Elevated Nitrogen Pressures Research Articles

Synthesis and Sensing Properties of N-doped ZnO Nanorod Arrays on Quartz

AbstractUsing different pressures of nitrogen, N-doped ZnO nanorod arrays of various densities have been synthesized on quartz substrates by pulsed laser deposition techniques. The nanorods grow preferentially perpendicular to the quartz surface. X-ray diffraction patterns revealed some degradation of the crystal structure at elevated nitrogen pressures. High concentrations of nitrogen doping in ZnO nanorods were estimated by X-ray photoelectron spectroscopy. Raman scattering spectra confirmed the wurtzite structure of N-doped ZnO nanorods. A prototype sensor based on the N-doped ZnO nanorod arrays demonstrates a linear dependence of the conductivity with operating temperature and pressure of a test gas pollutant.

Dynamic properties of nitrogen alloyed 1045 iron-carbon-steel

A new high-pressure melting process enables nitrogen addition to ferritic Fe-C-alloys. A commercial-grade steel alloy, 1045, was cast under elevated nitrogen pressure, resulting in solid solution (austenite, ferrite, and martensite) and a porous microstructure. The retained high nitrogen concentrations in the Fe-C-alloy resulted in a substantial improvement in both quasistatic and dynamic compression strength. Using Planar-Plate-Impact-Technique (PPI) together with a VISAR and Split-Hopinson-Pressure-Bar-Technique (SHPB) the dynamic properties have been determined in the strain rate regime 10 3 s -1 - 10 6 s -1 . With the PPI-Technique the Hugoniot-Elastic-Limit and the spall strength as well as the U S -U p and the stress-strain relation have been determined. An unusual strong slope of the U S -u p -relation and a very low spall strength have been observed for both nitrogen contents. In this paper, the properties of the Fe-C-N alloy with these two nitrogen contents will be compared to those of nitrogen alloyed austenitic steels.

Corrosion resistance of duplex and gradient CrN x coated H13 steel

The electrochemical behavior of H13 steel coated with duplex and gradient CrN x coatings deposited by cathodic arc deposition has been studied. The substrate material was coated with CrN by cathodic arc deposition technique. Duplex layers of CrN x , which normally include an interlayer approximately 100–200 nm of Cr under the main CrN coating, were prepared; gradient CrN x coating were produced with continuous elevated nitrogen pressure. The X-ray photoelectron spectroscopy (XPS) was applied to characterize the chemical composition, and the glancing angle X-ray diffraction (GAXRD) was used to examine the crystallographic structure. The potentiodynamic polarization was examined by Zahner IM6e electrochemical workstation in a 0.5 M H 2SO 4 solution at ambient temperature, and the corrosive surface was detected by scanning electron microscopy (SEM). It was shown that the gradient coating could enhance the corrosion performance of CrN x coated H13 steel. The corrosion resistance improvement was not only attributed to the increase in thickness, but also to the internal microstructure and phase composition. Gradient CrN x coating produced in this work was proved to be particularly promising in terms of corrosion resistance, owing to its incontinuous pinholes and different composition: Cr, Cr 2N in inner part and CrN in surface. The results showed that the gradient coating had an improved electrochemical performance than duplex CrN coating.

Properties of Boron Carbonitride Prepared by Self-propagating High-Temperature Synthesis

Boron carbonitride is prepared by self-propagating high-temperature synthesis in a filtration combustion regime at elevated nitrogen pressures. The combustion rate is found to depend on the starting-mixture composition and nitrogen pressure. The chemical and phase compositions of the synthesized materials are determined by chemical analysis, x-ray diffraction, thermogravimetry, and IR spectroscopy. Data are presented on the electrical resistivity and strength of the materials. Conditions are optimized for producing conducting boron carbonitride materials.

Characterization of Fe–C–N alloys’ mechanical properties

Abstract Melting a Fe–0.9wt.%C alloy under elevated nitrogen pressures produced carbide- and nitride-free alloys. The combination of high interstitial concentrations, approaching 2 wt.%, and duplex bcc-Fe/fcc-Fe microstructure resulted in alloys with compression yield strength in excess of 3 GPa while retaining over 20% ductility and with wear properties comparable to white cast irons. Statistical analysis showed a strong correlation between microstructure/nitrogen concentration and mechanical properties: hardness, strength, and wear. By the combination of both carbon and nitrogen, this process for the first time extended the interstitial concentration in iron to 2 wt.%.

Characterization of Fe-C/N Steel

A new processing technique produced iron-carbon-nitrogen solid-solution alloys. The technique reduced the time necessary for nitrogen addition by adding nitrogen under elevated nitrogen pressure to a Fe-C alloy at the point of melting. Carbon and nitrogen levels in excess of 0.5wt% resulted in solid-solution Fe-C/N alloys with no carbide or nitride precipitates, but with the retention of fcc-Fe phase. The knee of the Time-Temperature-Transformation curve shifted to longer times. Hardness, compression strength, and wear resistance increased with increasing nitrogen concentration and were superior to iron-carbon alloys without the nitrogen addition.

Carbothermal Preparation of β-SiAlON Powder at Elevated Nitrogen Pressures

β-SiAlON powders have been prepared by carbothermal nitridation of kaolin or SiO 2 plus Al 2O 3 mixtures at 1450-1800°C and elevated nitrogen pressures (1-50 bar). Powders with widely variable particle size and morphology were obtained depending on temperature, pressure, gas-flow rate, and raw-material type. An elevated nitrogen pressure effectively reduced SiO loss and suppressed SiC formation. Pure β-SiAlON powder of composition Si 2·8Al 3·2O 3·2N 4·8 and with a fine particle size (∼1 μm) was produced at 1650°C, 50 bar N 2 and a high gas-flow rate for 2 h. The nitridation rate was hampered by an increase of the CO concentration in the system.

Nitrogen Addition to an O-1 Tool Steel

A new processing technique makes nitrogen alloying possible by adding nitrogen under elevated nitrogen pressure to prealloyed Fe-C ingots during continuous casting, producing a whole new class of precipitation-free, iron–carbon–nitrogen alloys. When both carbon and nitrogen bulk concentration levels exceeded 0.5 wt%, a duplex fcc-/(bcc-bct-) Fe microstructure resulted that is iron carbide- and nitride-free. With increasing carbon and nitrogen concentrations, there was an increase in the retained fcc-Fe phase. In cooling rate studies, increasing carbon and nitrogen concentrations shifted the knee of the fcc-Fe-to-bcc-Fe phase time–temperature–transformation (T–T–T) curve to longer times. Hardness, compression strength, and wear resistance increased with increasing carbon and nitrogen concentrations and were superior to iron–carbon alloys without the nitrogen addition.

High-Pressure Processing and Characterization of Fe–High-C/N Alloys

A new process has been developed that results in (i) enhanced nitrogen addition to ferritic iron–carbon alloys and (ii) melt-casting in a single operation. This new processing technique enables Fe–C alloys to retain high nitrogen interstitial concentrations and to reduce significantly, and possibly eliminate, carbide formation. In this study two commercial-grade, steel alloys were cast under elevated nitrogen pressures, resulting in solid solution (austenite, ferrite, and martensite) high-carbon and high-nitrogen iron alloys that were, within detection limits, carbide- and nitride-free. These alloys were subsequently thermally processed to transform part of the retained austenite to martensite. The microstructure and mechanical properties of the alloys were studied as a function of carbon and nitrogen composition and as a function of thermal processing. The retain high nitrogen concentrations in these cast and processed iron–carbon alloys resulted in a substantial improvement in compression strengths.

Carbothermal synthesis of aluminium nitride at elevated nitrogen pressures

This paper is the first part of an investigation of carbothermal nitridation of alumina at elevated nitrogen pressure. The effect of some process parameters on nitridation rate was studied at different temperatures (1300–1700 °C) and pressures (0.1–5 MPa). The experiments were performed in a graphite furnace permitting a controlled nitrogen flow through the sample holder, and continuous measurement of CO in the outlet gas. The products were characterized by X-ray diffraction, BET and elemental analyses. The nitridation rate was found to be a function of the process parameters. For a suitable temperature and pressure, e.g. 1600 °C and 1 MPa, pure AIN was obtained after just 1 h, with very low residual oxygen content (0.4 wt%). At a high gas-flow rate (⩾5 I min−1), the optimum range of pressure was 0.5–1 MPa at 1550–1700 °C. The nitridation was hampered by even small amounts of CO added to the system. No intermediate phases formed under the experimental conditions studied. Thermodynamic calculations on the Al-O-C-N system were performed in a search for explanations of the results.

Carbothermal synthesis of aluminium nitride at elevated nitrogen pressures

The particle size and morphology of AlN product powders from carbothermal synthesis have been studied by scanning electron microscopy, surface-area measurement and microarea elemental analysis. AlN powders with widely variable grain size and morphology could be produced by varying the pressure, gas-flow rate and alumina type. Discrete AlN particles (∼0.5-3 μm) were obtained by using fine γ-Al 2 O 3 as starting material at 1600 o C over the pressure and gas-flow rate ranges 0.1-5 MPa and 1-10 l min -1 . When coarse α-Al 2 O 3 was used as raw material, large aggregates of AlN particles normally formed, unless the reaction was run at high nitrogen pressure (5 MPa) and low gas-flow rate (1 l min -1 )

Si‐C‐O‐N High‐Pressure Equilibria and ΔGfo for Si2ON2

Phase relations in the Si‐C‐O‐N system were studied by subjecting mixtures of carbon and silica (C/SiO2= 2 to 3) to high temperatures (up to 2000 K) and elevated nitrogen pressures (0.5 to 2.0 MPa) in a gas autoclave. The condensed‐phase assemblages obtained at different temperatures, pressures, and N/Si ratios are reported and compared with thermodynamic calculations. Values of ΔGfo(Si2ON2) consistent with the experimental data are ‐397 kJ/mol at 1780 K and ‐369 kJ/mol at 1890 K. The influence on the synthesis of seeding the starting material is discussed.

Stability of Phases in the Si‐C‐N‐O System

The stability of the phases in equilibrium is calculated for the Si‐C‐N‐O system in order to analyze and predict the reactions in ceramic whisker formation and sintering of silicon nitride composites. Equilibria among SiC, Si3N4, Si2N2O, SiO2, Si, and the gas phase are evaluated at different carbon activities, nitrogen pressures, and temperatures. Phase stability diagrams are constructed as a function of nitrogen and oxygen pressures for two levels of carbon activity. Silicon nitride becomes a stable phase with increasing nitrogen pressure or decreasing carbon activity and temperature, whereas silicon carbide becomes a dominant phase at lower nitrogen pressures or at higher temperatures when carbon activity is unity. The maximum sintering temperature of the SiC/Si3N4 composite is higher with an elevated nitrogen pressure or a reduced carbon activity.

Physical and mechanical properties of cubic δ-VN 1− x

Compact samples of cubic δ-VN 1- x were prepared by the reaction of vanadium metal with pure nitrogen in the pressure range 10 kPa ⩽ p(N 2) ⩽ 4 MPa and at temperatures T from 1353 to 1923 K. The samples had compositions between VN 0.765 and VN 0.996 and lattice parameters between 0.4097(4) and 0.4135(3) nm. In this range the lattice parameter is a linear function of the nitrogen-to-metal ratio. The pressure isotherm at 1723 K as a function of composition was established. Near-stoichiometric VN 1− x can only be prepared under elevated nitrogen pressures. The microhardness HV 0.1 of VN 1− x was measured. Hardness decreases significantly with increasing nitrogen content. Within the composition range investigated the vanadium sublattice is fully occupied at substoichiometric compositions, whereas the occupancy decreases just perceptibly as stoichiometry is approached. The occupancy of the nitrogen sites is a linear function of the [N] [V] ratio.

Studies in the System CaO‐Al2O3‐SiO2H2O: III, New Data on the Polymorphism of Ca2SiO2 and Its Stability in the System CaO‐SiO2‐ H2O

The nature of the low‐temperature inversions γ‐α' and α'‐β was investigated by various techniques: hydrothermal and “dry” quenching runs, differential thermal analysis at atmospheric and elevated nitrogen pressures, X‐ray diffractometer patterns obtained at elevated temperatures, “static” pressure techniques, and infrared absorption spectrometry. A revised energy‐temperature diagram is presented for Ca2SiO4, with the transition γ' to α' taking place at about 725°C. and the α'‐β transition, although not reversible at an exact temperature, taking place at about 670° C. At low water pressures (2000 lb. per sq. in.) the inversion γ‐α' was placed at 675°C. Attempts to extrapolate the value obtained at 2000 lb. per sq. in. to obtain a more accurate reversible inversion temperature at atmospheric pressure, although limited in accuracy by the reliability of heat‐of‐transition data, would indicate a temperature of about 725° C. at atmospheric pressure. Three new compounds, 8CaO.3SiO2 ‐3H2O (X), 6CaO 3SiO2.H2O (Y), and 9CaO‐6SiO2 H2O (Z), were found to be stable above 700°C. at H2O pressures greater than 7500 lb. per sq. in.