The effect of growth conditions,point defects and hydrogen on the electronic structure and properties of p-type (Al,N) codoped ZnO: A first principles study

The effects of point defects, hydrogen, and growth conditions on the electronic structure and properties of the (Al,N) codoped p-type ZnO have been investigated using the first principles method. The obtained results showed that the Al_Zn–N_O–V_Zn complex is a shallow acceptor that can play an important role in achieving the p-type conductivity in the (Al,N) codoped ZnO films. Our results showed also that the electrical conductivity type in the (Al,N) codoped ZnO films strongly depends on the donor/acceptor concentrations ratio. The codoped ZnO films prepared under both Zn-rich and O-rich growth conditions with a donors/acceptors ratio of 1:2 have a p-type conductivity, while those prepared with a ratio of 1:1 cannot be p-type unless if they are prepared under O-rich conditions. The achieved p-type quality depends also on the used nitrogen doping source. To prepare p-type ZnO film of high quality using the (Al,N) codoping method, the use of NO or NO₂ is recommended. The presence of donor defects such as oxygen vacancies and hydrogen will significantly affect the electronic properties of the (Al,N) codoped ZnO films, and if the concentration of these defects in the sample is high enough, the material can be easily converted to n-type.     

Optimised phase compositions and improved microwave dielectric properties based on calcium tin silicates

The Ca_(1+2y)Sn_(1-x)Si_(1+y)O_(5-2x+4y) low-permittivity microwave dielectric ceramics were prepared through solid-state reaction at 1350–1450 °C for 5 h. The relations between microwave dielectric properties and phase compositions for non-stoichiometric Ca_(1+2y)Sn_(1-x)Si_(1+y)O_(5-2x+4y) ceramics have been investigated. A single CaSnSiO₅ phase with abnormally positive temperature coefficient of resonant frequency (τ_f = + 62.5 ppm/°C) was synthesised at 1450 °C. This composition was an effective τ_f compensator of CaSiO₃ and Ca₃SnSi₂O₉ phases with typically negative τ_f value. The CaSiO₃ second phase was related to the Sn deficiency in the CaSn_(1-x)SiO_(5-2x) (0 < x < 1.0) composition, whereas the Ca₃SnSi₂O₉ second phase was obtained by controlling the Ca:Sn:Si ratios on the basis of the Ca_(1+2y)SnSi_(1+y)O_(5+4y) (0 < y < 1.0) composition. A promising low-permittivity millimetre-wave ceramic with most excellent microwave dielectric properties (ε_r = 10.2, Q×f = 81,000 GHz and τ_f = −4.8 ppm/°C) was produced from the Ca_(1+2y)SnSi_(1+y)O_(5+4y) (y = 0.4) ceramic.     

Synthesis and properties of cinnamic acid series organic UV ray absorbents–interleaved layered double hydroxides

Organic ultraviolet (UV) ray absorbents, cinnamic acid (CA) and p-methoxycinnamic acid (PMOCA) were intercalated into Zn₂Al layered double hydroxides (Zn₂Al-LDHs) by co-precipitation reaction. The organic–inorganic nanocomposites, Zn₂Al-LDH/CA and Zn₂Al-LDH/PMOCA were obtained. The samples showed excellent UV ray absorption ability and their catalytic activity for the air oxidation of castor oil greatly decreased when the organic UV ray absorbents were intercalated in the layers of the Zn₂Al-LDHs. The studies suggested that Zn₂Al-LDH/organic UV absorbent nanocomposites might be used as safe sunscreen materials.     

Photocatalytic evolution of hydrogen over mesoporous TiO2 supported NiO photocatalyst prepared by single-step sol–gel process with surfactant template

Photocatalytic activity of mesoporous titania supported nickel oxide photocatalyst synthesized by single-step sol–gel (SSSG) process combined with surfactant-assisted template method was investigated for hydrogen evolution from an aqueous methanol solution, in comparison with one prepared by conventional incipient wetness impregnation (IWI) method. In single-step sol–gel process, nickel precursor was introduced into the titania sol prepared with the aid of a surfactant template behaving as pore-controlling agent to attain meso-scaled pore. The single-step sol–gel photocatalyst was experimentally found to enhance the photocatalytic evolution of hydrogen rather than the impregnated one. The optimum level of nickel loading in photocatalytic activity test for single-step sol–gel method was slightly higher than that for incipient wetness impregnation method. Characterization results demonstrated the significant modification of physical characteristics of the single-step sol–gel photocatalyst, anticipated to relating to the observation of higher photocatalytic hydrogen evolution activity.     

Evaluation of accelerated test methods for determining alkali-silica reactivity of concrete aggregates

For assessing the applicability of a newly proposed Chinese accelerated mortar bar test (CAMBT) to overseas aggregates and determining the appropriate aggregate size fraction for the test, the influence of aggregate particle size on ASR expansion was studied at 0.15–0.80 mm, 1.25–2.50 mm and 2.5–5.0 mm size fractions on nine aggregates from a range of sources. Correlation between expansions in the CAMBT and in the accelerated mortar bar test (AMBT), and correlations between the two accelerated tests and the Concrete Prism Test (CPT) were examined. The results indicate that, for most aggregates tested, 0.15–0.80  mm is not the most sensitive aggregate size to expansion in the CAMBT, especially at early period before 10 days. The 1.25–2.50 mm size fraction of all the nine aggregates, gives the highest early expansion (first 10 days). Correlation between expansions in the CAMBT and expansions in the AMBT is satisfactory. However, the correlations in expansions of both AMBT and CAMBT with the CPT are very poor. A better correlation between expansions in the modified CAMBT and in the CPT is obtained when 2.5–5.0 mm aggregate particles was used, but further tests are necessary to establish the full reliability of the test.     

The effect of zirconium addition on the microstructure and properties of chopped carbon fiber/carbon composites

Carbon/carbon composites containing zirconium were prepared using chopped carbon fiber, mesophase pitch and Zr powder by the traditional process including molding, carbonization, densification and graphitization. The influence of Zr on the microstructure and properties of the composites were investigated. Results show that Zr can improve the interface bonding, promote more perfect and larger crystallites and enhance the conductive/mechanical properties of the composites. The high in-plane thermal conductivity of 464 W/(m K) and excellent bending strength of 83.6 MPa was obtained for a Zr content of 13.9 wt% at heat treatment temperature(HTT) of 2500 °C. However the conductive/mechanical properties of the composites decrease dramatically for an higher HTT of 3000 °C. SEM micrograph of the fracture surface for the composites shows that lower disorder crystallite arrangement of fiber and carbon matrix come into being in the composites during HTT of 3000 °C, which should be responsible for the low properties. Correlation between the content of Zr and the microstructure and properties are discussed.     

The current detour effect observed on materials with random microstucture: experimental evidence from Li3xLa2/3−xTiO3 studied by impedance spectroscopy

Impedance spectroscopy (IS) has been used to study the influence on the low frequency part of the impedance diagrams of the microstructure of a fast ionic conductor, Li_3xLa_2/3−xTiO₃ with x = 0.10 (named hereafter LLTO). This oxide has been synthesised by sol–gel method. After synthesis, the powder of LLTO displays a large distribution of grain size and agglomerates. The grain size distribution and the porosity of the ceramic have been changed by heat-treatment from 600 °C to 1200 °C in air. The impedance spectra of these ceramics, recorded at different temperatures from room temperature (RT) to 400 °C, show a low-frequency depressed arc, which is characteristic of the grain boundary response of the ceramic. Its shape depends strongly on the heat-treatment of the ceramic, and therefore, on its microstructure. It is a simple arc when the pellet is well sintered but becomes very complex for non-sintered ceramics with high resistive grain boundary and pores. The observed “fish” shape indicates the presence of current “detours effect” in the material. This effect means that current detours around blocking grain boundary and/or pores occur to lower the impedance. Consequently, the brick layer model (BLM), which assumes an ideal microstructure, and then no current “detours effect”, can not be used to analyse these impedance data.     

Synthesis and electrochemical properties of layered Li[Ni0.333Co0.333Mn0.293Al0.04]O2−zFz cathode materials prepared by the sol–gel method

The cathode-active materials, layered Li[Ni_0.333Co_0.333Mn_0.293Al_0.04]O_2−zF_z (0 ≤ z ≤ 0.1), were synthesized from a sol–gel precursor at 900 °C in air. The influence of Al–F co-substitution on the structural and electrochemical properties of the as-prepared samples was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and electrochemical experiments. The results showed that Li[Ni_0.333Co_0.333Mn_0.293Al_0.04]O_2−zF_z has a typical hexagonal structure with a single phase, the particle sizes of the samples tended to increase with increasing fluorine content. It has been found that Li[Ni_0.333Co_0.333Mn_0.293Al_0.04]O_1.95F_0.05 showed an improved cathodic behavior and discharge capacity retention compared to the undoped samples in the voltage range of 3.0–4.3 V. The electrodes prepared from Li[Ni_0.333Co_0.333Mn_0.293Al_0.04]O_1.95F_0.05 delivered an initial discharge capacity of 158 mAh⁻¹ g and an initial coulombic efficiency is 91.3%, and the capacity retention at the 20th cycle was 94.9%. Though the F-doped samples had lower initial capacities, they showed better cycle performances compared with F-free samples. Therefore, this is a promising material for a lithium-ion battery.     

A foam engulfment model for lost foam casting of aluminum

In lost foam casting of aluminum, pressure equilibrium between the liquid metal and the decomposing foam can produce a variety of different shapes for the metal flow front, ranging from convex to concave. In extreme cases, the flow front can become so strongly concave that small pieces of the foam pattern begin to break off inside the concave hollow of the flow front and become enveloped by the advancing liquid metal. When this happens, the entire mechanism of foam decomposition changes from steady ablation to a more chaotic motion in which the metal seems to “chew” its way through the pattern, creating large bubbles of vaporizing polymer liquid in its wake. These bubbles usually lead to undesirable anomalies in the final casting. In most cases, the nonlinear equations that govern the shape of the flow front depend on a single nondimensional number, which relates the onset of the engulfing motion to specific material, geometric, and process parameters. Numerical solutions to these equations are obtained for several special cases. These solutions help to explain a number of experimental observations that until now have been poorly understood.