Energy levels and crystal-field parameters for Pr and Nd ions in rare earth (RE) tellurium oxides RE2Te4O11 revisited - Ascent/descent in symmetry method applied for triclinic site symmetry

Triclinic site symmetry presents considerable challenges in the studies of energy levels and crystal-field parameters (CFPs) for rare earth (RE) ions in crystals. Assignment of the spectral features to the irreducible representations of low symmetry point groups is difficult and may be unreliable. Fittings of large number of CFPs, even if the number of the available experimental energy levels is sufficient, often yield multiple solutions with relatively low and close rms deviations. These challenges call for better methods of fitting and analysis of CFPs. In this paper, we employ the ascent/descent in symmetry (ADS) method and three computational approaches: (i) a crystallographic data analysis program to identify approximated higher symmetry, (ii) the superposition model (SPM) analysis, and (iii) the pseudosymmetry axes method (PAM) for the combined coordination factors obtained using SPM. As a case study, the experimental CF splittings of Pr³⁺ and Nd³⁺ ions at triclinic C₁ sites in tellurium oxides RE₂Te₄O₁₁ are re-analyzed. Two alternative ADS chains C₁ ↔ C₂ ↔ D₂ and C₁ ↔ C_s ↔ C_2v were independently obtained. For each approximation of the actual C₁ symmetry of the REO₈ polyhedron in RE₂Te₄O₁₁, the ligands’ positions are determined and used in SPM to calculate CFP sets, which are used as starting for additional fittings. Comparative analysis of the fitted and calculated CFP sets enables assignment of appropriate axis system to the fitted CFP sets, thus avoiding the ambiguities occurring in previous ADS applications. The very good compatibility and consistency of the newly determined CFP sets indicate high reliability of both approaches. These CFP sets were used to simulate magnetic susceptibility data for polycrystalline samples of Pr₂Te₄O₁₁ and Nd₂Te₄O₁₁. Wider applications of the proposed procedures in optical spectroscopy studies of low symmetry systems may improve reliability of the CFP sets reported in literature.     

Ascorbate electro-oxidation by modified electrodes: Polypyrrole and polypyrrole/Ni(OH)2 composite thin films

The present paper describes the utilization of polypyrrole and the composite of polypyrrole doped with nickel hydroxide modified electrodes toward the catalytic oxidation of ascorbate. Films were potentiostatically deposited onto a glassy carbon surface and Fluor-doped tin oxide glass for different times. The physical characterization was performed using the low angle X-ray diffraction technique. Furthermore, the films were electrochemically characterized using cyclic voltammetry. The X-ray diffraction results show the existence of different polymorphic phases of nickel hydroxide in the polymer matrix, and the β-Ni(OH)₂ phase appears to be dominant. The cyclic voltammetry profile in KOH solution shows the presence of two redox peaks that are related to the Ni^II/Ni^III and Ni^III/Ni^II couples, at approximately 0.5 and 0.35 V, respectively. The reversible electro-oxidation of ascorbate was observed on the surface of the polypyrrole and composite films. The analytical curves obtained using voltammetric techniques show a linear relationship between the faradaic current and the increase of the ascorbic acid concentration. The sensitivity of these films, which is obtained from the slope of the analytical curves, shows that the composite film is more electroactive than the polypyrrole film: 133.4 mA L mol^− 1 cm^− 2 and 83.8 mA L mol^− 1 cm^− 2, respectively. The rate constants of the catalytic ascorbate electro-oxidation were also reported, where the mean values were found to be 217.74 M^− 1 s^− 1 and 54.37 M^− 1 s^− 1, for the composite and polypyrrole films, respectively. The low cost of polypyrrole doped with Ni(OH)₂ composite electrodes presents a more selective and high sensitivity to determine ascorbic acid concentration.     

气氛中硫和氯导致Fe-15Cr-10Al合金表面氧化铝膜退化

研究了Fe-15Cr-10Al合金在700℃还原性H_2-CO_2和H_2-HCl-H_2S-HCl混合气氛中的腐蚀行为.结果表明,添加到H_2-CO_2气氛中的微量HCl和H_2S导致合金表面氧化铝膜退化,在合金表面生成了铁铬铝混合氧化物层.这种退化与腐蚀过程中生成的硫化物和氯化物密切相关.计算混合气氛中平衡时的氯势、氧势和硫势预测了合金与气氛可能发生的反应,并解释了腐蚀机制.     

Structural and electrochromic properties of molybdenum doped vanadium pentoxide thin films by sol-gel and hydrothermal synthesis

Molybdenum-doped vanadium pentoxide (Mo-doped V₂O₅) thin films with doping levels of 3-10 mol% were prepared by dip-coating technique from a stable Mo-doped V₂O₅ sol synthesized by sol-gel and hydrothermal reaction. The Mo-doped V₂O₅ films had a layered V₂O₅ matrix structure along c-axis orientation with Mo⁶⁺ as substitutes. Values of the inserted and extracted charge density of 21.4 and 21.3 mC·cm^− 2 and the transmittance variation (ΔT at 640 nm) between anodic (+ 1.0 V) and cathodic (− 1.0 V) colored states of 41% were observed for the films with 5 mol% Mo⁶⁺ doping. Above this dopant concentration, the charge capacity and ΔT decreased. The enhancement of the electrochemical and electrochromic properties of the films is related to changes in the electronic properties of V₂O₅ films due to the creation of energy levels in the band gap of V₂O₅ by the Mo doping, accompanied by the reduction of the forbidden-band width and the increase of the conductivity.     

Self-doping Effects on the Superconducting Properties of Cu0.5Tl0.25M0.25Ba2Ca2Cu3O10−δ (M = Bi, Hg, Nb, Pd, Li, Na, K)

The effect of self-doping and substitution of elements of higher and lower electronegativity, such as Bi, Hg, Nb, Pd, Li, Na, K, on the superconducting properties of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ with x=0.25 is investigated. These experiments demonstrated that the elements of lower electronegativity such as Li, Na, and K can easily liberate their outer most s-electron that could be supplied to the conducting CuO₂ planes of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ superconductor, and as a result, we get enhanced superconducting properties. However, highly electro-negative elements hinder the transfer of carriers from charge reservoir layer to the conducting CuO₂ planes and promote inferior superconducting properties. In the present studies, we have investigated the effect of post-annealing in nitrogen and oxygen atmospheres for optimizing the carriers in conducting CuO₂ planes of Cu_0.5Tl_0.5−x M _x Ba₂Ca₂Cu₃O_10−δ (M=Bi,Hg,Nb,Pd,Li,Na,K) superconductor. These studies are important since the density of carriers in the conducting CuO₂ planes determines the Fermi-vector k _F and Fermi velocity v _F of the carriers, which ultimately brings about the final superconducting state of the system.      

Morphologies and electrochemical properties of 0.6Li2MnO3·0.4LiCoO2 composite cathode powders prepared by spray pyrolysis

Nanosized 0.6Li₂MnO₃·0.4LiCoO₂ composite cathode powders are prepared by spray pyrolysis. The micron-sized composite powders are converted into nanosized powders by a simple milling process. The mean sizes of the composite powders measured from the TEM images increase from 20 to 170 nm when the post-treatment temperatures increase from 650 to 900 °C. The Brunauer–Emmett–Teller surface areas of the composite powders post-treated at 650 and 900 °C are 24 and 3 m² g⁻¹, respectively. The XRD patterns indicate that the layered composite powders post-treated at 800 and 900 °C have high crystallinity and low cation mixing. The mean crystallite sizes of the powders, measured from the (003) peak widths of the XRD patterns using Scherrer's equation, are 35 and 56 nm at post-treatment temperatures of 800 and 900 °C, respectively. The initial discharge capacities of the 0.6Li₂MnO₃·0.4LiCoO₂ composite are 262, 267, 264, and 263 mAh g⁻¹ when the post-treat temperatures of the powders are 650, 700, 800, and 900 °C, respectively. The discharge capacity of the composite powders post-treated at 900 °C abruptly decreases from 263 to 214 mAh g⁻¹ by the seventh cycle and then slowly decreases to 198 mAh g⁻¹ with increasing cycle number, up to 30.     

Interplay of low and highT c phases in Bi-Sr-Ca-Cu-O superconductor

In the Bi-Sr-Ca-Cu-O system stringent conditions of heat-treatment lead to the formation of a mixture of both the low and highT _c phases and obtaining a single-phase material becomes extremely difficult. This study reports preparation of samples with single superconducting transitions at ∼ 75 K and ∼ 108 K; the compositions of which correspond ton=2,3 in the series Bi₂Sr₂Ca _n−1Cu _n O_{4 + 2n} . X-ray diffraction studies show that the lowerT _c material is a relatively pure phase while the higherT _c phase only co-exists with the lowerT _c phase. The most obvious effect of doping the system with lead is to make the reaction take place faster and thereby increase the volume fraction of the 110K phase.     

Callindra haematocephata and Peltophorum pterocarpum flowers as natural sensitizers for TiO2 thin film based dye-sensitized solar cells

We have studied the performance of dye-sensitized solar cells employing natural dye extracted from the flowers Callindra haematocephata and Peltophorum pterocarpum as sensitizers for TiO₂ photoanode. The extracts have shown appreciable absorption in the visible region. FTIR studies indicated the presence of anthocyanins and β-carotene in the flowers of C. haematocephata and P. pterocarpum respectively. The extracts were anchored on TiO₂ film deposited on transparent conductive glass (FTO) which were used as photoanode. The dye coated TiO₂ film electrode, Pt counter electrode and electrolyte (I⁻₃) assembled into a cell module was illuminated by a light source with intensity 100 mW/cm² to measure the photoelectric conversion efficiency of the DSSCs. From the J-V characteristic curves of cells, the parameters related to the solar cell performance were determined. The conversion efficiency of the DSSC employing natural dye extract from the flower C. haematocephata and P. pterocarpumwere was found as 0.06% and 0.04%, with open-circuit voltage (V_OC) of 370 mV & 400 mV, short-circuit current density (J_SC) of 0.25 mA/cm² & 0.15 mA/cm², fill factor (FF) of 0.70 & 0.71 and P_max of 65 & 45 μW cm⁻² respectively. The extract of the flower C. haematocephata exhibited better photosensitization action compared to the flower of P. pterocarpum.     

The impact of Ti and temperature on the stability of Nb5Si3 phases: a first-principles study

Abstract

Nb-silicide based alloys could be used at T > 1423 K in future aero-engines. Titanium is an important additive to these new alloys where it improves oxidation, fracture toughness and reduces density. The microstructures of the new alloys consist of an Nb solid solution, and silicides and other intermetallics can be present. Three Nb₅Si₃ polymorphs are known, namely αNb₅Si₃ (tI32 Cr₅B₃-type, D8_l), βNb₅Si₃ (tI32 W₅Si₃-type, D8_m) and γNb₅Si₃ (hP16 Mn₅Si₃-type, D8₈). In these 5–3 silicides Nb atoms can be substituted by Ti atoms. The type of stable Nb₅Si₃ depends on temperature and concentration of Ti addition and is important for the stability and properties of the alloys. The effect of increasing concentration of Ti on the transition temperature between the polymorphs has not been studied. In this work first-principles calculations were used to predict the stability and physical properties of the various Nb₅Si₃ silicides alloyed with Ti. Temperature-dependent enthalpies of formation were computed, and the transition temperature between the low (α) and high (β) temperature polymorphs of Nb₅Si₃ was found to decrease significantly with increasing Ti content. The γNb₅Si₃ was found to be stable only at high Ti concentrations, above approximately 50 at. % Ti. Calculation of physical properties and the Cauchy pressures, Pugh’s index of ductility and Poisson ratio showed that as the Ti content increased, the bulk moduli of all silicides decreased, while the shear and elastic moduli and the Debye temperature increased for the αNb₅Si₃ and γNb₅Si₃ and decreased for βNb₅Si₃. With the addition of Ti the αNb₅Si₃ and γNb₅Si₃ became less ductile, whereas the βNb₅Si₃ became more ductile. When Ti was added in the αNb₅Si₃ and βNb₅Si₃ the linear thermal expansion coefficients of the silicides decreased, but the anisotropy of coefficient of thermal expansion did not change significantly.     

Evolution of mechanical and electrical properties of tin-lead and lead free solder to copper joint interface

Cu-(Sn37Pb) and Cu-(Sn3.5Ag0.5Cu) solder joints were prepared at the same reflow temperature of 230 °C. The microstructural observation of the solder assemblies in scanning and transmission electron microscopes confirmed the presence of η-Cu₆Sn₅ in case of the former, and Cu₃Sn + η-Cu₆Sn₅ for the latter in the reaction zone. The findings are correlated with the electrical and mechanical properties of the joints. Lead free solder-Cu joint exhibited lower reaction zone thickness and improved electrical conductivity (0.28 × 10⁶Ω^− 1 cm^− 1) and shear strength ∼ 68MPa compared to conventional lead-tin solder-Cu joint. The latter showed electrical conductivity and shear strength of 0.22 × 10⁶Ω^− 1 cm^− 1 and ∼ 55 MPa, respectively. The difference in reaction zone thickness is explained on the basis of melt superheat, with Sn being the primary diffusing species in the intermetallic layer.     

Effect of praseodymium substitution on the growth and properties of Y1−x Pr x Ba2Cu3O7−δ (o <x < 0·4) single crystals

In the present paper, a modified self-flux technique has been successfully employed for the growth of pure and praseodymium substituted (partially) large single crystals of high temperature superconducting Y_1−x Pr _x Ba₂Cu₃O_7−δ (x = 0·0,0·2,0·4). Typical sizes of the platy and bulky crystals of pure YBCO(123) material are ≈ 2 × 2 × 0·1 mm³ and 4 × 1 × 1 mm³, respectively. In case of Pr-substitution, the typical sizes of platy and bulky crystals of Y_0·8Pr_0·2Ba₂Cu₃O_7−δ and Y_0·6Pr_0·4Ba₂Cu₃O_7−δ materials are ≈ 2 × 3 × 0·1 mm³ and 5 × 1 × 1 mm³ and ≈ 1 × 1·5 × 0·1 mm³ and 7 × 0·2 × 0·1 mm³, respectively. The morphology and growth habit of the as-grown single crystals and the critical transition temperature (T _c) of the oxygenated crystals were found to depend on the Pr-content. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Comparative study on the properties of amorphous carbon layers deposited from 1-hexene and propylene for dry etch hard mask application in semiconductor device manufacturing

Amorphous carbon layers (ACLs) were prepared by plasma enhanced chemical vapor deposition (PECVD) from 1-hexene (C₆H₁₂) and propylene (C₃H₆) as a carbon source at different temperatures for dry etch hard mask of semiconductor devices manufacturing process. The deposition rate of ACL deposited at 550 °C from C₆H₁₂ and C₃H₆ was 5050 Å/min and 6360 Å/min. Although the deposition rate of ACL deposited from C₆H₆ was lower than that from C₃H₆, normalized deposition rate of ACL deposited from C₆H₁₂ was 1.64 times higher than that from C₃H₆. The relative amount of hydrocarbon contents measured by FTIR (Fourier transformation infrared) and TDS (thermal desorption spectroscopy) was decreased with the increase of deposition temperature. Raman results showed that the numbers and size of graphite cluster of ACLs deposited from each source were increased with the increase of deposition temperature. The extinction coefficient of ACL deposited at 550 °C from C₆H₁₂ was 0.51 and that from C₃H₆ was 0.48. The density of ACL deposited at 550 °C from C₆H₁₂ was 1.48 g/cm³ and that from C₃H₆ was 1.45 g/cm³. The dry etching rate of ACL deposited at 550 °C from C₆H₁₂ was 1770 Å/min and that from C₃H₆ was 1840 Å/min. The deposition rate, dry etch rate and the amount of hydrocarbon contents of ACLs deposited from each carbon source were decreased with the increase of deposition temperature but extinction coefficient and density were increased with the increase of deposition temperature. We concluded that the variation behavior of the deposition characteristics and film properties of ACLs from C₆H₁₂ with the increase of deposition temperature was the same as those of ACLs from C₃H₆. The high density and low dry etch rate of ACL from C₆H₁₂ can be explained by less hydrocarbon incorporation during deposition and these properties are more favorable for the dry etch hard mask application in semiconductor device fabrication.     

Notch stress concepts for the fatigue assessment of welded joints - Background and applications

Among modern fatigue design concepts for welded structures, the linear-elastic notch stress concept gains increasing industrial acceptance. There are two variants of this concept, one for thick walled (t ? 5 mm) welded joints with the reference radius r_ref = 1.00 mm, which is already included in the fatigue design recommendations of the IIW and applied for the assessment of big welded structures, and one for thin walled (t < 5 mm) welded joints with the reference radius r_ref = 0.05 mm, which is more and more used in the automotive industry.The concept with r_ref = 1.00 mm is based on the micro-support theory of Neuber with the fictitious radius r_ref = 1.00 mm, derived by Radaj. The background of the concept with r_ref = 0.05 mm is the relationship between the stress-intensity factor and the notch stress according to Creager and Paris as well as Irvin’s theory of crack blunting. Besides these two theories, the definition of both of these radii has also an experimental background; they are observed in many welded joints.In the present paper, first the background and then different applications of both concept variants are described: the application of the reference radius of r_ref = 1.00 mm for MAG-welded offshore K-nodes (t = 30 mm) and sandwich panels for ship decks (t = 5 mm), and the application of r_ref = 0.05 mm for spot-welded automotive doors (t = 1 mm) and MAG-welded automotive trailing links (t = 3-4 mm). The sandwich panels were evaluated additionally with r_ref = 0.05 mm. Calculations and experimental results are compared and the reliability of the notch stress concept variants underlined. Additionally, recommendations for the slope of design lines distinguishing between thin and thick dimensions are given, i.e. k = 3.0 and 5.0 (normal stress, shear stress) for thick and stiff structures, k = 5.0 and 7.0 for thin and flexible structures.     

Spectroscopic behavior of Nd in a new microcrystalline ZnY4W3O16 tungstate

In the framework of systematic investigations of the optical properties of a new tungstate ZnY₄W₃O₁₆ in which Y³⁺ cations allow for rare-earth ions doping, the compounds doped with neodymium Nd³⁺ ion were synthesized, analyzed and considered as a possible laser material. The crystalline stoichiometric sample of the chemical formula ZnNd₄W₃O₁₆ and their yttrium diluted analogues 0.5, 1, 5 and 10 mol% Nd³⁺-doped ZnY₄W₃O₁₆ were obtained by a solid-state reaction. X-ray powder diffraction analysis as well as the IR spectra were used to the structural characterization of the compounds. Electron microscopy has been used to analyze the grain size, the presence of aggregates, and the type of boundary between the microcrystals. In order to study spectroscopic properties, the high resolution absorption and emission spectra at room and low temperature were measured in visible and IR regions. The radiative transition probabilities in those tungstates were calculated and analyzed. Based on the 4 K absorption spectra in the range of ⁴I_9/2 → ²P_1/2 transition the number of metal sites occupied by the dopants was determined. The strong fluorescent emission involving the ⁴F_3/2 → ⁴I_9/2, ⁴I_11/2, ⁴I_13/2 transitions at 298 and 77 K were observed under pulsed laser and xenon lamp excitation. The dynamics of the Nd³⁺ excited states were characterized by decay times measurements and compared to earlier reported data for neodymium tungstates.     

硅酸镓镧晶体谐振器的高频振动分析

利用Mindlin板理论分析了硅酸镓镧晶体板强耦合的厚度剪切振动和弯曲振动，获得了硅酸镓镧晶体板高频振动的色散关系、频谱关系和振动模态位移图。数值计算结果表明，Mindlin板理论可以获得硅酸镓镧晶体板厚度剪切振动的一阶精确截止频率，无需修正系数。基于石英晶体谐振器设计晶片最佳长厚比的选取方法，确定了硅酸镓镧晶片的最佳尺寸，避免了厚度剪切振动模态和弯曲振动模态的强耦合。通过绘制硅酸镓镧晶体板在最佳尺寸时的各振动模态位移图，发现厚度剪切振动模态是主振模态，具有很好的能陷效应。Mindlin板理论在硅酸镓镧晶体板高频振动的应用分析可以指导硅酸镓镧晶体谐振器的实际研发。     

Structure, morphology and optical properties of nanocrystalline yttrium oxide (Y2O3) thin films

Yttrium oxide (Y₂O₃) thin films were grown onto Si(1 0 0) substrates using reactive magnetron sputter-deposition at temperatures ranging from room temperature (RT) to 500 °C. The effect of growth temperature (T_s) on the growth behavior, microstructure and optical properties of Y₂O₃ films was investigated. The structural studies employing reflection high-energy electron diffraction RHEED indicate that the films grown at room temperature (RT) are amorphous while the films grown at T_s = 300-500 °C are nanocrystalline and crystallize in cubic structure. Grain-size (L) increases from ∼15 to 40 nm with increasing T_s. Spectroscopic ellipsometry measurements indicate that the size-effects and ultra-microstructure were significant on the optical constants and their dispersion profiles of Y₂O₃ films. A significant enhancement in the index of refraction (n) (from 2.03 to 2.25) is observed in well-defined Y₂O₃ nanocrystalline films compared to that of amorphous Y₂O₃. The observed changes in the optical constants were explained on the basis of increased packing density and crystallinity of the films with increasing T_s. The spectrophotometry analysis indicates the direct nature of the band gap (E_g) in Y₂O₃ films. E_g values vary in the range of 5.91-6.15 eV for Y₂O₃ films grown in the range of RT-500 °C, where the lower E_g values for films grown at lower temperature is attributed to incomplete oxidation and formation of chemical defects. A direct, linear relationship between microstructure and optical parameters found for Y₂O₃ films suggest that tuning optical properties for desired applications can be achieved by controlling the size and structure at the nanoscale dimensions.     

Nano Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Induced Fluxoid Jumps and Low Field Enhanced Critical Current Density in MgB<Subscript>2</Subscript> Superconductor

Nano particle of Fe₃O₄ (nFe₃O₄) up to 6 at% were doped in the superconducting MgB₂ samples. Despite the strong ferromagnetic nature of Fe₃O₄, both the ac susceptibility and the resistivity measurements show that up to 4 at% of Fe₃O₄, T _c =38 K is not changed, whereas for 6% T _c decreases by 6 K. This indicates that a low concentration of Fe does not substitute either the Mg or B sites and probably occupies the intergrain spaces. For 0.5% doped Fe₃O₄, an increase in J _c with respect to the pure MgB₂ samples is observed in the lower field and temperature regions (H<2 T and 20 K) indicating an enhanced flux pinning and the magnetic activation, i.e., the interaction between the magnetic dipole of Fe ion and the vortices is weak in comparison to the effective pinning potential. Whereas, at H>2 T, J _c of the doped samples is always less than that of MgB₂, and the activation is dominant in comparison with the effective pinning potential provided by the doping. Flux jumps are observed in lower T and H regions for the samples doped up to 1% nFe₃O₄ only. Magnetization plots of higher Fe content samples exhibited clear paramagnetic background. Mossbauer measurements for the higher (4, 6 at%) nFe₃O₄ doped MgB₂ samples show that at RT, the hyperfine field for both samples is ∼100 kOe and ∼120 kOe at 90 K. This means that the nFe₃O₄ particles decompose and form possibly an intermetallic Fe-B phase in the matrix.     

Effect of mechanical milling and heat treatment on the structure and magnetic properties of gas atomized Mn-Al alloy powders

Ferromagnetic Mn-Al alloy powders were fabricated by mechanical milling and heat treatment with gas-atomized powders. Different processes, i.e., heat treatment before ball milling and ball milling before heat treatment, result in different microstructures and magnetic properties of the powders. It was found that H_c increased and M_r decreased with the size reduction regardless of the sequence of heat treatment and ball milling. However, tendency of the change in H_c and M_r depended on the sequence. Further annealing of the powders ball-milled after heat treatment resulted in slight decrease of H_c and large increase of M_r. The magnetic properties, M_r = 41.2 emu/g, H_c = 3.1 kOe, were obtained from the powders ball-milled for 5 h after heat treatment at 650 °C for 20 min, and subsequent annealing at 280 °C for 20 min.     

Formation of Nd2Fe14B hydride by milling of anhydride particles in toluene in a closed reactor

When milling micrometer thin Nd₂Fe₁₄B platelets, of an average 1–2 mm diameter, in toluene in a closed reactor, part of the toluene decomposes at the surface of the platelets and yields nascent hydrogen and carbon/low hydrocarbons. The hydrogen diffuses into the Nd₂Fe₁₄B platelets and the carbon forms a thin surface passivation layer of the platelets, forming the stable Nd₂Fe₁₄BH _x ,x ≤ 5, hydride at room temperature. On heating in a calorimeter, the hydrogen desorbs off the sample with a well-defined endotherm between 370 and 425 K. An N₂ gas atmosphere, if used during the heating, facilitates the H-desorption process with the modified kinetic parameters. For example, the enthalpy of the H-desorption ΔH and the related activation energyE _a have the measured values ΔH = 153 J/g andE _a = 58·2 kJ/mol in argon and ΔH = 256 J/g andE _a = 41·6 kJ/mol in N₂. It is argued that N₂ gas has a fast reaction with the H atoms desorbing off the thin sample platelets and forms NH₃ gas with an instantaneous decrease of the total external gas pressure at the sample. This supports the fast desorption of H atoms in the sample with the modified desorption kinetics in N₂ gas.     

Plasma properties of CrOx films synthesized by a cathodic arc evaporation process

The plasma in a cathodic arc evaporation process used for the deposition of Cr_1−xO_x films was studied by an optical emission spectroscopy (OES). With the introduction of Ar and oxygen into the chamber at deposition pressures from 0.7 Pa to 2.7 Pa, high density of evaporated chromium catalyzes the decomposition of oxygen reactive gas, and induces the formation of Cr_1−xO_x films. Optical emission spectra including atomic and ionized Cr, excited and ionized oxygen revealed that excitation, ionization and charge transfer reactions of the Cr-O plasma occurred during the Cr_1−xO_x deposition process. A simplified empirical model which incorporates the relevant atomic processes in the gas phase with the chemical composition and deposition rate of the deposited Cr_1−xO_x coating was developed. Rhombohedral Cr₂O₃ and tetragonal CrO₂ were observed in the Cr_1−xO_x coatings deposited at higher pressure than 1.3 Pa. The Cr_1−xO_x coating depicted a dense and compact microstructure with well-attached interface.