e of CzEH-PPV and OxdEH-PPV samples was lower than that of MEH-PPV sample, but more balanced. The quantum efficiency of the LED by using CzEH-PPV or OxdEH-PPV materials was 10 times higher than that prepared from MEH-PPV. The balance of the μ_h and μ_e plays an important role for the quantum efficiency. We analyze the balance of the μ_h and μ_e and the relatively low mobilities of CzEH-PPV and OxdEH-PPV samples in terms of the heavier effective mass due to the asymmetric dipole distribution in the side chains. The results of photocurrent of the systems qualitatively agreed with the result of the electrical measurements. From AC impedance measurement of the LEDs, we observed that the relaxation time of MEH-PPV was shorter than that of OxdEH-PPV sample because of the higher mobility of MEH-PPV sample.     

Effect of heat treatment and welding on mechanical properties of low-alloy molybdenum alloys in a wide temperature range

The short-term strength and ductility of rolled thin sheets from low-alloy molybdenum alloys TsM-10, TsM-6, TsM-12, and MI-5 in deformed, polygonized, recrystallized, and cast states is studied in the temperature range 20–2000°C. It is shown that the high-temperature strength of these alloys can be increased by a factor of 1.1–1.6 by annealing at 1150–1500°C for 1 h. Welded joints of molybdenum alloys are shown to have a lower strength than the base metal at 20–1500°C but commonly have a comparable strength in the high-temperature region. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 7, pp. 27–32, July, 2000.     

Magnetocaloric effect over a wide temperature range in GdxHo3−xAl2 compounds with successive magnetic transitions

The Gd_xHo_3−xAl₂ (x = 0.5–2.5) compounds undergo two successive magnetic transitions: a second-order magnetic transition from paramagnetism (PM) to ferromagnetism (FM) around T_C followed by a FM to ferrimagnetism (FIM)-like transition at T_t. The T_C increases linearly with the increase of Gd content due to the de Gennes factor of Gd being larger than that of Ho, while the T_t remains nearly constant around 45 K independent of the variation of Gd content. The working temperature range is expanded by substituting Ho with Gd atoms due to the higher transition temperature of Gd₃Al₂ and the different composition dependence of successive magnetic transitions, and thus leading to an enhanced refrigerant capacity (RC). For a relatively low field change of 2 T, a giant RC value of 272 J/kg with a wide working temperature range of ∼80 K is obtained for x = 1.0 compound due to the partial overlap of successive ΔS_M peaks. This is probably the largest RC value so far reported in the temperature range of liquid natural gas (111 K). Consequently, the large ΔS_M and giant RC suggest that Gd_xHo_3−xAl₂ compounds could be good candidates of magnetic refrigerants in a wide temperature range.     

宽温域下LaF3对镍基自润滑材料摩擦学性能的影响

为增强材料在宽温域下服役的稳定性及减少摩擦与磨损对材料性能的影响,采用Ni60合金粉末作为基体材料,利用放电等离子烧结技术(SPS)制备出了不同LaF3含量的镍基自润滑复合材料。利用HT-1000型高温摩擦磨损试验机、GBS-SmartWLI白光干涉三维轮廓仪、SEM、XRD等对复合材料在大气环境下的宽温域摩擦学性能进行了研究。结果表明,所制备的镍基自润滑材料在宽温域下具有优良的摩擦学性能。复合材料摩擦系数与磨损率从200到800℃的范围内保持在一个较低水平。当LaF3的含量为6%(质量分数)时,平均摩擦系数在200到800℃之间为0.189到0.288且变化范围最小,在600℃下表现最低为0.189,平均磨损率在10-5mm3/N·m数量级,综合表现出最优的摩擦学性能。这是因为LaF3含量为6%的材料在高温下形成了SrSiO3、SrMoO4、La6MoO12等盐类,所形成的盐转移膜一方面防止了对偶材料表面直接接触,另一方面减小了接触薄层的抗剪切强度,从而显著地提高了宽温域环境下材料的稳定性。随着测试温度增加到800℃,由于高温氧化作用使得复合材料摩擦系数略微上升,但仍然保持在0.2左右的较低水平。     

Ag/ZnO复合材料在氮气光固定中的应用：构造肖特基势垒以实现有效的电荷载流子分离

将氮气(N2)光催化还原为氨(NH3)是一种可持续的能源生产方法。等离激元共振光催化剂能够通过表面等离激元共振效应实现太阳能的有效转化,也因此受到越来越广泛的关注。然而,热载流子往往会在催化固氮的过程中发生重新结合。本研究将具有等离激元共振效应的Ag纳米粒子与ZnO半导体复合(Ag/ZnO)并应用于氮气光固定。与ZnO相比,Ag/ZnO在氮气光固定的催化活性得到了提高,室温下氨生成速率达到120 μmol.gcat.^-1.h^-1。进一步的机理研究表明在Ag纳米颗粒与ZnO的界面处形成了肖特基势垒,这大幅度促进了光生电子-空穴对的分离。Ag纳米粒子通过表面等离激元共振效应生成热载流子,所形成的肖特基势垒则促进了电子从Ag向ZnO转移。此外,ZnO中的富电子Zn ⁺可能作为活性位点以吸附和活化氮气分子,从而促进氮气光固定的进行。     

Microstructure evolution and age-hardening response in Mg-Sn-Sm alloys under a wide range of Sm/Sn ratio

The microstructure evolution and age-hardening response for different Sm/Sn ratios (0–2.55, in wt.%) of Mg-Sn-Sm alloys were investigated. The second phase formation in as-cast alloys and the Mg₃Sm precipitates formed in aged alloys were characterized using XRD, FESEM and HAADF-STEM with EDS techniques. Results indicate that the Sm/Sn ratio has a great influence on the phase constitution, α-Mg grain size and age-hardening response. With the increment of Sm/Sn ratio, Mg₄₁Sm₅ and thermally stable MgSnSm phases precipitate. When the Sm/Sn ratio is about 1.19, the secondary dendrite arm spacing of α-Mg grains significantly decreases. Furthermore, the alloy with Sm/Sn ratio up to 2.55 exhibits the highest age-hardening response, the hardness value increases from 52 HB at solution-treated condition to 74 HB at peak-aged condition (ageing at 220 °C for a short time of 4 h). This is attributed to the large volume fraction of needle-like Mg₃Sm precipitates formed in the α-Mg matrix during ageing treatment, which results in a significant precipitation strengthening effect.

     

Morphological study of 16-year patinas formed on copper in a wide range of atmospheric exposures

Much information is available on the atmospheric corrosion of copper and patina formation mechanisms in the short, mid and even long term. However, studies of the structure and morphology of patina layers are less abundant and mostly deal with patinas formed in the atmosphere over a small number of years. The present study concentrates on the structure and morphology of corrosion product films formed on copper after long-term atmospheric exposure (13-16 years) in five Spanish atmospheres of different types: rural, urban, industrial and marine (mild and severe). Characterisation has been performed by X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS). Long-term copper corrosion is higher in industrial and marine atmospheres and lower in rural and urban atmospheres. In all cases a decrease in the corrosion rate with exposure time is observed. The formation of antlerite [Cu₃SO₄(OH)₄] is seen in more acidic conditions and in specimen areas subject to a high time of wetness. The presence of nantokite (CuCl), which is not generally mentioned in field studies, has been detected under the cuprite layer very close to the base copper.     

Crystal and magnetic structures of Laves phase compound NdCo2 in the temperature range between 9 and 300 K

The crystal and magnetic structures of the Laves phase compound NdCo₂ in the temperature range from 9 to 300 K are determined by Rietveld refinement technique, using high-resolution neutron powder diffraction data. The compound crystallizes in space group above the magnetic ordering temperature T_C (≈100 K), in space group I4₁/amd below T_C and in space group Imma below the tetragonal–orthorhombic structural/magnetic transitions at T_M ≈ 42 K. The assignment of the space groups to the crystal structures of NdCo₂ in different temperature ranges complies with the reported Mössbauer studies. Detailed information of the crystal and magnetic structures of NdCo₂ at different temperatures are reported.     

Oxidation characteristics of Ti-4.37 wt.% Ta alloy in the temperature range 1258–1473 K

Oxidation kinetics of Ti-4.37 wt.% Ta (1.19 at.%) alloy in either air or oxygen have been investigated in the temperature range 1258–1473 K and at three pressure levels: 0.013, 0.133, and 1.0 bar. X-ray data reveal only TiO₂ (rutile) as the main oxide and, in addition, TiO₂, S-TiN, and -TiN at the metal surface for oxidation in air at 1273 K. The -TiN phase was not detected at higher temperatures. Electron microprobe analyses are used to determine the concentration profiles of Ti, Ta, O, and N across the oxide scale and in the metal. Microhardness traverses follow the oxygen distribution in the alloy. The oxygen diffusion coefficient,D = 60.8 exp(–235.1/RT), as determined from weight-gain measurements, is in reasonable agreement with that calculated from microhardness data. The oxidation mechanisms for oxidation of the alloy in air or oxygen are proposed.The existence of interstitial Ti cations in the oxide is suggested.Presently on academic leave at Max-Planck-Institut für Metallforschung, Institut für Werk-stoffwissenschaften, Stuttgart, German Federal Republic.     

Optical and electrical properties of TiO2/Au/TiO2 multilayer coatings in large area deposition at room temperature

TiO2/Au/TiO2 multilayer thin films were deposited at polymer substrate at room temperature using dc (direct current) magnetron sputtering method.By varying the thickness of each layer,the optical and electrical properties of the TiO2/Au/TiO2 multilayer films can be tailored to suit different applications.The thickness and optical properties of the Au layer and the quality of the Au-dielectric interfaces are critical for the electrical and optical performance of the Au-dielectric multilayer thin films.At the thickness of 8 nm,the Au layer forms a continuous structure having the lowest resistivity and it must be thin for high transmittance.The multilayer stack can be optimized to have a sheet resistance of 6 Ω/sq.at a transmittance over 80% at 680 nm in wavelength.The peak transmittance shifts towards the long wavelength region when the thickness of the two TiO2 (upper and lower) layers increases.When the film thickness of the two TiO2 film is 45 nm,a high transmittance value is obtained for the entire visible light wavelength region.     

Electrochemical characteristics of zirconium oxide treated Type 304 stainless steels of different surface oxide structures in high temperature water

Electrochemical polarization analyses were conducted to investigate the impact of different oxide structures on corrosion resistance of ZrO₂-treated Type 304 stainless steel specimens in high temperature water. All specimens were pre-oxidized in high temperature water containing either 300 ppb dissolved oxygen or 50 ppb dissolved hydrogen, followed by a hydrothermal deposition treatment with ZrO₂ nanoparticles. Experimental results revealed that the corrosion potentials and corrosion current densities on the ZrO₂-treated specimens were lower than those on the untreated ones, and the decreases in these parameters were more distinct on the ZrO₂-treated specimens with oxides developed under dissolved oxygen condition.     

Colossal and constant magnetoresistance over a large temperature range between 230 and 4.2 K in La0.8Li0.2MnO3 prepared by a partial melting technique

La_0.8Li_0.2MnO₃ was prepared by a partial melting technique. The Li-doped LaMnO₃ compound has a rhombohedral perovskite structure and shows a wide semiconductor-to-metal transition between 230 and 175 K in resistance versus temperature. Nearly constant magnetoresistance, Δρ/ρ₀, above 20–60% was achieved with application of magnetic fields from 1 to 8 Tesla (T) over a very large temperature range from the ferromagnetic transition at 230–4.2 K. As a comparison, a partial melted La_0.7Ca_0.3MnO₃ shows a very narrow ferromagnetic transition around 230 K which is the same as that of single crystals and epitaxy thin films.