The cryogenic thermal expansion and mechanical properties of plasma modified ZrW2O8 reinforced epoxy

In this article, epoxy resin reinforced by negative thermal expansion material, ZrW₂O₈, was fabricated. The surface modification of ZrW₂O₈ particles was performed via plasma enhanced chemical vapor deposition (PECVD) process. As a result, a thin film was uniformly deposited on the surfaces of the ZrW₂O₈ particles, leading to an improvement of compatibility and dispersion of ZrW₂O₈ fillers inside epoxy matrix. Moreover, the coefficients of thermal expansion (CTEs) of the composite material containing 0-40 vol.% fillers were studied under cryogenic temperatures. The results showed a significant reduction in thermal expansion with increasing ZrW₂O₈ content. The cryogenic mechanical properties of ZrW₂O₈/epoxy composites were also investigated, showing the properties were improved by adding ZrW₂O₈ to certain content. In addition, the mechanical strength and modulus of the composite were observed significantly higher at cryogenic temperature than that at room temperature because of the thermal shrink effect and the frozen epoxy matrix.     

The electrical resistance and thermal conductivity of Ti 15V–3Cr–3Sn–3Al at cryogenic temperatures

Ti 15V–3Cr–3Sn–3Al, sometimes referred to as Ti 15-3-3-3 or “Magic Titanium”, is a candidate material for components requiring high mechanical strength and low thermal conductivity at cryogenic temperatures. The electrical resistance of Ti 15-3-3-3 was measured between 230 mK and room temperature, and the thermal conductivity between 230 mK and 7.7 K. A superconducting transition was observed at T_C = 3.89 ± 0.01 K. Below the superconducting transition temperature, the thermal conductivity was fitted to a function of the form λ(T<T_C)=α·T·e^-β·T_C/T, where α = 0.043 ± 0.002 W/(m K²) and β = 0.27 ± 0.01. Above T_C, the thermal conductivity of Ti 15-3-3-3 was fitted to a function of the form λ(T > T_C) = γ · T^δ, where and δ = 0.4 ± 0.05. The thermal conductivity of Ti 15-3-3-3 is compared with other materials commonly used for the construction of thermally isolating support structures. Ti 15-3-3-3 is shown to exhibit one of the lowest ratios of thermal conductivity to mechanical strength and is thus particularly well suited for such applications.     

Study of an Optical Device Used to Homogenize a Laser Beam. Application to Emissivity Measurements on Semitransparent Materials at High Temperature

On an experimental apparatus designed to measure the directional spectral emissivity of semitransparent materials at high temperature (2000°C), a specific optical device (kaleidoscope) is mounted in order to homogenize the energy distribution of a CO₂ laser beam, which is used as a heating source. An objective of this work, for operating in the best conditions, is to obtain a uniform temperature of the tested sample. This study, based on Fourier optics, focuses on a square aperture kaleidoscope. A model was developed to simulate the energy distribution of the laser beam, at any transverse plane on the optical path. The final objective is to simulate the energy distribution on the sample surface to optimize the homogenization device taking into account the surface temperature gradient induced by the local energy distribution. To validate this model, quantitative comparisons of theoretical simulations and experimental thermal spots are performed.     

Characterization and corrosion behavior of electroless Ni–P/nano-SiC coating inside the CO2 containing media in the presence of acetic acid

In this research, Ni–P and Ni–P/nano-SiC coatings were applied on the X70 steel substrate successfully without any surfactant. Then, CO₂ corrosion in the presence of acetic acid (HAc) was investigated using electrochemical techniques. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) techniques were used for surface analyses of the coatings. The electrochemical behavior of corrosion was investigated using polarization test and electrochemical impedance spectroscopy (EIS). XRD pattern of Ni–P/nano-SiC coating was very similar to that of Ni–P coating. EDS results demonstrated the presence of SiC particles in the coating. SEM images confirmed the presence of SiC nano-particles with almost uniform distribution in the coating. The corrosion current density was less in the Ni–P and Ni–P/nano-SiC coated samples than uncoated X70 steel. Ni–P/nano-SiC coated sample had the most corrosion resistance because of less effective metallic area available for corrosive media. The overall protection mechanism of Ni–P and Ni–P/nano-SiC coatings was achieved by formation of a layer of adsorbed hypophosphite anions (H₂PO₂⁻).     

Structural characterization and novel optical properties of defect chalcopyrite ZnGa2Te4 thin films

Stoichiometric thin film samples of the ternary ZnGa₂Te₄ defect chalcopyrite compound were prepared and characterized by X-ray diffraction technique. The elemental chemical composition of the prepared bulk material as well as of the as-deposited film was determined by energy-dispersive X-ray spectrometry. ZnGa₂Te₄ thin films were deposited, by conventional thermal evaporation technique onto highly cleaned glass substrates. The X-ray and electron diffraction studies revealed that the as-deposited and the annealed ZnGa₂Te₄ films at annealing temperature t_a ≤ 548 K are amorphous, while those annealed at t_a ≥ 573 K (for 1 h), are polycrystalline. The optical properties of the as-deposited films have been investigated for the first time at normal incidence in the spectral range from 500 to 2500 nm. The refractive index dispersion in the transmission and low absorption region is adequately described by the Wemple–DiDomenico single oscillator model, whereby, the values of the oscillator parameters have been calculated. The analysis of the optical absorption coefficient revealed an in-direct optical transition with energy of 1.33 eV for the as-deposited sample. This work suggested that ZnGa₂Te₄ is a good candidate in solar cell devices as an absorbing layer.     

Effect of different treatments of copper surface on its total hemispherical absorptivity bellow 77 K

Total hemispherical absorptivity of copper surfaces treated with standard industrial methods was measured in dependence on the temperature of thermal radiation, varying from 25 K to 300 K. The sample temperature was typically from 5 K to 40 K and did not exceed 70 K. Usability of chemical and mechanical Cu surface finishing as well as Cu plating with Ni and Au for cryogenic design is discussed. As an example of practical application of our results, the cryogenic design of a LN₂ trap is presented.     

Optical,thermal and microhardness studies on dichloridobis(1-ethyl-2,6-dimethylpyridinium-4-olate-κO) zinc(II)

A newly engineered host–guest hydrogen bonded metal–organic coordination compound, dichloridobis(1-ethyl-2,6-dimethylpyridinium-4-olate-κO)zinc(II) (EDMPZC), [C₁₈H₂₆Cl₂N₂O₂Zn] has been designed and synthesized. Single crystals of dimensions (5 × 5 × 2 mm³) have been grown by slow evaporation technique. The unit cell dimensions and morphology are identified from single crystal XRD analysis. Further, it has been characterized by FT-IR absorption, FT-NMR spectroscopy, elemental analyses (CHN and XRF) and their thermal stability investigated following TG/DTA and DSC techniques. The thermal transport properties, thermal effusivity (e), thermal diffusivity (α), thermal conductivity (K) and heat capacity (C_p) have been measured by the photopyroelectric technique at room temperature. The laser damage threshold of the grown crystal was measured using Q-switched Nd:YAG laser (1064 nm, 10 ns, 10 Hz). The mechanical stability of the crystal has been studied from the Vicker's microhardness measurement. The UV absorption edge is 262 nm with a wide optical transmittance window covering the UV–Vis–NIR region and the optical band gap of the compound is found to be 3.5 eV.     

Heat transfer performance of cryogenic oscillating heat pipes for effective cooling of superconducting magnets

The cryogenic oscillating heat pipe (OHP) for conduction cooling of superconducting magnets was developed and the function was demonstrated successfully. OHP is a highly-efficient heat transfer device using oscillating flow of two-phase mixture. The working fluids that are employed in the present research are Nitrogen, Neon and Hydrogen, and the operating temperatures are 67–91 K, 26–34 K and 17–27 K, respectively. The estimated effective thermal conductivities from the measurement data of the OHP were higher than one of the solids such as copper at low temperature. These results revealed that the cryogenic OHP can enhance the performance of cooling system for magnets.     

Experimental study of the influences of degraded vacuum on multilayer insulation blankets

The paper presented experimental investigation on the heat transfer of MLI with different rarefied gases at different pressures. The investigations were carried out using an innovative static liquid nitrogen boil-off rate measurement system in the case of the small temperature perturbations of cold and warm boundaries. The heat fluxes for a number of inert and some polyatomic gases have been analyzed at different heat transfer conditions ranging from molecular to continuum regime, apparent thermal conductivities of the multilayer insulation were measured over a wide range of temperature (77 K–300 K) and pressure (10⁻³–10⁵ Pa) using the apparatus. The experimental results indicated that under degraded vacuum condition, the influences of rarefied gas on the MLI thermal performance very depend on the gas rarefaction degree which impacted by the MLI vacuum degree. Under the condition of molecular regime heat transfer, the MLI thermal performance was greatly influenced by gas energy accommodation coefficients (EAC), when under the continuum regime, the performances depend on the thermal conductivity of rarefied gas itself. Compared to the results of N₂, Ar, CO₂, Air and He as interstitial gases in the MLI, Ar was the better selection as space gas because of its low EAC and thermal conductivity characteristics on the different vacuum condition ranging from high pressure to vacuum. So different residual gases can be utilized according to the vacuum level and gas energy accommodation coefficient, in order to improve the insulation performance of low vacuum MLI.     

Degradation behaviour of sputtered Co–Al coatings on superalloy

Degradation behaviour of sputtered Co–Al coatings on Superni-718 substrate has been investigated. Cyclic high temperature oxidation tests were conducted on uncoated and coated samples at peak temperatures of 900 °C for up to 100 thermal cycles between the peak and room temperatures. The results showed that a dense scale formed on the coated samples during thermal cycling at the peak temperature of 900 °C. The external scale exhibited good spallation resistance during cyclic oxidation testing at both temperatures. The improvement in oxide scale spallation resistance is believed to be related to the fine-grained structure of the coating. Nanostructured Co–Al coatings on Superni-718 substrate were deposited by DC/RF magnetron sputtering. FE-SEM/EDS, AFM, and XRD were used to characterize the morphology and formation of different phases in the coatings, respectively. The Co–Al coating on superalloy substrate showed better performance of cyclic high temperature oxidation resistance due to its possession of β-CoAl phase as Al reservoir and the formation of Al₂O₃ and spinel phases such as CoCr₂O₄ and CoAl₂O₄ in scale. The oxidation results confirmed an improved oxidation resistance of the Co–Al coating on superalloy as compare to bare substrate in air at 900 °C temperature up to 100 cycles.     

Ambi-site substitution of Mn in lanthanum germanate apatites

A neutron diffraction study at 4 K of the Mn doped lanthanum germanate apatite-type oxide ion conductor of nominal starting composition “La_9.5Mn_0.5(GeO₄)₆O_2.75” is reported. The structure was refined in space group P6₃/m, although high thermal displacement parameters were observed for the oxide ion sites (particularly O3, and O4). Reduced thermal displacement parameters were obtained by splitting the O3 site, and allowing the O4 oxygen to move off site, which may indicate local regions of lower symmetry within the structure. In addition, the data suggested ambi-site substitution of Mn, with it being present on both the Ge site and the La site. Assuming no change in La:Mn:Ge ratio, a composition of La_9.18Mn_0.28(GeO₄)_5.8(MnO₄)_0.2O₂ was determined. As such there are nominally no interstitial oxide ions, but rather cation vacancies on the La site. Therefore, the high conductivity for this sample is most likely related to the introduction of Frenkel-type defects at higher temperature, as previously proposed for other apatite-type systems containing vacancies on the La site.     

CO2 absorption and desorption of Bi2O3-La2O3 powders prepared by mechanical synthesis

In order to obtain CO₂-absorbents to eliminate CO₂ concentration locally, Bi₂O₃-La₂O₃ mixed powders were prepared by mechanical alloying (MA) method using a planetary ball-milling machine. CO₂-absorption and desorption properties were checked by TG-DTA for the obtained powder samples. As a result, the sample shown by (Bi₂O₃)_1−x(La₂O₃)_x [x≤0.50] was found to form α-Bi₂O₃-solid solution with repeated CO₂-adsorption and desorption around 400- 500 °C. Absorbed and desorbed CO₂ contents varied with MA time: the 72 h MA’ed sample had a larger CO₂ content than the 24 h MA’ed sample. The performance depended on the sample composition, and (Bi₂O₃)_0.70(La₂O₃)_0.30 was found to have the highest performance in the present system.     

Experimental investigations on cryogenic cooling by liquid nitrogen in the end milling of hardened steel

Milling of hardened steel generates excessive heat during the chip formation process, which increases the temperature of cutting tool and accelerates tool wear. Application of conventional cutting fluid in milling process may not effectively control the heat generation also it has inherent health and environmental problems. To minimize health hazard and environmental problems caused by using conventional cutting fluid, a cryogenic cooling set up is developed to cool tool–chip interface using liquid nitrogen (LN₂). This paper presents results on the effect of LN₂ as a coolant on machinability of hardened AISI H13 tool steel for varying cutting speed in the range of 75–125 m/min during end milling with PVD TiAlN coated carbide inserts at a constant feed rate. The results show that machining with LN₂ lowers cutting temperature, tool flank wear, surface roughness and cutting forces as compared with dry and wet machining. With LN₂ cooling, it has been found that the cutting temperature was reduced by 57–60% and 37–42%; the tool flank wear was reduced by 29–34% and 10–12%; the surface roughness was decreased by 33–40% and 25–29% compared to dry and wet machining. The cutting forces also decreased moderately compared to dry and wet machining. This can be attributed to the fact that LN₂ machining provides better cooling and lubrication through substantial reduction in the cutting zone temperature.     

Synthesis, structure and magnetic properties of Sr2Fe1−xGaxMoO6 (0 ≤ x ≤ 0.6) double perovskites

Polycrystalline Sr₂Fe_1−xGa_xMoO₆ (0 ≤ x ≤ 0.6) materials have been synthesized by solid state reaction method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature dependent NPD data shows that the compounds crystallize in the tetragonal symmetry in the space group I4/m. The anti-site (AS) defects concentration increases with Ga doping, giving rise to highly B-site disordered materials. Ga doping at the Fe-site decreases the cell volume. The evolution of bond lengths and the cation oxidation states was determined from the Rietveld refinement data. The saturation magnetization and Curie temperature decreased with the increasing Ga content in the samples. Low temperature neutron diffraction data analysis and magnetization measurements confirm the magnetic interaction as ferrimagnetic in the sample.     

An alternative gas sensor material: Synthesis and electrical characterization of SmCoO3

Single-phase perovskite SmCoO₃ was prepared by a wet-chemical synthesis technique using metal-nitrates and citric acid; after its characterization by thermal analyses and X-ray diffraction, sintering at 900 °C in air, gave single phase and well crystallized powders. The powders were mixed with an organic solvent to prepare a slurry, which was deposited on alumina substrates as thick films, using the screen-printing technique. Electrical and gas sensing properties of sintered SmCoO₃ films were investigated in air, O₂ and CO₂, the results show that sensitivity reached a maximum value at 420 °C, for both gases. Dynamic tests revealed a better behavior of SmCoO₃ in CO₂ than O₂, due to a fast response and a larger electrical resistance change to this gas. X-ray diffraction made on powders after electrical characterization in gases, showed that perovskite-type structure was preserved.     

Structural characterization of PDMS–TEOS–CaO–TiO2 hybrid materials obtained by sol–gel

Hybrid materials of the polydimethylsiloxane (PDMS)–Tetraethyl orthosilicate (TEOS) system have already been widely described by several authors, being their potential for biological applications largely studied. For some bone related applications, the addition of Ca to these systems is commonly accepted as necessary for the enhancement of bioactivity. However Ca is commonly introduced in the form of calcium nitrate, which requires a heat treatment step in order to eliminate the nitrate residues. The aim of the present work is to produce a PDMS–SiO₂–CaO–TiO₂ hybrid material through a sol–gel process, for potential biomedical applications. Calcium nitrate was used as the calcium source and to avoid the presence of nitrate residues in the final product, a heat-treating step at 400 °C was performed. Titanium was added to the hybrid system in order to enhance the thermal stability of the materials subjected to this temperature. TGA, FT-IR and ²⁹Si NMR techniques were used to understand the role of the calcium and titanium in the structure formation, and the way they influence the thermal stability of the PDMS–SiO₂ system. A final material containing calcium, with no traces of nitrate and the maintenance of a significant amount of methyl groups was successfully obtained.     

Isothermal oxidation behavior of reactive hot-pressed TiN–TiB2 ceramics at elevated temperatures

Isothermal oxidation behavior of reactive hot-pressed TiN–TiB₂ ceramics with various TiN/TiB₂ molar ratios of 2/1, 1/1 and 1/2 was evaluated in the temperature range of 500–800 °C in air. TiN–TiB₂ ceramics have a relative density of 97–98.6%. The oxidation weight gains of TiN–TiB₂ ceramics depend upon the composition, oxidation temperature and exposure time. The structure and morphology of oxidized layers of TiN–TiB₂ ceramics were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). During isothermal oxidation of TiN–TiB₂ ceramics, anatase and rutile-TiO₂ form as the oxidized products at 500 °C. However, phase transformation from anatase to rutile occurs at temperatures between 500 and 600 °C, and therefore rutile-TiO₂ becomes the only crystalline phase after oxidation at temperatures of 600–800 °C for 10 h. The oxidation mechanism was proposed with reference to thermodynamically feasible oxidation reactions. The influence of composition on oxidation behavior of TiN–TiB₂ ceramics varies with temperature.     

Microemulsion-assisted solvothermal synthesis of Nd2(CO3)3·8H2O microstructures

Microstructures of Nd₂(CO₃)₃·8H₂O with various morphological structures and sizes were successfully synthesized using the microemulsion-assisted solvothermal method. The obtained products were characterized by X-ray diffraction (XRD), differential scanning calorimetry and thermal gravimetric analysis (DSC-TGA), scanning electron microscope (SEM), transmission electron microscope (TEM) and electron diffraction (ED). The results showed that pyramid-like and spherical Nd₂(CO₃)₃·8H₂O microstructures were synthesized depending on the reaction time and reaction temperature. Moreover, the reaction time and temperature also played important roles in controlling the morphologies and sizes of the resulting Nd₂(CO₃)₃·8H₂O microstructures.     

Improvement in the hydrogen-storage properties of Mg by the addition of metallic elements Ni, Fe, and Ti, and an oxide Fe2O3

Pure Mg was employed as a starting material instead of MgH₂ in this work. The magnesium prepared by mechanical grinding under H₂ (reactive mechanical grinding) with transition elements or oxides showed relatively high hydriding and dehydriding rates when the content of additives was about 20 wt.%. Ni, Fe and Ti were chosen as metallic transition elements to be added. Fe₂O₃ was selected as an oxide to be added. Samples Mg–14Ni–2Fe₂O₃–2Ti–2Fe were prepared by reactive mechanical grinding, and their hydrogen storage properties were examined and compared with those of a pure Mg sample prepared by reactive mechanical grinding under the same conditions. The Mg–14Ni–2Fe₂O₃–2Ti–2Fe sample showed much better hydrogen storage properties than the pure Mg sample. The as-milled Mg–14Ni–2Fe₂O₃–2Ti–2Fe sample did not require the activation. This sample absorbs 4.26 wt.% H for 5 min, and 4.41 wt.% H for 10 min, and 4.56 wt.% H for 60 min at n = 2. It desorbs 1.13 wt.% H for 10 min, 2.67 wt.% H for 30 min, and 3.32 wt.% H for 60 min at n = 2.     

Stripe correlation of tilts and superconductivity in La2CuO4+Σ

In single crystal La₂CuO_4+Σ,Σ ≈ 0.015, c-axis interstitial oxygen ordering (staging) in the oxygen-rich phase of the phase-separated sample was observed through satellite peaks in the neutron scattering which first appear at 250 K. If the sample is cooled through the temperature region around 210 K over a period of several hours, these satellite peaks show splitting in the a*-direction. This first-order phase transition involves a periodic one-dimensional modulation of the in-plane order. Microscopic modeling leads to a picture where the excess oxygen, and therefore also the doped holes, order in stripes along the a-direction in addition to staging along the c-direction. This ordering in stripes can be regarded as a continuation of the phase separation but now on a local scale, leading to a local increase in oxygen and hole density. This increase can account for the increase of the superconductingT _c of 2.5 K observed in our sample and also for many previously reported annealing experiments.