Structural and luminescence properties of silica powders and transparent glass‐ceramics containing LaF3:Eu3+ nanocrystals

In this work, silica powders and transparent glass‐ceramic materials containing LaF₃:Eu³⁺ nanocrystals were synthesized using the low‐temperature sol‐gel technique. Prepared samples were characterized by TG/DSC analysis as well as X‐ray diffraction and IR spectroscopy. The transformation from liquid sols toward bulk powders and xerogels was also examined and analyzed. The optical behavior of prepared Eu³⁺‐doped sol‐gel samples were evaluated based on photoluminescence excitation (PLE: λ_em = 611 nm) and emission (PL: λ_exc = 393 nm, λ_exc = 397 nm) spectra as well as luminescence decay analysis. The series of luminescence lines located within reddish‐orange spectral scope were registered and identified as the intra‐configurational 4f⁶‐4f⁶ transitions originated from Eu³⁺ optically active ions (⁵D₀ → ⁷F_J, J = 0‐4). Moreover, the R/O‐ratio was also calculated to estimate the symmetry in local framework around Eu³⁺ ions. The luminescence spectra and double‐exponential character of decay curves recorded for fabricated nanocrystalline sol‐gel samples (τ₁(⁵D₀) = 2.07 ms, τ₂(⁵D₀) = 8.07 ms and τ₁(⁵D₀) = 0.79 ms, τ₂(⁵D₀) = 9.76 ms for powders and glass‐ceramics, respectively) indicated the successful migration of optically active Eu³⁺ ions from amorphous silica framework to low phonon energy LaF₃ nanocrystal phase.     

Structural,magnetic and dielectric properties of Cr substituted yttrium iron garnets

Chromium substituted polycrystalline Yttrium Iron Garnets (Y₃Fe_5–xCr_xO₁₂ with x = 0 to 0.5) were prepared in single‐phase form with lattice constant in the range of 12.3775 Å to 12.3560 Å. All samples exhibit ferrimagnetic transition with transition temperature (T_c) in the range of 547 K for x = 0 to 494 K for x = 0.5. The saturation magnetization value at room temperature is found to increase with Cr concentration that is, from 24.8 to 26.6 emu/g and this is attributed to the preferential occupation of Cr³⁺ ions at octahedral site of Fe³⁺ ions. The frequency dependence of impedance data shows the relaxation and thermal activation of charge carriers across grains and grain boundaries. The complex impedance spectra were modeled by considering equivalent circuits having contributions from the resistance and constant phase element due to grains and grain boundaries and capacitance across grain boundaries. The dielectric constant is found to increase from 20 to 52 as the Cr concentration is increased and it is attributed to hopping of charge carriers across Fe²⁺–Fe³⁺ centers. The Arrhenius plots of relaxation time of charge carriers and conductivity show an anomaly in the vicinity of ferrimagnetic transition temperature and it highlights the presence of magneto‐electric coupling.     

Large Electrostrictive Strain in (Bi0.5Na0.5)TiO3–BaTiO3–(Sr0.7Bi0.2)TiO3 Solid Solutions

Relaxor ferroelectrics (0.94 ? x)(Bi_0.5Na_0.5)TiO₃–0.06BaTiO_3?x(Sr_0.7Bi_0.2□_0.1)TiO₃ (BNT–BT–xSBT) (0 ≤ x ≤ 0.5), were prepared by a solid‐state reaction process, and their structures were characterized by the transmission electron microscopy and Raman spectroscopy. The BNT–BT–0.3SBT has a very high electrostrictive strain S = 0.152% with hysteresis‐free behavior, much more than the reported S in other ferroelectrics. S–P² profiles perfectly follow the quadratic relation, which indicates a purely electrostrictive effect with a high electrostrictive coefficient (Q₁₁) of 0.0297 m⁴/C². Even, its Q₁₁ keeps at a high level in the temperature range from ambient temperature to 180°C. The field‐induced large electrostrictive strain of BNT–BT–0.3SBT was attributed to the existence of ferroelectric nanodomains.     

Large electric‐field‐induced strain and enhanced piezoelectric constant in CuO‐modified BiFeO3‐BaTiO3 ceramics

In this work, we fabricated the (1‐x)BiFeO₃‐xBaTiO₃+y‰ mol CuO ceramics by the modified thermal quenching technique. The pure perovskite phase was formed and a morphotropic phase boundary (MPB) was observed in the ceramics with x = 0.30‐0.33. The addition of CuO can significantly enhance the density of the BiFeO₃‐BaTiO₃ material. Importantly, an enhanced piezoelectric constant (d₃₃=165 pC/N), a large electric‐field‐induced strain (?S = 0.54%: peak to peak strain) and a large piezoelectric actuator constant (d₃₃*=449 pm/V) together with a high Curie temperature (T_C) of 503°C were observed in the ceramics with x = 0.30 and y = 5. As a result, the enhanced piezoelectricity and large electric‐field‐induced strain could significantly stimulate further researches in BFO‐based ceramics.     

Preparation of ferric oxide powders by ultrasonic-assisted impinging stream reaction

In this paper, Fe₃O₄ powder was prepared by the chemical precipitation method using impinging stream technology. The influences of feed flow rate, L/D, reactant concentration, ratio of iron ion concentration, reaction temperature, and impact time on the size and distribution of particles were investigated by the orthogonal experiment method. The microstructure and morphology of Fe₃O₄ powders were characterized by scanning electron microscopy, X-ray diffractometer, and granulometer. The results showed that when ratio of iron ion concentration c(Fe²⁺):c(Fe³⁺) = 0.75, reactant concentration c(Fe³⁺) = 0.4 mol · L⁻¹, feed flow rate Q = 800 L · h⁻¹, L/D = 3, reaction temperature T = 20°C, impact time t = 50 min, the prepared Fe₃O₄ had an average particle size of 1.815 μm and the most uniform distribution. The influence of ultrasonic enhancement on the mixing process and powder preparation in an impinging stream reactor was investigated. The size and distribution of the powders were significantly affected by ultrasonic enhancement. With the increase in ultrasonic power, the particle size of the powders decreased and the distribution became narrower. The particle size was reduced by 68.78%, and the particle distribution range was narrowed by 84.34% under ultrasound enhancement. This study promised the effective utilization of ultrasonic cavitation in the optimization of experimental equipment and the preparation of ultrafine powder, which provides a basis for process optimization of powder preparation.     

Characterization of the aging behavior of raw epoxidized natural rubber with a rubber processing analyzer

Tests of the strain sweep, frequency sweep, and stress relaxation for raw epoxidized natural rubber were carried out with a rubber processing analyzer. The results showed that the complex viscosity, η*, decreased with the prolongation of the aging time in the region of Newtonian flow, but in the region of non‐Newtonian flow, the decrement of η* with a rising shear rate decreased with the prolongation of the aging time. The torque (S′) response from the strain sweep indicated that aging brought about an obvious decrease in the increment of S′ with rising strain in the linear viscoelastic region and a small increase in the slope of the plateau on the curve of the S′ response in the nonlinear viscoelastic region. The stress relaxation rate constants k and b, calculated according to the equations S_t = S₀e^?kt and S_t = S₁t^?b (where S_t, S₀, and S₁ are the stresses at relaxation time t, t = 0, and t = 1, respectively), increased, and the stress relaxation time obtained directly from the rubber processing analyzer shortened with the prolongation of the aging time. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1277–1281, 2006     

Polarization behavior of sol‐gel‐derived relaxor Ba(Zr,Ti)O3 films

Films of the relaxor ferroelectric BaZr_0.25Ti_0.75O₃ (0.25‐BZT) were synthesized via a sol‐gel route to investigate the effect of film thickness on the dielectric properties and for comparison with normal ferroelectric BaTiO₃ (BT). The as‐prepared films on Nb‐doped SrTiO₃ (Nb–ST) displayed a (100) orientation; thinner films had stronger (100) orientations. Microwave dielectric measurements up to a few GHz quantified the polarizations, that is, the dipole contribution, ε_dipole, the combination of the ionic and electronic polarizations, ε_ionic+el., and the total contribution, ε_total. The ε_dipole in the relaxors at a film thickness of t=630 nm was 360, which was double that for the normal ferroelectric BT (ε_dipole=180) at t=735 nm. The larger apparent permittivity of the BZT therefore originated from the larger ε_dipole of the polar nanoregions (PNRs), while the nanograins of BT with few domain walls led to a comparably smaller ε_dipole. The volume ratio of the surface and film‐substrate interface lacking the dipole interactions increased with the reduction in the film thickness, leading to the significant depression in the permittivity for both specimens. The difference in the thickness dependence of the dielectric properties of the sol‐gel derived relaxor BZT and the normal ferroelectric BT films was attributed to the different origins of their dipole contribution, that is, the PNRs and ferroelectric domains, respectively.     

Novel Series of Low‐Firing Microwave Dielectric Ceramics: Ca5A4(VO4)6 (A2+ = Mg,Zn)

Using a conventional solid‐state reaction Ca₅A₄(VO₄)₆ (A²⁺ = Mg, Zn) ceramics were prepared and their microwave dielectric properties were investigated for the first time. X‐ray diffraction revealed the formation of pure‐phase ceramics with a cubic garnet structure for both samples. Two promising ceramics Ca₅Zn₄(VO₄)₆ and Ca₅Mg₄(VO₄)₆ sintered at 725°C and 800°C were found to possess good microwave dielectric properties: ε_r = 11.7 and 9.2, Q × f = 49 400 GHz (at 9.7 GHz) and 53 300 GHz (at 10.6 GHz), and τ_f = ?83 and ?50 ppm/°C, respectively.     

Impact of Ba2+ on Structure and Piezoelectric Properties of PMN–PZN–PNN–PT Ceramics Near the Morphotropic Phase Boundary

The effect of using Ba²⁺ to modify the structure and dielectric and piezoelectric properties of Pb_(1?x)Ba_x(Mg_1/3Nb_2/3)_m(Zn_1/3Nb_2/3)_y(Ni_1/3Nb_2/3)_nTi_zO₃(x = 0–0.15) ceramics near the morphotropic phase boundary has been investigated. It is found that increasing the content of Ba²⁺ leads to a sequence of structural transformations from heterophase state (tetragonal and pseudocubic phases) to a cubic state via an intermediate pseudocubic phase. The evolution of the dielectric and piezoelectric properties (sequences of transformations: normal ferroelectric →relaxor ferroelectric → paraelectric) is shown. It is stated that ceramics with x = 0.025–0.050 possess the optimal combination of functional parameters for use in low‐frequency receivers and actuators. They are characterized by high values of small‐signal and large‐signal piezoelectric coefficients d₃₃ of 621 pC/N and ~1500 m/V (at E = 5 kV/cm), respectively, and also by increased values of dielectric permittivity ε/ε₀ and tunability coefficient (К = [(ε/ε₀(E = 0) ? ε/ε₀(E≠0))/ε/ε₀(E = 0)]·100%), equal to ~7000 and ~80%, respectively (at E = 20 kV/cm). It is shown that for the creation of high‐power piezoelectric transducers it is necessary to use ceramics with x = 0.125, which differs with high values of the mechanical quality factor Q_m and ε/ε₀ (1406 and 10 890, respectively).     

Application of nonisothermal cure kinetics to the interaction of poly(ethylene terephthalate) with alkyd resin varnish

Linear and nonlinear viscoelasticity of an adhesion material for integrated circuit chips were investigated to control the processability. The material consists of acrylic polymer (AP) and epoxy oligomer (EP). EP content in AP/EP blend is 70 vol %. From the linear viscoelasticity, it was found that the iso‐free volume state of AP/EP blend was 20°C lower than that of AP and the entanglement molecular weight M_e of AP/EP was three times higher than that of AP. Nonlinear stress relaxation modulus G(t,γ) showed that the time‐strain separability, G(t,γ) = G(t)h(γ), was applicable at long time above a characteristic time τ_k, where G(t) is linear relaxation modulus and h(γ) is the damping function. The τ_k value was estimated to be 10 s for AP/EP and below 0.1 s for AP at an iso‐free volume state. (h(γ) for AP and AP/EP behaved like a usual linear homopolymer.) The time evolution of the elongational viscosity η_E(t) of each sample showed that AP/EP system exhibited strong strain hardening at , although AP did not show strain hardening at strain rate measured, when the data were compared at an iso‐free volume state. These results strongly suggest that the strain hardening behavior of AP/EP is attributable to enhancement of the stretch of AP polymer chains by diluting EP oligomer chains. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Structure,Microwave Dielectric Properties,and Low‐Temperature Sintering of Acceptor/Donor Codoped Li2Ti1−x(Al0.5Nb0.5)xO3 Ceramics

The structure, microwave dielectric properties, and low‐temperature sintering behavior of acceptor/donor codoped Li₂TiO₃ ceramics [Li₂Ti_1?x(Al_0.5Nb_0.5)_xO₃, x = 0–0.3] were investigated systematically. The x‐ray diffraction confirmed that a single‐phase solid solution remained within 0 < x ≤ 0.2 and secondary phases started to appear as x > 0.2, accompanied by an order–disorder phase transition in the whole range. Scanning electron microscopy observation indicated that the complex substitution of Al³⁺ and Nb⁵⁺ produced a significant effect on the microstructural morphology. Both microcrack healing and grain growth contributed to the obviously enhanced Q×f values. By comparison, the decrease of ε_r and τ_f values was ascribed to the ionic polarizability and the cell volume, respectively. Excellent microwave dielectric properties of ε_r ~ 21.2, Q×f ~ 181 800 GHz and τ_f  ~ 12.8 ppm/°C were achieved in the x = 0.15 sample when sintered at 1150°C. After 1.5 mol% BaCu(B₂O₅) additive was introduced, it could be well sintered at 950°C and exhibited good microwave dielectric properties of ε_r ~ 20.4, Q×f ~ 53 290 GHz and τ_f ~ 3.6 ppm/°C as well. The cofiring test of the low‐sintering sample with Ag powder proved its good chemical stability during high temperature, which enables it to be a promising middle‐permittivity candidate material for the applications of low‐temperature cofired ceramics.     

High strain (0.4%) Bi(Mg2/3Nb1/3)O3‐BaTiO3‐BiFeO3 lead‐free piezoelectric ceramics and multilayers

The relationship between the piezoelectric properties and the structure/microstructure for 0.05Bi(Mg_2/3Nb_1/3)O₃‐(0.95‐x)BaTiO₃‐xBiFeO₃ (BBFT, x = 0.55, 0.60, 0.63, 0.65, 0.70, and 0.75) ceramics has been investigated. Scanning electron microscopy revealed a homogeneous microstructure for x < 0.75 but there was evidence of a core‐shell cation distribution for x = 0.75 which could be suppressed in part through quenching from the sintering temperature. X‐ray diffraction (XRD) suggested a gradual structural transition from pseudocubic to rhombohedral for 0.63 < x < 0.70, characterized by the coexistence of phases. The temperature dependence of relative permittivity, polarization‐electric field hysteresis loops, bipolar strain‐electric field curves revealed that BBFT transformed from relaxor‐like to ferroelectric behavior with an increase in x, consistent with changes in the phase assemblage and domain structure. The largest strain was 0.41% for x = 0.63 at 10 kV/mm. The largest effective piezoelectric coefficient (d₃₃^*) was 544 pm/V for x = 0.63 at 5 kV/mm but the largest Berlincourt d₃₃ (148 pC/N) was obtained for x = 0.70. We propose that d₃₃^* is optimized at the point of crossover from relaxor to ferroelectric which facilitates a macroscopic field induced transition to a ferroelectric state but that d₃₃ is optimized in the ferroelectric, rhombohedral phase. Unipolar strain was measured as a function of temperature for x = 0.63 with strains of 0.30% achieved at 175°C, accompanied by a significant decrease in hysteresis with respect to room temperature measurements. The potential for BBFT compositions to be used as high strain actuators is demonstrated by the fabrication of a prototype multilayer which achieved 3 μm displacement at 150°C.     

Structure and microwave dielectric properties of ALn4(MoO4)7 (A = Ba,Sr, Ca,Ln = La,Pr, Nd,and Sm) ceramics

ALn₄(MoO₄)₇ (A = Ba, Sr, Ca, Ln = La, Pr, Nd, Sm) ceramics are prepared by solid state ceramic route and the structural properties have been studied using powder X-ray diffraction and laser Raman spectroscopy. All the ceramics under study are phase pure except BaLn₄(MoO₄)₇ (Ln = Pr, Nd, Sm). Scanning electron micrographs of the sintered ceramics show closely packed microstructure with phase homogeneity. BaLa₄(MoO₄)₇ ceramic has a maximum density of 4.5 g/cm³ at 710°C together with ԑ_r = 11.8, Q_u x f = 39 300 GHz, and τ_f = −68 ppm/^oC. SrLa₄(MoO₄)₇ ceramic exhibited a maximum density of 4.4 g/cm³, ԑ_r = 11.7, Q_u x f = 44 200 GHz, and τ_f = −83 ppm/^°C at 740°C whereas CaLa₄(MoO₄)₇ ceramic possess a maximum density of 4.2 g/cm³, ԑ_r = 11.4, Q_u x f = 30 200 GHz and τ_f = −90 ppm/^oC at 750°C at microwave frequencies. The chemical compatibility of the BaLa₄(MoO₄)₇, SrLa₄(MoO₄)₇ and CaLa₄(MoO₄)₇ ceramics with silver electrode have been studied using powder X-ray diffraction of the co-fired samples and is further examined with energy dispersive X-ray spectroscopy.     

Low‐Temperature‐Fired ReVO4 (Re = La,Ce) Microwave Dielectric Ceramics

We report a series of ReVO₄ (Re = La, Ce) microwave dielectric ceramics fabricated by a standard solid‐state reaction method. X‐ray diffraction and scanning electron microscopy measurements were performed to explore the phase purity, sintering behavior, and microstructure. The analysis revealed that pure and dense monoclinic LaVO₄ ceramics with a monazite structure and tetragonal CeVO₄ ceramics with a zircon structure could be obtained in their respective sintering temperature range. Furthermore, LaVO₄ and CeVO₄ ceramics sintered at 850°C and 950°C for 4 h possessed out‐bound microwave dielectric properties: ε_r = 14.2, Q × f = 48197 GHz, τ_f = ?37.9 ppm/°C, and ε_r = 12.3, Q × f = 41 460 GHz, τ_f = ?34.4 ppm/°C, respectively. The overall results suggest that the ReVO₄ ceramics could be promising materials for low‐temperature‐cofired ceramic technology.     

Temperature stable K0.5(Nd1−xBix)0.5MoO4 microwave dielectrics ceramics with ultra‐low sintering temperature

K_0.5(Nd_1?xBi_x)_0.5MoO₄ (0.2 ≤ x ≤ 0.7) ceramics were prepared via the solid‐state reaction method. All ceramics densified below 720°C with a uniform microstructure. As x increased from 0.2 to 0.7, relative permittivity (?_r) increased from 13.6 to 26.2 commensurate with an increase in temperature coefficient of resonant frequency (TCF) from – 31 ppm/°C to + 60 ppm/°C and a decrease in Qf value (Q = quality factor; f = resonant frequency) from 23 400 to 8620 GHz. Optimum TCF was obtained for x = 0.3 (?15 ppm/°C) and 0.4 (+4 ppm/°C) sintered at 660 and 620°C with ?_r ~15.4, Q_f ~19 650 GHz, and ?_r ~17.3, Q_f ~13 050 GHz, respectively. Ceramics in this novel solid solution are a candidate for ultra low temperature co‐fired ceramic (ULTCC) technology.     

Effect of BiMeO3 on the Phase Structure,Ferroelectric Stability,and Properties of Lead‐Free Bi0.5(Na0.80K0.20)0.5TiO3 Ceramics

The effects of BiMeO₃ (Me = Fe, Sc, Mn, Al) addition on the phase transition and electrical properties of Bi_0.5(Na_0.80K_0.20)_0.5TiO₃ (BNKT20) lead‐free piezoceramics were systematically investigated. Results showed that addition of BiFeO₃ into BNKT20 induces a phase transition from tetragonal–rhombohedral coexisted phases to a tetragonal phase with the observation of enhanced piezoelectric properties (d₃₃ = 150 pC/N for 0.02BiFeO₃). BiScO₃, BiMnO₃, and BiAlO₃ substitutions into BNKT20 induce a phase transition from coexistence of ferroelectric tetragonal and rhombohedral to a relaxor pseudocubic with a significant disruption of the long‐range ferroelectric order, and correspondingly adjusts the ferroelectric–relaxor transition point T_F–R to room temperature. Accordingly, large accompanying normalized strains of 0.34%–0.36% are obtained near the ferroelectric–relaxor phase boundary, and the mergence of large strain response can be ascribed to a reversible field‐induced ergodic relaxor‐to‐ferroelectric phase transformation. Moreover, our study also revealed that the composition located at the ferroelectric–relaxor phase boundary where the strain response is consistently derivable shifts to a BNKT20‐rich composition as the tolerance factor t of the end‐member BiMeO₃ increases, and this relationship is expected to provide a guideline for designing high‐performance (Bi_0.5Na_0.5)TiO₃‐based materials by searching the ferroelectric–relaxor phase boundary.     

Effect of BBS‐Based Frit on the Low Temperature Sintering and Electrical Properties of KNN Lead‐Free Piezoceramics

Using BaO–B₂O₃–SiO₂ (BBS)‐based frit as sintering aid, the K_0.49Na_0.51NbO₃ (KNN) + x wt% BBS (x = 1.0, 1.5, 2.0 and 2.5) lead‐free piezoelectric ceramics were successfully fabricated by solid‐state reaction method under low‐sintering temperature of 1000°C. The effect of BBS frit doping amount on the structure and electrical properties of the ceramics was investigated. The KNN ceramics with 1.5 wt% BBS frit showed optimal properties as follows: piezoelectric constant d₃₃ = 108 pC/N, planar electromechanical coupling coefficient k_p = 41%, mechanical quality factor Q_m = 225, relative dielectric constant ε_r = 410, dielectric loss tanδ = 0.57% and Curie temperature T_c = 400°C. This ceramic sample should be a good lead‐free candidate for actuators or high temperature sensors application due to its ultra‐low tanδ, relatively high Q_m and T_c.     

Structural relaxation in chalcogenide glasses

The structural relaxation of chalcogenide glasses is discussed within Tool–Narayanaswamy–Moynihan (TNM) formalism. The TNM parameters for more than 70 different glassy compositions are compared on the basis of the relaxation rate defined as R_f(ΔT) = −(dT_f/dlogt)_i at the inflection point of the isothermal relaxation curve plotted on a logarithmic timescale. The R_f(ΔT) depends on the TNM parameter ß and the parameter σ, combining the nonlinearity parameter x, the effective activation energy h^* or the fragility m. It is shown that R_f(10) estimated at 10 K below T_g is useful for the prediction of structural relaxation kinetics in different amorphous materials. The chalcogenide glasses are, for example, compared with oxide glasses and organic polymers. For all these materials, the R_f(10) versus σ plot shows a well-defined pattern that is thoroughly discussed.     

Normal‐Relaxor‐Diffuse Ferroelectric Phase Transition and Electrical Properties of Bi(Mg1/2Ti1/2)3–PbZrO3–PbTiO3 Solid Solution Ceramics Near the Morphotropic Phase Boundary

Perovskite‐type xBi(Mg_1/2Ti_1/2)O₃–(0.56 ? x)PbZrO₃–0.44PbTiO₃ (xBMT–PZ–PT) ternary solid solution ceramics were synthesized via a conventional solid‐state reaction method. The phase transition behaviors, dielectric, ferroelectric, and piezoelectric properties were investigated as a function of the BMT content. The X‐ray diffraction analysis showed that the tetragonality of xBMT–PZ–PT was enhanced with increasing the BMT content, and a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases was identified approximately in the composition of x = 0.08. In addition, the dielectric diffuseness and frequency dispersion behavior were induced with the addition of BMT and a normal‐relaxor‐diffuse ferroelectric transformation was observed from the PZ‐rich side to the BMT‐rich side. The electrical properties of xBMT–PZ–PT ceramics exhibit obviously compositional dependence. The x = 0.08 composition not only possessed the optimum properties with ε^T₃₃/ε₀ = 1450, Q_m = 69, d₃₃ = 390 pC/N, k_p = 0.46, P_r = 30 μC/cm², E_c = 1.4 kV/mm, T_c = 325°C, and a strain of 0.174% (d₃₃^* = 436 pm/V) under an electric field of 4 kV/mm as a result of the coexistence of two ferroelectric phases near the MPB, but also owned a good thermal‐depolarization behavior with a d₃₃ value of >315 pC/N up to 290°C and a frequency‐insensitive strain behavior.