Lifu Hei

Energy consumption of electrooxidation systems with boron-doped diamond electrodes in the pulse current mode

A pulse current technique was conducted in a boron-doped diamond (BDD) anode system for electrochemical wastewater treatment. Due to the strong generation and weak absorption of hydroxyl radicals on the diamond surface, the BDD electrode possesses a powerful capability of electrochemical oxidation of organic compounds, especially in the pulse current mode. The influences of pulse current parameters such as current density, pulse duty cycle, and frequency were investigated in terms of chemical oxygen demand (COD) removal, average current efficiency, and specific energy consumption. The results demonstrated that the relatively high COD removal and low specific energy consumption were obtained simultaneously only if the current density or pulse duty cycle was adjusted to a reasonable value. Increasing the frequency slightly enhanced the COD removal and average current efficiency. A pulse-BDD anode system showed a stronger energy saving ability than a constant-BDD anode system when the electrochemical oxidation of phenol of the two systems was compared. The results prove that the pulse current technique is more cost-effective and more suitable for a BDD anode system for real wastewater treatment. A kinetic analysis was presented to explain the above results.

Optical characterization of single-crystal diamond grown by DC arc plasma jet CVD

Optical centers of single-crystal diamond grown by DC arc plasma jet chemical vapor deposition (CVD) were examined using a low-temperature photoluminescence (PL) technique. The results show that most of the nitrogen-vacancy (NV) complexes are present as NV− centers, although some H2 and H3 centers and B-aggregates are also present in the single-crystal diamond because of nitrogen aggregation resulting from high N2 incorporation and the high mobility of vacancies under growth temperatures of 950–1000°C. Furthermore, emissions of radiation-induced defects were also detected at 389, 467.5, 550, and 588.6 nm in the PL spectra. The reason for the formation of these radiation-induced defects is not clear. Although a Ni-based alloy was used during the diamond growth, Ni-related emissions were not detected in the PL spectra. In addition, the silicon-vacancy (Si-V)-related emission line at 737 nm, which has been observed in the spectra of many previously reported microwave plasma chemical vapor deposition (MPCVD) synthetic diamonds, was absent in the PL spectra of the single-crystal diamond prepared in this work. The high density of NV− centers, along with the absence of Ni-related defects and Si-V centers, makes the single-crystal diamond grown by DC arc plasma jet CVD a promising material for applications in quantum computing.

Preparation of nano-diamond films on GaN with a Si buffer layer

Gallium nitride (GaN) has been widely used in electronic and optoelectronic devices because of its unique electrical properties. However, its low thermal conductivity and the high thermal boundary resistance at the interface between GaN and substrates such as Si and Al2O3preventefficient heat dissipation from the heated regions, which limits the further development of GaN-based high power devices. Diamond, with the highest thermal conductivity, has been considered to be one of the most promising heat sink materials. However, it is hard to prepare a diamond film on a GaN substrate because there is a high thermal expansion coefficient difference and also a large lattice mismatch between them. An approach to prepare a nano-diamond film on a GaN substrate by incorporating a Si buffer layer has been proposed. A GaN substrate decomposes significantly from 560 to 680°C when exposed to ahydrogen plasma for 5 min and no adhesive nano-diamond film can be directly grown on it. This decomposition is significantly suppressed by the presence of a Si buffer layer and a nano-diamond film about 2 μm thick can be deposited on a GaN substrate by microwave chemical vapor deposition using CH4 as the carbon source. With an optimum Si layer of 10 nm, the adhesive force between the nano-diamond film and the GaN substrate reaches 10N, which is ascribed to the complete conversion of the Si layer to a silicon carbidetransition layer during the deposition.

Factors Affecting the Fracture Strength of Freestanding Diamond Films

As an emerging brand new type of engineering material for a variety of important high technology applications, the deep understanding of the mechanical behavior of freestanding diamond films has become an emergent task of vital importance. Unfortunately the mechanical behavior of this brand new material is not fully understood. Effects that affect the fracture strength are still not very clear except that the fracture strength of freestanding diamond films is only depended on the grain size (film thickness), and is insensitive to the microscopic defects, and the strength is considerably higher when the nucleation side is in tension than that when the growth side is in tension. However, this is not the full story. Based on the experimental date accumulated in USTB (University of Science and Technology Beijing) for high power dc arcjet diamond films, other factors that may affect the fracture strength of freestanding diamond films are discussed in detail. Effects of the quality of the diamond films, impurities (nitrogen, hydrogen, non-diamond carbon etc.), film morphology and texture, on fracture strength are discussed. Effects of the deposition parameters are explained. Advantages for small amount of nitrogen addition, and for the use of higher substrate temperatures in the increase in fracture strength are demonstrated, which have already been applied in the mass production of tool grade freestanding diamond films. It is hoped that the present paper will be helpful for those who wish to understand and use this brand new type of engineering material

Thermal Stress in Free-standing Diamond Films with Cr Interlayer Destroyed

Thermal stress in large area free-standing diamond films was remarkable during the post-deposition cooling of direct current (DC) arc plasma jet chemical vapor deposition (CVD) process. In this research, the stress release caused by delamination of Cr interlayer was of great importance to ensure the integrity of free-standing diamond film. The effects of Cr interlayer on Mo substrate, namely composite substrate, on thermal stress were investigated. Thermo-mechanical coupling analysis of the thermal stress was applied by finite element analysis (FEA) using ANSYS code. It was found that the interlayer could be destroyed first by the large thermal stress, and then the stress could be released and the probability of diamond film crack initiation would be reduced. The stress concentration at the bent edge of diamond film was also discussed. In addition, diamond films deposited on Mo substrates with and without Cr interlayer were prepared by DC arc plasma jet CVD system and experimental measurements were used to characterize these films. It was found that composite substrate could be an effective method of growing free-standing crack-free diamond films by DC arc plasma jet CVD system when there is no special requirement to the film strength.