Extrinsic Point Defects in Low-Positive Thermal Expansion Al2W3O12 and Their Effects on Thermal and Optical Properties.

A2M3O12-type ceramics are potentially useful in a variety of applications due to their peculiar thermal and mechanical properties. In addition, their intrinsic coefficients of thermal expansion can be finely tuned through different mechanisms. Despite the great influence of extrinsic point defects on physical properties, only a few reports have dealt with their relationship to thermal expansion and thermal conductivity. Extrinsic oxygen vacancies in orthorhombic Al2W3O12, in different concentrations, were formed through heat treatments in argon or hydrogen atmospheres. X-ray powder diffraction, diffuse reflectance spectroscopy, and Raman and electron paramagnetic resonance spectroscopies were used to study the as-formed vacancies, and X-ray photoelectron spectroscopy was employed to propose a charge compensation mechanism. It was found that the intrinsic coefficient of thermal expansion of orthorhombic Al2W3O12 was severely affected by extrinsic oxygen vacancies. Thermal expansion was decreased up to 40% (from 25 to 400 °C) with respect to the extrinsic-point-defect-free counterpart. Unit-cell volumes of defective orthorhombic Al2W3O12 were larger, while their W-O bonds were weaker, likely leading to higher lattice flexibility and enhanced low-energy transverse acoustic modes. Extrinsic oxygen vacancies could be an additional mechanism for fine-tuning the intrinsic coefficients of thermal expansion in A2M3O12-type ceramics and in other framework structures built through two or threefold linkages.

Phase Transition and Coefficients of Thermal Expansion in Al2-xInxW3O12 (0.2 ≤ x ≤ 1).

Materials from theA2M3O12 family are known for their extensive chemical versatility while preserving the polyhedral-corner-shared orthorhombic crystal system, as well as for their consequent unusual thermal expansion, varying from negative and near-zero to slightly positive. The rarest are near-zero thermal expansion materials, which are of paramount importance in thermal shock resistance applications. Ceramic materials with chemistry Al2-xInxW3O12 (x = 0.2-1.0) were synthesized using a modified reverse-strike co-precipitation method and prepared into solid specimens using traditional ceramic sintering. The resulting materials were characterized by X-ray powder diffraction (ambient and in situ high temperatures), differential scanning calorimetry and dilatometry to delineate thermal expansion, phase transitions and crystal structures. It was found that the x = 0.2 composition had the lowest thermal expansion, 1.88 × 10-6 K-1, which was still higher than the end member Al2W3O12 for the chemical series. Furthermore, the AlInW3O12 was monoclinic phase at room temperature and transformed to the orthorhombic form at ca. 200 °C, in contrast with previous reports. Interestingly, the x = 0.2, x = 0.4 and x = 0.7 materials did not exhibit the expected orthorhombic-to-monoclinic phase transition as observed for the other compositions, and hence did not follow the expected Vegard-like relationship associated with the electronegativity rule. Overall, compositions within the Al2-xInxW3O12 family should not be considered candidates for high thermal shock applications that would require near-zero thermal expansion properties.

Data supporting micromechanical models for the estimation of Young's modulus and coefficient of thermal expansion of titanate nanotube/Y2W3O12/HDPE ternary composites.

This article presents several micromechanical models to predict the Young's modulus and the coefficient of thermal expansion of titanate nanotube/Y2W3O12/HDPE composites. The equations and assumptions of the selected micromechanical models are described in detail for this ternary system. Data of the elastic constants, coefficient of thermal expansion of composite components and other associated parameters, obtained either by literature survey or processing of literature information, are compiled in this work. For further interpretation of the data presented in this article, please see our research article entitled "The effect of titanate nanotube/Y2W3O12 hybrid fillers on mechanical and thermal properties of HDPE-based composites" (Pontón et al., 2019).

Crystal Structure and Properties of Imidazo-Pyridine Ionic Liquids.

Computational studies were performed on novel protic ionic liquids imidazolium-[1,2-a]-pyridine trifluoroacetate [ImPr][TFA] synthesized by the reaction of imidazo-[1,2a]-pyridine (ImPr) with trifluoroacetic acid (TFA), and on fused salt imidazolium-[1,2-a]-pyridine maleamic carbonate [ImPr][Mal] synthesized by reaction of ImPr with maleamic acid (Mal). Synthesis was performed as one-pot reactions, which applies green chemistry tenets. Both these compounds begin to decompose at 180°C. Our computational studies suggest another thermal reaction channel, in which [ImPr][Mal] can also thermally polymerizes to polyacrylamide which then cyclizes. This is thermal product remains stable up to 700 degrees, consistent with our thermogravimetric studies. [ImPr][TFA] exhibited good conductivity and ideal ionic behavior, as evaluated by a Walden plot. X-ray crystallography of [ImPr][TFA] revealed a tightly packed system for the crystals as a result of strong ionic interaction, pi-stacking, and fluorine-CH interactions. Both synthesized compounds exhibited some CO2 absorptivity, with [ImPr][Mal] outperforming [ImPr][TFA] in this regard. The quantum chemistry based computational methods can shed light on many properties of these ionic liquids, but they are challenged in fully describing their ionic nature. © 2017 Wiley Periodicals, Inc.

Origins of ultralow thermal conductivity in bulk [6,6]-phenyl-C61-butyric acid methyl ester (PCBM).

Bulk PCBM has exceptionally low thermal conductivity, 0.07 W m(-1) K(-1) at room temperature. We show that its ultralow thermal conductivity is an intrinsic property. Based on results for thermal conductivity and heat capacity measurements down to <2 K, along with Raman spectroscopy and dilatometry, a new model for minimum thermal conductivity was developed. In the model the thermal energy is transferred between entities of phonons oscillating in a range of frequencies, and limited by the atomic density and the phonon mean speed. The model accurately represents the low thermal conductivity for both PCBM and C60/C70.

Determination of phase stability of elemental boron.

Boron is an important element, used in applications from superhard materials to superconductors. Boron exists in several forms (allotropes) and, surprisingly, it was not known which form (α or ß) is stable at ambient conditions. Through experiment, we quantify the relative stability of α-boron and ß-boron as a function of temperature. The ground-state energies of α-boron and ß-boron are nearly identical. For all temperatures up to 2000 K, the complicated ß-boron structure is more stable than the simpler α-boron structure at ambient pressure. Below 1000 K, ß-boron is entropically stabilized with respect to α-boron owing to its partially occupied sites, whereas at higher temperatures ß-boron is enthalpically stabilized with respect to α-boron. We show that α-boron only becomes stable on application of pressure.

Magnetic phase transitions and magnetic entropy in the XY antiferromagnetic pyrochlores (Er1-x Y x )2Ti2O7.

We present the results of experimental determination of the heat capacity of the pyrochlore Er2Ti2O7 as a function of temperature (0.35-300 K) and magnetic field (up to 9 T), and for magnetically diluted solid solutions of the general formula (Er1-x Y x )2Ti2O7 (x≤0.471). On either doping or increase of magnetic field, or both, the Néel temperature first shifts to lower temperature until a critical point above which there is no well-defined transition but a Schottky-like anomaly associated with the splitting of the ground state Kramers doublet. By taking into account details of the lattice contribution to the heat capacity, we accurately isolate the magnetic contribution to the heat capacity and hence to the entropy. For pure Er2Ti2O7 and for (Er1-x Y x )2Ti2O7, the magnetic entropy as a function of temperature evolves with two plateaus: the first at [Formula: see text], and the other at [Formula: see text]. When a very high magnetic field is applied, the first plateau is washed out. The influence of dilution at low values is similar to the increase of magnetic field, as we show by examination of the critical temperature versus critical field curve in reduced terms.

Influence of guest loading on thermal properties of NaxSi136 clathrates.

Thermal properties of a series of type II clathrates of the formula NaxSi136 with 0 < x < 24 and Na guests occupying the Si cages have been investigated over the temperature range from 2 to 300 K. Heat capacity and thermal conductivity results show that the structure is remarkably responsive to the loading of Na guests. The response is phononic: the host lattice expands in a non-monotonic way, and first stiffens, then relaxes at low loading into the larger Si28 cages (x < 9), then stiffens again as the Na concentration increases further. The response is also electronic, through changes in electronic properties as additional Na is loaded into the smaller Si20 cages at high loading (x > 9). In total, the influence of the guest loading illustrates the complexities of structure-property relations in a guest-host system.

Novel method to produce single-walled carbon nanotube films and their thermal and electrical properties.

Single-walled carbon nanotube films are promising candidates for applications requiring transparent conductors due to their low sheet resistance and high transparency in the visible region. Vacuum filtration is a common and easy to implement technique to produce such films but it is complicated by the need to transfer the films to desired substrates. Here we report conditions under which single-walled carbon nanotube films produced by vacuum filtration detach from the filter membrane upon submersion into water, providing a facile method to transfer filtration-produced nanotube films to desired substrates. Sheet resistance and transparency measurements show that these films are competitive with other high conductivity films made through more cumbersome procedures. Films post-treated with nitric acid or made with acid pre-treated nanotubes have superior performance to those made with high-purity nanotubes without any acid treatment. Thermal imaging by scanning thermal microscopy indicates that heat dissipation by the film is comparable to that of a glass substrate.