Search

A series of compounds of Co1-xMgxMoO4 compositions has been prepared by a conventional ceramic route. The members of the whole solid solution exhibit a reversible first-order phase transition which was probed by using thermal expansion and low-temperature reflectivity techniques. Whereas the α → β transition temperature evolves linearly on warming from 435 to 200 °C with x going from 0 to 0.9, the β → α transition temperature variation falls down on cooling from -40 °C to -140 °C going from CoMoO4 to Co0.1Mg0.9MoO4 with an asymptotic evolution. The phase transition temperatures have been explained on the basis of a crystal polarization effect under substitution of Mg for Co. Thus, from an applicative point of view, new thermochromic pigments with tunable transition temperatures are here proposed. [ABSTRACT FROM AUTHOR]

The thermochromic phase transition of CuMo[sub1-x]W[subx]O[sub4] oxide was delayed by the deposit of SiO[sub2] shells. The phase transition temperature was investigated by optical reflectivity versus temperature. The effect of the shell thickness on the transition temperature is established. The cyclability of the phenomenon is also discussed. [ABSTRACT FROM AUTHOR]

The members of the CuMo1 - xWxO4 series (0 ≤ x < 0.1) undergo a first-order phase transition at normal pressure, which can be induced by temperature. The two allotropic forms exhibit two distinguishable colors, green for the high-temperature form (a) and brownish-red for the low temperature one (γ), which opens up a new market for user-friendly temperature indicators. From X-ray diffraction and microprobe analyses as from optical properties, the tungsten substitution rate for molybdenum is limited to 12%. Beyond, a third, parasitic wolframite-type phase, CuMo0.6W0.4O6, systematically crystallizes besides the α/γCuMo0.9W0.104 compounds. Within the CuMo1 - xWxO4 solid-solution domain, the dependence of the transition temperatures was followed by calorimetry, optical reflectivity, and magnetism. On the basis of these measurements, the transition is characterized for all of the chemical compositions by a hysteresis loop of about 90 K in width with a temperature transition strongly dependent on the tungsten content. Namely, the γ → α transition can occur between 260 and 360 K, and the α → γ transition between 175 and 275 K as a function of x. The control of the α/γ transition temperatures with x is related to the larger propensity of tungsten compared to molybdenum, to adopt a tetrahedral environment. [ABSTRACT FROM AUTHOR]

Mn 2+ doped Zn 3 (PO 4 ) 2 phosphate exhibits a rich thermal history with numerous events. Following the dehydration of Zn 3 (PO 4 ) 2 ⋅4H 2 O hopeite at temperatures up to T = 400 °C, various allotropic forms α/δ → γ → β can be stabilized with irreversible transitions for a 600 °C ⩽ T ⩽ 1000 °C temperature range. Furthermore, in these different networks, a transition metal can occupy distorted tetrahedral (α/δ forms) and pentahedral sites as well as octahedral sites for the high-temperature γ and β forms that correspond to non-centrosymmetric environments. Depending on the thermal history of the samples, two different types of emission spectra were obtained. By increasing the annealing temperature, the green emission associated with Mn 2+ in a Td environment progressively disappears in favor of the red one that corresponds to Mn 2+ stabilized in fivefold and sixfold coordination sites. There is also a dependence of the emission color on the excitation wavelength arising from the differences in the excitation spectra of Mn 2+ in these two types of sites. Excitations in the oxygen-manganese charge transfer band at 250 nm or in the 3d–3d absorption region at 420 nm give rise to the red or green emission at 420 nm, the green one being enhanced for an excitation at 420 nm. The α/δ → γ transition at 600 °C ⩽ T ⩽ 900 °C can easily be monitored in the emission spectra obtained after an excitation at 250 nm with a rather good chromatic contrast. The γ → β high-temperature transition at 900 °C ⩽ T ⩽ 1000 °C can be more easily followed by in the emission spectra obtained after an excitation at 420 nm. By taking into account both these emission spectra, various trichromatic coordinates can be calculated and allow subsequent phase transitions. The chromaticity of Zn 3 (PO 4 ) 2 : Mn 2+ will strongly change for 600 °C ⩽ T ⩽ 1000 °C, and one can consider this phosphate as an interesting thermal sensor. [ABSTRACT FROM AUTHOR]

Abstract: Irreversible thermochromism over a wide temperature range has been observed from the decomposition of a mixture of a barium carbonate and a metal oxide. The control of the reaction temperature can be predicted from the calculation of the Madelung energy of the barium/transition metal mixed oxide formed consecutively with the decarbonatation. Moreover, the Madelung energy of this formed mixed oxide may be predicted from bond valence considerations. This study offers a simple predictive approach to propose temperature indicators with significant optical contrast and a thermochromic temperature varying between 400 and 1200°C. [Copyright &y& Elsevier]