Ohmic contact properties of p-type surface conductive layer on H-terminated diamond films prepared by DC arc jet CVD

With the advantages of high deposition rate and large deposition area, polycrystalline diamond films prepared by direct current (DC) arc jet chemical vapor deposition (CVD) are considered to be one of the most promising materials for high-frequency and high-power electronic devices. In this paper, high-quality self-standing polycrystalline diamond films with the diameter of 100 mm were prepared by DC arc jet CVD, and then, the p-type surface conductive layer with the sheet carrier density of 1011-1013 cm?2 on the H-terminated diamond film was obtained by micro-wave hydrogen plasma treatment for 40 min. Ti/Au and Au films were deposited on the H-terminated diamond surface as the ohmic contact electrode, respectively, afterwards, they were treated by rapid vacuum annealing at different temperatures. The properties of these two types of ohmic contacts were investigated by measuring the specific contact resistance using the transmission line method (TLM). Due to the formation of Ti-related carbide at high temperature, the specific contact resistance of Ti/Au contact gradually decreases to 9.95 × 10?5 Ω·cm2 as the temperature increases to 820℃. However, when the annealing temperature reaches 850℃, the ohmic contact for Ti/Au is degraded significantly due to the strong diffusion and reaction between Ti and Au. As for the as-deposited Au contact, it shows an ohmic contact. After annealing treatment at 550℃, low specific contact resistance was detected for Au contact, which is derived from the enhancement of interdiffusion between Au and diamond films.     

Optoelectronic and electrochemical behaviour of γ-CuI thin films prepared by solid iodination process

A simple and efficient solid iodination method has been proposed for the fabrication of p-type γ-CuI thin films.The structural,morphological,optical,electrical and electrochemical properties have been investigated in order to serve as an effective hole-transporting layer in solid-state solar cells.The fabricated films exhibited p-type conductivity with resistivity of 7.0×10~(-2)Ωcm,the hole concentration of ～1.13×10~(19)cm~(-3)and the mobility of 18.34 cm~(-2)V~(-1)s~(-1).The cyclic voltammetry result shows a maximum specific capacitance of 43 mF/cm~2 at a scan rate of 10 mV/s.The cyclic stability and capacitance retention were found to be 99.7%.These findings demonstrate that γ-CuI film can be a potential candidate for multiple applications,such as a hole transporting material for solid-state solar cells and electrochemical supercapacitor.     

High-performance doping-free carbon-nanotube-based CMOS devices and integrated circuits

Ballistic n-type carbon nanotube (CNT)-based field-effect transistors (FETs) have been fabricated by contacting semiconducting single-walled CNTs (SWCNTs) using Sc or Y. The n-type CNT FETs were pushed to their performance limits through further optimizing their gate structure and insulator. The CNT FETs outperformed n-type Si metal-oxide-semiconductor (MOS) FETs with the same gate length and displayed better downscaling behavior than the Si MOS FETs. Together with the demonstration of ballistic p-type CNT FETs using Pd contacts, this technological advance is a step toward the doping-free fabrication of CNT-based ballistic complementary metal-oxide-semiconductor (CMOS) devices and integrated circuits. Taking full advantage of the perfectly symmetric band structure of the semiconductor SWCNT, a perfect SWCNT-based CMOS inverter was demonstrated, which had a voltage gain of over 160. Two adjacent n- and p-type FETs fabricated on the same SWCNT with a self-aligned top-gate realized high field mobility simultaneously for electrons (3000 cm² V^?1 s^?1) and holes (3300 cm² V^?1 s^?1). The CNT FETs also had excellent potential for high-frequency applications, such as a high-performance frequency doubler.     

Conductive and corrosion behaviors of silver-doped carbon-coated stainless steel as PEMFC bipolar plates

Ni–Cr enrichment on stainless steel SS316L resulting from chemical activation enabled the deposition of carbon by spraying a stable suspension of carbon nanoparticles; trace Ag was deposited in situ to prepare a thin continuous Ag-doped carbon film on a porous carbon-coated SS316L substrate. The corrosion resistance of this film in 0.5 mol·L-1 H₂SO₄ solution containing 5 ppm F- at 80℃ was investigated using polarization tests. The results showed that the surface treatment of the SS316L strongly affected the adhesion of the carbon coating to the stainless steel. Compared to the bare SS316L, the Ag-doped carbon-coated SS316L bipolar plate was remarkably more stable in both the anode and cathode environments of proton exchange membrane fuel cell (PEMFC) and the interface contact resistance between the specimen and Toray 060 carbon paper was reduced from 333.0 mΩ·cm2 to 21.6 mΩ·cm2 at a compaction pressure of 1.2 MPa.     

Influence of modulation period for β-Cu2+xSe/SiC nano-multilayer films on their thermoelectric properties

In this study, β-Cu_2+xSe/SiC nano-multilayer films with different modulation period were successfully deposited on SiO₂/Si substrates by sputtering alternately using Cu–Se and SiC targets. The deposited films were observed on both surface and cross-section, and the thermoelectric properties were studied. The results show that both carrier concentration and mobility at room temperature decreased with the reducing modulation period for the nano-multilayer films. The conductivity slightly decreased and Seebeck coefficient greatly increased with the reducing modulation period. As a result of competition, the power factor of the nano multilayer films increased with the reducing modulation period because the positive effect of the Seebeck coefficient exceeded the negative effect of the conductivity. In the case of β-Cu_2+xSe/SiC nano multilayer film with the smallest modulation periods (210 ?nm), the power factor reached 0.39 ?mWm^?1K^?2 and 0.59 ?mWm^?1K^?2 at room temperature and 325 ?°C, respectively. The enhanced power factor for nano multilayer films is attributed to the scattering process at the β-Cu_2+xSe/SiC layer interface, which reduces the carrier concentration and the mobility. It is concluded that the thermoelectric properties of β-Cu_2+xSe films can be effectively improved by designing nano multilayer structure.     

Effects of boron on the microstructure and thermal properties of Cu/diamond composites prepared by pressure infiltration

Diamond reinforced copper (Cu/diamond) composites were prepared by pressure infiltration for their application in thermal management where both high thermal conductivity and low coefficient of thermal expansion (CTE) are important. They were characterized by the microstructure and thermal properties as a function of boron content, which is used for matrix-alloying to increase the interfacial bonding between the diamond and copper. The obtained composites show high thermal conductivity (>660 W/(m·K)) and low CET (<7.4×10-6 K-1) due to the formation of the B₁₃C₂ layer at the diamond-copper interface, which greatly strengthens the interfacial bonding. Thermal property measurements indicate that in the Cu-B/diamond composites, the thermal conductivity and the CTE show a different variation trend as a function of boron content, which is attributed to the thickness and distribution of the interfacial carbide layer. The CTE behavior of the present composites can be well described by Kerner’s model, especially for the composites with 0.5wt% B.     

Kinetic study on the direct nitridation of silicon powders diluted with α-Si3N4 at normal pressure

Silicon nitride (Si₃N₄) powders were prepared by the direct nitridation of silicon powders diluted with α-Si₃N₄ at normal pressure. Silicon powders of 2.2 μm in average diameter were used as the raw materials. The nitriding temperature was from 1623 to 1823 K, and the reaction time ranged from 0 to 20 min. The phase compositions and morphologies of the products were analyzed by X-ray diffraction and scanning electron microscopy, respectively. The effects of nitriding temperature and reaction time on the conversion rate of silicon were determined. Based on the shrinking core model as well as the relationship between the conversion rate of silicon and the reaction time at different temperatures, a simple model was derived to describe the reaction between silicon and nitrogen. The model revealed an asymptotic exponential trend of the silicon conversion rate with time. Three kinetic parameters of silicon nitridation at atmospheric pressure were calculated, including the pre-exponential factor (2.27 cm·s^?1) in the Arrhenius equation, activation energy (114 kJ·mol^?1), and effective diffusion coefficient (6.2×10^?8 cm²·s^?1). A formula was also derived to calculate the reaction rate constant.     

自持金刚石厚膜上沉积N掺杂ZnO薄膜的生长及电学特性

采用金属有机化合物气相沉积(MOCVD)两步生长法在自持化学气相沉积(CVD)金刚石厚膜的成核面上制备ZnO薄膜, 并研究了薄膜的生长特性和电学特性. 结果表明, 在基片温度为600 ℃时沉积得到的ZnO薄膜表面均匀, 取向较一致, 为c轴取向生长. 其载流子迁移率为3.79 cm²/(V·s).      

Selective interfacial bonding and thermal conductivity of diamond/Cu-alloy composites prepared by HPHT technique

Cu-based and Cu-alloy-based diamond composites were made by high-pressure-high-temperature (HPHT) sintering with the aim of maximizing the thermal conductivity of the composites. Improvements in interfacial bonding strength and thermo-physical properties of the composites were achieved using an atomized copper alloy with minor additions of Co, Cr, B, and Ti. The thermal conductivity (TC) obtained exhibited as high as 688 W·m-1·K-1, but also as low as 325 W·m-1·K-1. A large variation in TC can be rationalized by the discrepancy of diamond-matrix interfacial bonding. It was found from fractography that preferential bonding between diamond and the Cu-alloy matrix occurred only on the diamond {100} faces. EDS analysis and Raman spectra suggested that selective interfacial bonding may be attributed to amorphous carbon increasing the wettability between diamond and the Cu-alloy matrix. Amorphous carbon was found to significantly affect the TC of the composite by interface modification.     

Photocatalytic activity of ferric oxide/titanium dioxide nanocomposite films on stainless steel fabricated by anodization and ion implantation

A simple surface treatment was used to develop photocatalytic activity for stainless steel. AISI 304 stainless steel specimens after anodization were implanted by Ti ions at an extracting voltage of 50 kV with an implantation dose of 3 × 1015 atoms·cm?2 and then annealed in air at 450℃ for 2 h. The morphology was observed by scanning electron microscopy. The microstructure was characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The photocatalytic degradation of methylene blue solution was carried out under ultraviolet light. The corrosion resistance of the stainless steel was evaluated in NaCl solution (3.5 wt%) by electrochemical polarization curves. It is found that the Ti ions depth profile resembles a Gaussian distribution in the implanted layer. The nanostructured Fe₂O₃/TiO₂ composite film exhibits a remarkable enhancement in photocatalytic activity referenced to the mechanically polished specimen and anodized specimen. Meanwhile, the annealed Ti-implanted specimen remains good corrosion resistance.     

Positronium annihilation in oxygen and nitrogen mixture

Formed in silica aerogels, positronium annihilation in oxygen and nitrogen mixture is studied by a simple and convenient method. The results show that ortho-positronium (o-Ps) lifetime is significantly shortened when some oxygen is introduced into nitrogen gas and that the o-Ps collisional quenching rate for an oxygen molecule is (28.0±0.4) μs^?1·amagat^?1 (1 amagat = 2.69×10²⁵ m^?3). It is found that the o-Ps annihilation rate in oxygen of 1atm at room temperature is (25.5±0.5) μs^?1 and that the effective number of electrons per oxygen molecule available for 2γ annihilation of positron in the o-Ps is (34.6±0.4).     

Interdiffusion in the Fe2O3-TiO2 system

Interdiffusion in the Fe₂O₃-TiO₂ system was investigated by the diffusion couple method in the temperature range of 1323 to 1473 K. The diffusion concentration curves of Ti⁴⁺ cations were obtained by electron probe microanalysis, according to which the Boltzmann-Matano method optimized by Broeder was used to calculate the interdiffusion coefficients. The interdiffusion coefficients almost increased linearly with the mole fraction of Ti⁴⁺ cations increasing, and they were in the range of 10^?12–10^?11cm²·s^?1. The increase of temperature could also lead to the increase of the interdiffusion coefficients at a constant concentration of Ti⁴⁺ cations. It was also found that the thickness growth of the diffusion layer obeyed the parabolic rate law.     

High quality p-type ZnO film growth by a simple method and its properties

P-type ZnO：N films have been grown successfully by chemical vapor deposition （CVD） using Zn4（OH）2（O2CCH3）6·2H2O as the solid source material and ZnNO3 as the doping source material. XPS, Hall-effect measurement and PL spectra were employed to analyze the structural, electrical and optical properties and study the influence of substrate temperature on the film. Results showed that with a lower substrate temperature, the film exhibited p-type conduction and its resistivity decreased when the substrete temperature increased. When the substrates temperature was 400℃, p-type ZnO films were obtained with carrier concentration of ＋5.127×10^17 cm^-3, resistivity of 0.04706 Ω· cm and Hall mobility of 259 cm^2/（V·s）; they still exhibited p-type conduction after a month. When the substrate temperature was too high, the film was transformed from p-type to n-type conduction.     

Cu2O-templated fabrication of Ni(OH)2·0.75H2O hollow tubes for electrocatalytic oxygen evolution reaction

Cu₂O is an ideal template material for the preparation of transition metal hydroxide/oxyhydroxides with oxygen evolution reaction (OER) enhanced catalytic performance. Here, inspired by Pearson's principle, Cu₂O wires were prepared and used as a sacrificial template to prepare Ni(OH)₂·0.75H₂O hollow tubes (Ni(OH)₂ HTs) with highly improved surface roughness. Benefiting from unique structural advantages, the Ni(OH)₂ HTs showed excellent catalytic activity, rapid kinetics and a long-term stability as the OER catalyst, where an overpotential of only 207 ?mV was required to drive a current density of 10 ?mA ?cm^?2, an ideal kinetics with a Tafel slope as 79.8 ?mV dec^?1 was calculated, and no obvious attenuation in chronoamperometry was discovered after operation for 24 ?h. This paper provides a novel template-assisted strategy to prepare high-performance transition metal-based OER catalysts possessing hollow and tubular structures.     

Preparation and properties of C/C-SiC brake composites fabricated by warm compacted-<Emphasis Type="Italic">in situ</Emphasis> reaction

Carbon fibre reinforced carbon and silicon carbide dual matrix composites (C/C-SiC) were fabricated by the warm compacted-in situ reaction. The microstructure, mechanical properties, tribological properties, and wear mechanism of C/C-SiC composites at different brake speeds were investigated. The results indicate that the composites are composed of 58wt% C, 37wt% SiC, and 5wt% Si. The density and open porosity are 2.0 g·cm⁻³ and 10%, respectively. The C/C-SiC brake composites exhibit good mechanical properties. The flexural strength can reach up to 160 MPa, and the impact strength can reach 2.5 kJ·m⁻². The C/C-SiC brake composites show excellent tribological performances. The friction coefficient is between 0.57 and 0.67 at the brake speeds from 8 to 24 m·s⁻¹. The brake is stable, and the wear rate is less than 2.02×10⁻⁶ cm³·J⁻¹. These results show that the C/C-SiC brake composites are the promising candidates for advanced brake and clutch systems.     

The contribution of root respiration to soil CO2 efflux in Puccinellia tenuiflora dominated community in a semi-arid meadow steppe

Rising atmospheric CO2 and temperature are altering ecosystem carbon cycling. Grasslands play an important role in regional climate change and global carbon cycle. Below-ground processes play a key role in the grassland carbon cycle because they regulate …     

A novel Pd/silicalite-1 composite membrane for hydrogen separation

A novel Pd/silicalite-1 composite membrane supported on the macroporous tubular stainless steel substrate was successfully fabricated by electroless plating at 303 K. The structure, morphology and gaseous permeability of the membrane were detected by X-ray diffractiometry （XRD）, scanning electron microscopy （SEM） and single-gas permeation test, respectively. Results confirm the formation of a thin, smooth, and continuous Pd/silicalite-1 composite membrane. The obtained composite membrane shows a high H2 permeance of 1.15×10^-6 mol. m^-2. s^-1. Pa^-1 with moderate H2 selectivity of 250 for H2/N2 at 773 K, at 0.1 MPa pressure drop, suggesting the potential application for H2 separation.     

Thermal damage and ablation behavior of plasma sprayed Ba2SmTaO6 coatings irradiated by high power continuous laser

Ba₂SmTaO₆ laser protection coatings of ≈200 ​μm thickness were deposited onto stainless steel surfaces by air plasma spraying, and the laser irradiation resistance of the coatings was investigated. For laser irradiation with a laser power density less than 1000 ​W/cm², the coatings kept intact. For a laser power density exceeding 1500 ​W/cm², the Ba₂SmTaO₆ coatings underwent recrystallization, grain growth occurred, and certain spray morphology features disappeared by melting. In the case of a laser power density of 2000 ​W/cm² applied for 10s, the incident laser parameter was beyond the coatings protection threshold, and the coating peeled off. The samples back surface temperature kept unchanged within the first 1s of laser irradiation, indicating that Ba₂SmTaO₆ coatings have excellent laser protection capability and can limit the rise of the substrate temperature. However, the low thermal conductivity of Ba₂SmTaO₆ leads to a detrimental laser energy concentration at the beginning of the laser irradiation period on the sample front surface, resulting in a rapid increase of the surface temperature up to the melting point.     

Enhanced electrochemical properties of LaFeO3 with Ni modification for MH-Ni batteries

In this work, we synthesized LaFeO₃-xwt%Ni (x=0, 5, 10, 15) composites via a solid-state reaction method by adding Ni to the reactants, La₂O₃ and Fe₂O₃. Field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopy (EDS) results revealed that Ni powders evenly dispersed among the LaFeO₃ particles and apparently reduced their aggregation, which imparted the composites with a loose structure. Moreover, the Ni formed a conductive network, thus improving the conductivity of the composites. The maximum discharge capacity of the LaFeO₃ electrodes remarkably increased from 266.8 mAh·g-1 (x=0) to 339.7 mAh·g-1 (x=10). In particular, the high-rate dischargeability of the LaFeO₃-10wt%Ni electrode at a discharge current density of 1500 mA·g-1 reached 54.6%, which was approximately 1.5 times higher than that of the pure LaFeO₃. Such a Ni-modified loose structure not only increased the charge transfer rate on the surface of the LaFeO₃ particles but also enhanced the hydrogen diffusion rate in the bulk LaFeO₃.     

氮化硼纤维填充聚乙烯醇导热复合材料的制备

以聚乙烯醇（PVA）为基体,选用六方氮化硼纤维（BN fiber）作为导热填料,通过溶液共混的方法制备导热复合材料。结合X射线衍射仪（XRD）、扫描电子显微镜（SEM）以及导热测试结果,探究填料的微观形貌以及与基体的界面相容性对于提升复合材料导热性能的影响。结果表明：BN fiber对于提升复合材料的面内导热率有很好的效果,而且采用过氧化氢（H₂O₂）溶液进行表面改性,可以有效改善界面相容性;当经过1 400℃热处理再经过表面改性的BN fiber（BN fiber-1400-H₂O₂）的填充量为5%（质量分数）时,复合材料的面内导热率达到了1.32 W·m^-1·K^-1,为纯PVA体系的629%,相比于表面改性前提升了60%。