Meijun Yang

Morphology and mechanical behavior of diamond films fabricated by IH-MPCVD

Morphology of diamond films has been controlled via intermediate frequency induction heated microwave plasma chemical vapor deposition (IH-MPCVD), which was transformed with various substrate temperatures (Tsub = 923–1123 K) and CH4/H2 ratios (ηc = 0.5–2 vol%). The coupling effects of Tsub and ηc on the structure of diamond films have been studied. At ηc = 0.5 vol%, the sp3/sp2 ratio of diamond films reached 98% at 1073 K, surface roughness (Rms) increased from 50 to 85 nm with increasing Tsub, the maximum hardness (Ha) reached 84 GPa at 973 K, and the maximum Youngs modulus (E) reached 642 GPa at 1023 K. The residual stress (σ) was calculated as a function of Tsub and ηc. The quality factor (Q), combining microstructure and mechanical behavior, has been creatively defined to evaluate the quality of diamond films.

Structural study of epitaxial NdBa2Cu3O7−x films by laser chemical vapor deposition

Epitaxial NdBa2Cu3O7−x films were prepared on (100) LaAlO3 single crystal substrates by laser chemical vapor deposition (laser CVD). The effect of deposition temperature on preferred orientation, crystallinity, microstructure, deposition rate of films was investigated. The preferred orientation of the NdBCO films changed from a, c-axis to c-axis, then back to a, c-axis, as the deposition temperature was increased from 993 to 1093 K. The highly c-axis-oriented epitaxial NdBa2Cu3O7−x film with critical transition temperature of 87 K was obtained at Tdep = 1033 K, with the in-plane epitaxial orientation relationship of NdBCO [100]∥LAO [010] and NdBCO [010]∥LAO [001]. The growth orientation varied from a-axis to c-axis as the film thickness increased in the case of the a, c-co-oriented NdBCO film. The contribution of thickness-dependent strain relaxation to preferred orientation of the NdBCO films was also discussed.

Morphological Evolution of Vertically Standing Molybdenum Disulfide Nanosheets by Chemical Vapor Deposition

In this study, we demonstrated the chemical vapor deposition (CVD) of vertically standing molybdenum disulfide (MoS2) nanosheets, with an unconventional combination of molybdenum hexacarbonyl (Mo(CO)6) and 1,2-ethanedithiol (C2H6S2) as the novel kind of Mo and S precursors respectively. The effect of the distance between the precursor’s outlet and substrates (denoted as d) on the growth characteristics of MoS2, including surface morphology and nanosheet structure, was investigated. Meanwhile, the relationship between the structure characteristics of MoS2 nanosheets and their catalytic performance for hydrogen evolution reaction (HER) was elucidated. The formation of vertically standing nanosheets was analyzed and verified by means of an extrusion growth model. The crystallinity, average length, and average depth between peak and valley (Rz) of MoS2 nanosheets differed depending on the spatial location of the substrate. Good crystalized MoS2 nanosheets grown at d = 5.5 cm with the largest average length of 440 nm, and the highest Rz of 162 nm contributed to a better HER performance, with a respective Tafel slope and exchange current density of 138.9 mV/decade, and 22.6 μA/cm2 for raw data (127.8 mV/decade and 19.3 μA/cm2 for iR-corrected data).

Elimination of Voids at Interface of β-SiC Films and Si Substrate by Laser CVD

Void-free β-SiC films were deposited on Si(001) substrates by laser chemical vapor deposition using hexamethyldisilane (HMDS) as the precursor. The effect of the time of introducing HMDS, i e, the substrate temperature when HMDS introduced (Tin), on the preferred orientation, surface microstructure and void was investigated. The orientation of the deposited SiC films changed from <001> to random to <111> with increasing Tin. The surface showed a layer-by-layer microstructure with voids above Tin ⩾ 773 K, and then transformed into mosaic structure without voids at Tin= 298 K. The mechanism of the elimination of voids was discussed. At Tin =298 K, Si surface can be covered by an ultrathin SiC film, which inhibits the out-diffusion of Si atoms from substrate and prohibites the formation of the voids.

Enhanced Thermoelectric Performance of Non-equilibrium Synthesized Fe0.4Co3.6Sb12-xGex Skutterudites via Randomly Distributed Multi-scaled Impurity Dots

The p-type Ge doped Fe0.4Co3.6Sb12-xGex skutterudites with multi-scaled impurity dots (500 nm-2 mm) were successfully prepared by using melt-quenching (MQ) and subsequent spark plasma sintering (SPS) technique. Compared with traditional method, the new technology significantly shortened the processing time from several days to less than 24 hours. The phase of impurity dots was demonstrated to be CoSb through analysis of X-ray diffraction (XRD) and energy-dispersive spectrum (EDS). Impurity dots were induced by Ge substitution of Sb in the non-equilibrium synthesized process. Due to the abandonment of the long reaction of annealing crystallization, a few of Ge atoms would fail to substitute Sb site of skutterudite in this non-equilibrium synthesized process, leading to that the multi-scaled impurity dots randomly distributed in the matrix of skutterudite Fe0.4Co3.6Sb12-xGex. The combination of multi-scaled impurity dots scattering long wavelength heat-carrying phonons and the point defect scattering short and middle wavelength heat-carrying phonons dramatically made the 22.2% reduction of lattice thermal conductivity. As a result, compared with unsubstituted sample of Fe0.4Co3.6Sb12, the maximum ZT value was increased by 30.5%. Thus, the two marked features of this new synthesis process, the shortened preparation time and the enhanced thermoelectric performance, would make a promising commercial application in the future.

Catalytic Decomposition of Nitric Oxide by LaCoO3 Nano-particles Prepared by Rotary CVD

Catalytic direct decomposition of NO by perovskite-type catalysts has attracted much attention for the various possible components and the unique structure. LaCoO3 nanoparticles were precipitated on α-Al2O3 micro powders by rotary chemical vapor deposition (rotary CVD) and its catalytic performance for the decomposition of NO was investigated. LaCoO3 nano-particles with 100 nm in average diameter and 1.5% in mass were uniformly dispersed on α-Al2O3 powder. The conversion of NO increased with increasing temperature from 400 to 950 °C, and reached 28.7% at 950 °C. The gas velocity of transformed NO on LaCoO3 nano-particles catalyst per mass unit was 7.7 mL/(g min), showing a good catalytic activity over the calculated results of pure catalysts. After five times of aging performance experiments, the NO conversion kept the same value, showing a good aging performance and thermal stability.

Thickness dependence of structure and superconductivity of the SmBa2Cu3O7 film by laser CVD

SmBa2Cu3O7 (SmBCO) superconductive films with thicknesses in the range from 0.47 to 2.72 μm were prepared on single-crystalline LAO (100) substrates by laser chemical vapor deposition (laser CVD). The effect of thickness on the orientation, microstructure, and superconductivity of SmBCO films was investigated. The preferred orientation of the SmBCO film changed from c-axis to a- and c-axis co-orientation with an increase in thickness from 0.47 to 2.72 μm. The SmBCO film with 1.06 μm thickness exhibited excellent c-axis epitaxial orientation with a minimum full width at half maximum of 1.07° for φ-scan and an in-plane epitaxial orientation relationship of SmBCO[100]//LAO[010] and SmBCO[010]//LAO[001]. The highest critical transition temperature and critical current density were 89.5 K and 1.92 MA cm−2 (corresponding to the critical current of 204 A cm−1-W) with a deposition rate of 7.6 μm h−1.