Stability to moisture for chemically vapour-deposited boron nitride

Chemically vapour-deposited boron nitride (CVD-BN) plates prepared by use of the BCl₃-NH₃-H₂ gas system were investigated as to their stability to moisture. Infrared (IR) spectroscopic measurement, chemical analysis and thermal gravimetric analysis were used in this study. The synthesis conditions of CVD-BN plates have a large influence on their stability to moisture. The stability of CVD-BN plates prepared under a total gas pressure (P _tot) of 10 to 60 torr degraded as the deposition temperature (T _dep) was lowered. The CVD-BN plates with transparent and isotropic properties, which were prepared at below 1400° C and above 10 torr, showed poor stability to moisture, The CVD-BN plates synthesized under 5 torr had high moisture-resistance, even at aT _dep as low as 1400° C. An IR absorption spectral study revealed that the unstable species existing in CVD-BN plans had changed to ammonium borate hydrates by reacting with moisture in the atmosphere. The stability to moisture for CVD-BN plates degraded as the deposition roe was raked, especially for the CVD-BN plates prepared at 1400° C.     

Synthesis and structure of chemically vapour-deposited boron nitride

Chemically vapour-deposited boron nitride (CVD-BN) plates have been synthesized on a graphite substrate by the reaction of the BCl₃-NH₃-H₂ gas system in a deposition temperature (T _dep) range from 1200 to 2000° C, with a total gas pressure (P _tot) which was varied from 5 to 60 torr. The effects ofP _tot andT _dep on the crystal structure and the microstructure of the CVD-BN plate were investigated. Turbostratic BN(t-BN) was deposited above 10 torr, at anyT _dep in the range investigated. The interlayer spacing (c ₀/2), the crystallite size (Lc) and the preferred orientation (PO) were strongly affected byT _dep. The t-BN obtained at lowT _dep had largec ₀/2 and smallLc andPO. AsT _dep increased,c ₀/2 tended to decrease whereasLc increased and thec-plane of the crystallites became oriented parallel to the deposition surface. At aP _tot of 5 torr, a mixture of t-BN and h-BN (hexagonal BN) was deposited at anyT _dep above 1700° C, and two kinds of t-BN different inc ₀/2 co-deposited at aT _dep below 1600° C. Moreover, it was indicated that r-BN (rhombohedral BN) was included in the deposits obtained at aP _tot of 5 torr and aT _dep of 1500 to 1600° C.     

Preparation of amorphous Si3N4-C plate by chemical vapour deposition

Chemical vapour deposition of a Si-N-C system has been studied by using SiCl₄, NH₃, H₂ and C₃H₈ as source gases at deposition temperatures (T _dep) of 1100 to 1600° C, and total gas pressures (P _tot) of 30 to 100 torr. To control the amount of carbon in these deposits the propane gas flow rate [FR(C₃H₈)] was varied from 0 to 200 cm³ min^–1. Homogeneous plate-like amorphous deposits were successfully prepared atT _dep=1100 to 1300° C,P _tot=30 to 70 torr andFR(C₃H₈)=25 to 100 cm³ min^–1. The deposits were composed of amorphous silicon nitride and carbon and the carbon content increased up to 10 wt% with increasingFR(C₃H₈). The surfaces of the deposits had a pebble-like structure.     

Preferred orientation of AlN plates prepared by chemical vapour deposition of AlCl3 + NH3 system

Aluminium nitride (AlN) plates about 1 mm thick (maximum) were prepared by chemical vapour deposition (CVD) at the maximum deposition rate of 430 nm s⁻¹ using AlCl₃, NH₃ and H₂ gases at deposition temperatures,T _dep, of 873–1473 K. The effects of deposition conditions on the preferred orientation, morphology and micro-structure were investigated. WhenT _dep was less than 1073 K, the resulting CVD AlN plates contained some impurity chlorine and the aluminium content exceed the nitrogen content. WhenT _dep exceeded 1173 K, no chlorine was detected, and the Al/N atomic ratio matched the stoichiometric value. The lattice parameters (a=0.311 nm,c=0.4979 nm) and density (3.26×10³ kgm⁻³) were in agreement with values reported previously. The crystal planes oriented parallel to the substrates changed from (1 1 ˉ2 0) to (1 0 ˉ1 0) to (0001) with increasing total gas pressure (P _tot) and decreasingT _dep. This tendency is discussed thermodynamically and is explained by the change of supersaturation in the gas phase.     

Density and deposition rates of amorphous CVD-Si3N4 including carbon

Amorphous Si₃N₄ containing uniformly distributed carbon was prepared by chemical vapour deposition [Am. CVD-(Si₃N₄-C)] using SiCl₄ vapour and NH₃, H₂ and C₃ H₈ gases at deposition temperatures (T _dep) of 1100 to 1300° C and at total gas pressures (P _tot) of 30 to 70 torr. The density of Am.CVD-(Si₃N₄-C) is between 2.80 and 3.00 g cm⁻³, depending upon the deposition conditions. Rate of growth in thickness increases with increasingT _dep andP _tot, and has the largest value of 0.6 mm h⁻¹ atT _dep=1300° C,P _tot=70 torr and propane gas flow rates [FR(C₃H₈)] of 0 to 20 cm³ min⁻. The activation energy of the formation decreases from 38 to 20 kcal mol⁻¹ with increasingP _tot andFR(C₃H₈).     

Preferred orientation of TiB2 plates prepared by CVD of the TiCl4 + B2H6 system

Titanium diboride (TiB₂) plates (about 1 mm maximum thickness) were prepared by chemical vapour deposition (CVD) using a TiCl₄, B₂H₆ and H₂ system at deposition temperatures,T _dep of 1323–1773 K. The B/Ti atomic ratio in the deposits was 2, and the composition is strictly stoichiometric. Chlorine was not detected. The measured lattice parameters werea=0.3029 nm andc=0.3229 nm. Density is in close agreement with the theoretical value (4.50 g cm⁻³). Preferred orientation of the CVD TiB₂ plates varies mainly with total gas pressures,P _tot. AtP _tot=4 kPa the (1 0 0) plane and atP _tot=40 kPa the (1 1 0) plane is preferably oriented parallel to the substrates. The effect ofP _tot on the preferred orientation is discussed thermodynamically, and explained by supersaturation in the gas phase.     

Deposition rates of titanium nitride plates prepared by chemical vapour deposition of TiCl4+NH3 system

Titanium nitride plates (TiN_x,x = 0.74–1.0, about 2 mm thick maximum) were prepared by chemical vapour deposition (CVD) using TiCI₄, NH₃ and H₂ as source gases. The effects of CVD conditions, i.e. gas molar ratio (m _N/Ti = NH₃/TiCI₄) and deposition temperature (T_dep), on deposition rates and surface morphology were examined, and the deposition mechanism of the CVD-TiN_x plates was discussed. The relationship between m_N/Ti and deposition rates showed a maximum peak at certainm _N/Ti, and this maximum peak shifted to lowerm _N/Ti with increasing_{T dep}. The activation energy for the formation of CVD-TiN_x plates was about 80 kJ mol^–1 in the lower temperature range. The decomposition reaction of NH₃ gas could be associated with the rate-controlling step. At higher temperatures, the diffusion process may be the rate-controlling step, and a large amount of powder (mainly NH₄Cl) was formed in the gas phase. The highest deposition rate obtained in the present work was 1.06×10^–7 ms^–1 (0.38 mmh^–1) atT _dep = 1773 K andm _N/Ti = 0.87.     

Morphology and preferred orientation of titanium nitride plates prepared by chemical vapour deposition

Thick titanium nitride (TiN _x ; x = 0.74–1.0) plates (up to 2 mm thick) were prepared by chemical vapour deposition using TiCl₄, NH₃ and H₂ as source gases at a total gas pressure, P _tot, of 4 kPa, deposition temperatures, T _dep, from 1373–1873 K, and NH₃/TiCl₄, m _N/Ti, gas molar ratio from 0.17–1.74. The effects of deposition conditions on morphology, preferred orientation and composition of CVD-TiN _x plates were investigated. Surface morphology changed from faceted to nodular texture with increasing m _N/Ti and T _dep. The faceted and nodular deposits showed columnar and shell-like fracture cross-sections, respectively. The composition (x = N/Ti) increased with increasing m _N/Ti and T _dep below m _N/Ti = 1.0, and was constant above m _N/Ti = 1.0. Three kinds of preferred orientations were observed: (100) orientation at low T _dep, (110) orientation at intermediate T _dep and low m _N/Ti, and (111) orientation at high T _dep and high m _N/Ti. This tendency is discussed thermodynamically, and explained as being due to changes in the degree of supersaturation in the gas phase.     

P+ implantation and annealing effects on theT c in BiSrCaCuO films

TheT _c changes related to the microstructure as a function of annealing temperature for the BiSrCaCuO (BSCCO) film implanted with 170 keV P⁺ at two different doses were studied. The BSCCO films were prepared by d.c. sputtering on MgO substrates. For the film implanted at a dose of 5×10¹⁵ cm^–2 post-implantation annealing at 600–800°C enabled theT _cs of the film to be completely recovered. For the film implanted at a dose of 1.0×10¹⁷cm^–2 theT _cs were only partly recovered after 600°C annealing. On further annealing at 700°C the superconductivity of the film disappeared. TEM examination showed that significant amount of CaP, Ca₃P₂, and some unknown phases were formed. It is considered that the significant amounts of these phases formed during post-implantation annealing renders the recovery of the superconductivity of the P⁺-implanted BSCCO film difficult.     

Chemical vapour-deposited silicon nitride

The massive amorphous and crystalline pyrolytic silicon nitride (Py-Si₃N₄) have been prepared under various conditions of production using SiCl₄, NH₃ and H₂. The structural features such as the crystal structure, lattice parameters, grain size, preferred orientation and infra-red absorption are related to the deposition conditions, namely deposition temperature (T_dep) and total gas pressure (P _tot). The X-ray and electron diffraction data for the crystalline Py-Si₃N₄ show that onlyα-Si₃N₄ is deposited even at a T_dep of 1500° C. The lattice parameter c increases with increasingT _dep andP _tot, while the lattice parameter a is independent ofT _dep andP _tot. The parameter c depends strongly upon the oxygen content. All the massive Py-Si₃N₄ reveal the strong (110), (210) and (222) orientations, depending uponP _tot andT _dep. Fine grained Py-Si₃N₄ (about 1μm) is obtained in the amorphous-crystalline boundary region. The infra-red absorption spectra indicate the stretching vibrations of Si-N compounds. The structure of the massive amorphous Py-Si₃N₄ is also discussed.     

Preparation of SiB4±x and SiB6 plates by chemical vapour deposition of SiCl4 + B2H6 system

SiB_4±x and SiB₆ plates were prepared by chemical vapour deposition (CVD) using SiCl₄, B₂H₆ and H₂ gases under the conditions of deposition temperatures (T _dep) from 1323–1773 K, total gas pressures (P _tot) from 4–40 kPa and B/Si source gas ratio (m _B/Si=2B₂H₆/SiCl₄) from 0.2–2.8. The effects of CVD conditions on the morphology, structure and composition of the deposits were examined. High-purity and high-density SiB_4±x and SiB₆ plates about 1 mm thick were obtained at the deposition rates of 71 and 47 nm s⁻¹, respectively. The lattice parameter, composition and density of CVD SiB_4±x plates were dependent on their non-stoichiometry. The lattice parameter,a, was 0.6325 nm, butc ranged from 1.262–1.271 nm.The B/Si atomic ratio ranged from 3.1–5.0, and the density ranged from 2.39–2.45×10³ kg m⁻³. The CVD SiB₆ plates showed constant values of lattice parameters (a=1.444 nm,b=1.828 nm,c=0.9915 nm), composition (B/Si=6.0) and density (2.42×10³ kg m⁻³), independent of CVD conditions.     

Preparation and densification of superconducting YBa2Cu3Ox ceramics

The solid-state reaction method to form the superconducting oxide YBa₂Cu₃O _x was studied. It was found that the starting cupric and yttrium components accelerated the decomposition of the BaCO₃ component. At a constant heating rate of 10 ° Cmin^–1 in thermogravimetric analysis, the temperature of complete decomposition,T _f, was lowered from greater than 1000 ° C in pure BaCO₃ to between 915 and 985 ° C. The effectiveness in decreasingT _f can be ranked in the order of oxalate, carbonate and oxide. The highest sintered density achieved in this study was 6.03 g cm^–3 (/_th = 95%) at 990 ° C and 5.85 g cm^–3 (/_th = 92%) at 960 ° C. The source of cupric ion had the largest effect on densification. The use of cupric carbonate resulted in a consistently high Archimedes density of about 6.00gcm^–3 and large dimensional shrinkage of about 20% at 990 ° C for 12h. In contrast, the use of cupric oxide gave the lowest density and smallest shrinkage. Within the same powder lot, higher sintered density and smaller dimensional shrinkage were observed in samples with higher initial green density and compaction pressure. However, the data suggested that the enhanced densification and higher density achieved by the use of cupric carbonate and oxalate cannot be accounted for by the different physical characteristics of the powders and the mechanics of powder compaction, measured collectively by the green density.     

Effects of Al2O3 addition on the sinterability and ionic conductivity of nasicon

The effects Al₂O₃ doping on the sintering behaviour and ionic conductivity of nasicon have been investigated using formulations of Na_3+2x Zr₂Si₂Al _x P_1–x O₁₂ where Al³⁺ ions were substituted to P⁵⁺ sites within the range 0x0.2. The density of Na_3+2x Zr₂Si₂Al _x P_1–x O₁₂ was increased with increasing amount of Al₂O₃ doping due to the enhanced liquid-phase sintering. The relative density of Na_3.4Zr₂Si₂Al_0.2P_0.8O₁₂ reached a maximum value of 96% when sintered at 1200°C for more than 7 h. The maximum conductivity of 0.24 ^–1 cm^–1 at 300°C was obtained for the composition with x=0.1 when sintered at 1200°C for 10 h.     

In(Ga)As quantum dots on Ge substrate

The effects of growth temperature and deposition thickness on the formation, size, density, and uniformity of InAs and In_0.5Ga_0.5As islands grown by metalorganic vapor phase epitaxy on a germanium substrate are investigated. Atomic force microscopy images show InAs islands when 0.8–2.0 monolayers are deposited. At the nominal deposition thickness of 2.0 monolayers an island density of 2.5×10¹⁰ cm^–2 is achieved. InAs islands covered with a GaAs layer show low-temperature luminescence at around 1.15 eV. The In_0.5Ga_0.5As islands grown at 550 °C show a maximum density of 3.5×10¹⁰ cm^–2 at a nominal three monolayers deposition thickness.     

Influence of different process parameters on physical properties of fluorine dopes indium oxide thin films

Fluorine-doped indium oxide films were prepared by the spray pyrolysis technique. The physical properties of these films were investigated with respect to various process parameters, namely variation of dopant concentration (in the solution), deposition temperature (T _s), carrier gas (air) flow rate and the thickness of the film. The best films had a Hall mobility of the order of 28 cm²V^–1 s^–1 and a carrier density of 2.7 × 10²⁰ cm^–3. These films were deposited at T _s=425 °C at an air flow rate of 71 min^–1 for an atomic ratio of fluorine to indium of 72%. The electrical resistivity of these films was of the order of 10^–4 cm and the average transmission in the visible range was found to be 80–90%. The films were polycrystalline, n-type semiconductors with [400] as a preferred orientation. The preferred orientation changes from [400] to [222] depending upon the process parameters.     

Thermodynamics for the preparation of SiC-C nano-composites by chemical vapour deposition

SiC-C nano-composites covering every possible combination of carbon and SiC were prepared by chemical vapour deposition. The specific compositions of the deposits were controlled by changing the Si/C molar ratio in the source gases at deposition temperatures (T _dep) of 1673 to 1873 K and total gas pressures (P _tot) of 6.7 to 40 k Pa using the SiCl₄-C₃H₈-H₂ system. The prediction, based on the thermodynamic calculation on composition, morphology and deposition rate, was compared with experimental results. The optimal deposition conditions predicted by the calculations were nearly in agreement with the experimental results.     

The thermal diffusivity of ice and water between −40 and + 60° C

The thermal diffusivity _s of triply-distilled deionised water, and ^L of single-crystal ice along the c-axis, have been measured by Angström's method. The temperature range covered was –40 to +60° C. The results for water compare well with published data for the thermal conductivity, but for ice there are unexplained discrepancies. The linear relationships _s=(8.43–0.101 T) 10^–3 cm²/sec and _L=(1.35+0.002 T) 10^–3 cm²/sec where T° C is the temperature, fit the data obtained.     

Plasma surface treatment of AISI 4140 steel for improved corrosion resistance

Alloy AISI 4140 steel was plasma-nitrided in a low-pressure abnormal glow discharge in 75% N₂+25% H₂ atmosphere (P=3 torr andT = 540°C for 2 h) using a home-made laboratory reactor. The corrosion behaviour was evaluated in a chloride environment by potentiostatic and potentiodynamic polarization as a function of thermal treatment and cooling cycle. Results from potentiodynamic polarization in NaCl (3%) indicated the presence of a low anodic dissolution current of about 1 A cm^–2 for ionitrided steel having layers thickness >5 m as compared to 0.1 A cm^–2 for untreated steel. In addition, the open circuit potential shifted to noble potential clearly indicating that the ionitridation process is a very efficient process for corrosion protection of AISI 4140 steel. Typical results obtained from samples oxidized during the cooling cycle showed an improvement in the corrosion resistance. The potential for pitting initiation and propagation as well as for pitting protection has been determined in a chloride environment. The use of electrochemical techniques as a control of standard parameters has been suggested.     

The Density of UO2–ZrO2 Alloys

The density of a UO₂–ZrO₂ melt (atomic ratio U/Zr = 1.528) is experimentally measured by a pycnometric method in the temperature range of 2973–3373 K. The found temperature dependence of density has the form (T) = (7.0 ± 0.01) – (4.5 ± 0.4) × 10^–4 × (T – 2973 K), g/cm³. The temperature dependence measured enables one to calculate the values of the density of UO₂–ZrO₂ melts depending on the temperature and composition for any atomic ratio U/Zr.     

The structural degradation and lamellar to rod transition in the Bi-Cd eutectic during unidirectional growth

The Bi-Cd eutectic system is a prototype quasi-regular eutectic in which the bismuth-rich phase has a volume fraction of 57%. It shows a high degree of regularity but, clearly, is not a normal (regular) eutectic. Microstructural observations of unidirectionally-grown specimens show that the minor cadmium-rich phase degrades at small temperature gradients and small growth rates. However, the structural refinement resulting from rapid freezing or chemical addition is found to be analogous to that of the F/NF eutectics. A lamellar rod transition has been achieved at intermediate growth rates by adding 2.0 wt % Sn as a modifier to the eutectic alloy. However, this was accompanied by the bismuth phase showing cellular facets of the solid-liquid interface.Nomenclature G _L temperature gradient in the melt ahead of the solid/liquid interface (° C cm^–1) - G _S temperature gradient in the solid behind the solid-liquid interface (° C cm^–1) - R growth rate of solid (cm sec^–1) - S cooling rate (° C sec^–1, ° C h^–1) - K _S thermal conductivity in the solid (W m^–1 K^–1) - K _L thermal conductivity in the melt (W m^–1 K^–1) - L latent heat of fusion (J mol^–1) - T temperature difference, undercooling (° C) - K ₁ constant in Equation 2 - K ₂ constant in Equation 3 - D diffusion coefficient of solute in solid (m² sec^–1) - C solubility in solid (wt %, at %) - M molecular weight (g mol^–1) - density (g cm^–3) - interfacial energy, surface tension (J mm^–2) - R gas constant, 8.314J mol^–1 K^–1 - r radius of curvature (m) - T temperature (K) - t time (sec) - F faceted - NF non-faceted