High-temperature DTA Research Articles

High-temperature phase stability and phase transformations of Niobium-Chromium Laves phase: Experimental and first-principles calculation studies

The high-temperature phase stability of the NbCr2 Laves phase has not yet been established beyond disagreement. The well accepted C14 phase field has been eliminated from the Nb-Cr phase diagram in a recent study that claimed that the NbCr2 alloy shows the C15 structure up to melting. The present study was conducted to address these ambiguities. The experimental investigations show the presence of C14, C15, and C36 structures in the as-cast alloy. Differential scanning calorimetry studies of the as-cast alloy show the transformation of the eutectic mixture of bcc + NbCr2 to single-phase NbCr2 at 1338 K. An in-situ X-ray diffraction (XRD) study of as-cast alloy shows a phase transformation of C36 → C14 below 1273 K. Density functional theory (DFT) calculations confirm two phase transformations: (a) a metastable transformation of C36 → C14 at 783 K and (b) an equilibrium transformation of C15 → C14 at 1971 K, which was validated by high temperature DTA and TEM studies. Further, extended DFT calculations on supercells with four compositions of NbCr1.9, NbCr2, NbCr2.1, and NbCr2.2 also corroborated the presence of the C14 phase, and consequently, the Nb-Cr phase diagram is modified. This finding resolves the controversy regarding the existence of an equilibrium C14 phase field before melting.

Liquidus projection and thermodynamic modeling of the Cr-Fe-Nb ternary system

A liquidus projection and a thermodynamic assessment of the Cr-Fe-Nb ternary system are proposed for the first time. The construction of the liquidus projection was obtained through evaluation of the microstructure of as cast samples and high temperature DTA measurements of equilibrated samples. Thermodynamic modeling was then done via the Calphad method based on DFT calculation for the description of the energy behavior of the phases and experimental evidence.

Experimental investigation and thermodynamic modeling of the ZrO2–MnOx system

Solid state phase equilibria of the ZrO2–MnOx system as well as the equilibria including liquid were investigated at air and argon atmospheres in the whole compositional range using high temperature DTA, X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX). The influence of the oxygen partial pressure on solubility of Mn in the c-ZrO2 phase, on the c-ZrO2 low temperature limit as well as on the L⇄c-ZrO2+MnOx eutectic composition was shown. The thermodynamic parameters of the liquid phase as well as of the ZrO2 solid solutions were assessed by CALPHAD method using the obtained experimental results. The calculated phase diagrams of the ZrO2–MnO system at air (0.21bar) and Ar (1·10−4bar) conditions were compared with the experimental results obtained in this work and those available in the literature. The calculated results were found to be in reasonable agreement with the experimental data.

Experimental investigation and thermodynamic modeling of the ZrO2–MgO–Al2O3 system

Solid state equilibria of the ZrO2–MgO–Al2O3 system as well as the equilibria including liquid were investigated in the whole compositional range using high temperature DTA, X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX). Isothermal sections at 1523, 1873 and 2023 K were constructed and the formation of the ternary X-phase in the MgO-rich region was confirmed. According to our results it forms by the peritectic reaction MgO + Spinel + L ⇄ X at 2114 K and decomposes by the X ⇄ F + MgO + Spinel reaction at 1894 K. The ternary mixing parameter of liquid was assessed using the results of melting experiments. The obtained liquid thermodynamic description was used to derive enthalpy and entropy values of the X phase. The calculated liquidus and solidus representing 5 invariant processes as well as calculated isothermal and vertical sections show good consistency with the results obtained in this work and those available in the literature.

To melting relations in the La2O3–Y2O3–Al2O3 system

Melting in the La 2 O 3 –Y 2 O 3 –Al 2 O 3 system was investigated using high temperature DTA and microstructures were studied by electron microscopy (SEM/EDX). The experimental results are consistent with calculations based on liquid phase description without ternary interactions and solid phases with limited mutual solubility of La and Y. The thermodynamic description was used to calculate vertical sections at 20 mol% Al 2 O 3 and at fixed ratio of La 2 O 3 /Y 2 O 3 =1 as well as solidus surface. • Melting relations in the La 2 O 3 –Y 2 O 3 –Al 2 O 3 system are investigated up to 2273 K. • Vertical sections and solidus surface are calculated. • Experimental data are reproduced by calculations. • Thermodynamic description of the La 2 O 3 –Y 2 O 3 –Al 2 O 3 system is proved.

Experimental Investigation and Thermodynamic Modeling of the ZrO2–MgO System

a b s t r a c t Solid state phase equilibria of the ZrO2–MnOx system as well as the equilibria including liquid were investigated at air and argon atmospheres in the whole compositional range using high temperature DTA, X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDX). The influence of the oxygen partial pressure on solubility of Mn in the c-ZrO2 phase, on the c-ZrO2 low temperature limit as well as on the L c-ZrO2 + MnOx eutectic composition was shown. The thermodynamic parameters of the liquid phase as well as of the ZrO2 solid solutions were assessed by CALPHAD method using the obtained experimental results. The calculated phase diagrams of the ZrO2–MnO system at air (0.21 bar) and Ar (1·10 −4 bar) conditions were compared with the experimental results obtained in this work and those available in the literature. The calculated results were found to be in reasonable agreement with the experimental data.

Effect of solidification path on the microstructure of Al2O3–Y2O3–ZrO2 ternary oxide eutectic ceramic system

The solidification path of the Al2O3–Y2O3–ZrO2 ternary oxide eutectic composite ceramic is determined by a high temperature DTA and laser floating zone (LFZ) directional solidification method to investigate the effect of solidification path on the microstructure of the ternary oxide. The DTA and microstructure analyses show that the YAG or Al2O3 tends to form as primary phase under the unconstrained solidification conditions, and then the system enters ternary eutectic solidification during cooling from 1950 °C at rate of 20 °C/min. The as-solidified composite ceramic shows a divorced irregular eutectic structure consisting of Al2O3, YAG and ZrO2 phases with a random distribution. The primary phases are however completely restrained at the directional solidification conditions with high temperature gradient, and the ternary composite by LFZ presents well coupled eutectic growth with ultra-fine microstructure and directional array. Furthermore, the eutectic transformation and growth mechanism of the composite ceramic under different solidification conditions are discussed.

MP-37 MEMS技術を用いた微小熱分析法の研究 : 高温度域への拡張(ポスターセッション)

The cantilever type MEMS calorimeter enables thermal analysis and mass measurement for micro- to nano-gram level samples. In this study, to extend the MEMS-based thermal analysis to high temperature range, a heat-resistant MEMS calorimeter has been developed. The new calorimeter equips a resistance temperature detector, a heater for temperature scan and a strain gauge for detecting the mechanical oscillation of the cantilever, and those resistance devices are made of platinum thin film. A thermal response test of the calorimeter showed fast temperature scanning ability of over 10^4K/s. From the experiment of high temperature scan, it was confirmed that the annealing of the calorimeter is necessary to get the reproducible high temperature resistance characteristics. Moreover, a high temperature DTA for tiny aluminum sample showed clear endothermic and exothermic peaks which correspond to melting and solidification of micro-gram level aluminum. The heat-resistant MEMS calorimeter performed an extension of the analytical temperature range up to 800℃.

Acidic Salts of the Decaoxodiperiodic Acid: NiH4I2O10· 6H2O, ZnH4I2O10· 6H2O andMgH4I2O10· 6H2O; Crystal Structures, Vibrational Spectra and Thermal Decomposition

By crystallization from strongly acidic aqueous solutions, nickel and zinc tetrahydrogen-decaoxodiperiodate hexahydrate, NiH4I2O10·6H2O and ZnH4I2O10·6H2O, have been obtained and found to be isotypic to CuH4I2O10·6H2O (P21/c, no. 14) with the lattice parameters a = 1070.3, b = 544.0, c = 1187.2 pm, β = 112.6° for the Ni compound and a = 1073.3, b = 545.3, c = 1189.5 pm and β = 112.5° for the Zn compound. The crystal structure of MgH4I2O10·6H2O has been reinvestigated and contrary to former results found to be isotypic to the Cu compound as well. IR and Raman spectra are given and analyzed with respect to the internal vibrations of the H4I2O10 2− ion and the hydrogen bonding system. According to high-temperature Raman spectra and DTA and TG measurements, the compounds decompose in the temperature range up to 300 °C via unknown salts with the anions H4IO6 2− and IO4 − to the corresponding iodates. In the case of the Ni compound the exothermic decomposition to the iodate occurs in two steps.

Crystal structure and vibrational spectra of BaH 4I 2O 10·2H 2O

By crystallization from strongly acidic aqueous solutions barium-tetrahydrogen-decaoxodiperiodate-dihydrate, BaH 4I 2O 10·2H 2O has been obtained (S.G. C2/ c, No. 15) with the lattice constants a = 12.728(3), b = 7.987(2), c = 9.459(2), and β = 94.07(3). IR and Raman spectra are given and analysed with respect to the internal vibrations of the H 4 I 2 O 10 2 - ion and the hydrogen bond system. According to high temperature Raman spectra and DTA and TG measurements, the compound decomposes via unknown salts with the anion IO 4 - above 230 °C to the corresponding iodate which above 575 °C starts to disproportionate to the periodate Ba 5(IO 6) 2.

Research on the phase decomposition of Lu xY 1−xAlO 3:Ce crystals at high temperatures

Cerium-doped lutetium ortho-aluminate LuAlO 3:Ce crystal is a good scintillator with excellent properties, while the single crystal is very difficult to grow and the phase decomposition mechanism remains unknown. In this paper, based on the analyses of high-temperature DTA and X-ray diffraction (XRD), it is supposed that LuAlO 3:Ce melts at 1905 °C, it tends to decompose when the temperature goes above 1200 °C, and the decomposition products are Lu 3Al 5O 12:Ce and Lu 4Al 2O 9:Ce. This decomposition is an irreversible endothermic reaction. This phenomenon is suggested to be caused by the unfitness of the radius of Lu 3+ ion for the perovskite-type structure of LuAlO 3, and this is the intrinsic cause for the instability of LuAlO 3:Ce. Introducing ions with larger radius (such as Y 3+) into LuAlO 3:Ce crystal may increase its phase stability.

The influence of ultrasound on the thermal behaviour of clay minerals

The aim of this work is to compare the influence of sonication on the thermal behaviour of kaolinites (KGa-1, KGa-2), pyrophyllite, talc and muscovite. Sonication produces a significant increase of the specific surface area due to particle size reduction. As sonication time increases and the particle size decreases, the thermal behaviour of these clay minerals is strongly modified. Thus, there is a significant increase of the weight loss at low temperature which is related to the loss of some outer hydroxyl groups and protonated hydroxyls whose proportion in the minerals increase as the new surface generated increases. Additionally, the original endothermic dehydroxylation effects of all these minerals shift to lower temperatures. Disordered kaolinite shows the smallest shift (9 °C) while muscovite presents the largest (184 °C). A linear relation has been established between the temperature shift of the dehydroxylation and the percentage of increase in the specific surface area. The influence of sonication on the high temperature DTA effects of kaolinite, pyrophyllite, and muscovite is also studied.

High-temperature DTA studies of A xM F 3 compounds ( [formula omitted], Rb, Cs; [formula omitted], Cr; and [formula omitted

A x VF 3 ( A = K , Rb, Cs; x = 0 – 1.0 ) and A x CrF 3 compounds were sealed inside high-temperature DTA molybdenum sample cups using electron beam welding techniques. Melting temperatures of the A x VF 3 compounds ranged from 1050 to 1400 °C; those of A x CrF 3 from 850 to 1200 °C. Reconstructive transitions were observed for A x VF 3 compounds between 650 and 950 °C, and for A x CrF 3 compounds between 550 and 825 °C. A displacive transition was observed to occur in VF 3 at 490 °C. Order–disorder transitions were determined by X-ray diffraction techniques. In the A x VF 3 compounds, electronic ordering and ordering of partially filled A + sites were observed to occur between 100 and 300 °C. Ordering phenomena in the A x CrF 3 compounds (in addition to electronic ordering and ordering of partially filled A + sites) included Jahn–Teller cooperative ordering. These compounded transitions occurred between 300 and 500 °C.

Crystal structure, phase transition and anisotropic thermal expansion of barium zirconium diorthophosphate, BaZr(PO 4) 2

The crystal structure of BaZr(PO 4) 2 at 298 K was determined from conventional X-ray powder diffraction data using direct methods, and it was further refined by the Rietveld method. The structure was monoclinic (space group C2/ m, Z = 2 ) with a = 0.85629 ( 3 ) nm , b = 0.53082 ( 2 ) nm , c = 0.78956 ( 2 ) nm , β = 93.086 ( 1 ) ° and V = 0.35836 ( 2 ) nm 3 . Final reliability indices were R wp = 8.21 % , R p = 5.64 % and R B = 2.92 % . The atom arrangement is similar to that of yavapaiite (KFe(SO 4) 2), however, these crystal structures differ distinctly in the coordination numbers of barium and potassium atoms; the former is tenfold coordinated, whereas the latter is sixfold coordinated. The powder specimens were also examined by high-temperature XRD and DTA to reveal the occurrence of a phase transition from monoclinic to orthorhombic at 732 K during heating. Upon cooling the reverse transition occurred at 710 K. The monoclinic crystal expanded almost one-dimensionally along [503] during the heating process. The orthorhombic phase also showed a tendency to expand one-dimensionally along the c-axis above 732 K.

Phase relations in the BaOTiO 2− δ system under highly reducing conditions

The phase relations in the BaO–TiO 2− δ system within the concentration range of 14–65 mol% BaO under reducing conditions were investigated using high temperature DTA and X-ray powder diffraction. The stable compounds under these conditions were determined, together with the temperatures of the invariant reactions and the liquidus boundaries. The phase relations in this BaO–TiO 2–Ti 2O 3 system under elevated temperatures and reducing atmospheres are represented in the form of quasi-binary phase diagrams.

Constitution and superconductivity of Y(Ni xPd 1− x) 2B 2C alloys

The effect of alloying on constitution and superconducting properties of intermetallic compounds of nominal composition Y(Ni x Pd 1− x ) 2B 2C has been studied by means of microscopic inspections, electron microprobe and high temperature DTA. The Ni-content y within the tetragonal Y(Ni y Pd 1− y ) 2B 2C-phase is found to exceed the nominal Ni-content x of the samples. The Y(Ni y Pd 1− y ) 2B 2C-phase is formed by a peritectic reaction and the formation temperature decreases with rising Pd-content of the samples. Furthermore, the Y(Ni y Pd 1− y ) 2B 2C-phase fraction is reduced and vanishes for x<0.3 (i.e. y<0.62). In contrast to the metastable superconducting YPd 2B 2C-phase the Y(Ni y Pd 1− y ) 2B 2C-phase with y≥0.62 is stable after annealing up to 1300 °C. The lattice parameters of the tetragonal unit cell both increased with Pd-content. The increasing Pd-content causes a drop of the superconducting transition temperature from 15.5 ( x=1.0) to 10.4 K ( x=0.3).

On the stability and Czochralski growth of Sm 2+ in solid solutions of Sr 1− xPb xB 4O 7

Solid solutions of Sr 1− x Pb x B 4O 7 were produced by a high-temperature solid state reaction. Formation of solid solutions was confirmed by X-ray powder diffraction analysis. High-temperature optical microscopy and DTA approve a binary phase diagram of Sr 1− x Pb x B 4O 7 (1≥ x≥0). The existence of Sm 2+ in Sr 1− x Pb x B 4O 7 was investigated by luminescence spectroscopy. For x≤0.5, mostly Sm 2+ entered the Sr position in ceramic samples. Crystal growth experiments showed that for x≥0.5, e.g. Sr 0.45Pb 0.55B 4O 7, only Sm 3+ was incorporated.

Utilization of the waste phosphogypsum for the Portland cement clinker production

This paper presents the results of investigation on the utilization of the phosphogypsum for the Portland cement clinker production. Characterization of the phosphogypsum and raw materials for rotary kilns was performed. Phosphogypsum, in quantities of 1%, 2%, 3% and 5%, was added to raw meal from regular production of rotary kiln and then the homogenization of raw meals and their chemical characterization was done. High temperature DTA and thermo‐microscopic investigations have been performed also. Raw meals without and with the addition of phosphogypsum have been laboratory sintered. Chemical composition of Portland cement clinkers has been defined and potential clinker phase composition has been calculated according to Bogue's formula from the chemical composition. Results are very encouraging regarding the energy consumption and Portland cement clinker quality. Phosphogypsum could be successfully applied as mineralizer in cement production, because with its addition, better composition of Portland cement clinker at the same sintering conditions could be obtained.

Magnetic ordering and superconductivity in Tb xY 1− xNi 2B 2C

The effect of alloying on magnetic ordering and superconducting properties of pseudoquaternary Tb x Y 1− x Ni 2B 2C intermetallic compounds has been investigated by susceptibility and resistivity measurements on polycrystalline samples. The single phase nature of the samples was proven by high-temperature DTA, X-ray and electron microprobe methods. Antiferromagnetic ordering and a depression of the Neél temperature with decreasing Tb content have been revealed for x>0.45. For lower Tb concentrations x<0.4, a superconducting transition has been observed. The superconducting transition temperature T C increases with increasing Y fraction. The magnetic susceptibility χ( T) of TbNi 2B 2C shows a maximum at T N=15.5 K, passes through a minimum with decreasing temperature, and rises toward a second maximum near 5.5 K. This second maximum was ascribed to weak ferromagnetic ordering which has been confirmed by magnetic hysteresis effects. For decreasing Tb content the ferromagnetic ordering temperature was found to shift to lower temperatures and to disappear for x<0.7.

Study of Jahn-Teller phase transitions in nonstoichiometric RMnO3+x orthomanganites ( R = La, Nd, Sm, Eu )

Abstract High temperature X-ray structural analysis, dilatometric, DTA and elastic properties investigations have been performed for the RMnO3+x orthomanganites ( R = La → Eu). These compounds show first-order transitions to their orbital disordered structures. The orbital ordered and disordered phases coexist in a wide temperature interval. The transition temperature decreases with increasing oxygen content and increases with the rare earth atomic number. In the RMnO3+x compounds ( R = Nd → Dy) the shape memory effect (SME) is observed. The SME is due to O I ↔ O transformation plasticity. The character of shape recovery is shown to depend on oxygen content and the orthorhombic distortion value of the unit cell.