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.     

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.     

Crystallization studies on AgI-Ag2O-MoO3 superionic system synthesized by melt quenching and mechanical milling

Crystallization in the melt-quenched (MQ) and mechanically milled (MM) superionic systems has been thoroughly investigated using differential scanning calorimetry, X-ray diffraction and electrical conductivity measurements. It is observed that the two systems obey different crystallization processes. The conventionally melt-quenched samples exhibit only one crystallization peak near 112 °C, whereas, the mechanochemically synthesized samples show two well-separated crystallization peaks at T_cl∼75-97 °C and T_c2∼132±2 °C. The higher value of electrical conductivity in the mechanochemically synthesized samples (∼10⁻² Ω⁻¹ cm⁻¹ at 300 K) than the melt-quenched samples is attributed to the higher value of disorder (entropy) in the former.     

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.     

Metastable phases in Gallium-Indium eutectic alloy particles and their size dependence

In this paper, submicrometer-sized Ga-In eutectic alloy particles were dispersed into polymethylmethacrylate (PMMA) matrix by ultrasonic vibration and sedimentation method. The solidification and melting processes of Ga-In eutectic alloy particles were studied by differential scanning calorimeter (DSC). Four endothermal peaks with the onset temperature located at 16, −11, −22, and −27 °C were observed in DSC heating curves, which corresponded to the melting process of the stable Ga-In phase α-Ga(In) and three metastable phases of β-Ga(In), δ-Ga(In) and γ-Ga(In), respectively. The stable phase α-Ga(In) can only be formed when the size of alloy particle was larger than 0.58 μm. Conversely, metastable phases β-Ga(In), δ-Ga(In) and γ-Ga(In) are mainly formed. The result shows that phase structures in Ga-In eutectic alloy are size dependent.     

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.     

Solid-phase crystallization of amorphous Si films on glass and Si wafer

When amorphous silicon films deposited on glass by physical or chemical vapor deposition are annealed, they undergo crystallization by nucleation and growth. The growth rate of Si crystallites is the highest in their 〈111〉 directions along or nearly along the film surface. The directed crystallization is likely to develop the 〈110〉//ND or 〈111〉//ND oriented Si crystallites. As the annealing temperature increases, the equiaxed crystallization increases, which in turn increases the random orientation. When amorphous Si is under a stress of the order of 0.1 GPa at about 540 °C, the tensile stress increases the growth rate of Si grains, whereas the compressive stress decreases the growth rate. However, the crystal growth rate increases with the increasing hydrostatic pressure, when the pressure is of the order of GPa at 530–540 °C. These phenomena have been discussed based on the directed crystallization model advanced before, which has been further elaborated.     

The orientational glass transition in pentachloronitrobenzene: a study by differential scanning calorimetry and thermally stimulated currents

The orientational disorder that is a feature of the crystalline pentachloronitrobenzene above ∼−82 °C, can be frozen by cooling to produce an orientational glass. The number of degrees of freedom frozen on cooling, or released on heating, in this orientational glass transition is low, so that the heat capacity change associated with this transition is expected to be small. In the present work, we show that the calorimetric signature of this orientational glass transition is in fact very weak. Conversely, since the molecular motions associated with this relaxation drag strong dipoles, the technique of thermally stimulated depolarisation currents (TSDC) provides a very strong signature of this transition. The orientational glass transition in pentachloronitrobenzene was studied by TSDC and, from this study, it was shown that this orientational glass belongs to the class of very strong glasses in the fragility scale proposed by Angell.     

Kinetics of the apparent isothermal and non-isothermal crystallization of the α-Fe phase within the amorphous Fe81B13Si4C2 alloy

Kinetics of the apparent isothermal and the non-isothermal crystallization of α-Fe phase within the amorphous Fe₈₁B₁₃Si₄C₂ alloy were investigated by an X-ray diffraction (XRD) and by a differential scanning calorimetry (DSC). It was established that the apparent isothermal crystallization of α-Fe phase within amorphous Fe₈₁B₁₃Si₄C₂ alloy could be described by the Johnson-Mehl-Avrami (JMA) kinetic model (with parameter n_iso=4.0). The apparent isothermal crystallization process includes a constant rate of nucleation and three-dimensional growth of nuclei. The results of X-ray diffraction (XRD) data of the isothermally crystallized samples confirmed the above established kinetic model. From the kinetic analysis of the non-isothermal crystallization of the α-Fe phase within this amorphous alloy, it was concluded that the autocatalytic two-parameter Šesták-Berggren (SB) reaction model (with kinetic exponents M=0.72 and N=1.02) describes well the studied process under the given conditions. The non-isothermal crystallization process involves the constant nucleation rate of stable nuclei with additional secondary two-dimensional (surface) nucleation and overlapping of the growing nuclei on account of the non-isothermal activation.     

DSC study of annealing and phase transformations of C60 and C70 polymerized under pressures in the range 9.5-13 GPa

C₆₀ and C₇₀ fullerenes polymerized under pressures between 9.5 and 13 GPa and temperatures between 670 and 1850 K were investigated by differential scanning calorimetry (DSC) in the range 240-640 K. Endothermal heat effects were observed with a peak maximum just below 540 K, a temperature characteristic for breakdown of (2+2) intermolecular links in dimers, 1D and 2D polymers. Exothermal effects, starting from 380 K, were observed for the first time in polymeric fullerenes. These effects are attributed to relaxation processes and to breakdown of other types of intermolecular bonds such as common four-sided rings and (3+3) interlinks.     

Characteristic dielectric parameters of para-, ferro- and antiferro-electric phases of (S)-4-(1-methylheptyloxycarbonyl)phenyl-4′-(6-pentanoyloxyhex-1-oxy)biphenyl-4-carboxylate

Frequency and temperature dependence of dielectric parameters of a liquid crystalline compound (S)-4-(1-methylheptyloxycarbonyl)phenyl-4′-(6-pentanoyloxyhex-1-oxy)biphenyl-4-carboxylate under planar orientation of the molecules have been investigated in the frequency range 1 Hz-10 MHz. This compound possesses smectic paraelectric (SmA*), ferroelectric (SmC*) and antiferroelectric (SmC_A*) phases. Dielectric spectroscopy suggests the existence of a relaxation mechanism in the SmA* phase, which behaves as a soft mode. In the SmC* phase two relaxation modes are observed. One mode continues from the SmA* phase with decreasing dielectric strength and the other has characteristics of the Goldstone mode. Two dielectric relaxation modes have been observed for the SmC_A* phase. These two modes are related to the antiferroelectric ordering and the helical structure of the SmC_A* phase.     

Enhanced magnetocaloric effect in a Co-doped Heusler Mn50Ni37Co3In10 unidirectional crystal

A high-pressure optical zone-melting technique was employed to grow a Mn-rich Heusler Mn₅₀Ni₃₇Co₃In₁₀ unidirectional crystal in the present study. It was found that the Co-doped Mn₅₀Ni₃₇Co₃In₁₀ unidirectional crystal showed a low magnetic hysteretic loss and a widened working temperature interval in the vicinity of the martensitic transformation. The inverse magnetic entropy change (∆S_M) reached 7.84 Jkg⁻¹K⁻¹ around 237.5 K under a magnetic field change of 30 kOe, and the corresponding effective refrigeration capacity (RC_eff) was about 127.2 Jkg⁻¹. The experimental results demonstrated a high potential to develop high-performance Mn-rich Heusler Mn–Ni–In magnetocaloric materials by means of Co doping in combination with the high-pressure optical zone-melting fabrication technique.     

Calorimetric study of characteristic temperatures and crystallization behavior in Ge-As-Se-Te glass system

Results of differential scanning calorimetry of high purity Ge_xAs_40−xSe₄₀Te₂₀ (x=0-40) chalcogenide glasses are reported. The glass transition temperatures and crystallization behavior were studied under non-isothermal conditions at different heating rates (2.5-35 K/min). The glass transition temperature changes from 140 °C up to 320 °C with increasing the Ge content in Ge_xAs_40−xSe₄₀Te₂₀ glass. The studied glasses with x≤35 have no exothermal peaks of crystallization, indicating their high glass-forming ability. The glass of Ge₄₀Se₄₀Te₂₀ composition has one-stage glass transition and double-stage crystallization process during phase change. The activation energy of the glass transition (E_g), the activation energy of crystallization (E_c), the Avrami exponent (n), the frequency factor (K₀) and the crystallization criteria of Ge₄₀Se₄₀Te₂₀ glass were determined.     

Effects of annealing temperature on structure and magnetic properties of amorphous Fe61Co27P12 nanowire arrays

Arrays of Fe₆₁Co₂₇P₁₂ nanowire with an aspect ratio about 70 were prepared in anodic aluminum oxide templates by electrodeposition. The influences of annealing temperature on structure and magnetic properties of Fe₆₁Co₂₇P₁₂ nanowires were studied. When the specimens were annealed below 400 °C, there are no obvious changes in structure except relaxation. With the annealing temperature increasing from 400 to 600 °C, the Fe-Co phase is detected by X-ray diffraction and Mössbauer spectra. The crystalline fraction and hyperfine field can be derived from Mössbauer spectra. The room temperature magnetic hysteresis loops show that the coercivity and squareness of the nanowire arrays in parallel to the wire axis increase with the increasing of annealing temperature, which mainly attributes to the strengthening of anisotropy.     

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.     

Discontinuity in heat capacity of Fe0.5Co0.5(110) alloy thin films

In this work we calculate heat capacity of alloy thin films of FeCo on the surface of the plane (110), using three parameters, the concentration x(i), the lattice long range order parameter t(i) and the magnetic order parameter σ(i), being i the number of layers of the thin film. The formulations reported by Hill [1] in the context of small particles and Valenta's model [2] can be applied to the film structure when we treat a thin film as a system divided into subsystems equivalent to two-dimensional parallel layers. The FeCo bulk alloy is completely homogeneous while a thin film have spatial discontinuities in their surfaces. We consider three ferromagnetic thin films formed by 11, 15 and 19 layers in the Helmholtz's free energy, which is minimized applying their first partial derivatives with respect to chemical composition, long range order parameter and magnetic order parameter. We calculate internal energy and heat capacity as a function of temperature and we verify that have two jumps as are reported in literature for the bulk; there are many results of bulk or surface effects of FeCo, but no enough results about ferromagnetic FeCo thin films and this fact does this work interesting.     

Internal stress dependent structural transition in ferromagnetic Ni-Mn-Ga nanoparticles prepared by ball milling

Ni-Mn-Ga nanoparticles were prepared by ball milling technique. X-ray diffraction pattern of the milled powders has a broad peak near the location of the prominent peak for the Heusler phase of Ni₂MnGa, indicating very disordered structures with small particle sizes. Structural properties of milled Ni-Mn-Ga particles recover to those of the bulk state after appropriate annealing temperature. It is worth noting that particles with size above 50 nm exhibit martensitic transformation. The average internal stress was calculated to be 2.83-1.13 MPa stored in the distorted lattice. Saturation magnetization of the milled sample increases with annealing temperature due to re-crystallization and grain growth.     

Monte Carlo simulation of Prussian blue analogs described by Heisenberg ternary alloy model

Within the framework of Monte Carlo simulation technique, we simulate magnetic behavior of Prussian blue analogs based on Heisenberg ternary alloy model. We present phase diagrams in various parameter spaces, and we compare some of our results with those based on Ising counterparts. We clarify the variations of transition temperature and compensation phenomenon with mixing ratio of magnetic ions, exchange interactions, and exchange anisotropy in the present ferro-ferrimagnetic Heisenberg system. According to our results, thermal variation of the total magnetization curves may exhibit N, L, P, Q, R type behaviors based on the Néel classification scheme.     

Experimental constraints on the phase diagram of elemental zirconium

The phase diagram of zirconium metal has been studied using synchrotron X-ray diffraction and time-of-flight neutron scattering at temperatures and pressures up to 1273 K and 17 GPa. The equilibrium phase boundary of the α-ω transition has a dT/dP slope of 473 K/GPa, and the extrapolated transition pressure at ambient temperature is located at 3.4 GPa. For the ω-β transition, the phase boundary has a negative dT/dP slope of 15.5 K/GPa between 6.4 and 15.3 GPa, which is substantially smaller than a previously reported value of −39±5 K/GPa in the pressure range of 32-35 GPa. This difference indicates a significant curvature of the phase boundary between 15.3 and 35 GPa. The α-ω-β triple point was estimated to be at 4.9 GPa and 953 K, which is comparable to previous results obtained from a differential thermal analysis. Except for the three known crystalline forms, the β phase of zirconium metal was found to possess an extraordinary glass forming ability at pressures between 6.4 and 8.6 GPa. This transformation leads to a limited stability field for the β phase in the pressure range of 6-16 GPa and to complications of high-temperature portion of phase diagram for zirconium metal.