Liquid Nitrogen Medium Research Articles

Analysis of surface discharge activity in epoxy nanocomposites in liquid nitrogen under AC voltage

A marginal increase in surface discharge inception voltage is observed with increase in percentage of clay in epoxy nanocomposites in liquid nitrogen medium. The flashover voltage of epoxy nanocomposites increases with increase in percentage of clay in epoxy resin up to 3 wt% and above which a slight reduction is observed but much higher than the pure epoxy resin.The intercalated and exfoliated properties were analyzed through TEM studies.Contact angle marginally vary with percentage of clay in epoxy resin. The surface variations of epoxy nanocomposites analysed through surface roughness measurement and by attenuated total reflectance Fourier transform infra-red spectroscopy (ATR-FTIR).The rise time of injected current pulse due to surface discharge process is measured to be 800 ps. The UHF signal generated due to surface discharge (at the point of inception and propagation) formation on epoxy resin with different percentage of clay in liquid nitrogen medium have dominant frequency near 1 GHz. The PRPD pattern obtained at the point of inception of surface discharges, the magnitude of discharges is nearly same in both half cycle of the applied AC voltage. Surface discharge occurs in the rising part of the applied AC voltage. When the applied voltage magnitude is increased, the higher magnitude discharges occurs at the positive half cycle of the applied AC voltage.

Single-walled nanohorns and other nanocarbons generated by submerged arc discharge between carbon electrodes in liquid argon and other media

Arc discharge between two graphite electrodes submerged in different liquid media yields various dimensional nanocarbon structures such as 1D carbon nanotubes and 2D graphene. Single-walled carbon nanohorns (SWNHs) prepared by submerged arc discharge in liquid nitrogen medium are found to have nitrogen impurities. Here, we report the structure and properties of pure and nitrogen-doped SWNHs obtained by submerged arc discharge in a liquid argon medium. The absence of an XPS N 1s signal, which is present in nanohorns obtained in liquid nitrogen, indicate that the nanohorns are free from nitrogen impurities. Raman spectra show a strong defect-induced D band and current–voltage characteristics show a slight nonlinear behavior. N2 adsorption of pure SWNHs shows type-IV isotherms with a surface area of 300 m2 g−1. Adsorption of CO2 and H2 in pure SWNHs has also been measured. Arc discharge in other liquid media such as water, ethanol, dimethylformamide (DMF), n-methyl pyrrolidone (NMP), formamide, benzene, heptane and acetone yields different nanocarbon structures including multi-walled carbon nanotubes (MWNTs), few-layer graphene, carbon onions and carbon nanoparticles.

Experimental study on the stability of graphitic C 3N 4 under high pressure and high temperature

The stability and decomposition of graphitic C 3N 4 ( g-C 3N 4) were studied in the pressure and temperature range of 10–25 GPa and up to 2000 °C by multi-anvil experiments and phase characterization of the quenched products. g-C 3N 4 was found to remain stable at relatively mild temperatures, but decomposes to graphite and nitrogen at temperatures above 600–700 °C and up to 15 GPa, while it decomposes directly to diamond (plus nitrogen) above 800–900 °C and between 22 and 25 GPa. The estimated decomposition curve for g-C 3N 4 has a positive slope (~ 0.05 GPa/K) up to ~ 22 GPa, but becomes inverted (negative) above this pressure. The diamond formed through decomposition is characterized by euhedral crystals which are not sintered to each other, but loosely aggregated, suggesting the crystallization in a liquid (nitrogen) medium. The nitrogen release from the graphitic C N framework may also play an important role in lowering the activation energy required for diamond formation and enhancing the grain growth rate. No phase transition of g-C 3N 4 was found in the studied P–T range.

Structure evolution of pure iron upon low-temperature deformation under high pressure

Structure evolution of iron (99.97% purity) deformed by shear under pressure at 80 K in a medium of liquid nitrogen has been investigated. It has been found that, along with dislocation slip, twinning and development of deformation microbands become operative mechanisms of low-temperature deformation. This led to specific type of inhomogeneity of the structure in which, up to ultimately attained degrees of deformation, low-angle misorientations are retained and, unlike room-temperature deformation, no homogeneous submicrocrystalline (SMC) structure is formed. Twinning contributes to the refinement of structure elements that are more than 1 μm in size; the further refinement occurs by the dislocation-disclination mechanism and goes to the steady-state stage.

Effect of strengthening friction treatment on the chemical composition, structure, and tribological properties of a high-carbon steel

The structure and chemical composition of nanocrystalline layers formed on the surface of a steel U8 with 0.83 wt % C (quenched, as well as quenched and tempered at 200°C) under the conditions of frictional loading by a hard-alloy indenter in different media (gaseous and liquid nitrogen, air) have been investigated by the methods of transmission electron microscopy, X-ray diffraction analysis, nuclear reactions, Rutherford back scattering, and wave- and energy-dispersive microanalyses. Maximum levels of defectiveness (high density of dislocations and point defects) and microhardness of the nanocrystalline structure have been attained upon friction treatment of the low-temperature tempered steel in a liquid-nitrogen medium because of deformation localization in a thin surface layer, intensification of deformation-induced dissolution of the ɛ carbide phase, and saturation of the layer with nitrogen and oxygen atoms, the latter dissolved in the liquid nitrogen as an impurity. A comparative analysis of the in-depth distribution of microhardness in frictionally strengthened surface layers has been performed for the steel with initial structures of tetragonal (untempered) and low-tempered (tempered at 200°C) martensite. A markedly larger depth of strain hardening has been attained upon friction treatment in the quenched untempered steel due to effective development of deformation-induced dynamic aging of high-carbon martensite even at small deformations. It has been established that the strengthening upon deformation of the surface by a sliding indenter exerts a positive influence on the tribological properties (wear rate and friction coefficient) of the steel under the conditions of frictional heating of different intensity.

Mechanodynamic diffusion of nitrogen molecules into armco-iron under its deformation in liquid nitrogen medium

Data on mechanodynamic penetration of nitrogen molecules are obtained under deformation of armco-iron samples. It is shown that molecular nitrogen diffuses into the surface layer of samples under their deformation in a liquid nitrogen medium, and the nitrogen concentration compares well with the helium concentration in samples deformed in a liquid helium medium and in some cases exceeds the latter.

Breakdown strength of solid dielectrics in liquid nitrogen

The objective of this research was to determine if a relationship could be found between dielectric strength and other properties of electrical insulating materials in the medium of liquid nitrogen on an empirical basis by using variables predicted by basic theory. A simple equation of the form E=A+Blog (rhov/xir tandelta) to predict the dielectric strength of a solid insulating material in liquid nitrogen medium has been proposed. The equation requires the values of volume resistivity (rhov), relative permittivity (xir) and loss tangent (tandelta) in the medium of liquid nitrogen, which may be obtained easily by low voltage non-destructive measurements. The values of electric strength calculated using this equation for Crepe paper, Kraft paper, varnished paper, pressboard, presspahn, mica, bakelite and asbestos are quite in agreement with the experimentally measured values. It is expected that the equations obtained will help the designers as a handy tool for quick estimation of breakdown strength of solid dielectrics dipped in liquid nitrogen.

The effect of quenching medium on the wear behaviour of a Ti–6Al–4V alloy

The aerospace alloy, Ti–6Al–4V is a difficult material to machine, and, in general, shows poor wear resistance due to the soft, ductile properties of the alloy. In this study, the Ti–6Al–4V alloy has been heat treated to a temperature above and below the β-transus temperature and then quenched using a medium of oil, water or liquid nitrogen to change the surface wear behaviour of the alloy. The results showed that no significant change in microstructure and surface properties was achieved when the alloy was heated to 750 °C and then quenched in liquid nitrogen. However, when the alloy was heated to 1,000 °C (above the β-transus), the hardness of the titanium alloy significantly increased from 400 VHN to about 800 VHN, but the wear resistance of the alloy did not improve. In fact, the wear resistance of the alloy decreased as the surface hardness increased, and this change in wear behaviour was attributed to a change in the mechanism of wear from plastic deformation to brittle-fracture of the surface.

New-types of apparatus for producing microcapsules and microgranules.

Two types of simple compact apparatuses for microencapsulation and microgranulation have been developed. The vibration-type apparatus is a better fit for producing gel-like microgranules of 1.5-3.0 mm in diameter to be used in production of food (fodder) products and additives and microgranulated cell culture. The spray apparatus is fit for operation in two modes: drop-forming (particles of 100-500 microm in diameter) and spray modes (particles of 10-50 microm in diameter). The apparatus is most efficient when used in the mode of forming the particles directly in the medium of liquid nitrogen with successive lyophilic drying (without preliminary defrosting). It is used for microgranulating the live virus materials with their virulence being preserved.

Thermoelectric influence on the strength of alloy ABM-1 in a medium of liquid nitrogen

We investigated the change of the structural state when a current pulse was passed through the high-modulus alloy ABM-1 situated in a medium of liquid nitrogen. Current pulses heat the material and bring about thermal stresses that lead to a reduction of the regions of coherent scattering. A change of the structural state causes a deviation of the dependence U (I a)from linearity. It was discovered that thermoelectric treatment results in the formation of a stable amorphous layer at least 10-μn thick on the surface of the alloy.

Friction behaviour and wear resistance of diamond-like carbon films under cryogenic temperatures

A study has been made of the friction behaviour and wear resistance in liquid nitrogen medium of pin-and-disc pairs with diamond-like carbon (DLC) coatings deposited onto disc surfaces of varying initial roughness, and in a number of cases onto pin surfaces. The dependence of the coefficient of friction on initial roughness has been shown to be extreme. Peculiarities of microrelief formation at the contact surface and the composition of the wear debris for the DLC-coated mating pairs were observed. Possible mechanisms of friction interaction for diamond-like coatings at cryogenic temperatures are considered.

Estimating the strength of three-layer carbon-reinforced-plastic panels using the acoustic-emission method

The results of investigation of the strength of three-layer panels with facings of laminar carbon-reinforced plastic and honeycomb aluminum filler are presented. Acoustic-emission signals were recorded during the cooling of specimens to 77°K, and also during their compression at both room temperature, and in a liquid-nitrogen medium. Mechanical characteristics and typical AE diagrams were obtained during the tests. Regression analysis using different functional relationships indicated a stable correlation between failing load and total AE count.

Change in the structural state of titanium in electrical heating in liquid nitrogen

Electrical heating in a liquid nitrogen medium alters the structural state of polycrystalline titanium. It is shown by the methods of resistometry, X-ray photography, and metallography that, in treatment by current, grains grow by collective recrystallization, and formation of rutile associated with active diffusion of oxygen is observed.

A freeze method of obtaining ceramic based on fused quartz

The technology of producing materials based on fused quartz by freezing the blank in a liquid nitrogen medium has been studied.