Various Temperatures In Vacuum Research Articles

Up-scalable preparation of nano zirconium carbide powder in liquid polymeric precursor and its pyrolysis mechanism

Nano size ZrC powder was prepared by liquid polymeric precursor method. Zirconium n-butoxide (Zr(O n Bu) 4 ) and benzoylacetone (BA) were mixed directly with different molar ratios to synthesize transparent liquid zirconium carbide single-source precursors. The carbon content in the precursor could be changed by adding different amount of BA. X-ray pure ZrC was obtained when the molar ratio of BA/Zr(O n Bu) 4 was 4.6:1. The viscosity of the precursor was very low (<8 mPa s) without the addition of solvents. Zirconium carbide powders were fabricated by the pyrolysis at 800 °C in argon and subsequent heating at various temperatures in vacuum for carbothermal reduction reaction. The pyrolysis behavior, phase composition and transformation, and microstructure of the as-fabricated ZrC powders were analyzed. The gases of CH 4 , CO and CO 2 released due to decomposition and evaporation of the organic component and transformation from ZrO 2 to ZrC during pyrolysis resulted in total 60–70% mass loss. The average grain size of the synthesized X-ray pure ZrC powders was less than 30 nm. Meanwhile, the pyrolysis mechanism of nano zirconium carbide powder was deduced.

An improvement of photocatalytic activity of TiO2 Degussa P25 powder

The photocatalytic activities of Degussa P25 powders annealed at various temperatures in vacuum and air were studied together with investigations of their compositions by XPS, of their crystal structures by XRD and of their specific surface areas by BET. It is shown that the photocatalytic activity of P25 powders was remarkably enhanced after vacuum annealing; the kinetic coefficient can be raised by 75% during annealing at 400°C. It is obvious that this enhancement is not related to the adhesion ability of the P25 powders.

Effects of Copper Interlayer and Annealing on Structure and Mechanical Properties of Diamond-Like Carbon Films by Cathode Arc Evaporation

Diamond-like carbon (DLC) bilayer films with Cu interlayer were prepared on silicon substrate by direct-current and pulsed cathode arc plasma technique, and annealed at various temperatures in vacuum. Structure, morphology and mechanical properties of the bilayer films were investigated by Raman spectroscopy, Auger electron spectroscopy, scanning electron microscopy and atomic force microscopy, surface profilometer and Vickers sclerometer. The results show that Cu interlayer changes the bilayer microstructure, including the thickness and element distribution of diffusion layer, the relative fraction of sp3/sp2bonding and growth model of bilayer. A simple three-layer model was used to describe the interdiffusion between Cu and C layer. Cu interlayer could be more effective against graphitization upon annealing. Morphological characteristics of the films were studied by analyzing the surface features of substrate. Cu/DLC bilayer exhibits highly dispersed nano-agglomerates with smaller size on the surface due to low surface energy of Cu interlayer. The stress and hardness of the films were affected accordingly. Cu/DLC bilayer shows a relatively high hardness at low annealing temperature but the stress almost no change. By changing Cu interlayer and annealing temperature, excellent DLC films could be designed for the protective, hard, lubricating and wear resistant coatings on mechanical, electronic and optical applications.

Structure and mechanical properties of diamond-like carbon films with copper functional layer by cathode arc evaporation

Using direct-current and pulsed cathode arc plasma technique, diamond-like carbon (DLC) bilayer films with Cu functional layer (interlayer and surface layer) were prepared on silicon substrate and annealed at various temperatures in vacuum. The effects of Cu functional layer and annealing temperature on the structure, morphology and mechanical properties of the films were investigated. The results showed that Cu functional layer changed the bilayers microstructure, including the size of sp2 carbon clusters and the thickness and element distribution of diffusion layer. A simple three-layer model was used to describe the interdiffusion between Cu and C layers. Cu interlayer facilitated the graphitic rearrangement in bilayer films due to weak Cu/C bonds and Cu surface layer could be more effective against graphitization. Cu/DLC and DLC/Cu bilayers showed highly dispersed nano-agglomerates on the surface, and the particle size and surface roughness of the bilayers were largely affected by annealing temperature. The hardness and stress of the bilayer were dependent on the location of Cu functional layer. DLC/Cu bilayer possessed a relatively higher hardness and a lower stress at low annealing temperature of 100–400°C. The results may be of interest for studying the relaxation of stress in the DLC bilayer films and can be used in advancing the development of DLC based engineering materials with high mechanical properties.

Effect of Sb incorporation on the dark conductivity and photoconductivity of Se 75In 25 glassy alloy thin films

In the present paper current–voltage ( I– V) characteristics have been studied at various temperatures in vacuum evaporated thin films of Se 75In 25− x Sb x (where x=0, 5, 10 and 15) glassy alloys. Ohmic behavior is observed at low electric fields, while at high electric fields ( E∼10 4 V/cm) current becomes superohmic. An analysis of the experimental data confirms that due to large currents dielectric breakdown occurs at high voltages which may prohibit the SCLC mechanism in Se 75In 25 sample. Such type of behavior is not observed when the third element Sb as an impurity is incorporated in the Se 75In 25 binary glassy alloy. In case of samples with 5–15 at% of Sb, the experimental data are found to fit well with the theory of space charge limited conduction (SCLC). Density of defect states (DOS) near Fermi-level is determined for these samples by applying the theory of an SCLC. Temperature and intensity dependence of the photoconductivity in the aforesaid glassy systems has been also examined. The variation in DOS could be correlated with the photoconductivity results obtained. The observed discontinuity at 10 at% of an Sb could be correlated with the coordination number and chemically ordered network model (CONM).

Effect of Bi incorporation on defect state density in a-Se85−xTe15Bix thin films

Current voltage ( I–V ) characteristics have been studied at various temperatures in vacuum evaporated thin films of a-Se 85− x Te 15 Bi x ( x= 0, 1, 2, 3, 4, 5) glassy alloys. Ohmic behavior is observed at low electric field while at high electric field ( E ∼10 4 V/cm) current becomes superohmic. An analysis of the experimental data reveals that in case of samples with 0–2 at% Bi, the experimental data fit well with the theory of space charge limited conduction (SCLC) for uniform distribution of localized states in the mobility gap. Such type of behavior is not observed at higher concentration of Bi in the present glassy system. Due to high conductivity in these samples, joule heating due to large currents may prohibit the SCLC mechanism. Density of localized defect states near Fermi level is determined for these samples by fitting the data in the theory of SCLC. The results indicate that density of defect states increases with Bi addition to binary Se 85 Te 15 alloy. This is explained in terms of electronegativity difference of Bi as compared to host elements.

Blue/White Emission from Hydrogenated Amorphous Silicon Carbide Films Prepared by PECVD

AbstractPhotoluminescence (PL) from hydrogenated silicon carbide (SiC:H) films is studied at room temperature. The films were deposited by plasma-enhanced chemical vapor (PECVD) technique with and without substrate bias using methyltrichlorosilane as a main precursor. After the deposition the samples were annealed at various temperatures in vacuum. The films were characterized by atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The samples deposited without substrate bias (series A) were amorphous, whereas the samples deposited with negative substrate bias -100V (series B) were nanocrystalline. The one-peak (470 nm) and double-peak (415 and 437 nm) PL structures of the as-deposited samples A and B were observed, respectively. Annealing strongly enhanced intensity of PL of the samples B and trandformed PL spectrum from double-peak into broad featureless band with intensity at about 470 nm. The blue PL in as-deposited films B is supposed to be assigned to the radiative recombination in the sites located at the nanocrystallite surface, whereas the photo excitation of carries mostly occurs in nanocrystallite cores. A further increase in annealing temperature causes hydrogen effusion, which leads to an increase of the concentration of non-raidative recombination centers associated with dangling-bonds and as a result, to the quenching of PL.

Enhancement of magnetic moment of Co-doped ZnO films by postannealing in vacuum

The Co-doped ZnO thin films were prepared on c-cut sapphire substrates by magnetron cosputtering, and then annealed at various temperatures in vacuum. Magnetic measurements indicate that all the films are ferromagnetic at room temperature and the magnetization of the annealed Zn0.88Co0.12O films is increased about one order of magnitude in comparison with the corresponding as-deposited one. The enhancement of magnetization is possibly due to the fact that the generation of oxygen vacancies during annealing in vacuum increases the carrier (electron) concentration, which is consistent with the mechanism of carrier-induced ferromagnetism. Optical spectrometry indicates that Co2+ enters the tetrahedral sites of the wurtzite structure of ZnO host and substitutes for Zn2+.

Study of defect states in a-Se85Te15−xPbx thin films by space charge limited conduction mechanism

Current–Voltage (I–V) characteristics have been studied at various temperatures in vacuum evaporated thin films of a-Se85Te15−xPbx (x = 0, 2, 4, 6) alloys. These characteristics show that, at low electric fields, an ohmic behaviour is observed. However, at high electric fields (E ∼ 104 V/cm), the current becomes superohmic. At high fields, in case of samples having 0 and 2 at% of Pb, the experimental data fits well with the theory of space charge limited conduction (SCLC) in case of uniform distribution of localized states in the mobility gap. Such type of behaviour is not observed at higher concentration of Pb in the present glassy system due to high conductivity. In these samples, joule heating due to large currents may prohibit the measurement of SCLC. Using the theory of SCLC for the uniform distribution of the traps, the density of localized defect states near Fermi level is calculated for these compositions. The results indicate that the density of defect states near Fermi level increases on addition of Pb to binary Se85Te15alloy. This is explained in terms of electronegativity of Pb as compared to host elements.

Synthesis of Ti 3SiC 2 from powder blend of Ti, Si and TiC

Titanium silicon carbide Ti 3SiC 2 was synthesized from a powder blend of Ti, Si and TiC at a molar ratio of Ti:Si:TiC = 2:2:3. The powder blend was pressed and shaped into a cylinder and then sintered at various temperatures in vacuum. X-ray diffraction analysis was conducted on the cylinders sintered at various temperatures. Results of the XRD analysis revealed that Ti 5Si 3, the most thermodynamically stable phase in Ti–Si system, was preferentially formed by the reaction between Ti and Si powders followed by the formation of TiSi 2 from remaining Ti and Si powders in a relatively low temperature range. These silicides reacted with TiC to form Ti 3SiC 2 at higher temperatures. The final phases in the cylinders sintered at temperatures over 1729 K were Ti 3SiC 2 and Ti 5Si 3, which well agrees with those estimated by a Ti–Si–C phase diagram. On the other hand, final phases in disks sintered under pressures of 14 MPa or more by using a carbon mold were found to be Ti 3SiC 2 and TiC, which suggests the equilibrium changes with the pressure.

Effect of annealing temperature on structural and electrical properties of tantalum nitride thin film resistors deposited on SiO2∕Si substrates by dc sputtering technique

Tantalum nitride thin films were deposited on SiO2 (600 nm)/Si substrates at 200 °C and a nitrogen/argon flow ratio of 3% by dc sputtering technique and then were annealed at various temperatures in vacuum and nitrogen ambients. For the films annealed in both vacuum and nitrogen ambients, the effect of annealing temperature on structural and electrical properties of the films was systematically investigated. Crystallinity of the films was significantly improved as annealing temperature increased. Temperature coefficient of resistance (TCR) of the films was varied from a negative value to a positive value with increasing annealing temperature in both vacuum and nitrogen ambients. The variation of TCR with annealing temperature in both vacuum and nitrogen ambients was due to the improved crystallinity rather than a nitrogen concentration in the films. Irrespective of annealing temperature, the films annealed at various temperatures do not exhibit the irreversibility of resistance.

Thermally and Molecularly Stimulated Relaxation of Hot Phonons in Suspended Carbon Nanotubes

The high-bias electrical transport properties of suspended metallic single-walled carbon nanotubes (SWNTs) are investigated at various temperatures in vacuum, in various gases, and when coated with molecular solids. It is revealed that nonequilibrium optical phonon effects in suspended nanotubes decrease as the ambient temperature increases. Gas molecules surrounding suspended SWNTs assist the relaxation of hot phonons and afford enhanced current flow along nanotubes. Molecular solids of carbon dioxide frozen onto suspended SWNTs quench the nonequilibrium phonon effect. The discovery of strong environmental effects on high current transport in nanotubes is important to high performance nanoelectronics applications of 1D nanowires in general.

Nanoscale Analysis of Multilayer Interfaces of W/Al2O3/Ti/Cu

W, Al2O3 and Ti films were deposited onto a Cu substrate by means of the rf magnetron sputtering method. After deposition, the foils were annealed at various temperatures in vacuum and the interfaces of the films were observed by a field-emission transmission electron microscopy (FE-TEM), after preparing a cross-sectional thin foil using a focused ion beam (FIB) machine. After annealing the foil at 473 and 623 K, no reaction phases were identified at each interface of W/Al2O3, Al2O3/Ti and Ti/Cu-substrate. However, from the results of compositional analysis at the interface of Al2O3/Ti bilayer, after heat-treatment at 623 K, the formation of an oxide layer was suggested even though it was not clearly observed. On the other hand, after heat-treatment at 823 K, the formation of CuTi2, Cu3Ti2 and Cu4Ti phases were identified at the interface of Ti/Cu bilayers from the compositional analysis of reaction layers after heat-treatment at different temperatures, and the diffusion coefficients and activation energies in the phases were evaluated. In this paper, the influence of heat-treatment on the interfacial behavior of multilayer are discussed on the basis of nanoscale analysis by EDS and HRTEM images.

Interdiffusion at the TiN/Ni interface in the cermet composed of TiN and Ni

In order to investigate the interdiffusion at the TiN/Ni interface in the cermets based on Al2O3 and nickel with a TiN interlayer, an independent experiment was designed and carried out. Pure TiN powder and pure nickel powder were selected as the starting materials. The cermet composed of TiN and nickel was manufactured by the powder metallurgy process including cold compacting and pressureless sintering at various temperatures in vacuum. During sintering, titanium atoms diffuse into nickel lattices forming homogeneous solid solution. This leads to the expansion of the lattice constant. The higher the sintering temperature, the more titanium atoms dissolve, and the more obvious is the expansion which occurs. Only a small amount of nickel diffuses into TiN and dissolves in it. The amount of the nickel is so small that the X-ray diffraction patterns show a stable crystal structure of TiN. © 1998 Kluwer Academic Publishers

Solid or liquid phase sintering of nanocrystalline WC/Co hardmetals

Nanocrystalline WC/Co hardmetal powder has been sintered at various temperatures in vacuum and HIP sintered under different pressures. In addition, hot stage optical microscopy (HSM) and differential thermal analysis (DTA) was performed on a similar powder. All the results indicate that the matrix of the nanopowder starts to melt at about 1150 °C; however, the melting is confined to locations that are still nanocrystalline. With increasing temperature, more material melts and at about 1330 °C the melting of the matrix is complete. High density can be achieved below this temperature, but not below 1200 °C. Compared with previous sintering results on micron sized powder, the conclusion is that the nanopowder densifies mostly in the solid state, while the latter mostly in the liquid state.

Misorientation and Surrounding-Atmosphere Dependence of Ductility at High Temperatures of Cu Bicrystals with Dispersed SiO2 Particles.

Orientation-controlled Cu bicrystals with disperesed SiO2 particles were tensile tested at various temperatures in vacuum and Ar-gas atmospheres. Two kinds of bicrystals having [001] twist 12°and 30°grain boundaries were employed. 30°bicrystal specimens fractured intergranularly at around the intermediate-temperature range, although 12°ones fractured transgranularly at all the temperatures. The elongation to fracture was smaller when deformed in Ar atmosphere than in vacuum almost at all the temperatures irrespective of the bicrystal specimens. The above results could be reasonably understood by considering the difference of the mean grain-boundary strength, the inherent grain-boundary strength, and the effect of gas atmosphere which would be responsible for the acceleration of void formation and growth.

Growth and structure of Ti films deposited on chemically polished MgO(100) substrates

The growth and structure of Ti films deposited at various temperatures in vacuum of about 10 −6 Pa on chemically polished MgO(100) substrates with heat treatment at 900°C for 30 min and without heat treatment have been studied by means of transmission electron microscopy. It is found that the thin layer, whose thickness is up to about 5 nm, is composed of fcc TiH x and TiO y crystallites at an early stage of deposition. The growth of hcp Ti crystallites becomes dominant when the substrate temperature is raised to above 800°C. The 50 nm thick Ti films heated at 800°C for 30 min immediately after being deposited at room temperature are found to be far better defined epitaxial films with only (10·1) orientation than those deposited at 800°C. The 50 nm thick Ti films deposited at room temperature on as chemically polished MgO(100) substrates without heat treatment change from amorphous to epitaxially grown ones with (10·1) orientation when heated at temperatures above 800°C for 30 min in vacuum. This shows that the heating of Ti films deposited at room temperature on as chemically polished MgO (100) substrates without heat treatment is more effective for the epitaxial growth.

Stoichiometry of Ta–N Film and Its Application for Diffusion Barrier in the Al3Ta/Ta–N/Si Contact System

In order to obtain a stable contact structure applicable to Si–LSI thin film technology, we have produced the contact structure of Al3Ta/Ta–N/Si by interposing the Ta–N film as a diffusion barrier between Al3Ta film and Si substrate. The contact structure was heat-treated at various temperatures in vacuum, and the behavior of mass transport across the interfaces of both Al3Ta/Ta–N and Ta–N/Si caused by the heating process was examined by Auger depth analysis. Also, on the basis of X-ray diffraction and XPS analysis, the barrier properties of the Ta–N film were examined with respect to the crystalline state and the chemical bonding state of the film. It is revealed that the thermal stability of this contact structure is closely related to the stoichiometry of the Ta–N film used as a diffusion barrier.

Surface-segregation analysis of NiSe by auger electron spectroscopy

Segregation studies of nickel selenide (NiSe) have been carried out at various temperatures in vacuum by Auger electron Spectroscopy. The surface of NiSe is found to be preferentially enriched in selenium. An attempt is made to explain Se enrichment in NiSe on the basis of available theoretical models.

Outgassing of Nuclear Rocket Fuel Elements

In order to predict the gaseous environment in a nuclear rocket engine before and after operation in space, an apparatus has been developed for measuring the type, quantity, and outgassing rates of gases that are released from graphite fuel elements at various temperatures in vacuum. Data readout from a quadrupole residual gas analyzer (QRGA) and nude Bayard-Alpert type ion gages permit analysis of up to 12 gases simultaneously. The data are processed by a computer program which first calculates the partial pressures of the various gases from a least square fit to a set of 18 simultaneous equations (one for each spectrometer peak) with 12 unknowns (12 separate gases). The coefficients of the unknowns are obtained from the cracking patterns of individual gases. The program then calculates and tabulates the outgassing rate of each gas for each sampling time and produces plots of the outgassing as functions of time or temperature. The computer program also: (a) fits the outgassing rates to an analytical expression, prints out the constants for the equation, and draws the best curve of the specified form through the data points; (b) extrapolates the data to long outgassing times and prints out the outgassing rates and total quantity of each gas at selected times; (c) compiles the data from a number of runs and plots the outgassing rates at preselected times as a function of temperature; and (d) prints out control data such as cracking patterns, calibration constants, and various checks and comparisons to aid in the evaluation of the instrumentation. The experiment not only produces accurate and detailed outgassing data, but also makes possible a more accurate extrapolation of data to long times and other temperatures. The detailed results will also aid in understanding the mechanisms controlling outgassing behavior.