Near‐Zero Thermal Expansion in In(HfMg)0.5Mo3O12

In(HfMg)_0.5Mo₃O₁₂, which can be considered as a 1:1 mole ratio solid solution of the low‐positive thermal expansion material HfMgMo₃O₁₂ and the low‐negative thermal expansion (NTE) material In₂Mo₃O₁₂ was prepared. From DSC and XRPD results, we show that In(HfMg)_0.5Mo₃O₁₂ exists in a monoclinic (P2₁/a) structure at low temperature and undergoes a phase transition at ~425 K to an orthorhombic phase (Pnma), with an associated enthalpy change of 0.89 kJ mol^?1. Thermal expansion is large and positive in the low‐temperature monoclinic phase (average α_? = 16 × 10^?6 K^?1 and 20 × 10^?6 K^?1, from dilatometry and XRPD, respectively). Remarkably, this material has a near‐zero thermal expansion (ZTE) coefficient over the temperature range ~500 to ~900 K in the high‐temperature orthorhombic phase, both intrinsically and for the bulk sample. The average linear intrinsic (XRPD) value is α_? = ?0.4 × 10^?6 K^?1, and the average bulk (dilatometric) value is α_? = 0.4 × 10^?6 K^?1 with an uncertainty of ± 0.2 × 10^?6 K^?1. The slight difference between intrinsic and bulk thermal expansion is attributed to microstructural effects. XRPD results show that the thermal expansion is more isotropic than for the parent compounds HfMgMo₃O₁₂ and In₂Mo₃O₁₂.     

Mechanical and Thermal Properties of Yb2SiO5: A Promising Material for T/EBCs Applications

Yb₂SiO₅ is a promising material for thermal/environmental barrier coatings (T/EBCs), and its mechanical and thermal properties, which are essential to the coating design and applications, are investigated in this work. Yb₂SiO₅ has relatively high fracture toughness, bending and compressive strength, but low Young's modulus. It is also tolerant to damage, which is underpinned by grain delamination and cleavage along {100}, {001}, and {040} planes. The average linear coefficient of thermal expansion (CTE) is 6.3 × 10^?6 K^?1 (473–1673 K) and the anisotropic CTEs are: α_a = (2.98 ± 0.16) × 10^?6 K^?1,α_b = (6.51 ± 0.19) × 10^?6 K^?1, and α_c = (9.08 ± 0.16) × 10^?6 K^?1. The thermal conductivities are 2.3 and 1.5 W (m·K)^?1 at 300 and 1200 K, respectively. The unique combination of these properties warrants Yb₂SiO₅ promising for T/EBCs applications.     

Zero thermal expansion in cubic MgZrF6

Isotropic zero thermal expansion (ZTE) is rare but intriguing physical property in materials. Here, we report an isotropic ZTE property in a double ReO₃‐type compound of MgZrF₆, which exhibits a negligible value of coefficient of thermal expansion (α_l = ?7.94 × 10^?7 K^?1 (XRD), α_l = ?4.22 × 10^?7 K^?1 (dilatometry), 300‐675 K). The ZTE mechanism of MgZrF₆ is understood by the joint studies of temperature dependence of crystal structure and lattice dynamics. Interestingly, different magnitudes of atomic displacement parameters (ADPs) for the fluorine atoms in MZrF₆ (M = Ca, Ni, Mg) are found. The strong temperature sensitivity of ADPs demonstrates intensive transverse thermal vibration of fluorine atoms, which contributes essentially to the negative thermal expansion of CaZrF₆. By contrast, for NiZrF₆ with positive thermal expansion, the temperature response of ADPs is weak. Moderate transverse thermal vibration takes place in MgZrF₆, and ZTE appears. Furthermore, lattice dynamics of MgZrF₆ is studied by temperature‐dependent Raman spectroscopy, which reveals the ZTE mechanism. In particular, the F_2g and A_g modes, corresponding to the bending and stretching vibrations of fluorine atoms, respectively, neither soften nor harden over the whole temperature range, which is correlated with the isotropic ZTE property of MgZrF₆.     

Negative thermal expansion properties of Cu1.5Mg0.5V2O7

Cu_1.5Mg_0.5V₂O₇ was prepared by a solid state method. Its phase, microstructure, thermal expansion property, and Raman spectra were analyzed in detail. Results show that Cu_1.5Mg_0.5V₂O₇ maintains a monoclinic crystal structure and exhibits an excellent linear negative thermal-expansion property with coefficient of thermal expansion of ?8.72?×?10^?6?K^?1 over a wide temperature range of 153–673?K. The mechanism underlying the negative thermal expansion of Cu_1.5Mg_0.5V₂O₇ involves the coupling effect of the tetrahedron caused by the lateral vibration of the bridge oxygen atom and the tensile effect of the tetrahedron, The partial collapse caused by the loss of the oxygen atoms also plays an important role in the mechanism.     

Theoretical Prediction and Experimental Investigation on the Thermal and Mechanical Properties of Bulk β‐Yb2Si2O7

The thermal and mechanical properties of β‐Yb₂Si₂O₇ were investigated using a combination of first‐principles calculations and experimental investigations. Theoretically, anisotropic chemical bonding and elastic properties, weak interatomic (010) and (001) planes in the crystal structure, damage tolerance, and low thermal conductivity are predicted. Experimentally, preferred orientation, superior mechanical properties, and damage tolerant behavior for hot‐pressed bulk β‐Yb₂Si₂O₇ are approved. Slipping along the weakly bonded {010}, {001}, or {100} planes, grain delamination, buckling, and kinking of nanolaminated grains are identified as main mechanisms for damage tolerance. The anisotropic linear thermal expansion coefficients (CTEs) are: α_a = (3.57 ± 0.18) × 10^?6 K^?1, α_b = (2.49 ± 0.14) × 10⁶ K^?1, and α_c = (1.48 ± 0.22) × 10^?6 K^?1 (673–1273 K). A low thermal conductivity of ~2.1 W (m·K)^?1 at 1273 K has been confirmed. The unique combination of these properties endow it a potential candidate for thermal barrier coating (TBC)/environmental barrier coating of silicon‐based ceramics.     

Glass‐Ceramics in the System BaO–SrO–ZnO–SiO2 with Adjustable Coefficients of Thermal Expansion

Glasses from the system BaO–SrO–ZnO–SiO₂ with different Ba/Sr ratios were characterized regarding crystallization behavior as well as the thermal expansion of almost fully crystallized glasses. Depending on the SrO concentration, different crystalline phases precipitate from the glasses. Those with low SrO concentrations precipitate crystals with the structure of low‐temperature BaZn₂Si₂O₇ as one of the major phases. Higher SrO concentrations cause the formation of Ba_1?xSr_xZn₂Si₂O₇ solid solutions with the structure of high‐temperature BaZn₂Si₂O₇. Both, the low‐ as well as the high‐temperature phase exhibit very different thermal expansion behaviors ranging from a very high coefficient of thermal expansion in the case of the low‐temperature phase to a very low coefficient of thermal expansion in the case of the high‐temperature phase. The glass‐ceramics with the highest and that with the lowest coefficient of thermal expansion measured between 100°C and 800°C show a difference of 7.9 × 10^?6 K^?1, which is caused solely by a substitution of BaO with SrO. In contrast, the maximum variation in the thermal expansion of the glasses was only 1.5 × 10^?6 K^?1. The microstructure of sintered and afterward crystallized glass powders was analyzed via scanning electron microscopy and showed crack‐free samples with low porosity.     

Non‐dispersive absorption of CO2 in parallel and cross‐flow membrane modules using EMISE

BACKGROUND: This paper reports an analysis of the mass transfer behaviour of CO₂ absorption in hollow fibre membrane modules in parallel and cross‐flow dispositions. The ionic liquid EMISE, 1‐ethyl‐3‐methylimidazolium ethylsulfate, is used to achieve a zero solvent emission process and the experimental results are compared with CO₂ permeation through the membrane, without solvent in the lumenside. RESULTS: Overall mass transfer coefficients K_{overall, CF} = (0.74 ± 0.02) × 10^?6 m s^?1 and K_{overall, PF} = (0.37 ± 0.018) × 10^?6 m s^?1 were obtained for cross‐flow and parallel flow, respectively. These values are one order of magnitude lower than the coefficient obtained in permeability experiments, K_{overall, PERM} = (6.16 ± 0.1) × 10^?6 m s^?1, indicating the influence of the absorption in the process. Including the specific surface and gas volume of each contactor in the analysis, a similar value of a first‐order kinetic rate constant, K_R = 2.7 × 10^?3 s^?1 is obtained, showing that the interfacial chemical reaction CO₂‐ionic liquid is the slow step in the absorption process. CONCLUSION: An interfacial chemical reaction rate constant K_R = 2.7 × 10^?3 s^?1, describes the behaviour of the CO₂ absorption in the ionic liquid EMISE using membrane contactors in parallel and cross‐flow dispositions. Copyright © 2012 Society of Chemical Industry     

Effects of Li Substitution on the Microstructure and Thermal Expansion Behavior of Pollucite Derived from Geopolymer

The purpose of this work was to study the role of lithium in cesium‐based geopolymers and the thermal evolution during heat treatment together with thermal expansion behavior of the resulting geopolymer ceramic. A series of lithium‐substituted cesium‐based geopolymers, Cs_(1?x)Li_xGP (where x = 0, 0.1, 0.2, and 0.3), were prepared. All the geopolymer samples were heated at 1300°C for 2 h and thermal evolution on heating was studied by a variety of techniques. Phase composition, microstructure evolution, and thermal expansion behaviors of the ceramics derived from the geopolymers were characterized. All the geopolymer specimens exhibited similar thermal evolutionary trends. With increases in lithium content, overall mass loss increased gradually due to the higher hydration energy of Li⁺ than Cs⁺. Thermal shrinkage of these specimens can be divided into four stages, i.e., structural resilience, dehydration, dehydroxylation, and sintering, according to the dilatometer results. The introduction of Li results in two‐step sintering behavior for the lithium‐substituted cesium‐based geopolymers. The average thermal expansion coefficient (CTE) of Cs_(1?x)Li_xGP ceramics decreased from 4.80 × 10^?6 K^?1 (x = 0) to 3.61 × 10^?6 K^?1 (x = 0.3) with increase in lithium substitution. The reason can be attributed to the presence of spodumene after thermal treatment, which has a relatively low thermal expansion coefficient compared with pollucite. Meanwhile, molten spodumene could serve as a buffer phase between pollucite crystals also conducive to the decline of CTE of this system.     

Crystal structure and thermal expansion of ammonium pentaborate NH<Subscript>4</Subscript>B<Subscript>5</Subscript>O<Subscript>8</Subscript>

Anhydrous ammonium pentaborate NH₄B₅O₈ has been synthesized by thermal dehydration of larderellite NH₄[B₅O₇(OH)₂] · H₂O at a temperature of 290°C for 7 h. The crystal structure has been determined from the X-ray powder diffraction data: a = 7.58667(5) Å, b = 12.00354(8) Å, c = 14.71199(8) Å, R _p = 6.23, R _wp = 7.98, R _B = 12.7, R _F = 8.95, and β-KB₅O₈ structure type. The double interpenetrating framework is formed by pentaborate groups, each consisting of a boron-oxygen tetrahedron and four triangles, in which all oxygen atoms are bridging. The thermal behavior of the NH₄B₅O₈ compound has been investigated using thermal X-ray diffraction. As for other pentaborates of this type, the thermal expansion of the NH₄B₅O₈ compound is anisotropic and reaches a maximum along the a axis. The thermal expansion coefficients are as follows: α _a = 39 × 10^?6, α _b = 6 × 10^?6, α _c = 20 × 10^?6, and α _V = 65 × 10^?6 °C^?1.     

Negative thermal expansion,electrical and mechanical properties of antiperovskite Mn3Zn0.5A0.5N (A = Sn,Ag, Ni)

Thermal expansion, electrical conductivity, hardness and friction property of Mn₃ZnN, which had been doped with Sn, Ag or Ni and sintered at 1223?K, were measured. X-ray diffraction analyses show that these compounds have a cubic antiperovskite Mn₃CuN-type structure and negative thermal expansion (NTE). The width of NTE operation temperature (ΔT) and the coefficient of thermal expansion are different when Mn₃ZnN was doped with different element. A giant NTE coefficient of ?81.00?×?10^?6?K^?1 is obtained from Mn₃Zn_0.5Ag_0.5N while ΔT is 18?K. A broad ΔT of 60?K is obtained from Mn₃Zn_0.5Sn_0.5N with thermal expansion coefficient of ?19.05?×?10^?6?K^?1. The results show that Mn₃Zn_0.5 A_0.5N (A?=?Sn, Ag, Ni) has good electrical conductivity. The electrical conductivity of Mn₃Zn_0.5Ag_0.5N is the largest among these compounds as 2.45?×?10³?S?cm^?1. The Vickers hardness of these compounds is more than 350?HV. The friction coefficients of Mn₃Zn_0.5Ag_0.5N, Mn₃Zn_0.5Ni_0.5N and Mn₃Zn_0.5Sn_0.5N are 0.5318, 0.4554 and 0.2336, respectively.     

First principles investigation on mechanical and thermal properties of α‐ and β‐YAlB4 ultra‐high temperature ceramics

Ultra‐high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties that makes them promising candidates for applications in extreme environments. Various UHTCs are needed due to diverse harsh conditions that UHTCs are faced with in different applications. Due to structural similarity to ZrB₂, possible high melting point and possible protective oxide scale formed in oxygen rich and water vapor environments, REAlB₄ (RE: rare‐earth) is suggested a good candidate for UHTCs. In the present work, temperature‐dependent mechanical and thermal properties of both α‐YAlB₄ (YCrB₄ type, space group Pbam) and β‐YAlB₄ (ThMoB₄ type, space group Cmmm) were investigated by first principles calculations in combination with quasi‐harmonic approach. Due to the structural similarity between α‐YAlB₄ and β‐YAlB₄, their properties are very similar to each other, which are approximately transverse isotropic with properties in (001) plane being almost the same and differing from properties out of (001) plane. The results reveal that resistance to normal strain in (001) plane (~460 GPa) is higher than that along [001] direction (~320 GPa) and thermal expansion in (001) plane (~10 × 10^?6 K^?1) is lower than that along [001] direction (~17 × 10^?6 K^?1), which is because the stiff boron networks are parallel to (001) plane. The average thermal expansion coefficient is around 12 × 10^?6 K^?1, which is fairly high among UHTCs and compatible with metallic frameworks. The combination of high thermal expansion coefficient and protective oxidation scale forming ability suggest that REAlB₄ is promising for practical applications not only as high‐temperature structural ceramic but also as oxidation resistant coating for alloys.     

A High‐Temperature Neutron Diffraction and First‐Principles Study of Ti 3 AlC 2 and Ti 3( Al 0.8 Sn 0.2) C 2

Herein, we report on the temperature‐dependent crystal structures of Ti ₃ AlC ₂ and Ti ₃ Al _0.8 Sn _0.2 C ₂ in the 373–1273 K temperature range, as determined by Rietveld analysis of high‐temperature neutron diffraction time‐of‐flight data. The compositions are 86(1) wt% Ti ₃ AlC ₂ and 14(1) wt% TiC _0.92(2) for the sample with no Sn , and 95(1) wt% Ti ₃( Al _0.8 Sn _0.2) C ₂ and 5(1) wt% Ti ₂ AlC for the solid solution with Sn . The average linear volumetric thermal expansion is 8.0(2) × 10^?6 K ^?1 for Ti ₃ AlC ₂ and 8.2(5) × 10^?6 K^?1 for Ti ₃( Al _0.8 Sn _0.2) C ₂. The average linear thermal expansion in the a and c directions, respectively, are 7.6(2) × 10^?6 K^?1 and 8.9(2) × 10^?6 K^?1 for Ti ₃ AlC ₂. For Ti ₃( Al _0.8 Sn _0.2) C ₂, the respective values are 8.0(5) × 10^?6 K^?1 and 8.6(6) × 10^?6 K^?1. In the case of the solid solution, the quadratic thermal expansion coefficients are also given. Detailed bond lengths analysis shows that the thermal expansions along the a and c directions are controlled by the thermal expansions of the Ti – C , and Ti – Al bond lengths, respectively. The atomic displacement parameters (ADPs) show that the Al and Sn atoms vibrate with a higher amplitude than the Ti and C atoms. Consistent with first‐principles calculations, the ADPs of the Al/Sn site(s) in Ti ₃( Al _0.8 Sn _0.2) C₂ are lower than the ADPs of Al in Ti ₃ AlC ₂.     

Electroreduction of hexavalent chromium by polyaniline

BACKGROUND: A plate‐gap model interpretation of enzymatic reaction kinetics and rotating disc voltammetry were applied for evaluation of the nature of the reaction of the electroreduction of Cr(VI) (as dichromate ions) on a polyaniline (PANI)‐modified glassy carbon (GC) electrode. RESULTS: The kinetic parameters (the maximal current (V_max) and Michaelis constant (K_M)) for electroreduction of Cr(VI) on the PANI‐modified GC electrode were determined as V_max = 0.34 × 10^?7 mol cm^?3 s^?1 and K_M = 0.47 × 10^?6 mol cm^?3. The reduction of dichromate is intensified by PANI film growth. CONCLUSION: To characterise the electroreduction of Cr(VI) on a PANI‐modified GC electrode, the kinetic parameters of the reaction were determined using a plate–gap model interpretation of enzymatic reaction kinetics and rotating disc voltammetry. The catalytic nature of Cr(VI) electroreduction on the PANI‐modified electrode has been shown. Copyright © 2009 Society of Chemical Industry     

Thermal Expansion of Rare‐Earth Pyrosilicates

The use of RE₂Si₂O₇ materials as environmental barrier coatings (EBCs) and in the sintering process of advanced ceramics demands a precise knowledge of the coefficient of thermal expansion of the RE₂Si₂O₇. High‐temperature X‐ray diffraction (HTXRD) patterns were collected on different RE₂Si₂O₇ polymorphs, namely A, G, α, β, γ, and δ, to determine the changes in unit cell dimensions. RE₂Si₂O₇ compounds belonging to the same polymorph showed, qualitatively, very similar unit cell parameters behavior with temperature, whereas the different polymorphs of a given RE₂Si₂O₇ compound exhibited markedly different thermal expansion evolution. The isotropy of thermal expansion was demonstrated for the A‐RE₂Si₂O₇ polymorph while the rest of polymorphs exhibited an anisotropic unit cell expansion with the biggest expansion directed along the REO_x polyhedral chains. The apparent bulk thermal expansion coeficcients (ABCTE) were calculated from the unit cell volume expansion for each RE₂Si₂O₇ compound. All compounds belonging to the same polymorph exhibited similar ABCTE values. However, the ABCTE values differ significantly from one polymorph to the other. The highest ABCTE values correspond to A‐RE₂Si₂O₇ compounds, with an average of 12.1 × 10^?6 K^?1, whereas the lowest values are those of β‐ and γ‐RE₂Si₂O₇, which showed average ABCTE values of ~4.0 × 10^?6 K^?1.     

Enhanced pyroelectric properties in (Bi0.5Na0.5)TiO3–BiAlO3–NaNbO3 ternary system lead‐free ceramics

High pyroelectric performance and good thermal stability of pyroelectric materials are desirable for the application of infrared thermal detectors. In this work, enhanced pyroelectric properties were achieved in a new ternary (1?x)(0.98(Bi_0.5Na_0.5)(Ti_0.995Mn_0.005)O₃–0.02BiAlO₃)–xNaNbO₃ (BNT–BA–xNN) lead‐free ceramics. The effect of NN addition on the microstructure, phase transition, ferroelectric, and pyroelectric properties of BNT–BA–xNN ceramics were investigated. It was found that the average grain size decreased as x increased to 0.03, whereas increased with further NN addition. The pyroelectric coefficient p at room temperature (RT) was significantly increased from 3.87 × 10^?8Ccm^?2K^?1 at x = 0 to 8.45 × 10^?8Ccm^?2K^?1 at x = 0.03. The figures of merit (FOMs), F_i, F_v and F_d, were also enhanced with addition of NN. Because of high p (7.48 × 10^?8Ccm^?2K^?1) as well as relatively low dielectric permittivity (~370) and low dielectric loss (~0.011), the optimal FOMs at RT were obtained at x = 0.02 with F_i = 2.66 × 10^?10 m/V, F_v = 8.07 × 10^?2 m²/C, and F_d = 4.22 × 10^?5 Pa^?1/2, which are superior to other reported lead‐free ceramics. Furthermore, the compositions with x ≤ 0.03 exhibited excellent temperature stability in a wide temperature range from 20 to 80°C because of high depolarization temperature (≥110°C). Those results unveil the potential of BNT–BA–xNN ceramics for infrared detector applications.     

Well‐functioned photovoltaics based on nanofibers composed of PBDT‐TIPS‐DTNT‐DT and graphenic precursors thermally modified by polythiophene,polyaniline and polypyrrole

Reduced graphene oxide nanosheets modified by conductive polymers including polythiophene (GPTh), polyaniline (GPANI) and polypyrrole (GPPy) were prepared using the graphene oxide as both substrate and chemical oxidant. UV–visible and Raman analyses confirmed that the graphene oxide simultaneously produced the reduced graphene oxide and polymerized the conjugated polymers. The prepared nanostructures were subsequently electrospun in mixing with poly(3‐hexylthiophene) (P3HT)/phenyl‐C71‐butyric acid methyl ester (PC₇₁BM) and poly[bis(triisopropylsilylethynyl)benzodithiophene‐bis(decyltetradecylthien)naphthobisthiadiazole] (PBDT‐TIPS‐DTNT‐DT)/PC₇₁BM components and embedded in the active layers of photovoltaic devices to improve the charge mobility and efficiency. The GPTh/PBDT‐TIPS‐DTNT‐DT/PC₇₁BM devices demonstrated better photovoltaic features (J_sc = 11.72 mA cm^?2, FF = 61%, V_oc = 0.68 V, PCE = 4.86%, μ_h = 8.7 × 10^?3 cm² V^–1 s^?1 and μ_e = 1.3 × 10^?2 cm² V^–1 s^?1) than the GPPy/PBDT‐TIPS‐DTNT‐DT/PC₇₁BM (J_sc = 10.30 mA cm^?2, FF = 60%, V_oc = 0.66 V, PCE = 4.08%, μ_h = 1.4 × 10^?3 cm² V^–1 s^?1 and μ_e = 8.9 × 10^?3 cm² V^–1 s^?1) and GPANI/PBDT‐TIPS‐DTNT‐DT/PC₇₁BM (J_sc = 10.48 mA cm^?2, FF = 59%, V_oc = 0.65 V, PCE = 4.02%, μ_h = 8.6 × 10^?4 cm² V^–1 s^?1 and μ_e = 7.8 × 10^?3 cm² V^–1 s^?1) systems, assigned to the greater compatibility of PTh in the nano‐hybrids and the thiophenic conjugated polymers in the bulk of the nanofibers and active thin films. Furthermore, the PBDT‐TIPS‐DTNT‐DT polymer chains (3.35%–5.04%) acted better than the P3HT chains (2.01%–3.76%) because of more complicated conductive structures. © 2019 Society of Chemical Industry     

The adiabatic compressibility of polyelectrolytes: Sodium and hydrochloride salts of acrylic acid–N-dimethylaminoethyl methacrylate copolymer

The results of adiabatic compressibility measurement for the 100% neutralized sodium and hydrochloride salts of three copolymers of acrylic acid and N-dimethylaminoethyl methacrylate, namely, AA-DAM 58, AA-DAM 43, and AA-DAM 33, are discussed. Similar to three unneutralized amphoteric polyelectrolytes, the ?K₂ and ?V₂'s for these salts are found to be concentration independent. Since in the former both anionic and cationic groups are present, fully neutralized sodium salts and hydrochloride salts of these polyelectrolytes are expected to show a decrease in ?K_2i⁰ and ?V_2i⁰ values due to maximum electrostriction. In fact, in case of AA-DAM 58 with excess amino group in the chain (58%), the experimentally obtained values are found to be ?9.6 × 10^?4 cc/bar/mole and 159.6 cc/mole, and ?3.3 × 10^?4 cc/bar/mole and 164.4 cc/mole for sodium and hydrochloride salts, respectively, which corresponds to a decrease of 7.1 × 10^?4 cc/bar/mole and 4.9 cc/mole for the sodium salt and 0.8 × 10^?4 cc/bar/mole and 0.1 cc/mole for the hydrochloride salt. In contrast, the hydrochloride salt of AA-DAM 43 shows an increase of 6.2 × 10^?4 cc/bar/mole and 2.2 cc/mole, respectively, while in case of AA-DAM 33, both the hydrochloride salt (56.2 × 10^?4 cc/bar/mole and 24.7 cc/mole) and the sodium salt (6.2 × 10^?4 cc/bar/mole and 6.2 cc/mole) show an increase. This increase has been ascribed to suppression of dissociation of carboxyl groups by the hydrochloric acid (common ion effect) added for neutralization. However, in all the three amphoteric copolymers, when neutralized with NaOH solution or HCI acid, the viscosity increases over that of the unneutralized copolymer.     

A High‐Temperature Neutron Diffraction Study of Nb2AlC and TiNbAlC

Herein, we report on the crystal structures of Nb₂AlC and TiNbAlC—actual composition (Ti_0.45,Nb_0.55)₂AlC—compounds determined from Rietveld analysis of neutron diffraction patterns in the 300–1173 K temperature range. The average linear thermal expansion coefficients of a Nb₂AlC sample in the a and c directions are, respectively, 7.9(5) × 10^?6 and 7.7(5) × 10^?6 K^?1 on one neutron diffractometer and 7.3(3) × 10^?6 and 7.0(2) × 10^?6 K^?1 on a second diffractometer. The respective values for the (Ti_0.45,Nb_0.55)₂AlC composition—only tested on one diffractometer—are 8.5(3) × 10^?6 and 7.5(5) × 10^?6 K^?1. These values are relatively low compared to other MAX phases. Like other MAX phases, however, the atomic displacement parameters (APDs) show that the Al atoms vibrate with higher amplitudes than the Ti and C atoms, and more along the basal planes than normal to them. When the predictions of the APDs obtained from density functional theory are compared to the experimental results, good quantitative agreement is found for the Al atoms. In case of the Nb and C atoms, the agreement was more qualitative.     

Borate melt structure: Temperature‐dependent B–O bond lengths and coordination numbers from high‐energy X‐ray diffraction

Borate melts containing <20 mol% Na₂O have been studied using high‐energy synchrotron X‐ray diffraction. Temperature dependencies of the mean B–O bond lengths are shown to vary strongly with soda content, by comparison to previous measurements on liquid B₂O₃ and Na₂B₄O₇. Whereas in liquid B₂O₃ linear thermal expansion of the BØ₃ units is observed, with coefficient α_BO = 3.7(2) × 10^?6 K^?1, this expansion is apparently slightly suppressed in melts containing <20 mol% Na₂O, and is dramatically reversed at the diborate composition. These effects are interpreted in terms of changes in the mean B–O coordination number, where the reaction BØ₄^? + BØ₃ ? BØ₃ + BØ₂O^? shifts to the right with increasing temperature. The empirical bond‐valence relationship is used to convert measured bond lengths, r_BO, to coordination numbers, n_BO, including a correction for the expected thermal expansion. This method is more accurate and precise than direct determination of n_BO from peak areas in the radial distribution functions. Gradients of Δn_BO/ΔT = ?3.4(3) × 10^?4 K^?1 close to the diborate composition, and Δn_BO/ΔT = ?0.3(1) × 10^?4 K^?1 for a 13(3) mol% Na₂O melt are observed, in reasonable agreement with Raman spectroscopic observations and thermodynamic modeling, with some quantitative differences. These observations go toward explaining isothermal viscosity maxima and changes in fragility across the sodium borate system.     

Physical Properties of La1−xEuxPO4,0 ≤ x ≤ 1, Monazite‐Type Ceramics

Synthetic La_1?xEu_xPO₄ monazite‐type ceramics with 0 ≤ x ≤ 1 have been characterized by ultrasound techniques, dilatometry, and micro‐calorimetry. The coefficients of thermal expansion and the elastic properties are, to a good approximation, linearly dependent on the europium concentration. Elastic stiffness coefficients range from 182(1) to 202(1) GPa for c₁₁ and from 53.8(7) to 61.1(4) GPa for c₄₄. They are strongly dependent on the density of the sample. The coefficient of thermal expansion at 673 K is 8.4(3)  × 10^?6 K^?1 for LaPO₄ and 9.9(3)  × 10^?6 K^?1 for EuPO₄, respectively. The heat capacities at ambient temperature are between 101.6(8) J·(mol·K)^?1 for LaPO₄ and 110.1(8) J·(mol·K)^?1 for EuPO₄. The difference between the heat capacity of LaPO₄ and the Eu‐containing solid solutions is dominated by electronic transitions of the 4f‐electrons at temperatures above 75 K.