Order—Disorder phase transformation in ZrS2 single crystals

Order-disorder phase transformation has been observed in ZrS₂ single crystals on annealing them at a temperature of 400 ± 10°C. Loss of sulphur, resulting in its deficiency, on annealing of crystals is thought to be the cause of the observed phase transformation. Evidence in its support, based on X-ray and electron diffraction results, is advanced.     

An investigation of phase transformation of titania slag using microwave heating

The influences of microwave heating on the phase transformation of titania slag were systematically investigated. The thermal stability, surface chemical functional groups and microstructure of the titania slag before and after microwave heating, at a temperature of 950?°C for 60 min, were also analyzed using thermogravimetry and differential thermal analysis (TG-DSC), Fourier transform infrared spectroscopy (FT-IR) spectrum and scanning electron microscope (SEM), respectively. The TG-DSC analysis revealed that the phase transformation of the titania slag from anatase TiO₂ to rutile TiO₂ occurred between 750 and 1000 °C. The FT-IR rustles demonstrate that the banding form of Ti⁴⁺, Ti³⁺ and Ti²⁺ ions and the methyl groups on the surface of the titania slag has changed and a new chemical bond Ti–OH was formed. The results of SEM showed that a large number of regulation rutile TiO₂ crystals were found on the surface of the microwave-treated samples and the synthetic rutile has been synthesized successfully using microwave heating.     

The discovery of a 2H–6H solid state transformation in ZnxCd1−xS single crystals

Single crystals of Zn_xCd_1?xS have been grown from the vapour phase at 1100°C in the presence of H₂S gas. X-ray diffraction studies of the as-grown crystals show that polytypism and stscking faults occur in Zn_xCd_1?xS crystals for x ? 0.94. It is observed that for 0.92 < x < 0.98 the 2H structure of Zn_xCd_1?xS crystals transforms to a disordered 6H structure on annealing in vacuum around 600°C. For 0.95 < x < 0.98 this 6H structure finally transforms to a disordered 3C structure on annealing further at higher temperatures around 800°C. The structural transformations occur through a non-random insertion of stacking faults, as revealed by the diffuse streak joining the X-ray diffraction maxima along the 10.L reciprocal lattice row. Experimental investigation of the diffuse intensity distribution, as recorded on a single crystal diffractometer from partially transformed single crystals, reveals that the mechanism of the transformation is very different from that reported for the same transformation in silicon carbide and cannot be described in terms of a single-parameter model of non-random deformation faulting.     

A Study of the Thermal Hysteresis in AgNO3

The reversible phase transformation of AgNO₃ is studied. Dielectric constant, d.c. resistivity, differential thermal analysis (DTA) and dilatometric measurements show the occurrence of a reversible phase transition II→I at 160°C with heat of transformation H = 0.78 kcal/mol. The thermal hysteresis in this reversible transformation is examined, the magnitude of the temperature hysteresis does not exceed 12°C. An acceptable agreement is observed between the measured values of the transition temperature obtained by three different experimental techniques. The dilatometric analysis shows that this transition is accompanied by thermal shrinkage with relative shrinkage coefficient 8 × 10^?4. Thermal analysis are also used to get thermodynamic and kinetic data of this phase transition. The temperature dependence of the dielectric constant and d.c. resistivity for single crystals as well as polycrystalline samples of AgNO₃ have clearly located and confirm the phase transitions II→I→II with a strong support to its thermal hysteresis character. The conduction mechanism is found to be activated by energy 0.12 eV for phase I and 0.36 eV for phase II. The observed thermal behaviour of the various measured parameters is attributed to orientational disorder of the nitrate group leading to an order-disorder phase transition which is reported here for first time in AgNO₃.     

Investigation of the phase transitions of ferroelectric KIO3 and KNbO3 crystals by optical diffraction

Optical diffraction is reviewed as a technique for investigation of the phase transitions in crystals with a multidomain structure. It has been used to study the phase transitions in KIO₃ and KNbO₃ single crystals. Strong optical diffraction bands resulted from electric domains in KNbO₃ crystals and their change with temperature were observed when a laser beam passed through the crystals. The diffraction patterns observed changed abruptly at 427°C, 223°C, and -50°C respectively, at which KNbO₃ crystals undergo structural phase transitions. It is considered that the change of the diffraction patterns with temperature is due to change of the electric domains in the crystals.     

The solid state phase transformation of potassium sulfate

Potassium sulfate single crystals that are grown from aqueous solutions lose, upon the first heating, up to 1% of mass that is assumed to be water. This mass loss occurs in the vicinity of the PT from orthorhombic to hexagonal K₂SO₄. Only in the first heating run of K₂SO₄ that has not yet released water, pretransitional thermal effects can be observed in the DTA curve. If K₂SO₄ crystals are grown from solutions containing 4 wt% Cd, Cu, or Fe, only Cu or Fe can be incorporated significantly with concentrations of several 0.1%. The phase transformation temperature measured for such solid solutions depends on the heating rate. For pure K₂SO₄, the phase transformation temperature is independent of heating rate 581.3 ^°C and the enthalpy of transformation is (5.8±0.2) kJ/mol.     

Ion implantation of sulphur into GaAs,GaP and ge monocrystals

The possibility of using ion implantation to form high concentration junctions in semiconductors has been explored for the specific case of sulphur in GaAs, GaP and Ge. The effects of ion dose, ion energy, crystal orientation and target temperature have been investigated by means of radiotracers and sectioning techniques.

It is shown that high concentration junctions can be formed using an incident ion having high electronic stopping cross-section and implanted along the <110< channeling directions of the crystals. A large increase in junction concentration may be obtained when the GaAs and GaP crystals are maintained at 150 °C during the implantation process, but this is not the case with Ge. Rutherford back-scattering of 1 MeV He⁺ ions has been used to measure the ion-bombardment induced damage in the crystals and to show how this damage can be annealed by heating the crystal during the implantation. The annealing, at temperatures up to 150 °C, is most effective in GaAs and least effective in Ge.     

Electrical conductivity in undoped and Mn2+-doped NaNO2 single crystals

Electrical conductivity studies in NaNO₂ single crystals with inherent impurities and also in crystals with added Mn²⁺ impurities have been reported. The heating conductivity runs of undoped and doped NaNO₂ crystals have been compared. The decrease in conductivity in cooling following a heating run has been attributed to the oxidation during heating leading to the bulk precipitation of impurities in the host. Above 170°C however the intrinsic defects are responsible for conduction. An anomaly is noticed in both the heating and cooling conductivity runs of the sample at about the Curie temperatures and has been found to show thermal hysteresis.     

Brillouin light scattering in LiKSO4 between 20 and 80°C

Elastic constants of LiKSO₄ crystals have been measured at room temperature and over the temperature range 20–80°C by Brillouin scattering. No anomaly has been found near 60°C, in disagreement with the results of a previous Brillouin study which suggested a new phase transition.     

In-situ and ex-situ study of crystallization behaviour of magnetron sputtered Ni63Zr37 amorphous thin film

ABSTRACT

The stages of crystallization of magnetron sputter-deposited Ni₆₃Zr₃₇ film with mostly amorphous structure have been investigated by differential scanning calorimetry (DSC) and in-situ annealing at 300°C by use of heating stage on a high-resolution transmission electron microscope (HRTEM). These results have been further confirmed by grazing incidence X-ray diffraction analyses of thin film specimens annealed ex-situ at 300°C for various durations. The temperature for crystallization found by DSC has been found to increase from 371°C to 434°C with an increase in heating rate from 3°C/min to 10°C/min, and the apparent activation energy for amorphous to crystalline transformation has been found as ～260.2?kJ/mol from the Kissinger plot. Studies on HRTEM using in-situ heating stage have shown the crystallization to occur on annealing at 300°C for ～10?min. Crystallization at a temperature lower than that found by DSC is attributed to structural relaxation with reduction of free volume due to thermal activation. It has been observed that Ni₃Zr forms first due to its large negative enthalpy of formation, and is followed by the formation of Ni-rich solid solution (Ni_ss) grains. HRTEM studies have shown grain rotation with the formation of partial dislocations at Ni₃Zr-Ni_ss interfaces as well as twinning followed by detwinning with dislocation formation in the Ni_ss matrix possibly to reduce the interfacial energy.     

Thermoluminescence of pretreated NaCl: Sr crystals

Thermal glow curves of thermally pretreated NaCl:Sr crystals have been recorded after irradiation with ultraviolet light at room temperature. The observed glow curves exhibit a prominent glow peak at 380°K only when the specimen is annealed and quenched from 750°C. This peak is attributed to the dipole clusters which disintegrate after heating the specimen beyond 380°K     

Effect of irradiation on CsNO3, RbNO3., and NaNO2 crystal structure

Studies of the structure changes in heated (20–200°C) and γ –irradiated (10⁹- 5×10⁹ R, E = 1.3 Mev, Tirrad. = 40°C) crystals CsNO₃, RbNO₃ and NaNO₂ have been carried out by means of X-ray diffraction methods. The investigation of temperature effect on the crystal structure has shown that the phase transformation in CsNO₃ and NaNO₂ are of continuous type while that in RbNO₃ is discontinuous. It has been found that crystal structures of CsNO₃ and NaNO₂ change under irradiation in the same way, as they change under heating, No changes of RbNO₃ crystal structure caused by irradiation have been found. Experimental results agree with an assumption that radiation-induced structure changes of high-temperature type can be observed only in the compounds, the phase transition of which is of continuous type.     

Recrystallization of hexagonal silicon carbide after gold ion irradiation and thermal annealing

Silicon carbide (SiC) single crystals with the 6H polytype structure were irradiated with 4.0-MeV Au ions at room temperature (RT) for increasing fluences ranging from 1?×?10¹² to 2?×?10¹⁵ cm^?2, corresponding to irradiation doses from ~0.03 to 5.3 displacements per atom (dpa). The damage build-up was studied by micro-Raman spectroscopy that shows a progressive amorphization by the decrease and broadening of 6H-SiC lattice phonon peaks and the related growth of bands assigned to Si–Si and C–C homonuclear bonds. A saturation of the lattice damage fraction deduced from Raman spectra is found for ~0.8?dpa (i.e. ion fluence of 3?×?10¹⁴ cm^?2). This process is accompanied by an increase and saturation of the out-of-plane expansion (also for ~0.8?dpa), deduced from the step height at the sample surface, as measured by phase-shift interferometry. Isochronal thermal annealing experiments were then performed on partially amorphous (from 30 to 90%) and fully amorphous samples for temperatures from 200 °C up to 1500 °C under vacuum. Damage recovery and densification take place at the same annealing stage with an onset temperature of ~200 °C. Almost complete 6H polytype regrowth is found for partially amorphous samples (for doses lower than 0.8 dpa) at 1000 °C, whereas a residual damage and swelling remain for larger doses. In the latter case, these unrelaxed internal stresses give rise to an exfoliation process for higher annealing temperatures.     

X-ray study on the phase transitions in triammonium hydrogen disulfate crystals and deuterated crystals

The structural phase transitions in triammonium hydrogen disulfate crystals and deuterated crystals below room temperature have been studied by X-ray diffraction. Three phases are observed in the temperature range from 25°C down to — 160°C. The space groups in three different phases are identified as C2/c, P2/n (or Pn), and C2 for (NH₄)₃H(SO₄)₂ and (ND₄)₃D(SO₄)₂ crystals. No isotope effect on the structural phase transitions in these crystals could be detected by these studies. The occurrence of structural phase transitions caused by the reorientation of SO₄ groups and/or the shift of oxygen atoms from the sulfate atom in the SO₄ group are suggested from the diffraction photographs.     

Piezoelectric activity and phase transitions in (PMN)0.69(PT)0.31 single crystal

Piezoelectric and dielectric investigations have been performed on a (PMN)_0.69(PT)_0.31 single crystal. Low frequency (100?Hz) dielectric permittivity measurements revealed distinct anomaly at 129°C (T _εmax) corresponding to the structural transformation from the tetragonal to cubic phase. Two other anomalies have been detected at 90 and 96°C. After preliminary polarization in the d.c. electric field, switched on above T _εmax and switched off inside the tetragonal phase, the piezoelectric activity has been observed in function of temperature. Values of the piezoelectric resonance frequencies changed markedly at 96°C (on cooling) and 124°C (on heating) showing clear softening of the elastic properties near these temperatures. Values of the piezoelectric and electromechanical coupling coefficients obtained were respectively of the order of 800?pCN^?1 (d ₃₁) and 0.35?(k ₃₁). Piezoelectric activity was detected tens of degrees above the temperature T _εmax and disappeared at temperature at which the dispersion of the dielectric permittivity due to the presence of polar nanoregions is negligible. It was found that strong softening of the elastic properties accompanies phase transitions to the tetragonal and monoclinic phase.     

Observation of insulating–insulating monoclinic structural transition in macro‐sized VO2 single crystals

VO₂ single crystals with unprecedented quality, exhibiting a first‐order metal–insulator transition (MIT) at 67.8 °C and an insulator –insulator transition (IIT) at ～49 °C, are grown using a self‐flux evaporation method. Using synchrotron‐based X‐ray microdiffraction analysis, it is shown that the IIT is related to a structural phase transition (SPT) from the monoclinic M2 phase to the M1 phase upon heating while the MIT occurs together with a SPT of M1 to the rutile R phase. All previous reports have shown that VO₂ exists in the M1 phase at room temperature in contrast to the M2 phase observed in this work. We suggest that internal strain inside single crystal VO₂ may generate the previously unobserved IIT and the unusual room temperature structure. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A redox reaction model for self-heating and aging prediction of Al/CuO multilayers

Sputter-deposited Al/CuO multilayers capable of highly energetic reactions have been the subject of intense studies for tunable initiation and actuation. Designing high performance Al/CuO-based initiator devices definitively requires reliable prediction of their ignition and reaction kinetics including self-heating or ageing as a function of heating rate and environmental conditions. The paper proposes a heterogeneous reaction model integrating an ensemble of basic mechanisms (oxygen diffusion, structural transformations, polymorphic phase changes) that have been collected from recent experimental investigations. The reaction model assumes that the rate of reaction is limited by the transport of oxygen across the growing layer of Al₂O₃ separating Al and CuO. Importantly, we show that the model predicts reasonably all exotherms through a wide range of temperature (ambient – 1000°C), all resulting from a pure diffusion process as experimentally observed for such Al/CuO multilayers. The model shows how the temperature ramp affects the structure of the multilayer and especially the growth of alumina-based interfacial regions. It highlights the importance of the interfacial chemistry evolution such as the native mixture of Al_xCu_yO_z transformation into a thin amorphous alumina, and the polymorphic phase transformation of this latter. The first one occurring at ～350°C results in a loss of continuity of the interface leading to the accelerated redox reaction whereas the second one occurring between 500 and 600°C produces a denser barrier to oxygen diffusion leading to the stop of redox reaction. We finally use the model to simulate thermal annealing as usually performed in accelerated ageing experiments. We theoretically observe and experimentally validate that a two weeks exposure of the multilayers at 200°C starts degrading the multilayers thermal properties whereas when the temperature remains below 200°C, the material keeps its entire integrity.     

Specific features of the formation of the ferroelectric phase in polytypes of TlGaSe2 crystals

The polytypism of layered crystals of thallium gallium diselenide TlGaSe₂ has been found to substantially affect the temperature of phase transformations and the mechanism of formation of the polar state in these ferroelectrics. In particular, it is shown that the phase transition observed in the C-TlGaSe₂ polytype is an improper ferroelectric phase transition occurring at a temperature T _c ≈ 108 K, whereas the phase transition observed in the 2C-TlGaSe₂ polytype is a proper ferroelectric phase transition occurring at a higher temperature T _c ≈ 111 K. It is concluded that the elucidation of the polytype of a particular sample is a necessary stage of investigation of the TlGaSe₂ crystals.     

Exploring the annealing temperature impacting on the magnetic coupling of nanometer soft grain and microstructure hard grain nanocomposites

Nanocomposites of 70% MgFe₂O₄ and 30% BaFe₁₂O₁₉ have been prepared by the mechano-chemical coprecipitation method. The prepared samples were annealed at different temperatures (400, 600, 800, and 1000 °C). The thermal gravimetric analysis (TGA) measurements of the as prepared samples has detected three weight losses attributed to evaporation, the decomposition of citric acid and nitrate combustion and the crystal transformation. The crystal growth of the soft phase has been proved by X-ray diffraction (XRD) and high resolution transmission electron microscope (HRTEM) measurements. The bonds have been detected by FT-IR measurements and are assigned to the two ferrite phases constituting the nanocomposites, which means that there are no intruder phases that have been detected. The vibrating sample magnetometer measurements (VSM) have displayed a gradually increasing of the saturation magnetization (Ms) with the annealing temperature, while the coercivity (H_c) and remnance (M_r) have exhibited a maximum value at the annealing temperature 600 °C. The ferromagnetic resonance (FMR) and VSM measurements have clarified the magnetic ordering changing inside the nanocomposites at the system transformation from the as prepared amorphous/hexagonal phase to the spinel/hexagonal phase with increasing annealing temperature. The porosity (P), the anisotropy (K) and the line width (ΔH) calculated parameters have been used to study the agreement between the FMR and VSM magnetic measurements.     

Transformations of iodine species inside elliptical channels of AlPO4‐11 crystals at low temperature: a Raman study

Raman spectra of iodine species confined in one‐dimensional elliptical channels of AlPO₄‐11 (AEL) crystals have been studied from room temperature down to −196 °C. As temperature decreases, thermal fluctuations of individual iodine molecules confined in AEL channels are slowed down and they prefer to rotate to channel axis direction, which increases the population of iodine molecules along channel axis (i.e., lying molecules and chains). Such temperature‐driven orientation transformation of iodine molecules is found to be reversible upon heating up to room temperature. The experimental observations are in good agreement with our theoretical simulations by molecular dynamics on low density iodine‐filled AEL crystals. We thus provide a new way to modulate the orientation of iodine molecules in nanochannels, which may have implications in low‐temperature‐sensitive nanoscale devices. Copyright © 2015 John Wiley & Sons, Ltd.