SiC/SiO2 coating for improving the oxidation resistive property of carbon nanofiber

The SiC/SiO₂ deposition was performed to improve the oxidation resistive properties of carbon nanofiber (CNF) from electrospinning at elevated temperatures through sol-gel process. The stabilized polyacrylonitrile (PAN) fibers were coated with SiO₂ followed by heat treatment up to 1000 and 1400 °C in an inert argon atmosphere. The chemical compositions of the CNFs surface heat-treated were characterized as C, Si and O existing as SiC and SiO₂ compounds on the surface. The uniform and continuous coating improved the oxidation resistance of the carbon nanofibers. The residual weight of the composite was 70-80% and mixture of SiC, SiO₂ and some residual carbon after exposure to air at 1000 °C.     

Crystallization of hydrogenated amorphous silicon carbon films with laser and thermal annealing

The crystallization of silicon rich hydrogenated amorphous silicon carbon films prepared by Plasma Enhanced Chemical Vapor Deposition technique has been induced by excimer laser annealing as well as thermal annealing. The excimer laser energy density (E_d) and the annealing temperature were varied from 123 to 242 mJ/cm² and from 250 to 1200 °C respectively. The effects of the two crystallization processes on the structural properties and bonding configurations of the films have been studied. The main results are that for the laser annealed samples, cubic SiC crystallites are formed for E_d ≥ 188 mJ/cm², while for the thermal annealed samples, micro-crystallites SiC and polycrystalline hexagonal SiC are observed for the annealing temperature of 800 and 1200 °C respectively. The crystallinity degree has been found to improve with the increase in the laser energy density as well as with the increase in the annealing temperature.     

Preparation and characterization of pulsed laser deposition (PLD) SiC films

Si K-edge XAFS was used to characterize a stoichiometric SiC film prepared by pulsed KrF laser deposition. The film was deposited on a p-type Si(1 0 0) wafer at a substrate temperature of 250 °C in high vacuum with a laser fluence of ∼5 J/cm². The results reveal that the film contains mainly a SiC phase with an amorphous structure in which the Si atoms are bonded to C atoms in its first shell similar to that of crystalline SiC powder but with significant disorder.     

Preparation and electrochemical characterization of cobalt-manganese oxide as electrode materials for electrochemical capacitors

Cobalt-manganese oxide materials (CMOs) were prepared by chemical method and heat treated at 150, 400, 600, 800 and 1000 °C, respectively. The physical and electrochemical properties of the materials were characterized. The heat treatment process leads to the removal of water molecules adsorbed on the surface of CMO particles (below 400 °C) and the progressive reduction of Mn and Co ions from Mn⁴⁺ and Co³⁺ to Mn³⁺/Mn²⁺ and Co²⁺, respectively (440-1000 °C). CMOs obtained by treatment below 800 °C have poor crystallinity and a highly crystallized tetragonal phase by treatment at 1000 °C. The ratio of Mn and Co in CMOs is found by EDX analysis to be about 2:1. The electrochemical testing results indicate that the high crystallization of CMO is disadvantageous for the energy storage as electrode material of electrochemical capacitors. However, for CMOs with poor crystallinity, relatively high specific capacitances can be obtained. The incorporation of protons and ions into the CMO's lattice during electrochemical charge/discharge process leads to the distortion of crystal lattice and improvement of crystallinity of CMO. The XRD patterns show that negative electrode (NE) and positive electrode (PE) have tetragonal (Co, Mn)(Mn, Co)₂O₄ phase.     

Synthesis of praseodymium oxide nanofiber by electrospinning

Well-defined and uniform Pr₆O₁₁ nanofibers were synthesized by electrospinning of an aqueous sol-gel consisting of praseodymium nitrate hexa-hydrate and polyvinyl acetate. The synthesized Pr₆O₁₁ nanofibers mat was dried at 80 °C for 24 h under vacuum and finally annealed at 600 °C for 2 h in static air furnace. From crystalline properties, the synthesized Pr₆O₁₁ nanofibers XRD analysis revealed the typical cubic structure. The morphological observation showed that the synthesized Pr₆O₁₁ nanofibers composed of fibers length in several 100 nm and diameter of ∼20 nm. Similarly, transmission electron microscope (TEM) measurement revealed the good crystalline nature of the synthesized Pr₆O₁₁ nanofibers with the average diameter of ∼20 nm. Photoluminescence (PL) demonstrated a strong green-blue emission peak at 521 nm, suggesting that the Pr₆O₁₁ nanofiber exhibited good crystal quality with very less structural defects.     

Temperature and substrate dependence of structure and growth mechanism of carbon nanofiber

The carbon nanofibers were grown on Ni/Si and Ni/Ti/Si substrates in 1 atm CH₄ atmosphere at 640 °C and 700 °C by thermal chemical vapor deposition method. The carbon nanofibers were characterized by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectrometry for morphology, microstructure, and crystallinity. The electron emission property of carbon nanofibers was also investigated by current-voltage (I-V) measurement. The results showed that the solid amorphous carbon nanofibers could be grown on Ni/Si substrate at 640 °C through tip growth mechanism, the carbon nanotubes could be grown on Ni/Si substrate at 700 °C through tip growth mechanism, and the carbon nanotubes could be grown on Ni/Ti/Si substrate at 700 °C through root growth mechanism.     

Low-temperature oxidation of SiC surfaces by supercritical water oxidation

The oxidation process on silicon carbide (SiC) surfaces is important for wide bandgap power semiconductor devices. We investigated SiC oxidation using supercritical water (SCW) at high pressure and temperature and found that a SiC surface can be easily oxidized at low temperature. The oxidation rate is 10 nm/min at 400 °C and 25 MPa, equal to that of conventional thermal dry oxidation at 1200 °C. Low-temperature oxidation should contribute to improved performance in future SiC devices. Moreover, we found that SCW oxidation at 400 °C forms a much smoother SiO₂/SiC interface than that obtained by conventional thermal dry oxidation. A higher oxidation rate and smaller microroughness are achieved at a lower oxidation temperature owing to the high density of oxidizers under SCW conditions.     

Synthesis of silicon carbide films by combined implantation with sputtering techniques

Silicon carbide (SiC) films were synthesized by combined metal vapor vacuum arc (MEVVA) ion implantation with ion beam assisted deposition (IBAD) techniques. Carbon ions with 40 keV energy were implanted into Si(1 0 0) substrates at ion fluence of 5 × 10¹⁶ ions/cm². Then silicon and carbon atoms were co-sputtered on the Si(1 0 0) substrate surface, at the same time the samples underwent assistant Ar-ion irradiation at 20 keV energy. A group of samples with substrate temperatures ranging from 400 to 600 °C were used to analyze the effect of temperature on formation of the SiC film. Influence of the assistant Ar-ion irradiation was also investigated. The structure, morphology and mechanical properties of the deposited films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and nanoindentation, respectively. The bond configurations were obtained from IR absorption and Raman spectroscopy. The experimental results indicate that microcrystalline SiC films were synthesized at 600 °C. The substrate temperature and assistant Ar-ion irradiation played a key role in the process. The assistant Ar-ion irradiation also helps increasing the nanohardness and bulk modulus of the SiC films. The best values of nanohardness and bulk modulus were 24.1 and 282.6 GPa, respectively.     

Crystalline quality of 3C-SiC formed by high-fluence C-implanted Si

Carbon ions at 40 keV were implanted into (1 0 0) high-purity p-type silicon wafers at 400 °C to a fluence of 6.5 × 10¹⁷ ions/cm². Subsequent thermal annealing of the implanted samples was performed in a diffusion furnace at atmospheric pressure with inert nitrogen ambient at 1100 °C. Time-of-flight energy elastic recoil detection analysis (ToF-E ERDA) was used to investigate depth distributions of the implanted ions. Infrared transmittance (IR) and Raman scattering measurements were used to characterize the formation of SiC in the implanted Si substrate. X-ray diffraction analysis (XRD) was used to characterize the crystalline quality in the surface layer of the sample. The formation of 3C-SiC and its crystalline structure obtained from the above mentioned techniques was finally confirmed by transmission electron microscopy (TEM). The results show that 3C-SiC is directly formed during implantation, and that the subsequent high-temperature annealing enhances the quality of the poly-crystalline SiC.     

Formation of atomically smooth SiO2/SiC interfaces at ∼120 °C by use of nitric acid oxidation method

Conventional thermal oxidation of SiC requires heating at ∼1100 °C. In the present study, we have developed a method of oxidizing SiC at low temperatures (i.e., ∼120 °C) to form relatively thick silicon dioxide (SiO₂) layers by use of nitric acid. When 4H-SiC(0 0 0 1) wafers are immersed in 40 wt% HNO₃ at the boiling temperature of 108 °C and the boiling is kept for 5 h after reaching the azeotropic point (i.e., 68 wt% HNO₃ at 121 °C), 8.1 nm thick SiO₂ layers are formed on the SiC substrates. High resolution transmission electron microscopy measurements show that the SiO₂/SiC interface is atomically flat and the SiO₂ layer is uniform without bunching. When SiC is immersed in an azeotropic mixture of HNO₃ with water from the first, the SiO₂ thickness is less than 0.3 nm. The metal-oxide-semiconductor (MOS) diodes with the SiO₂ layer formed by the nitric acid oxidation method possess a considerably low leakage current density.     

Amorphous to crystalline phase transition in pulsed laser deposited silicon carbide

SiC thin films were grown on Si (1 0 0) substrates by excimer laser ablation of a SiC target in vacuum. The effect of deposition temperature (up to 950 °C), post-deposition annealing and laser energy on the nanostructure, bonding and crystalline properties of the films was studied, in order to elucidate their transition from an amorphous to a crystalline phase. Infra-red spectroscopy shows that growth at temperatures greater than 600 °C produces layers with increasingly uniform environment of the Si-C bonds, while the appearance of large crystallites is detected, by X-ray diffraction, at 800 °C. Electron paramagnetic resonance confirms the presence of clustered paramagnetic centers within the sp² carbon domains. Increasing deposition temperature leads to a decrease of the spin density and to a temperature-dependent component of the EPR linewidth induced by spin hopping. For films grown below 650 °C, post-deposition annealing at 1100 °C reduces the spin density as a result of a more uniform Si-C nanostructure, though large scale crystallization is not observed. For greater deposition temperatures, annealing leads to little changes in the bonding properties, but suppresses the temperature dependent component of the EPR linewidth. These findings are explained by a relaxation of the stress in the layers, through the annealing of the bond angle disorder that inhibits spin hopping processes.     

Silicon carbide nanocrystals growth on Si(1 0 0) and Si(1 1 1) from a chemisorbed methanol layer

SiC nanocrystals are grown at high temperature on Si(1 0 0) and Si(1 1 1) surfaces starting from a chemisorbed layer of methanol. The decomposition of this layer allows to have a well defined amount of carbon to feed SiC growth. Nanocrystals ranging from 10 nm to 50 nm with density from 100 μm⁻² to 1500 μm⁻² are obtained, and the total volume of produced SiC corresponds to carbon provided by the chemisorbed organic layer. Large differences in nanocrystal size and density, as well as in surface roughness, are observed depending on substrate orientation. The internal structure, crystallinity and epitaxy of nanocrystals grown on Si(1 0 0) are studied using cross-sectional transmission electron microscopy (XTEM), methanol adsorption and surface evolution using scanning tunnelling microscopy (STM). The joint application of XTEM and STM techniques allows a complete characterization of the geometry and chemical composition of these nanostructures.     

Preparation of anti-oxidative carbon fiber at high temperature

In this paper, carbon fibers with improved thermal stability and oxidation resistive properties were prepared and evaluated their physical performances under oxidation condition. Carbon fibers were coated with SiC particles dispersed in a polyacrylonitrile solution and then followed by pyrolyzed at 1400 °C to obtain the SiC nanoparticle deposition on the surface of the carbon fiber. The SiC coated carbon fiber showed extended oxidation resistive property as remaining 80-88% of the original weight even at high temperature 1000 °C under air, as compared with the control of zero weight at 600 °C. The effects of the coating conditions on the oxidation resistive properties of the coated fibers were studied in detail.     

The effect of hydrogen irradiation and annealing on the low-temperature growth of homoepitaxial ZnO layers grown on (0 0 0 1) ZnO substrates by plasma-assisted molecular beam epitaxy

Low-temperature growth of high quality homoepitaxial ZnO is realized by using hydrogen irradiation and annealing processes (modified method). By modified method, two-dimensional growth and atomically smooth surface with steps (terrace length ∼75 nm) are achieved at 400 °C. Furthermore, FWHMs of high resolution X-ray rocking curves for (0 0 0 2) and (10-11) reflections are evaluated as narrow as 21 arcsec, which indicates that high crystallinity of the ZnO grown at 400 °C by modified method is almost similar to that grown at 600 °C by conventional method. Photoluminescence results show the considerable improvement of optical properties, such as an emersion of free exciton (FX) and a decrease of donor bound exciton (D°X) linewidth, by using modified method, even at growth temperature as low as 400 °C.     

Origin of ohmic behavior in Ni, Ni2Si and Pd contacts on n-type SiC

Ni, Ni₂Si and Pd contacts were prepared on n-type 4H-SiC and annealed in the temperature range of 750-1150 °C. The annealed contacts were analyzed before and after acid etching, and different features were found in unetched and etched contacts. Carbon left on the SiC surface after the acid etching of Ni₂Si contacts annealed at 960 °C was highly graphitized. In nickel contacts, the graphitization of interface carbon began at 960 °C and increased after annealing at higher temperatures. In palladium contacts, the onset of the interface carbon graphitization was observed after annealing at 1150 °C. For all three types of metallization, the minimal values of contact resistivity were achieved only when the sharp first-order peak at 1585 cm⁻¹ and distinct second-order peak at ∼2700 cm⁻¹ related to the presence of graphitized carbon were detected by Raman spectroscopy after the acid etching of contacts. The properties of unannealed secondary contacts deposited onto etched primary contacts were similar to the properties of the primary contacts unless carbon was selectively etched. The results show that ohmic behavior of Ni-based and Pd contacts on n-type SiC originates from the formation of graphitic carbon at the interface with SiC.     

Effect of composition and deposition temperature on the characteristics of Ga doped ZnO thin films

ZnO films doped with Ga (GZO) of varying composition were prepared on Corning glass substrate by radio frequency magnetron sputtering at various deposition temperatures of room temperature, 150, 250 and 400 °C, and their temperature dependent photoelectric and structural properties were correlated with Ga composition. With increasing deposition temperature, the Ga content, at which the lowest electrical resistivity and the best crystallinity were observed, decreased. Films with optimal electrical resistivity of 2-3 × 10⁻⁴ Ω cm and with good crystallinity were obtained in the substrate temperature range from 150 to 250 °C, and the corresponding C_Ga/(C_Ga + C_Zn) atomic ratio was about 0.049. GZO films grown at room temperature had coarse columnar structure and low optical transmittance, while films deposited at 400 °C yielded the highest figure of merit (FOM) due to very low optical absorption despite rather moderate electrical resistivity slightly higher than 4 × 10⁻⁴ Ω cm. The optimum Ga content at which the maximum figure of merit was obtained decreased with increasing deposition temperature.     

Cleaning of SiC surfaces by low temperature ECR microwave hydrogen plasma

N-type 4H-SiC (0 0 0 1) surfaces were cleaned by low temperature hydrogen plasma in electronic cyclotron resonance (ECR) microware plasma system. The effects of the hydrogen plasma treatment (HPT) on the structure, chemical and electronic properties of surfaces were characterized by in situ reflection high energy electron diffraction (RHEED) and X-ray photoelectron spectroscopy (XPS). The RHEED results indicate that the structures of the films are strongly dependent on the treatment temperature and time. Significant improvements in quality of 4H-SiC films can be obtained with the temperature ranging from 200 °C to 700 °C for an appropriate treatment period. The XPS results show that the surface oxygen is greatly reduced and the carbon contamination is completely removed from the 4H-SiC surfaces. The hydrogenated SiC surfaces exhibit an unprecedented stability against oxidation in the air. The surface Fermi level moves toward the conduction band in 4H-SiC after the treatment indicating an unpinning Fermi level with the density of surfaces states as low as 8.09 × 10¹⁰ cm⁻² eV⁻¹.     

Thermoluminescent and stuctural properties of BaAl2O4:Eu,Nd,Gdphosphors prepared by combustion method

BaAl₂O₄:Eu²⁺,Nd³⁺,Gd³⁺ phosphors were prepared by a combustion method at different initiating temperatures (400–1200 °C), using urea as a comburent. The powders were annealed at different temperatures in the range of 400–1100 °C for 3 h. X-ray diffraction data show that the crystallinity of the BaAl₂O₄ structure greatly improved with increasing annealing temperature. Blue-green photoluminescence, with persistent/long afterglow, was observed at 498 nm. This emission was attributed to the 4f⁶5d¹–4f⁷ transitions of Eu²⁺ ions. The phosphorescence decay curves were obtained by irradiating the samples with a 365 nm UV light. The glow curves of the as-prepared and the annealed samples were investigated in this study. The thermoluminescent (TL) glow peaks of the samples prepared at 600 °C and 1200 °C were both stable at ∼72 °C suggesting that the traps responsible for the bands were fixed at this position irrespective of annealing temperature. These bands are at a similar position, which suggests that the traps responsible for these bands are similar. The rate of decay of the sample annealed at 600 °C was faster than that of the sample prepared at 1200 °C.     

Effect of different annealing temperature on Sb-doped ZnO thin films prepared by pulsed laser deposition on sapphire substrates

Influence of annealing temperature on the properties of Sb-doped ZnO thin films were studied. Hall measurement results indicated that the Sb-doped ZnO annealed at 950 °C was p-type conductivity. X-ray diffraction (XRD) results indicated that the Sb-doped ZnO thin films prepared at the experiments are high c-axis oriented. It was worth noting that p-type sample had the worst crystallinity. The measurements of low-temperature photoluminescence (PL) spectra indicate that the sample annealed at the temperatures of 950 °C showed strong acceptor-bound exciton (A⁰X) emission, and confirmed that it is related to Sb-doping by comparing with the undoped ZnO low-temperature PL spectrum.