Effects of niobium on thermal stability and corrosion behavior of glassy Cu-Zr-Al-Nb alloys

The corrosion behavior of Cu_95−xZr_xAl₅ (x=40, 42.5 and 45 at.%) in 1 N HCl, 3 mass% NaCl and 1 N H₂SO₄ solutions was studied. As Zr content increases, the corrosion resistance is slightly enhanced. In order to improve the corrosion resistance of the Cu-Zr-Al glassy alloy, Nb was selected to substitute Cu. Although the supercooled liquid region ΔT_x of the Cu-Zr-Al glassy alloys decreases with increasing Nb content, the alloys still retain high glass-forming ability and bulk glassy samples with 1.5 mm diameter can be obtained when up to 5 at.% Nb was added. It is found that the addition of Nb results in improvement of the corrosion resistance of the glassy Cu-Zr-Al alloys.     

Reversibility window in as-quenched Ge-As-S glasses

Thermoanalytical characteristics and Raman scattering of high purity sulfur and ternary bulk glasses Ge_xAs_xS_(100−2x) for x=4-22 at. % were studied. The intermediate phase characterized by vanishing of non-reversing heat flow ΔH_nr, i.e. so-called the thermally reversing window was found between mean coordination number 〈r〉∼2.28-2.47. Separated phase of non-crystalline cycloocta-S, manifesting itself by λ-transition at ∼155 °C, was found for glasses with sulfur content higher than ∼80 at.%. Raman spectra of studied Ge-As-S glasses showed different shapes in three different areas according to three distinct phases of network glasses-floppy, intermediate, rigid.     

A comparative study on the glass-forming ability of some alloys in the Sb-As-Se system by differential scanning calorimetry

The glass-forming ability and devitrification of alloys in the Sb-As-Se system have been studied by differential scanning calorimetry (DSC). A comparison of various simple quantitative methods to assess the level of stability of glassy materials in the above-mentioned system is presented. All these methods are based on the characteristic temperatures, obtained by heating of the samples in non-isothermal regime, such as the glass transition temperature, T_g, the temperature at which crystallization begins, T_in, the temperature corresponding to the maximum crystallization rate, T_p, or the melting temperature, T_m. In this work, a kinetic parameter K_r(T) is added to the stability criteria. The thermal stability of some ternary compounds of Sb_xAs_{0.60−(2x+y)}Se_0.40+x+y-type has been evaluated experimentally and correlated with the activation energies of crystallization by this kinetic criterion and compared with those evaluated by other criteria.     

Impingement effect on the glass-crystal transformation kinetics by using DSC under non-isothermal regime. Application to the crystallization of the several semiconducting alloys of the Sb-As-Se and Ge-Sb-Se glassy systems

A procedure has been developed for analyzing the evolution with time of the actual volume fraction transformed, for calculating the kinetic parameters and for analyzing the glass-crystal transformation mechanisms in solid systems involving formation and growth of nuclei. By defining an extended volume of transformed material and assuming spatially random transformed regions, a general expression of the extended volume fraction has been obtained as a function of the temperature. Considering the mutual interference of regions growing from separate nuclei (impingement effect) and from the above-mentioned expression, the actual volume fraction transformed has been deduced. The kinetic parameters have been obtained, assuming that the reaction rate constant is a time function through its Arrhenian temperature dependence. The theoretical method developed has been applied to the crystallization kinetics of a set semiconducting alloys, prepared in our laboratory, corresponding to the Sb-As-Se and Ge-Sb-Se glassy systems. The obtained values for the kinetic parameters agree satisfactorily with the calculated results by the Austin-Rickett kinetic equation, under non-isothermal regime. This fact allows to check the validity of the theoretical model developed.     

Characterization of explosives traces by the Nanocalorimetry

Thermal characterization of energetic materials is often a non-trivial task. Experiments on such materials employing differential scanning calorimetry (DSC) have been mainly performed using special high-pressure crucibles, in which the pressure uncontrollably changes during the measurement. Conducting constant pressure experiments would allow addressing the decomposition reaction kinetics in a more quantitative way. In addition, the explosives detection requires sensing much smaller amounts of samples than those used in the DSC measurements. Explosives in the state of traces typically provide the sample sizes in the nano-gram to pico-gram range.The present work has been carried out with the Nanocalorimeter (www.nanotlab.com), which can safely measure nano-gram-size samples. The Nanocalorimeter is operational in both DC and AC modes. In the DC mode, the device can perform heating ramps at ultra-fast heating rates (10³-10⁶ K/s) that are up to 1 million times faster than the conventional DSC. In the AC mode, which is analogous to the Modulated-DSC, the interval of the temperature modulation frequencies is also unmatched: the upper frequency bound reaches 3.0-10.0 kHz. Similar to the experimental setups described in the literature, the Nanocalorimeter employs a commercial gas sensor with integrated heating elements and thermocouples.In this work, a series of energetic materials of practical interest such as hexogen (cyclotrimethylenetrinitramine) and pentrite are characterized for the first time using ultra-fast heatings. The samples have been deposited on the sensor membrane using either micro-manipulation or spin-coating. Coupling of the Nanocalorimeter to a fast CCD camera was found quite useful to simultaneously visualize the processes occurring on a micro-second time-scale.     

Molecular alloys as phase change materials (MAPCM) for energy storage and thermal protection at temperatures from 70 to 85 °C

Molecular alloys, that combine a relatively high heat of melting with a suitable melting temperature adapted to the application temperature, are excellent materials for thermal protection and for thermal energy storage. Of special interest is the fact that, by making alloys of molecular materials; the range of melting can be adjusted over a range of temperatures. The present paper reports on the design of MAPCMs to be used for energy storage and thermal protection at temperatures from 70 to 85 °C. The aim is to use these materials for thermal protection in the catering sector in order to avoid proliferation of micro organisms; the minimal temperature required is higher than 65 °C. The work illustrates how some fundamental studies are helpful in choosing the right composition that is able to work at the temperature required for an application. Several molecular alloys using the n-alkanes are elaborated and characterized. The preparation of mixed crystals, their crystallographic and thermodynamic properties and stability, phase change behaviour, and their use in practical applications are reported.     

Effect of heat treatment on the optical and electrical transport properties of Ge15Sb10Se75 and Ge25Sb10Se65 thin films

The effect of heat treatment on the optical and electrical properties of Ge₁₅Sb₁₀Se₇₅ and Ge₂₅Sb₁₀Se₆₅ thin films in the range of annealing temperature 373-723 K has been investigated. Analysis of the optical absorption data indicates that Tauc's relation for the allowed non-direct transition successfully describes the optical processes in these films. The optical band gap (E_g^opt.) as well as the activation energy for the electrical conduction (ΔE) increase with the increase of annealing temperature (T_a) up to the glass transition temperature (T_g). Then a remarkable decrease in both the E_g^opt. and ΔE values occurred with a further increase of the annealing temperature (T_a>T_g). The obtained results were explained in terms of the Mott and Davis model for amorphous materials and amorphous to crystalline structure transformations. Furthermore, the deduced value of E_g^opt. for the Ge₂₅Sb₁₀Se₆₅ thin film is higher than that observed for the Ge₁₅Sb₁₀Se₇₅ thin film. This behavior was discussed on the basis of the chemical ordered network model (CONM) and the average value for the overall mean bond energy 〈E〉 of the amorphous system Ge_xSb₁₀Se_90−x with x=15 and 25 at%. The annealing process at T_a>T_g results in the formation of some crystalline phases GeSe, GeSe₂ and Sb₂Se₃ as revealed in XRD patterns, which confirms our discussion of the obtained results.     

Thermal behavior and nucleation kinetics of 1,5-dimethyl-2-nitroimino-1, 3, 5-triazinane crystal

Nitroguanidine derivatives have increasingly gained attention because of their high insecticidal activities and wide spectrum. In this paper, nitroguanidine derivative 1,5-dimethyl-2-nitroimino-1, 3, 5-triazinane was synthesized, and its crystal structure was determined by X-ray technique. The thermal behaviors of 1, 5-dimethyl-2-nitroimino-1, 3, 5-triazinane in a nitrogen atmosphere were also studied under non-isothermal conditions by thermogravimetry (TG) and differential scanning calorimetry (DSC) techniques. The TG and DSC studies showed that the sample started to melt at 408.1 K with high melting enthalpy of 121.3 J/g and was stable up to at least 423.2 K, which indicated that the sample could be effectively utilized for various devices below 423.2 K. The melting entropy of 1,5-dimethyl-2-nitroimino-1, 3, 5-triazinane calculated from melting point and melting enthalpy using Eq. (1) was 51.476 J mol⁻¹ K⁻¹. In addition, the nucleation parameters of 1,5-dimethyl-2-nitroimino-1, 3, 5-triazinane in ethanol, such as the radius of critical nucleus and the Gibbs free energy barrier, had also been investigated based on classical nucleation theory.     

The glassy state of the amorphous V2O5–NiO–TeO2 samples

The glass transition temperature dependence to heating rate and therefore the activation energy (ΔH^?) of the glass transition of (60-x)V₂O₅–xNiO–40TeO₂ oxide glasses with 0≤x≤20 (in mol%) were investigated at heating rates φ (=3 6, 9, 10 and 12 K/min) using differential scanning calorimetry (DSC). The heating rate dependence of T_g was used to investigate the applicability of different theoretical models describing the glass transition. Using the application of Moynihan and Kissinger et al. models to the present data, different values of (ΔH^?) at each different heating-rate regions were obtained. The fragility parameter (m=ΔH^?/R T_g) was ∼24.98 for x=10 mol%, suggesting that this glass may be considered as a rather strong glass (fragility index m∼>20 is an indication of fragile glass). Also the compositional dependence of T_g and ΔH^? was investigated.     

Dielectric, thermal and optical study of an unusually shaped liquid crystal

Dielectric measurements have been carried out for the determination of real and imaginary parts of the permittivity of a newly synthesized, unusually shaped liquid crystal. The sample has been investigated in the frequency range from 100 Hz to 10 MHz within a temperature range 80-130 °C. The dielectric measurements in the smectic A phase indicate a Cole-Cole type of dispersion, and the activation energy was found to be 5.5 meV by using the Arrhenius plot of relaxation time. In addition to this, thermal and optical transmittance studies have also been conducted in the above mentioned temperature range, and the temperature dependence of these parameters has been discussed in detail. The phase transition temperature obtained from a differential scanning calorimetry (DSC) study matches within 2 °C that was obtained from an optical transmittance study. The dielectric and optical behavior of the unusually shaped liquid crystal has been explained on the basis of a proposed theoretical model in which a sample possesses two different conformers having induced polarizations in opposite directions.     

Crystallization kinetics in phosphate sodium-based glass studied by DSC technique

In this work, differential scanning calorimetry (DSC) was used to study the mechanism of crystallization of 50P₂O₅-27.8Na₂O-16ZnO-6.2Al₂O₃ glass. DSC measurements were performed on bulk and powdered glasses with different particle size. The curve for bulk glass shows one crystallization peak while powdered glasses presented two distinct crystallization peaks. Based on DSC studies, the activation energies obtained were 336±6 and 213±3 kJ mol⁻¹, associated with first and second crystallization peaks, respectively. Analyzing the DSC parameters as a function of particle size, the Avrami n parameter suggests that the peak at low temperature may be associated with surface crystallization while the peak at high temperature is associated with bulk crystallization.     

Dielectric and AC ionic conductivity investigations in K3H(SeO4)2 single crystal

Optical observation under the polarizing microscope and DSC measurements on K₃H(SeO₄)₂ single crystal have been carried out in the temperature range 25-200 °C. It reveals a high-temperature structural phase transition at around 110 °C. The crystal system transformed from monoclinic to trigonal. Electrical impedance measurements of K₃H(SeO₄)₂ were performed as a function of both temperature and frequency. The electrical conduction and dielectric relaxation have been studied. The temperature dependence of electrical conductivity indicates that the sample crystal became a fast ionic conductor in the high-temperature phase. The frequency dependence of conductivity follows the Jonscher's universal dynamic law with the relation σ(ω)=σ(0)+Aωⁿ, where ω is the frequency of the AC field, and n is the exponent. The obtained n values decrease from 1.2 to 0.1 from the room temperature phase to fast ionic phase. The high ionic conductivity in the high-temperature phase is explained by the dynamical disordering of protons between the neighboring SeO₄ groups, which provide more vacant sites in the crystal.     

Effects of the substitution of gadolinium for neodymium on the crystal and magnetic properties of the Nd1−xGdxCo4B compounds

The crystal and magnetic properties of the Nd_1−xGd_xCo₄B compounds for 0?x?1 have been studied by X-ray powder diffraction, magnetization and differential scanning calorimetry (DSC) measurements. These compounds crystallize in a hexagonal CeCo₄B-type structure with the P6/mmm space group. The substitution of Gd for Nd leads to a decrease of the unit-cell parameter a and the unit-cell volume V, while the unit-cell parameter c remains almost constant. Magnetic measurements indicate that all samples are ordered magnetically below room temperature. The Curie temperatures determined by the DSC technique increase linearly as Nd is substituted by Gd. The saturation magnetization at 5 K decreases upon the Gd substitution up to x=0.6, and then increases again.     

Effect of plasticizer on ionic transport and dielectric properties of PVA-H3PO4 proton conducting polymeric electrolytes

Solid polymer electrolytes have attracted considerable attention due to their wide variety of electrochemical device applications. The present paper is focused on the effect of plasticizer to study the structural, electrical and dielectric properties of PVA-H₃PO₄ complex polymer electrolytes. XRD results show that the crystallinity decreases due to addition of plasticizer up to particular amount of polyethylene glycol (PEG) and thereafter it increases. Consequently, there is an enhancement in the amorphicity of the samples responsible for process of ion transport. This characteristic behavior can be verified by the analysis of the differential scanning calorimetry results. FTIR spectroscopy has been used to characterize the structure of polymer and confirms the complexation of plasticizer with host polymeric matrix. Electrical and dielectric properties have been studied for different wt% of plasticizer and their variations have been observed. The addition of PEG has significantly improved the ionic conductivity. The optimum ionic conductivity value of the plasticized polymer electrolyte film of 30 wt% PEG has been achieved to be of the order of 10⁻⁴ S cm⁻¹ at room temperature and corresponding ionic transference number is 0.98. The minimum activation energy is found to be 0.25 eV for optimum conductivity condition.     

Compositional dependence of the optical properties of novel Ge-Ga-Te-CsI far infrared transmitting chalcohalide glasses system

A systematic series of (Ge₁₅Ga₁₀Te₇₅)_1−x(CsI)_x (x=0, 5, 10, 15 at%) far infrared transmitting chalcohalide glasses were prepared by the traditional melt-quenching method. The physical, thermal and optical properties were determined. The allowed direct transition and indirect transition of samples were calculated according to the Tauc equation. The results show that glass transition temperatures (T_g) were in the range 133-175 °C, with ΔT values between 81 and 130 °C. The highest values of metallization criterion (0.244) and energy gap (1.191 eV) were obtained for (Ge₁₅Ga₁₀Te₇₅)₈₅(CsI)₁₅. When the dissolved amount of CsI increased from 0 to 15 at%, the direct optical band gap and indirect optical band gap were in the ranges 0.629-1.075 eV and 0.438-0.524 eV, respectively. The Ge-Ga-Te-CsI glasses have an effective transmission window between 1.7 and 25 μm, encompassing the region of interest for bio-sensing applications.     

Fluoride glasses in the InF3-GaF3-YF3-PbF2-CaF2-ZnF2 system

New glasses have been synthesized in a multicomponent system based on indium fluoride. Samples of a few mm in thickness were obtained. They are transparent and homogeneous. Main physical properties such as density, characteristic temperatures, density, thermal expansion and refractive index have been measured. The evolution versus composition is reported for samples with the formula: (35−x) InF₃-xGaF₃-10YF₃-25PbF₂-15CaF₂-15ZnF₂. T_g lies between 260 and 296 °C while melting starts around 480 °C. Glass samples are stable at room temperature. By comparison with other standard fluoride glasses, they exhibit higher refractive index and density.     

Incident-angle dependence of the longitudinal optic mode and the Doppler effect in silica glass

The infrared reflection spectra of silica glass were monitored at different incident angles of the wave. The density of states (DOS) and frequency ω_LO of the longitudinal optic (LO) mode were found to be the functions of the incident angle. The DOS of the LO mode increases with increasing incident angle as a whole. However, two regions can be divided in which one is below ∼30° and the other is above 30°. The frequency ω_LO of the LO mode increases linearly with incident angle. The earlier proposed relationship between the shifted frequency produced by the Doppler effect and the source frequency of the main transverse optic (TO) mode was investigated by studying the infrared spectra of the annealed glass capillaries. The shifted frequency was found to be linearly proportional to the source frequency of the TO mode, as depicted by the Doppler effect.     

UV absorption edge shift by doping alkali fluorides in fluoroaluminate glass

The effect on transmittance and reflectance in the vacuum ultraviolet (VUV) region by doping alkali fluorides (LiF, NaF and KF) has been investigated in a ternary fluoroaluminate (18BaF₂-37CaF₂-45AlF₃) glass. The absorption edge of the glass obeys the Urbach rule and was shifted monotonically towards higher energies by increasing the concentration of each alkali fluoride. The VUV reflection peak at 11.3 eV was not sensitive to the change of the concentration of dopants. The magnitude of the edge shift was 10-20 times larger than that expected from the additive law on the magnitude of absorption coefficients of the glass and alkali fluorides. An excitonic interaction similar to that observed in mixed crystals of alkali halides is suggested from the monotonic shift toward the absorption edges of the dopants. The weak sensitivity of the reflection peak upon doping supports that the excitonic state lies predominantly around the edge energies.     

Thermal stability and reaction properties of passivated Al/CuO nano-thermite

Thermal stability and reaction properties of Al-CuO system, a mixture of 50-200 nm aluminum nanoparticles passivated by nitrocellulose and 12 nm copper (II) oxide, were investigated with microstructure characterization, differential thermal analysis (DTA), and thermogravimetric analysis (TGA). Transmission electron microscopy observation confirmed that the passivation coating successfully hinders the oxidization. TGA revealed that the passivation shell does not influence the ignition temperature of the thermite reaction. Reaction chemistry of the nano-thermite was elucidated by heating the composite both in inert ambient and vacuum. It was found that the thermite reaction composes of three continuing steps: At 570 °C, Al is oxidized into Al₂O₃ by reacting with CuO, which forms Cu₂O and produces a significant amount of heat. Subsequently two endothermic reactions occur. Starting at 800 °C, alumina reacts with Cu₂O and forms CuAlO₂. Above this temperature CuAlO₂ will decompose and eventually produce alumina, Cu, and O₂ at 1000 °C. Since the nano-thermite reaction pathway differs greatly from bulk thermite reactions, these results are important to develop a nano-thermite platform that can be used for a novel low cost, low temperature, and copper based microjoining and advance IC packaging.     

Influence of framework silica-to-alumina ratio on the water adsorption and desorption characteristics of MHI-CaX/CaY zeolite

The influence of the framework SiO₂/Al₂O₃ ratio from 2.0 to 10.0 of commercial faujasite-type CaX/CaY zeolite produced by Mitsubishi Heavy Industries Ltd. (MHI) on the water adsorption and desorption characteristics was investigated. Not only the change in electronegativity of the zeolite but the change in pore-size distribution of the zeolite affects the water adsorption and desorption characteristics of the zeolite. We found great differences in isotherms of water between CaY7.0 (SiO₂/Al₂O₃=7.0) and CaY10.0. The differences are mainly caused by the considerable change in pore-size distribution. A step-wise variation was observed in the desorption isotherm of water from CaY10.0 at approximately P/P₀=0.4. This is due to the pore distribution of CaY10.0 being relatively poor in smaller micropores in zeolite structure, since a similar phenomenon is observed in the case of argon adsorption on CaY10.0. In the experiments using a fixed bed, an apparent dependency of HTO dehydration ratio on the flow rate of the purge gas is measured with the CaY10.0 zeolite, while the water desorption from other tested CaX/CaY zeolite is independent of the flow rate of helium purge gas. This indicates that the transfer step of water diffusion through a laminar film appeared as one of the rate-controlling steps in the water desorption from CaY10.0.