Phase transitions in R2 SnCl6-type crystals (R = NH3 C2H3, NH3C2H5 and N(CH3)4)

The thermal dilatation in (NH₃ ·CH₃) SnCl₆, (NH₃ · C₂H₅) SnCl₆ and [N(CH₃)] SnCl₆ was measured, and as the results it has turned out that (NH₃ ⁶·C₂H₅) SnCl₆ and [N(CH₃)₄]₂ SnCl₆ undergo the first order transitions at 128 K and 158 K, respectively. The low temperature phases of (NH · C₂H₅) SnCl₆ and [N(CH₃)₄]₂ SnC1₆ are found to be monoclinic and tetragonal, respectively, No phase transition was observed in (NH₃ ·CH₃)₂ SnCl₆ down to 77 K.     

Dieletric and relaxation behaviors of (PbMg1/3Nb2/3O3)0.67(PbTiO3)0.33 single crystal

Dielectric permittivity along the [111] direction has been measured as a function of temperature for a relaxor ferroelectric single crystal (PbMg_1/3Nb_2/3O₃)_0.67(PbTiO₃)_0.33 (PMN-33%PT). A sharp ferroelectric phase transition was observed near 425 K and 429 K for cooling and heating processes, respectively. As temperature decreases, a diffuse phase transition (which begins near 330 K upon cooling) was detected. In addition, the nature of the thermal hysteresis for the dielectric permittivity confirms that these transitions (near 330 and 425 K upon cooling) are diffuse first-order and first-order, respectively. The frequency-dependent dielectric data ε'₁₁₁ (ƒ, T) prove the existence of an electric dipolar relaxation process between 350 and 400 K. The activation energy, the Vogel-Fulcher temperature and attempt frequency corresponding to this relaxation process are also calculated.     

Surface properties and reactivity of Cu/γ-Al2O3 catalysts for NO reduction by C3H6: Influences of calcination temperatures and additives

The influences of calcination temperatures and additives for 10 wt.% Cu/γ-Al₂O₃ catalysts on the surface properties and reactivity for NO reduction by C₃H₆ in the presence of excess oxygen were investigated. The results of XRD and XPS show that the 10 wt.% Cu/γ-Al₂O₃ catalysts calcined below 973 K possess highly dispersed surface and bulk CuO phases. The 10 wt.% Cu/γ-Al₂O₃ and 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalysts calcined at 1073 K possess a CuAl₂O₄ phase with a spinel-type structure. In addition, the 10 wt.% La–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K possesses a bulk CuO phase. The result of NO reduction by C₃H₆ shows that the CuAl₂O₄ is a more active phase than the highly dispersed and bulk CuO phase. However, the 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K possesses significantly lower reactivity for NO reduction than the 10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K, although these catalysts possess the same CuAl₂O₄ phase. The low reactivity for NO reduction for 10 wt.% Mn–10 wt.% Cu/γ-Al₂O₃ catalyst calcined at 1073 K is attributed to the formation of less active CuAl₂O₄ phase with high aggregation and preferential promotion of C₃H₆ combustion to CO_x by MnO₂. The engine dynamometer test for NO reduction shows that the C₃H₆ is a more effective reducing agent for NO reduction than the C₂H₅OH. The maximum reactivity for NO reduction by C₃H₆ is reached when the NO/C₃H₆ ratio is one.     

Ferroelectricity in AgNbl-xTaxO3 solid solutions

Existence of the ferroelectric phase in AgNb_l-xTa_xO₃ solid solutions was confirmed. Temperature of the ferroelectric phase transition decreases from 348K for AgNbO₃ to 140K for AgNb_0.4Ta_0.6O₃ with increasing Ta concentration. Investigations of spontaneous polarization P_s, total pyroelectric coefficient p_t and dielectric permittivity were carried out for ceramic samples (0 ≤ X ≤ 0.6). Values of P_s, p_t and ε_max increase with increaseing Ta concentration. Values of P_s are of the order of 0.1 μC/cm².     

Ferroelectric and ferroelastic phase transitions in (CH3NH3)3Sb2Br9 crystals

(CH₃NH₃)₃Sb₂Br₉ crystals were studied by pyroelectric method in the temperature range 10 - 200 K. Pyroelectric measurements revealed reversal spontaneous polarization below 141 K along the a-axis. The optical observations showed that MABA possesses ferroelastic domains in all low temperature phases.     

Crystal Structure and Thermoelectric Properties of YAl3C3

The crystal structure of YAl₃C₃ was refined from laboratory X-ray powder diffraction data (Cu K α₁) using the Rietveld method. The crystal structure is hexagonal (space group P 6₃ mc , Z =2) with lattice dimensions a =0.342157(4) nm, c =1.72820(1) nm, and V =0.175217(3) nm³. The final reliability indices were R _wp=9.94% ( R _wp/ R _e=1.18), R _p=7.36%, R _B=1.77%, and R _F=1.03%. The compound shows an intergrowth structure with electroconductive [YC₂] thin slabs separated by Al₄C₃-type [AlC] layers. This material had thermoelectric properties superior to those of the layered carbides Zr₂[Al_3.56Si_0.44]C₅, Zr₂Al₃C₄, and Zr₃Al₃C₅ in the temperature range of 500– 1073 K, with a maximal power-factor value of 1.96 × 10⁻⁴ W·(m·K²)⁻¹ at 1073 K.     

Phase transitions and domain structures in relaxor-based ferroelectric (PbZn1/3Nb2/3O3)0.915(PbTiO3)0.085 single crystal

Polarization-electric field (P-E) hysteresis loops and domain structures have been measured as a function of temperature in relaxor-based ferroelectric single crystal (PbZn_1/3Nb_2/3O₃)_0.915(PbTiO₃)_0.085 (PZN-8.5%PT). In order of increasing temperature, PZN-8.5%PT undergoes successive phase transitions: rhombohedra1 phase (below ∼375 K) → coexistence of rhombohedral and tetragonal phases (between ∼375 and ∼390 K) → tetragonal phase (between ∼390 and ∼420 K) → coexistence of tetragonal and cubic phases (between ∼420 and ∼460 K) → cubic phase (above ∼460 K). Phase coexistence suggests an inhomogeneous distribution of Ti⁴⁺ concentration in the PZN-8.5%PT crystal.     

Modification of Piezoelectric Characteristics of the Pb(Mg,Nb)O3-PbZrO3-PbTiO3 Ternary System by Aliovalent Additives

The effects of donor and acceptor doping on the piezoelectric properties of 18Pb(Mg_1/3Nb_2/3)O₃·39PbZrO₃·43PbTiO₃ternary solid solution have been investigated. The dopants influence in a pronounced way both the crystal structure and microstructure of the materials. The materials are transformed from the rhombohedral to the tetragonal structure, and the grain size is reduced when Nb ⁵⁺ cations are added. The crystal structures are transformed from the rhombohedral to the cubic structure, and grain growth is enhanced, when Mg ²⁺ cations are included. The values of K _P and K are maximized for 1-mol%-Nb⁵⁺-doped samples (K _P= 0.69, K = 1152); the Q _m factor and E _c are optimized for 4-mol%-Mg²⁺-doped samples ( Q _m= 1534, E _c= 0.56).     

Thermal conductivity of Pb (Mg1/3 Nb2/3) O3 under the influence of high electric field

Thermal conductivity data of Pb (Mg_1/3 Nb_2/3) O₃ (PMN) ceramics in the unpoled and poled state are presented. The measurements were performed in the temperature range from 2 K up to 100 K. The thermal conductivity exhibits glass like behaviour. The data from 2 K to 15 K can be explained in terms of the soft potential model. The thermal conductivity of the poled sample is enhanced in comparision to the unpoled one.     

Quantification of the in situ DRIFT spectra of Pt/K/γ-A12O3 NOx adsorber catalysts

A method to quantify DRIFT spectral features associated with the in situ adsorption of gases on a NO_x adsorber catalyst, Pt/K/Al₂O₃, is described. To implement this method, the multicomponent catalyst is analysed with DRIFT and chemisorption to determine that under operating conditions the surface comprised a Pt phase, a pure γ-Al₂O₃ phase with associated hydroxyl groups at the surface, and an alkalized-Al₂O₃ phase where the surface –OH groups are replaced by –OK groups. Both DRIFTS and chemisorption experiments show that 93–97% of the potassium exists in this form. The phases have a fractional surface area of 1.1% for the 1.7 nm-sized Pt, 34% for pure Al₂O₃ and 65% for the alkalized-Al₂O₃. NO₂ and CO₂ chemisorption at 250 °C is implemented to determine the saturation uptake value, which is observed with DRIFTS at 250 °C. Pt/Al₂O₃ adsorbs 0.087 μmol CO₂/m²and 2.0 μmol NO₂/m², and Pt/K/Al₂O₃ adsorbs 2.0 μmol CO₂/m²and 6.4 μmol NO₂/m². This method can be implemented to quantitatively monitor the formation of carboxylates and nitrates on Pt/K/Al₂O₃ during both lean and rich periods of the NO_x adsorber catalyst cycle.     

Structure and Electrical Properties of A-site-Deficient Perovskite Compounds in the La2/3TiO3-La1/3NbO3 System

The structure and electrical properties of an A-site-deficient perovskite compound found in the La_2/3TiO₃-La_1/3NbO₃ system were investigated. The composition of the perovskite compound seemed to be very close to La_0.633(Ti_0.90Nb_0.10)O₃. X-ray diffraction analysis revealed a superstructure with a doubled c -axis parameter, resulting from an ordered arrangement of the A-site cation vacancies. Impedance measurements on the compound showed that La_0.633(Ti_0.90Nb_0.10)O₃ had high ionic conductivity at relatively low temperature (<770 K) and increased electronic conduction at high temperature (>770 K). The bulk ionic conductivity was comparable with that of La_0.683(Ti_0.95Al_0.05)O₃, which has the highest ionic conductivity among the La-(Ti,Al)-O perovskite compounds.     

四元体系NaCl+NaBO2+Na2CO3+H2O 298.15 K 相平衡研究

柴达木盐湖中具有丰富的盐湖离子,对其中的一个四元体系水盐相图开展研究,采用等温溶解平衡法开展了298.15 K时四元体系NaCl+NaBO₂+Na₂CO₃+H₂O相平衡研究,测定了体系平衡液相组成及密度和折光率,绘制了四元体系NaCl+NaBO₂+Na₂CO₃+H₂O 298.15 K的相图及相应的物化性质图。研究发现NaCl+NaBO₂+Na₂CO₃+H₂O四元体系298.15 K 时包含2个共饱点（E₁、E₂）、5条溶解度曲线（AE₁、BE₁、CE₂、DE₂、E₁E₂）、4个结晶区（NaCl、NaBO₂·4H₂O、Na₂CO₃·7H₂O、NaCl·NaBO₂·2H₂O）。其中三元体系NaCl+NaBO₂+H₂O在298.15 K下产生了复盐NaCl·NaBO₂·2H₂O,通过研究发现该四元体系NaCl+NaBO₂+Na₂CO₃+H₂O在298.15 K下也具有NaCl·NaBO₂·2H₂O复盐区。     

三氯化铝催化有机硅单体副产共沸物歧化反应密度泛函理论研究

AlCl₃催化(CH₃)₃SiCl和SiCl₄的歧化反应体系中可能存在(CH₃)₃SiCl与SiCl₄生成(CH₃)₂SiCl₂与CH₃SiCl₃(反应1),(CH₃)₂SiCl₂与SiCl₄生成CH₃SiCl₃(反应2)和(CH₃)₃SiCl与CH₃SiCl₃生成(CH₃)₂SiCl₂(反应3)等反应。本文采用密度泛函理论对这3种反应进行了研究。结果发现在AlCl₃ 催化剂上,可能发生CH₃AlCl₂中间体生成的分步反应和反应物共吸附反应,对两种情况下3种反应的反应路径均进行了计算,发现3种反应均偏向于以双分子共吸附机理进行。3种反应的能垒排序是反应2 >反应1 >反应3。同时,热力学数据分析发现,在543.15～593.15 K之间,温度变化对反应2的摩尔Gibbs反应能变基本无影响,但在593.15 K时,反应1和反应3的Gibbs自由能变显著减小。这些计算结果显示,如果适当调控反应温度,就能够在保证反应1和3进行的同时,抑制反应2的进行,从而实现歧化产物组成的调控,有利于(CH₃)₂SiCl₂的生成。     

Dielectric Properties of Pb(Fe2/3W1/3)O3–PbTiO3 Solid-Solution Ceramics

The dielectric properties of (1− x )Pb(Fe_2/3W_1/3)O₃· x PbTiO₃ solid solutions were investigated from 10² to 10⁶ Hz in the temperature range 150–600 K. The phase transition of Pb(Fe_2/3W_1/3)O₃ (PFW) was shifted by PbTiO₃ (PT) additions to higher temperatures at a rate of 6.3 K/mol% of PT. The temperature dependence of dielectric permittivity showed a sharper transition as the PT content increased. Dielectric measurements in a wide temperature range showed the presence of a second set of dielectric peaks at higher temperatures (350–600 K), besides the ferroelectric–paraelectric phase transition. This second set of peaks vanished when the samples were annealed in nitrogen. The activation energy values for the second relaxation varied between 0.50 and 0.63 eV, in agreement with the conduction activation energy determined for each sample. This relaxation is apparently related to electron holes.     

Catalytic combustion of soot over alkali doped SrTiO3

The effect of introduction of alkalies (Me = Li, K, Cs) into SrTiO₃ on the physico-chemical properties of resulted materials and their catalytic activity in soot combustion was studied. Two groups of SrTiO₃ based perovskites were prepared: substituted in A-position of the structure (Sr_1 − xMe_xTiO₃, x = 0.05–0.2) and impregnated with the same amount of alkali metals. Prepared materials exhibit low specific surface area and perovskite structure, only these ones impregnated with the highest amount of Cs (K) show weak XRD signals of Me₂O. TPD-O₂ experiments show bimodal profiles of O₂ desorption curves with maximums corresponding to individual step of alkali nitrates thermal decomposition. It is supposed that second peak of O₂ desorption from impregnated SrTiO₃ can be related to reversible decomposition of MeNO₃. XPS shows that surface of SrTiO₃ substituted with K (Cs) is much richer in these elements than the surface of impregnated one. Prepared materials lower the temperature of soot ignition from 530 (inert) to 470 °C for SrTiO₃ and to 302–303 °C for Sr_0.8K_0.2TiO₃ and Sr_0.8K_0.2TiO₃, respectively. Substituted materials are more active in soot combustion than impregnated ones. A mechanism explaining effect of alkali metals nitrate addition to SrTiO₃ on its catalytic activity in soot combustion is proposed.     

Ultrasonic investigations of [(CH3)2NH2]5Cd3Cl11

The temperature dependences of ultrasound velocities along the crystallographic a, b and c axis in [(CH₃)₂NH₂]₅Cd₃Cl₁₁ were studied in the vicinity of phase transition at 178.5 K. The results obtained for the longitudinal and shear ultrasonic velocities confirmed clearly the existence of the second-order phase transition at this temperature and the quadratic relation between δV₄ and δV₆ and phase transition parameter was observed.     

Vaporization Studies of the La2O3–Ga2O3 System

The vaporization of the samples of the compositions Ga₂O₃+ LaGaO₃, LaGaO₃+ La₄Ga₂O₉, and La₄Ga₂O₉+ La₂O₃ was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O₂, Ga, GaO, Ga₂O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO₃( s ) and La₄Ga₂O₉( s ) at 1700 K from the component oxides were derived from the thermodynamic activities as −46.4 ± 4.7 and −99.2 ± 7.9 kJ·mol⁻¹, respectively. The results were compared with the literature data obtained using other methods.     

煤粉炉SNCR对SO3生成影响的数值模拟

基于详细化学反应机理,利用CHEMKIN-PRO软件中的平推流反应器研究了选择性非催化还原（SNCR）脱硝过程对SO₃生成作用,以及烟气中SO₂、NO、O₂和H₂O的体积浓度对SO₃生成量的影响。结果表明,氨的注入改变了SO₃生成机理和主要路径,明显促进烟气中SO₂向SO₃氧化。在NH₃的体积浓度为300 μl·L^-1、SO₂的体积浓度为2000 μl·L^-1、停留时间1.9 s内温度从1373 K降低至573 K时,生成的SO₃体积浓度大于10 μl·L^-1。随着SO₂体积分数的降低,生成的SO₃体积浓度减小,但转化率有所增加;此外生成的SO₃体积浓度随O₂体积浓度、NO体积浓度和停留时间的增加而增加,随着H₂O体积浓度的增加而减小。燃用高硫煤时,SNCR对SO₃的生成作用必须给予重视。     

Nb2O5-supported WO3: a comparative study with WO3/Al2O3

WO₃/Nb₂O₅-supported samples prepared by impregnation are characterised by X-ray diffraction (XRD), Raman spectroscopy and X-ray absorption spectroscopy (XAS) at the W–L₃ absorption edge, as well as temperature programmed reduction (TPR) and FT-IR monitoring of pyridine adsorption. Results are compared with those obtained for WO₃/Al₂O₃ samples prepared in the same conditions, showing that niobia is able to disperse tungsta better than alumina does. Formation of a crystalline WO₃ needs larger tungsten contents on niobia than on alumina, since tungsten solution into niobia is easier than into alumina. Raman and XAS spectra recorded under ambient conditions suggest that similar WO_x species are formed on both supports at tungsten contents 0.5–1 theoretical monolayers; however, TPR results for the low tungsten loaded samples indicate that, when reduction starts (always at temperatures higher than 700 K under H₂/Ar flow) there is a larger concentration of tetrahedral [WO₄] species on alumina, than on niobia. Samples with low tungsten loading have been tested in isopropanol decomposition and ethylene oxidation, following both processes by FT-IR of adsorbed species up to 673 K. Results show that adsorption of ethylene on WO₃/Nb₂O₅ yields acetaldehyde and acetate at 473 K, while this adsorption is non-reactive either on the supports or on WO₃/Al₂O₃. Isopropanol adsorbs dissociatively on both supports, leading to acetone and propene formation on tungsta–niobia, but only propene on tungsta–alumina, probably due to the larger reducibility of the tungsten-containing phases.     

The effect of oxygen concentration on the reduction of NO with propylene over CuO/γ-Al2O3

The effect of oxygen concentration on the pulse and steady-state selective catalytic reduction (SCR) of NO with C₃H₆ over CuO/γ-Al₂O₃ has been studied by infrared spectroscopy (IR) coupled with mass spectroscopy studies. IR studies revealed that the pulse SCR occurred via (i) the oxidation of Cu⁰/Cu⁺ to Cu²⁺ by NO and O₂, (ii) the co-adsorption of NO/NO₂/O₂ to produce Cu²⁺(NO₃⁻)₂, and (iii) the reaction of Cu²⁺(NO₃⁻)₂ with C₃H₆ to produce N₂, CO₂, and H₂O. Increasing the O₂/NO ratio from 25.0 to 83.4 promotes the formation of NO₂ from gas phase oxidation of NO, resulting in a reactant mixture of NO/NO₂/O₂. This reactant mixture allows the formation of Cu²⁺(NO₃⁻)₂ and its reaction with the C₃H₆ to occur at a higher rate with a higher selectivity toward N₂ than the low O₂/NO flow. Both the high and low O₂/NO steady-state SCR reactions follow the same pathway, proceeding via adsorbed C₃H₇---NO₂, C₃H₇---ONO, CH₃COO⁻, Cu⁰---CN, and Cu⁺---NCO intermediates toward N₂, CO₂, and H₂O products. High O₂ concentration in the high O₂/NO SCR accelerates both the formation and destruction of adsorbates, resulting in their intensities similar to the low O₂/NO SCR at 523–698 K. High O₂ concentration in the reactant mixture resulted in a higher rate of destruction of the intermediates than low O₂ concentration at temperatures above 723 K.