Low‐Temperature Relaxations Associated with Mixed‐Valent Structure in Sr2TiMnO6

The dielectric properties of Sr₂TiMnO₆ ceramic samples were investigated as functions of temperature (100 K ≤ T ≤ 320 K) and frequency (100 Hz ≤ f ≤ 10 MHz). Two thermally activated dielectric relaxations were observed. The sample was confirmed to possess multivalent states of Mn and Ti ions and the coexistence of electron holes and electrons. Our results revealed that both relaxations are bulk effect related to localized carriers hopping inside grains. It was suggested that the low‐temperature relaxation (LTR) can be related to dipolar effect due to electron holes, and the high‐temperature relaxation (HTR) was associated with the electrons hopping between Ti³⁺ and Ti⁴⁺ ions.     

The PZT system (PbTixZr1−xO3, 0≤x≤1.0): Dielectric response of solid solutions in broad temperature (10≤T≤1000 K) and frequency (10−2≤f≤107 Hz) ranges (Part 4)

Solid solution (SS) ceramics of the PZT (PbTi_xZr_1?xO₃, 0≤x≤1.0) system were studied in broad temperature (10≤T≤1000 K) and electric field frequency (10^?2≤f≤10⁷ Hz) ranges. Several groups of SS were distinguished, which differ by nonmonotonic behavior of dielectric parameters in the cryogenic temperature range and at Т>300 K, which results both from the defective state and from the polymorphism of SS. A conclusion is made on the expediency of use of the obtained data during the application of materials based on the PZT system in the broad range of external actions.     

Time–temperature dependent brittle fracture of viscoelastic solids

The Griffith formulation is used to study fracture behavior of poly(methyl methacrylate) (PMMA) as a function of strain rate and temperature over the range 4.4 × 10^?5 ≤ ε ≤ 4.4 × 10^?2 in./in./sec and 25°C ≤ T ≤ T_g, T_g being the glass transition temperature. It is found that the transition from brittle to ductile failure occurs abruptly at a temperature T_f which is dependent on strain rate and is approximately the same as the glass transition temperature of the material. The Griffith brittle fracture criterion is found to apply below T_f for all strain rates. The brittle fracture behavior is shown to obey the time–temperature equivalence principle in the same way as the material's other viscoelastic properties, having the same shift function.     

Orthorhombic‐pseudocubic phase transition and piezoelectric properties of (Na0.5K0.5)(Nb1−xSbx)‐SrZrO3 ceramics

0.96(Na_0.5K_0.5)(Nb_1?xSb_x)‐0.04SrZrO₃ ceramics with 0.0≤x≤0.06 were well sintered at 1060°C for 6 hours without a secondary phase. Orthorhombic‐tetragonal transition temperature (T_O‐T) and Curie temperature (T_C) decreased with the addition of Sb₂O₅. The decrease in T_C was considerable compared to that in T_O‐T, and thus the tetragonal phase zone disappeared when x exceeded 0.03. Therefore, a broad peak for orthorhombic‐pseudocubic transition as opposed to that for orthorhombic‐tetragonal transition appeared at 115°C‐78.2°C for specimens with 0.04≤x≤0.06. An orthorhombic structure was observed for specimens with x≤0.03. However, the polymorphic phase boundary structure containing orthorhombic and pseudocubic structures was formed for the specimens 0.04≤x≤0.06. Furthermore, a specimen with x=0.055 exhibited a large piezoelectric strain constant of 325 pC/N, indicating that the coexistence of orthorhombic and pseudocubic structures could improve the piezoelectric properties of (Na_0.5K_0.5)NbO₃‐based lead‐free piezoelectric ceramics.     

Crystal structure,densification, and microwave dielectric properties of Li3Mg2(Nb(1−x)Mox)O6+x/2 (0 ≤ x ≤ 0.08) ceramics

A novel system Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 (0 ≤ x ≤ 0.08) microwave dielectric ceramics were fabricated by the solid-state method. The charge compensation of Mo⁶⁺ ions substitution for Nb⁵⁺ ions was performed by introducing oxygen ions. The X-ray diffraction patterns and Rietveld refinements indicated Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 ceramics with single phase and orthorhombic structure. Micro-structure and density confirmed that the grain of Li₃Mg₂(Nb_(1-x)Mo_x)O_6+x/2 ceramics grew well. In addition, the permittivity of Li₃Mg₂(Nb_(1−x)Mo_x)O_6+x/2 ceramics with the same trend as density decreased slightly with increasing Mo⁶⁺ ions content. However, the Q*f and τ_f were obviously improved with an appropriate amount of Mo⁶⁺ ions. When x ≤ 0.04, the Q*f was closely related to the bond valence of samples, while when x ≥ 0.06, the Q*f was closely related to the density of samples. The variations of τ_f and oxygen octahedral distortion were the opposite. In conclusions, the Li₃Mg₂(Nb_0.98Mo_0.02)O_6.01 ceramic sintered at 1200°C for 6 hours exhibited outstanding properties: ε_r ~ 15.18, Q*f ~ 116 266 GHz, τ_f ~ −15.71 ppm/^oC.     

CuO‐added KNbO3‐BaZrO3 lead‐free piezoelectric ceramics with low loss and large electric field‐induced strain

CuO‐added (1‐x)KNbO₃‐x mol%BaZrO₃ ceramics with 0.0≤x≤7.0 were sintered at 960°C. Large double polarization vs electric field (P‐E) and sprout‐shaped strain vs electric field (S‐E) hysteresis curves were obtained from the specimens with x≤2.0. They exhibited large polarizing electric fields (E_P) owing to the presence of a large number of defect dipoles (P_Ds) that formed between Cu²⁺ ions and oxygen vacancies. Small double P‐E hysteresis curves were observed for the specimens with x≥3.0 with reduced E_P because of the decreased number of P_Ds and the presence of a polymorphic phase structure containing both orthorhombic and pseudocubic structures. In particular, the specimen with x=5.0 exhibited a large strain of 0.16% at 8.0 kV/mm with a small E_P of 1.2 kV/mm and good fatigue property: this specimen maintained a strain of 0.13% at 6.0 kV/mm after 10⁶ cycles of 3.0 kV/mm.     

Influence of iron oxide substitution on the magnetic interactions in sol–gel-derived 45S5 bioactive glass–ceramics

Magnetic interactions in sol–gel-derived bioactive magnetic glass–ceramics (MGCs) with compositions of (45 − x)SiO₂·24.5CaO·24.5Na₂O·6P₂O₅ xFe₂O₃ (2 ≤ x ≥ 15 wt.%) have been investigated using electron paramagnetic resonance (EPR), and temperature-dependent magnetic susceptibility and magnetization (M–T) techniques. EPR spectra of the MGC samples revealed strong composition dependence in the intensity and linewidth of resonance absorptions at g ≈ 2.0 and g ≈ 4.3. EPR linewidth analysis showed the dominance of dipole–dipole interaction in MGC samples with iron oxide content ≤4 wt.% and a crossover to super-exchange type interaction in samples with higher iron oxide content. Composition-dependent magnetic interaction in these MGC could be related to Fe²⁺ and Fe³⁺ ion concentrations using high-temperature magnetic susceptibility studies. Zero-field cooled and field cooled M–T curves indicate different magnetic behavior for MGC samples with x ≤ 6 and x ≥ 8 wt.% iron oxide. Although the former show weak magnetic behavior, the latter exhibit superparamagnetic behavior which could be correlated with the percentage of magnetic phases present in each sample. These studies reveal composition-dependent variation in dipolar and super-exchange type interaction in the samples which could help in assessing these MGC for biomedical applications.     

Performance Assessment of Some Isomers of Saturated Polycyclic Hydrocarbons for Use as Jet Fuels

The performance of jet fuel depends on the density (ρ), condensed phase heat of formation (▵_fH°(c)), and specific impulse (I_SP). Exo‐tricyclo[5.2.1.0(2,6)]decane (C₁₀H₁₆) or JP‐10 is now used as a suitable synthetic liquid jet fuel because it has the approximated values of ρ=1.1 g cm⁻³ and ▵_fH°(c)=− 123 kJ mol⁻¹ and a broad range between the melting and boiling points, i.e. T_bp–T_mp=196.2 K. This work introduces a suitable pathway for calculation of the values of ρ, ▵_fH°(c), and I_SP of 13 well‐known isomers of JP‐10 and a series of saturated polycyclic hydrocarbons with general formula of C_nH_n′ (5≤n≤12) in order to specify high performance jet fuels. Although 13 compounds have larger values of I_SP*ρ than JP‐10, only two compounds, tetraspiro[2.0.0.0.2.1.1.1]undecane and tetracyclo[3.2.0.0(2,7).0(4,6)]heptane, are suitable as jet fuels.     

Electrochemical impedance spectroscopy of oxygen and hydrogen evolution on amorphous alloys in 1 M KOH

The mechanisms of oxygen and hydrogen evolution on amorphous alloys G 14 (Fe₆₀Co₂₀Si₁₀B₁₀) and G 16 (Co₅₀Ni₂₅Si₁₅B₁₀) in 1 M KOH at T = 298 K and 333 K were studied by electrochemical impedance spectroscopy (EIS). Comparative measurements were carried out on polycrystalline Pt electrodes. Impedance spectra in the frequency range 10^?3 Hz ≤ f ≤ 10⁴ Hz were analyzed to determine the kinetic behaviour of amorphous alloys by application of transfer function analysis, using non-linear fit routines. The EIS-data are interpreted in terms of consecutive reaction mechanisms for both oxygen and hydrogen evolution.     

Thermoelectric performance of Ni,Co, and Fe nanoparticles incorporated into their metal borates glassy matrices

Here, we present our current attempt to intrinsically dope Ni⁰, Co⁰, and Fe⁰ nanoparticles within Ni^II-, Co^II-, and Fe^II-borate glassy matrices, respectively. The system was prepared by one-pot reaction of the desired M_T^II salt with excess NaBH₄ through an in-situ reduction and hydrolysis processes to afford metallic M_T⁰ nanoparticles dispersed into the M_T-BO₃ matrix. The composition and structural characteristics of these M_T⁰:M_T-BO₃ materials were identified by thermal oxidation, ATR-IR, X-ray powder diffraction, and magnetic techniques as glassy/amorphous borate matrices containing magnetic nanoparticles. The electrical conductivity (σ) of cold-pressed discs of these metal-doped composites shows that they behave as nonohmic semiconductors within the temperature range of 303 ≤ T ≤ 373 K suggesting a mixed electronic-ionic conduction. However, their thermal conductivity (κ) occurs through phonon lattice vibration dynamics rather than electronic. The σ/κ ratio shows a steep non-linear increase from 9.4 to 270 KV⁻² in Ni⁰:Ni-BO₃. In contrast, a moderate-weak increase is observed for Co⁰:Co-BO₃ and Fe⁰:Fe-BO₃ analogs. The obtained materials are examined for thermoelectric (TE) applications by determining their Seebeck coefficient (S) power factor (PF), figure of merit (ZT), and conversion efficiency (η%). All the TE data shows that Ni⁰:Ni-BO₃ (S, 80 μVK⁻¹; PF, 97.7 mWm⁻¹ K⁻¹; ZT 0.54; η, 2.15%) is a better TE semiconductor than the other two M_T⁰:M_T-BO₃. This finding shows that Ni⁰:Ni-BO₃ is a promising candidate to exploit low-temperature waste heat from body heat, sunshine, and small domestic devices for small-scale TE applications.     

On Criteria for Occurrence of Azeotropes in Isothermal and Isobaric Binary Systems

Criteria are developed for the occurrence of azeotropes in binary nonelectrolyte systems for both isothermal and isobaric situations in terms of liquid‐phase activity coefficients at infinite dilution (γ^∞). In the case of isothermal systems at temperature T, for a positive azeotrope, γ^∞_lvc≥ p*_mvc/p*_lvc where lvc refers to the less volatile component, mvc to the more volatile component, and p* to saturation vapour pressure at T; for a negative azeotrope, γ^∞_mvc≤ p*_lvc/p*_mvc. In the case of isobaric systems at pressure P, for a minimum‐boiling azeotrope, γ^∞_lvc(T_Bmvc) ≥ P/p*_lvc(T_Bmvc), where T_B refers to the boiling point at P; for a maximum‐boiling azeotrope, γ^∞_mvc (T_Blvc) ≤ P/p*_mvc(T_Blvc). The criteria are also given in terms of the parameters of selected correlations for the excess molar Gibbs function (g^E). Examples of the use of the criteria are provided. Various methods that generate values of γ^∞ can be used in conjunction with the criteria, for example, in screening procedures.     

Reactivity of disulfide peptide l-cystine with iodido (diethylenetriamine)platinum(II) and synthesis of products in the presence of an iodide ion

Substitution reactions of complex [PtI(dien)]⁺ (where, dien=diethylenetriamine ) with sulfur-containing peptide l-cystine has been studied in 1.0×10^?1 M aqueous perchlorate or acetate medium between 298≤T (K)≤323 and 2.30≤pH≤4.45 using a UV–visible spectrophotometer. Products obtained have characterized from their physico-chemical and spectroscopic methods at various pH and temperatures. From this characterization, products have indicated that [PtI(dien)]⁺ has formed a complex with l-cystine and acts as a bidentate ligand, through Pt–S bond at 2.30≤pH≤3.30 and through Pt–N and Pt–S bond of cystine in 3.95≤pH≤4.45. At 2.30≤pH≤3.30, ring opening and closing of dien have occurred at 308 and 323 K, respectively, and the same has happened at pH≥3.95. All reactions have followed the rate law – d[mixture]/dt=(k ₁+k ₂ [cystine]) [Pt(II)], where k ₂ denotes the second-order rate constant. Activation parameters E _a , Δ H ^# and Δ S ^# have been determined. Product formation and reversible and forward reaction rate constants have also been evaluated.     

Lattice dynamics of yttria: A combined investigation from spectrum measurements and first-principle calculations

C-type Y₂O₃ ceramics (relative density ~94%) were prepared at 1500 °C for 2 hours with 1% wt. ZnO as sintering aid. The cell parameters of Y₂O₃ from Rietveld refinements are a = 10.6113(1) Å, V = 1194.8(1) ų. The vibrational modes / lattice dynamics of Y₂O₃ were investigated using vibrational spectra (Raman and infrared reflection spectra) and first-principle (DFT) calculations. Eight of the 22 predicted first-order Raman modes and 12 of 16 predicted IR modes are observed and reliably assigned. For the observed vibrational modes, an excellent linearity (f_exp = 1.023f_theo, R² = 0.9999) between frequency from calculations (f_theo) and that from measurements (f_exp) is observed. Accordingly, the corrected frequency (f_cor) of vibrational modes, phonon band structure, and density of phonon states (DOPS) of Y₂O₃ are presented, in which, the frequency of phonons of Y₂O₃ is ≤625.2 cm⁻¹ (wavelength ≥16.0 μm) with a gap of 30.6 cm⁻¹ from 486.0 to 516.6 cm⁻¹ (wavelength 20.6 - 19.4 μm) at room temperature. The modes with f_theo ≥292.5 cm⁻¹ (f_cor ≥299.2 cm⁻¹) are dominated by the vibrations of O²⁻ (light atom vibrations) and the vibrational modes with f_theo ≤239.0 cm⁻¹ (f_cor ≤244.5 cm⁻¹) are dominated by the vibrations of both Y³⁺ and O²⁻ (co-vibrations). The three modes T_u⁽⁷⁾ at 301.6 cm⁻¹, T_u⁽¹⁰⁾ at 333.7 cm⁻¹, and T_u⁽¹²⁾ at 369.7 cm⁻¹ of Y-O stretch vibrations dominate the phonon dielectric constant and dielectric loss of Y₂O₃ with more than 85% contributions.     

Effects of Al doping upon ac susceptibility of Pr0.5Ca0.5MnO3

Pr_0.5Ca_0.5Mn_1?xAl_xO₃ (0≤x≤0.07) compounds were prepared by standard solid-state reaction. The ac susceptibility of the samples at low temperatures was investigated. The real component χ′ peak at the freezing temperature T_f is suppressed with increasing the frequency. χ′ shows a linear relation between T_f and the logarithm of the frequency. The normalized slope P=ΔT_f/T_fΔ log₁₀ω is much lower than for canonical insulating spin glass systems (0.06≤P≤0.08). The intensity of imaginary component χ″ at T_f for the x=0, 0.01, 0.02 samples increases with increasing frequency. The results of χ′ and χ″ suggest that the x=0, 0.01, 0.02 samples have a cluster glass ground state. The intensity of χ″ at T_f for the x=0.05, 0.07 samples decreases with increasing frequency, suggesting a phase separation ground state. The intensity of χ″ at T_f for the x=0.03, 0.04 samples decreases with increasing frequency for ω≤701 Hz and increases with further increase of frequency. This complex behavior is ascribed to the competition between the effects of large and small ferromagnetic clusters in the sample. The ground state of the x=0.03, 0.04 samples is the transition state from cluster glass (for the x=0.02 sample) to phase separation (for the x=0.05 sample). AFM cluster blocking and the spin blocking were observed in the same sample.     

Isothermal crystallization kinetics and subsequent melting behavior of β‐nucleated isotactic polypropylene/graphene oxide composites with different ordered structure

The effects of ordered structure on isothermal crystallization kinetics and subsequent melting behavior of β‐nucleated isotactic polypropylene/graphene oxide (iPP/GO) composites were studied using differential scanning calorimetry. The ordered structure status was controlled by tuning the fusion temperature (T_f). The results showed that depending on the variation of crystallization rate, the whole T_f range could be divided into three regions: Region I (T_f > 179 °C), Region II (170 °C ≤ T_f ≤ 179 °C) and Region III (T_f < 170 °C). As T_f decreased from Region I to Region III, the crystallization rate would increase substantially at two transition points, due to the variation of the ordered structure status. Calculation of Avrami exponent n indicated that the ordered structure induced the formation of two‐dimensional growing crystallites rather than three‐dimensional growing crystallites. Moreover, in the case of isothermal crystallization, the ordered structure effect (OSE) can also greatly increase the relative content of β‐phase (β_c). In Region II, OSE took place, resulting in evident increase of β_c, achieving 92.4% at maximum. The variation of the isothermal crystallization temperature (T_iso) had little influence on the T_f range (Region II) of the OSE. The higher T_f in Region II was more favorable for the formation of higher β_c. The ordered structure was favorable for the improvement of the nucleating efficiency of β‐nucleating agent (β‐NE), and was more effective for the improvement of lower β‐NE. © 2018 Society of Chemical Industry     

Synthesis and characterization of hybrid films from hyperbranched polyester using a sol–gel process

Hybrid organic/inorganic materials were prepared by an in situ sol–gel process using tetraethoxysilane (TEOS) in the presence of hyperbranched polyester. The influences of hyperbranched polyester molar mass as well as the amount of TEOS were examined. The condensation degree was characterized by solid state ²⁹Si NMR. The combination of solubility tests, calcination tests, SAXS and dynamic mechanical analysis allowed us to investigate the hybrid material nanostructure. The results show high compatibility between the inorganic silica phase and the organic polymer phase, due to the spherical shape of the hyperbranched polymer and its numerous hydroxyl groups. As a consequence, a continuous inorganic phase was formed even with a low silica precursor content without any macroscopic phase separation. These hybrid materials have a high T_g and high storage modulus even at an elevated temperature combined with improved thermal stability. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39830.     

Dielectric and Raman Spectroscopic Studies of Na0.5Bi0.5TiO3–BaSnO3 Ferroelectric System

A series of lead‐free perovskite solid solutions of (1 ? x) Na_0.5Bi_0.5TiO₃(NBT)—x BaSnO₃(BSN), for 0.0 ≤ x ≤ 0.15 have been synthesized using a high‐temperature solid‐state reaction route. The phase transition behaviors are studied using dielectric and Raman spectroscopic techniques. The ferroelectric to relaxor phase transition temperature (T_FR) and the temperature corresponding to maximum dielectric permittivity (T_m) are estimated from the temperature‐dependent dielectric data. Dielectric studies show diffuse phase transition around ~335°C in pure NBT and this transition temperature decreases with increase in x. The disappearance of x‐dependence of A₁ mode frequency at ~134 cm^?1 for x ≥ 0.1 is consistent with rhombohedral‐orthorhombic transition. In situ temperature dependence Raman spectroscopic studies show disappearance and discontinuous changes in the phonon mode frequencies across rhombohedral (x < 0.1)/orthorhombic (x ≥ 0.1) to tetragonal transition.     

Electrical Switching in Cu–As–Se Glasses

Electrical switching in Cu_xAs₄₀Se_60−x glasses has been studied over a wide composition range for 0≤x≤32. The glasses with lower Cu concentrations (x<15) do not exhibit switching, whereas glasses in the range 15≤x≤25 show a threshold type switching. The glasses in the range 26≤x≤29 exhibited an unusual switching from low-resistance to high-resistance state. For x≥30, the glasses are found to show a memory switching. The thermally crystallized samples indicate that the structural network is characterized by Cu₃AsSe₄ and As₂Se₃ for x<15 and by Cu₃AsSe₄ and Cu₂As₃ for x≥25. The composition range 15≤x≤20 is characterized only by Cu₃AsSe₄ structural units. The samples cooled from their melt show only the ternary Cu₃AsSe₄ for x≤20. For x>20, precipitates of “As” have also been observed along with Cu₃AsSe₄ and Cu₂As₃ phases. The present studies provided a unique way to understand the electrical switching exhibited by chalcogenide glasses based on the thermal model and the filament formation.     

Effects of crystalline and orientational memory phenomena on the isothermal bulk crystallization and subsequent melting behavior of poly(trimethylene terephthalate)

The effects of crystalline and orientational memory phenomena on the subsequent isothermal crystallization and subsequent melting behavior of poly(trimethylene terephthalate) (PTT) were investigated by studying the effect of prior melt‐annealing temperature, T_f, on the subsequent isothermal crystallization kinetics, crystalline structure and subsequent melting behavior of neat and sheared PTT samples. On partial melting, choices of the T_f used to melt the samples played an important role in determining their bulk crystallization rates, in which the bulk crystallization rate parameters studied were all found to decrease monotonically with increasing T_f. The decrease in the values of these rate parameters with T_f continued up to a critical T_f value (ie ca 275 °C for neat PTT samples and ca 280 °C for PTT samples which were sheared at shear rates of 92.1 and 245.6 s^?1). Choices of the T_f used to melt neat PTT samples had no effect on the crystal structure formed. The subsequent melting behavior suggested that the T_f used to melt both neat and sheared samples had no effect on the peak positions of the melting endotherms observed and that the observed peak values of these endotherms for all sample types studied were almost identical. Copyright © 2004 Society of Chemical Industry     

Structural analysis and electrode performance of Ce doped SrMnO3 synthesised by EDTA citrate complexing process

Abstract

Sr_1?xCe_xMnO₃ (SCM, 0·1≤x≤0·4) powders were synthesised by an ethylenediaminetetraacetic acid citrate complexing process, and their properties were investigated. The synthesised Sr_1?xCe_xMnO₃ powders showed a pure perovskite phase, whereas the composition with x?=?0·4 had second phases. The unit cell volumes increased with increasing Ce content because substituted Ce ions formed some Mn³⁺ ions, which have a larger ionic radius than Mn⁴⁺. The electrical conductivity improved with increasing Ce content up to x?=?0·3 (291 S cm^?1 at 750°C), revealing a double exchange interaction. Although the electrical conductivity was increased by doping Ce ions, the polarisation resistance increased due to the increase in lattice distortion with doping Ce content. The substitution of Ce ions for Sr in SrMnO₃ led to the formation of larger Mn³⁺ ions than Mn⁴⁺ ions and lattice distortion, which would affect the electrical and oxygen ion conductivity.