Yellow-green luminescence and extreme thermal quenching in the Sr6M2Al4O15:Eu2+ (M = Y,Lu, Sc) phosphor series

A series of Eu²⁺-substituted yellow-green emitting phosphors based on the compound, Sr₆M₂Al₄O₁₅ (M = Y, Lu, Sc) were identified as potential efficient phosphors based on their high calculated Debye temperatures (Θ_D > 450 K), which acts as a proxy for photoluminescent quantum yield (PLQY). The crystal structure contains corner-sharing [MO₆] octahedra and [AlO₄] tetrahedra leading to a highly connected, densely packed crystal structure. However, contrary to prediction, these compounds all showed a low PLQY (<6.5%) at room temperature. Temperature dependent luminescence measurements indicate that the photoluminescence is intense at 80 K but loses ≈90% of the emission intensity by room temperature, with the thermal quenching temperature (T₅₀) occurring well below room temperature. These results suggest that even though Debye temperature (Θ_D) is a valid proxy for PLQY, it does not describe thermal quenching.     

A comparison of the increased temperature accelerated degradation of Poly(d,l-lactide-co-glycolide) and Poly(l-lactide-co-glycolide)

Bioresorbable polymers composed of Poly(lactide), Poly(glycolide) and their related copolymers have become increasingly popular for the preparation of bone substitute constructs. In vitro tests assessing the degradative changes in physicochemical, mechanical, and biological properties of bioresorbable polymers are generally carried out at 37 °C, in pH 7.4 phosphate-buffered saline (PBS). However, long degradation times, varying from months to years make it difficult to assess these polymers at their late stages of degradation. An increased temperature accelerated degradation methodology, that simulates the long-term degradation of Poly(d,l-lactide-co-glycolide) and Poly(l-lactide-co-glycolide), has been validated in this study. Samples were degraded in PBS, under sterile conditions. Degradation temperatures of 47 °C, 57 °C and 70 °C were selected and compared to physiological temperature, 37 °C. At predetermined time intervals, samples were retrieved and evaluated for changes in mass, swelling, molecular weight, crystallinity, and thermal properties. The results from this study suggest that the degradation mechanism at elevated temperatures is similar to that observed at 37 °C. It is recommended that 47 °C is adopted by the research community to accelerate the degradation of these polymers. It is hoped the application of this methodology could be used as a valuable tool, prior to the assessment of the long-term biocompatibility of these polymers.     

In vitro and in vivo degradation behaviors of thermosensitive poly(organophosphazene) hydrogels

Biodegradable and thermosensitive poly(organophosphazenes) with various substituents were synthesized and their hydrolytic degradation properties were investigated in vitro and in vivo. The aqueous solutions of all polymers showed a sol-gel phase transition behavior depending on temperature changes. The side groups of polymers significantly affected the polymer degradation and accelerated hydrolysis of polymers in the order of carboxylic acid > depsipeptide > without carboxylic acid and depsipeptide. The increased gel strength led to the decreased hydrolysis rate. The polymer hydrogels with 750 Da of α-amino-ω-methoxy poly(ethylene glycol) were rapidly decreased by dissolution. The polymer degradation was also influenced by pH and temperature. The in vivo behaviors of mass decrease of the polymer hydrogels were similar with the in vitro results. These results suggest that the biodegradable and thermosensitive poly(organophosphazenes) hold great potentials as an injectable and biodegradable hydrogel for biomedical applications with controllable degradation rate.     

Note on “Thermodynamic temperature differences from the ITS-90 for the correction of thermodynamic property data” by F. Pavese,P. Ciarlini,and P.P.M. Steur [1]

A recent paper by Pavese et al. presents formulas for calculating the differences (T ? T₉₀) between temperatures measured on the internationally recognized temperature scale, ITS-90 and the thermodynamic temperature.However, there has been a systematic, international process in place to critically review the differences (T ? T₉₀) and to disseminate the results in a convenient form. The process was undertaken by Working Group 4 (WG4) at the request of the Consultative Committee for Thermometry. WG4 recently published the results of its review including consensus values of (T ? T₉₀) and a user-friendly, representation of (T ? T₉₀). One of the authors of the Pavese et al. paper is member of WG4.A multiplicity of representations of (T ? T₉₀) is an undesirable barrier to the exchange of thermodynamic information and obscures the status of temperature standards. Thus, WG4 recommends that the chemical thermodynamics community use the values (T ? T₉₀) and their polynomial representation as published in the WG4 paper.     

Study on cloud points of Triton X-100-cationic gemini surfactants mixtures: A spectroscopic approach

This study investigates the effects of various cationic surfactants on the cloud point (CP) of the nonionic surfactant Triton X-100 (TX-100) in aqueous solutions. Instead of visual observation, a spectrophotometer was used for measurement of the cloud point temperatures. The values of CPs for Triton X-100 can be measured directly because TX-100 has an average number of oxyethylene units per molecule of p ≈ 9.5 and a CP = 66.0 °C. Quaternary ammonium dimeric surfactants (m-s-m, m = 10, 12, and 16, and s = 2, 6, and 10) were synthesized and used. The melting temperature T_M and the Krafft temperature T_K were measured for 1 wt% aqueous solutions of these synthesized surfactants. The melting temperature of the solid gemini surfactants increased with the carbon number of the alkyl chain. The results showed that additions of the gemini surfactants (which are infinitely miscible with water) to Triton X-100 increased the cloud point of the TX-100 solutions. All salts tested in these studies had a large effect on the CPs of nonionic surfactants due to their effect on water structure and their hydrophilicity. The effect of the alkyl chain length of the gemini surfactant on the CP of Triton X-100 is therefore more important than the spacer chain length.     

On the effect of heat on the chemical composition and dimensions of thermally-modified wood

Heat-induced weight loss (WL) and chemical and dimensional changes of small specimens of beech (Fagus sylvatica L.), Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L.) wood were examined after thermal modification in the 190-245 °C temperature range. Treated specimens exhibited reductions in their oven-dry weight in line with the severity of the treatment, with the effect of increasing the temperature of exposure being greater than extending the period of treatment. Wood polysaccharides were found to be distinctly more labile than the lignin constituent; the latter increased possibly as a result of repolymerisation reactions trapping some degradation products in the process. Specimens shrank in the transversal plane in a tangential to radial ratio of 2:1 regardless of the treatment regime, while their length increased marginally for WL < 10-12%. It is proposed that the thermal modification leaves the cell wall material in a permanent strained state.     

The polymorphism of CsThF5

Three new polymorphs of cesium thorium pentafluoride, CsThF₅, have been synthesized hydrothermally and structurally characterized. Of the structures of three polymorphs of CsThF₅ one is described in space group P4/nmm with a = 5.7358(8) and c = 7.4048(15) and two in the space group P2₁/c: one with a = 8.3600(17), b = 7.5780(15), c = 8.9912(18), β = 116.69(3) and the other with a = 8.4196(17), b = 14.645(3), c = 8.5017(17), β = 102.67(3). Comparison of these materials was made based on their structures and synthesis conditions. The formation of these species and other previously reported metal fluoride species in hydrothermal fluids appears to be dependent upon the concentration of the alkali fluoride mineralizer, pressure, temperature, and reaction time. Differential scanning calorimetry (DSC) studies show that all of the different phases of cesium thorium fluoride transition into the same monoclinic phase of CsThF₅ and eventually into the orthorhombic phase of CsTh₃F₁₃.     

An investigation of magnetocaloric effect and its implementation in critical behavior study of La1−xNdxMn2Si2 compounds

The magnetic properties of La_1−xNd_xMn₂Si₂ (x = 0.30–0.45) compounds are studied over the temperature range 5 K ≤ T ≤ 375 K. We report inverse and conventional magnetic entropy change values of La_1−xNd_xMn₂Si₂ (x = 0.35 and 0.40) compounds over the temperature range 5 ≤ T ≤ 375 K. In addition, we study critical behavior of La_1−xNd_xMn₂Si₂ (x = 0.35 and 0.40) compounds around their Curie temperatures. The field dependence of the magnetic entropy change is brought out and implemented to deduce the critical exponents. The critical behavior study shows that the magnetic interactions for the x = 0.35 sample have the same behavior below and above T_C. However, for the x = 0.40 sample has different behavior below and above T_C. Thus, the x = 0.40 sample behaves as a multiphase compound.     

High temperature dielectric loss of h-BN at X band and its dependence on the electron structure,defects and impurities

The dielectric loss of hexagonal boron nitride (h-BN) at high temperature ranging from 291 K to 2023 K at X band is investigated by experimental and theoretical analyses. The dielectric loss is relatively low below 1273 K, nevertheless, above this temperature, much rapid increase in the dielectric loss is observed with the increase of temperature. At 2023 K, the dielectric loss is found to be 40 times more than that at room temperature. Our calculation reveals that the rapid increase in the dielectric loss from 1273 K to 1800 K is mainly contributed by the boron vacancy (V_B) defects but from that 1800 K to 2023 K is mostly caused by the hopping phenomenon of the electrons. It is also found that the reason that V_B has the smallest energy among the defect is that its electronegativiities are smaller and its bonding ability is weaker compared to other defects. And the co-existence of defects and impurities is found to enhance the conduction loss by first principle calculations, which is due to that the bond of B1 atom and N1 atom becomes weak and the jumping distance of the impurity ions is increased when V_B and Na co-doped in the structure.     

Syntheses,structures and magnetic properties of the alkali oxonickelates(I) A3NiO2 (A = K,Rb, Cs)

New oxonickelates(I), Rb₃NiO₂ and Cs₃NiO₂, were prepared via the azide/nitrate route, starting from stoichiometric mixtures of azides, nitrates and NiO as precursors. The mixtures were heated steadily in a controlled heating regime up to 723 K and annealed at this temperature for 50 h (30 h for cesium compound) in specially designed containers with silver inlays. The crystal structures of Rb₃NiO₂ and Cs₃NiO₂ were solved and refined by X-ray powder methods. Room temperature α-Rb₃NiO₂ (P4₁2₁2, Z = 4, a = 6.2651(2) Å, b = 14.7438(3) Å; R_wp = 6.30%) and high temperature β-Rb₃NiO₂ (at 523 K P4₂/mnm, Z = 2, a = 6.2750(2) Å, b = 7.5088(3) Å; R_wp = 7.85%) were found to be isostructural to room and high temperature α- and β-K₃NiO₂, respectively. Cs₃NiO₂ crystallizes at room temperature isostructural with the β-K₃NiO₂ (P4₂/mnm, Z = 2, a = 6.4336(3) Å, b = 8.0844(4) Å; R_wp = 5.21%). A₃NiO₂ (A = K, Rb, Cs) are paramagnetic in the whole temperature range investigated. The magnetic susceptibility data have been evaluated by the Curie–Weiss law, where the calculated magnetic moments are as expected for a d⁹ system (μ = 1.73–2.20μ_B). Negative values of Weiss constants are indicative for antiferromagnetic interactions in this family of compounds.     

Thermodynamic properties of sublimation of the ortho and meta isomers of acetoxy and acetamido benzoic acids

This paper reports vapour pressures measured at several different temperatures using the Knudsen effusion method of ortho-acetoxybenzoic acid (aspirin) (341.1 to 361.1) K, meta-acetoxybenzoic acid (344.2 to 362.2) K, ortho-acetamidobenzoic acid (367.2 to 389.2) K, and meta-acetamidobenzoic acid (423.2 to 441.1) K. The experimental results enabled the determination of the standard molar enthalpies, entropies and Gibbs energies of sublimation, at T = 298.15 K, of the four compounds studied. DSC experiments yield results of the temperature and enthalpy of fusion. The experimental results were compared with literature ones for the para isomers of the acids acetoxybenzoic and acetamidobenzoic. Correlations involving temperature of fusion, and standard molar enthalpy and Gibbs energy of sublimation of several substituted benzoic acids were proposed. Those correlation equations allow a good estimative of volatility of benzoic acid derivatives from their enthalpies of sublimation and temperatures of fusion.     

Structural study of polymorphism and thermal behavior of CaZr(PO4)2

The crystal structure of CaZr(PO₄)₂ has been revised by ab initio Rietveld analysis of X-ray powder diffraction data. At room temperature, CaZr(PO₄)₂ crystallizes in the orthorhombic space group Pna2₁ (Z = 4). Differential thermal analysis suggests a reversible second order transition at 1000 °C confirmed by high temperature XRD analysis that brings out the existence of a high temperature form, very similar to the room temperature one, but more symmetrical (Pnma, Z = 4). Analysis of the crystal parameters evolution during heating reveals that CaZr(PO₄)₂ exhibits a quite low thermal expansion coefficient of 6.11·10⁻⁶ K⁻¹. This value stems from a combination of several mechanisms, including Coulombic repulsion and bridging oxygen rocking motion.     

Low temperature and high pressure thermoelastic and crystallographic properties of SrZrO3 perovskite in the Pbnm phase

The thermoelastic and structural properties of SrZrO₃ perovskite in the Pnma (Pbnm) phase have been studied using neutron powder diffraction at 82 temperatures between 11 K and 406 K at ambient pressure, and at sixteen pressures between 0.07 and 6.7 GPa at ambient temperature. The bulk modulus, derived by fitting the equation of state to a second order Birch-Murnaghan equation-of-state, 157(5) GPa, is in excellent agreement with that deduced in a recent resonant ultrasound investigation. Experimental axial compressional moduli are in agreement with those calculated from the elastic stiffness coefficients derived by ab-initio calculation, although the experimental bulk modulus is significantly softer than that calculated. Following low temperature saturation for temperatures less than 40 K, the unit cell monotonically increases with a predicted high temperature limit in the volume expansivity of ∼2.65 × 10⁻⁵ K⁻¹. Axial linear thermal expansion coefficients are found to be in the order α_b < α_c < α_a for all temperatures greater than 20 K with the b axis indicating a weak, low temperature negative expansion coefficient at low temperatures. The thermoelastic properties of SrZrO₃ can be approximated by a two-term Debye model for the phonon density of states with Debye temperatures of 238(4) K and 713(6) K derived in a self-consistent manner by simultaneously fitting the isochoric heat capacity and the unit cell volume. Atomic displacement parameters have been fitted to a modified Debye model in which the zero-point term is an additional refinable variable and shows the cations and anions have well separated Debye temperatures, mirroring the need for two Debye-like distributions in the vibrational density of states. The temperature dependence of the crystal structure is presented in terms of the amplitudes of the seven symmetry-adapted basis vectors of the aristotype phase that are consistent with space group Pbnm, thus permitting a direct measure of the order parameter evolution in SrZrO₃. The temperature variation of the in-phase tilt, which is lost at the phase transition at 973 K, is consistent with tricritical behaviour, in agreement with published results based on high temperature crystallographic data.     

On the structural and magnetoelectrical characterization of epitaxial SrRuO3 thin films grown on (001) SrTiO3 substrates

High-quality epitaxial thin films of the ferromagnetic metallic oxide SrRuO₃ (SRO) were fabricated by dc-sputtering at high oxygen pressure and their structural and magnetoelectrical properties were carefully studied. The films featured a Curie temperature T_C ～ 160 K and a magnetic moment of ～0.7 μ_B per Ru ion. The temperature dependent magnetization could be well described by the scaling relation M(T) ∝ (T_C ? T)^β with a critical exponent β = 0.53 over the entire ferromagnetic temperature range. A negative magnetoresistance, MR, on the order of a few percent was found up to room temperature. MR showed a maximum of ～4% right at T_C where a kink structure of the resistivity, ρ, at zero field was flattened out on magnetic field application. This ρ contribution could be related to scattering due to orientational disorder of the Ru magnetic moments which become aligned by an external magnetic field. In addition, an equally strong MR effect, related to localization phenomena, could be observed at lower temperature. Particularly, the second MR peak at ～35 K might be related to a Fermi-liquid to non-Fermi-liquid crossover. A scaling behavior dρ/dT ∝ |T ? T_C|^α was observed only above T_C. Here, values for the exponent α ≈ ?0.4 and α ≈ ?1.4 were obtained in zero field and in a field of 9 T, respectively. The commonly observed ρ minimum, appearing at low temperatures (～3 K in the present case), is correlated with the structural disorder of the SRO films and is believed to have its origin in quantum corrections to the conductivity (QCC).     

Ultrasonic velocity for an equimolar mixture of molten AgI and NaCl in the biphasic region

The ultrasonic velocities of a molten stratifying mixture composed of 0.5 AgI and 0.5 NaCl (the composition corresponding to the top of the miscibility gap) were measured along the saturation line for a wide temperature range via the pulse method to establish the characteristics of mixing salts with different chemical bonds. We show that the difference, Δu, between the magnitudes of the sound velocities for the coexisting phases decreases with increasing temperature and becomes zero at 1064 K. This temperature corresponds to the critical phase transition point, T_c. The temperature dependence of the sound velocity difference, Δu, is described by the equation Δu ≈ (T_c − T)^Θ, where Θ = 0.896, which is less than that found for alkali halide melts (Θ = 1.02), in which long-range Coulomb forces between ions prevail. The results are discussed in terms of the peculiarity of the chemical bond in silver iodide.     

Crystal structures and magnetic properties of the honeycomb-lattice antiferromagnet M2(pymca)3(ClO4), (M = Fe,Co, Ni)

We report on the syntheses, crystal structures, and magnetic properties of a series of transition metal coordination polymers M₂(pymca)₃(ClO₄), (pymca = pyrimidine-2-carboxylic acid, M = Fe (1), Co (2), and Ni (3)). These compounds are found to crystallize in a trigonal crystal system, space group P31m, with the lattice constants a = 9.727 Å and c = 5.996 Å for 1, a = 9.608 Å and c = 5.996 Å for 2, and a = 9.477 Å and c = 5.958 Å for 3 at room temperature. In these compounds, each pymca ligand connects to two M²⁺ ions, forming a honeycomb network in the ab plane. The temperature dependences of magnetic susceptibilities in these compounds show broad maxima, indicating antiferromagnetic interactions within two-dimensional honeycomb layers. We also observed an antiferromagnetic phase transition at low temperatures by magnetic susceptibility and heat capacity measurements. From the crystal structures and magnetic properties, we conclude that the compounds 1, 2, and 3 are good realizations of honeycomb-lattice antiferromagnets.     

Influence of thermal treatment on the glass transition temperature of thermosetting epoxy laminate

The influence of accelerated thermal treatment of thermosetting epoxy laminate on its glass transition temperature was studied. Lamplex^® FR-4 glass fibre-reinforced epoxy laminate (used for printed circuit board manufacturing) was used in these experiments. The composite was exposed to thermal treatments at temperatures ranging from 170 °C to 200 °C for times ranging from 10 to 480 h. The glass transition temperature (T_g) was analysed via dynamic mechanical analysis (DMA). It has been proven that the glass transition temperature rapidly decreases in reaction to thermal stress. The obtained T_g data were used for Arrhenius plots for different critical temperatures (T_g-crit. = 105–120 °C). From their slopes (?E_a/R), the activation energy of the thermal degradation process was calculated as 75.5 kJ/mol. In addition to this main relaxation mechanism, DMA also recorded one smaller relaxation process in the most aged samples. Microscopic analysis of the sample structure showed the presence of pronounced small regions of degradation both on the surface and in the inner structure, which are probably the causes of microscopic delamination and the smaller relaxation process.     

Effects of simulation conditions on antibacterial performance of polypropylene and polystyrene doped with HPQM antibacterial agent

2-Hydroxypropyl-3-piperazinyl-quinoline carboxylic acid methacrylate (HPQM) antibacterial agent in two different forms, solid form (HPQM-supported Neusilin absorbance; HPQM-neu) and liquid form (HPQM dissolved in deionized water; HPQM-water), were added to polypropylene (PP) and polystyrene (PS). The antibacterial performance against Escherichia coli and mechanical properties of the specimens were investigated. The effects of simulated conditions, including immersion in water (at room temperature and 80 °C), immersion in surfactant solution (at room temperature and 80 °C) and exposure to UV aging, on the antibacterial performance of specimens were also studied. The results showed that the optimal HPQM concentration for PP/HPQM-neu and PP/HPQM-water were 500 and 750 ppm, respectively, in order to reach 99.9% E. coli bacteria reduction, but those for PS/HPQM-neu and PS/HPQM-water were 1250 ppm. Addition of HPQM in both forms to PP and PS did not change the mechanical performance. HPQM-water was more appropriate and effective for antibacterial peformance for PP and PS than HPQM-neu. Immersion of all specimens in water and detergent solution (at room temperature and 80 °C) and UV aging decreased the antibacterial performance at different rates, the effect being very pronounced for the specimens with HPQM-neu.     

Effect of temperature on the excess molar volumes of some alcohol + aromatic mixtures and modelling by cubic EOS mixing rules

Densities of the (methanol + benzene), (ethanol + benzene), (methanol + chlorobenzene) and (ethanol + chlorobenzene) mixtures have been measured at six temperatures (288.15, 293.15, 298.15, 303.15, 308.15 and 313.15 K) and 101.33 kPa. Excess molar volumes V^E were determined and fitted by the Redlich–Kister equation. It was observed that in all cases V^E increases with rising of temperature. The values of limiting excess partial molar volumes have been calculated, as well. The obtained results have been analysed in terms of specific molecular interactions present in these mixtures taking into considerations effect of temperature on them. The correlation of V^E binary data was performed with the Peng–Robinson–Stryjek–Vera cubic equation of state (PRSV CEOS) coupled with the van der Waals (vdW1) and CEOS/G^E mixing rule introduced by Twu, Coon, Bluck and Tilton (TCBT). The experimental values of V^E were compared with those estimated by both mixing rules at the temperature range and on each temperature, separately.     

Magnetoelectric and magnetodielectric effect in Ba1−xSrxTiO3 and Co0.9Ni0.1Fe2−xMnxO4 composites

Ba_1−xSr_xTiO₃ (BST) is a ferroelectric material known to possess ferroelectric transition temperature T_c in the vicinity of room temperature ∼30 °C for x = 0.3. As the BST with x in the vicinity of 0.3 is elastically soft at 30 °C, the compositions with x = 0.2, 0.25 and 0.3 are chosen to investigate their possible magnetoelectric (ME) and magnetodielectric (MD) applications. The (CoNi) FeMn₂O₄ (CNFMO) is selected to be a piezo-magnetic phase. Here hydroxide co-precipitation route is adopted so as to synthesize BST and CNFMO of nearly 100 nm crystallite size. Starting with the BST and CNFMO powders, the composites yCNBST = yCNFMO + (1 − y) BST are synthesized for y = 0.3 and 0.4. The parent compositions of BST as well as the CNFMO are characterized for dielectric and magnetic properties to confirm the formation of the desire ferroelectric and magnetostrictive phases. The composites are investigated for the crystal structure, dielectric, magnetoelectric and magnetodielectric properties. The results show that the composite Ba_0.75Sr_0.25TiO₃–Co_0.9Ni_0.1Fe_1.7Mn_0.3O₄ exhibit excellent ME and MD properties simultaneously. The results on the ME and MD properties are understood in terms of the stress induced variations in polarization and dielectric constant ɛ respectively.