New dielectric material system of La(Mg1/2Ti1/2)O3–Ca0.6La0.8/3TiO3 for microwave applications

The microstructure and microwave dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics system with ZnO additions (0.5 wt.%) investigated by the conventional solid-state route have been studied. Doping with ZnO (0.5 wt.%) can effectively promote the densification and the dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics. 0.6La(Mg_1/2Ti_1/2)O₃–0.4Ca_0.6La_0.8/3TiO₃ ceramics with 0.5 wt.% ZnO addition possess a dielectric constant (_r) of 43.6, a Q × f value of 48,000 (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −1 ppm/°C sintering at 1475 °C. As the content of La(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 62,900 (GHz) for x = 0.8 is achieved at the sintering temperature 1475 °C. A parallel-coupled line band-pass filter is designed and simulated using the proposed dielectric to study its performance.     

Dielectric and magnetic properties of 0.7Bi(Fe1−xCrx)O3–0.1BaTiO3–0.2PbTiO3 solid solutions

0.7Bi(Fe_1−xCr_x)O₃–0.1BaTiO₃–0.2PbTiO₃ (x = 0, 0.1, 0.2, 0.3) solid solutions were prepared by the traditional ceramic process. X-ray diffraction results revealed that the samples with x = 0–0.3 showed pure perovskite structure. Frequency and temperature dependences of dielectric constants and dielectric loss of the samples were investigated. Both dielectric constant and the loss tangent increased at given frequencies (100 Hz–1 MHz), while the Curie temperature of the solid solutions decreased with increasing Cr content. Room temperature magnetic hysteresis loops indicated that an appropriate amount of Cr could improve magnetization of the solid solutions.     

Structural and piezoelectric properties of LiNbO3-modified BiScO3–PbTiO3 ceramics

Piezoelectric perovskite materials based on the solid solution (1 − x)BiScO₃–xPbTiO₃ (BSPT) have been attracting attention for their high Curie temperature (T_c = 450 °C) and excellent piezoelectric properties. The LiNbO₃ (LN), which has a T_c as high as 1150 °C, has been recently reported forming a phase pure perovskite solid solution with some perovskite structure compounds. In the current work, the effects of LN substitution on the structural and electrical properties of BSPT ceramics were investigated in the 0.36BiScO₃–0.64{(1 − x)PbTiO₃–xLiNbO₃} (BSPTLNx) system. The results of LN addition in the BSPT ceramics show significant enhancement of the piezoelectric properties. The piezoelectric constant d₃₃, planar electromechanical coupling coefficient and remnant polarization P_r values reached 465 pC/N, 0.57 and 48 μC/cm², respectively, for x = 0.04. The T_c gradually decreases with increasing LN content in the BSPTLNx system, due to the structure transform from the tetragonal to the rhombohedral. A typical relaxor behavior is also produced with the LN substitution in the BSPTLNx system.     

Crystal structure and microwave dielectric properties of a new A4B3O12-type cation-deficient perovskite Ba3LaTa3O12

A new microwave dielectric ceramic Ba₃LaTa₃O₁₂ has been prepared by conventional solid-state ceramic route. Through Rietveld refinement of X-ray powder diffraction data, the compound is identified as an A₄B₃O₁₂-type B-site cation-deficient perovskite with space group and lattice constants a = 5.7573(2) Å, c = 28.2386(2) Å, V = 810.62(4) Å³ and Z = 3. The microwave dielectric properties of Ba₃LaTa₃O₁₂ were investigated. The compound exhibits a relative dielectric constant (_r) of 36.8, a quality factor Q_u × f of 21,965 at 6.4040 GHz and a small negative temperature coefficient of resonant frequency (τ_f) of −49.6 ppm/°C.     

X-ray diffraction study of Ba0.985Na0.015Ti0.985Nb0.015O3, Ba0.6Na0.4Ti0.6Nb0.4O3 and Ba0.3Na0.7Ti0.3Nb0.7O3 compositions

The Ba_0.985Na_0.015Ti_0.985Nb_0.015O₃, Ba_0.6Na_0.4Ti_0.6Nb_0.4O₃ and Ba_0.3Na_0.7Ti_0.3Nb_0.7O₃ compositions of the (1 − x) BaTiO₃–xNaNbO₃ (BTNNx) system have been studied by X-ray diffraction and by measurements of dielectric properties. The specimens with composition BTNN (x = 0.015, 0.40 and 0.70) have been refined by the JANA program from X-ray powder diffraction data. Ceramic samples with composition (1 − x) BaTiO₃ + xNaNbO₃ (where x = 0.015, 0.40 and 0.70) were prepared by calcinations from appropriate mixture of BaCO₃, TiO₂, Na₂CO₃ and Nb₂O₅. The calcined powder was sintered at temperature range 1200–1400 °C. As the composition x increased from 0.015 (and 0.70), the ferroelectric ceramics (x = 0.015, FE) with tetragonal phase changed to the ferroelectric relaxors (RFE, x = 0.40). RFE ceramics showed a peculiar diffuse phase transition and dielectric relaxation at the low temperature (down to 180 K) due to a frustration between RFE and FE state. These ceramics present the classical ferroelectric character when 0 ≤ x < 0.075 and 0.55 < x ≤ 1 and relaxor character when 0.075 ≤ x ≤ 0.55.     

Structure, microstructure and electrical properties of (1 − x − y)Bi0.5Na0.5TiO3–xBi0.5K0.5TiO3–yBi0.5Li0.5TiO3 lead-free piezoelectric ceramics

The ternary system (1 − x − y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃–yBi_0.5Li_0.5TiO₃ (abbreviated to BNKLT-x/y) was synthesized by conventional oxide-mixed method. The phase structure, microstructure, and dielectric, ferroelectric and piezoelectric properties of the ceramics were investigated. The X-ray diffraction patterns showed that pure perovskite phase with rhombohedral structure can be obtained in all the ceramics. The grain size varied with x and y. The temperature dependence of dielectric constant and dielectric loss revealed there were two phase transitions which were from ferroelectric (tetragonal) to anti-ferroelectric (rhombohedral) and anti-ferroelectric to paraelectric (cubic). Either increasing x or y content can make T_m (the temperature at which dielectric constant _r reaches the maximum) increase. With the addition of Bi_0.5K_0.5TiO₃, the remanent polarization P_r increased but the coercive field E_c decreased. With the addition of Bi_0.5Li_0.5TiO₃, P_r increased obviously and E_c increased slightly. Due to the stronger ferroelectricity by modifying Bi_0.5K_0.5TiO₃ and Bi_0.5Li_0.5TiO₃, the piezoelectric properties were enhanced at x = 0.22 and y = 0.10, which were as follows: P_r = 31.92 μC/cm², E_c = 32.40 kV/cm, _r = 1118, tan δ = 0.041, d₃₃ = 203 pC/N and K_p = 0.31. The results show that the BNKLT-x/y ceramics are promising candidates for the lead-free materials.     

Studies on structural and electrical properties of Co1−xNixFe1.9Mn0.1O4 ferrite

Nickel-doped iron-deficient cobalt ferrite with small amount of manganese having the chemical composition Co_1−xNi_xFe_1.9Mn_0.1O₄, with x = 0.2, 0.4, 0.6 and 0.8, were prepared by standard double sintering ceramic method. The spinel phase formation was confirmed by X-ray diffraction (XRD). The DC resistivity measurements with temperature indicate a semiconducting behavior showing a linear decrease with increasing temperature and the doping of Ni enhances the resistivity. Maximum resistivity of the order of 10⁹ Ω cm was found for composition x = 0.8. Room temperature dielectric constant measurements with frequency (100 Hz to 1 MHz), show usual dielectric dispersion. Also, the variation of room temperature AC conductivity as a function of frequency were studied and explained by using Maxwell–Wagner two-layer model. The studies on dielectric constant (′), loss tangent (tan δ) and AC conductivity (σ_AC), at four different frequencies (viz., 1, 10, 100 kHz and 1 MHz), with temperature were made.     

Study of (1 − x)(Mg0.6Zn0.4)0.95Co0.05TiO3–xCa0.61Nd0.26TiO3 microwave dielectrics

The microwave dielectric properties and the microstructures of the (1 − x)(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–xCa_0.61Nd_0.26TiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. Ca_0.61Nd_0.26TiO₃ has a large positive temperature coefficient of resonant frequency. (Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃ possesses a negative temperature coefficient of resonant frequency. By appropriately adjusting the x value in the (1 − x)(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–xCa_0.61Nd_0.26TiO₃ ceramic system, a near-zero τ_f value can be obtained. A new microwave dielectric material of 0.8(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–0.2Ca_0.61Nd_0.26TiO₃ possesses the excellent dielectric properties of a dielectric constant of 28.6, a Q × f value of 80,600 GHz and a temperature coefficient of resonant frequency of 4.1 ppm/°C and has a lower sintering temperature of 1250 °C.     

Improved high-Q microwave dielectric resonator using ZnO-doped La(Co1/2Ti1/2)O3 ceramics

The microwave dielectric properties and the microstructures of ZnO-doped La(Co_1/2Ti_1/2)O₃ ceramics prepared by conventional solid-state route have been studied. Doped with ZnO (up to 0.75 wt%) can effectively promote the densification of La(Co_1/2Ti_1/2)O₃ ceramics with low sintering temperature. At 1320 °C, La(Co_1/2Ti_1/2)O₃ ceramics with 0.75 wt% ZnO addition possesses a dielectric constant (_r) of 30.2, a Q × f value of 73,000 GHz (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −35 ppm/°C.     

Formation and properties of BaxFe3−xO4 with spinel structure by mechanochemical reaction of α-Fe2O3 and BaCO3

Magnetic Ba_xFe_3−xO₄ (x  0.23) with spinel structure was fabricated by ball milling of mixture of BaCO₃ and nonmagnetic α-Fe₂O₃ powders, and the molar ratio of BaCO₃ and α-Fe₂O₃ is 1:6. In the milling process, a mechanochemical reaction took place between BaCO₃ and α-Fe₂O₃, and Ba cation incorporated into α-Fe₂O₃ with rhombohedral structure to form a α-(Fe,Ba)₂O₃ solid solution. The Ba content in the α-(Fe,Ba)₂O₃ increased with increasing milling time, when the Ba content exceeded a limited solubility, the α-(Fe,Ba)₂O₃ transformed into a phase of Ba_xFe_3−xO₄ with spinel structure, where the Ba cation occupied an octahedral site or tetrahedral site. The product obtained in the balling process was different from that prepared in the annealing process at atmospheric pressure, which was BaFe₂O₄ with orthorhombic structure. Accompanying the crystal structure transition from α-(Fe,Ba)₂O₃ to Ba_xFe_3−xO₄, the magnetic properties also changed from nonmagnetism into ferromagnetism. The saturation magnetization was 53.3 emu/g and coercivity was 113.7 Oe. The mechanism of transitions of the crystal structure was discussed in the present work.     

Investigation of the microwave dielectric properties of Ca1−xMgxLa4Ti5O17 ceramics for application in coplanar patch antenna

In this paper, the dielectric properties of Ca_1−xMg_xLa₄Ti₅O₁₇ ceramics at microwave frequency have been studied. The diffraction peaks of Ca_(1−x)Mg_xLa₄Ti₅O₁₇ ceramics nearly unchanged with x increasing from 0 to 0.03. Similar X-ray diffraction peaks of Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic were observed at different sintering temperatures. A maximum density of 5.3 g/cm³ can be obtained for Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic sintered at 1500 °C for 4 h. A maximum dielectric constant (_r) and quality factor (Q × f) of Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic sintered at 1500 °C for 4 h are 56.3 and 12,300 GHz (at 6.4 GHz), respectively. A near-zero temperature coefficient of resonant frequency (τ_f) of −9.6 ppm/°C can be obtained for Ca_0.99Mg_0.01La₄Ti₅O₁₇ ceramic sintered at 1500 °C for 4 h. The measurement results for the aperture-coupled coplanar patch antenna at 2.5 GHz are presented. With this technique, a 3.33% bandwidth (return loss <−10 dB) with a center frequency at approximately 2.5 GHz has been successfully achieved.     

Monoclinic phases and stress-relief conditions in (1 − x)Pb(Mg1/3Nb2/3)TiO3–xPbTiO3 solid solutions

A comparative study on heterophase states in perovskite-type solid solutions of (1 − x)Pb(Mg_1/3Nb_2/3)TiO₃–xPbTiO₃ is carried out for compositions near the morphotropic phase boundary. The conditions for mechanical stress relief at elastic matching of phases are analysed at x = const in a wide temperature range. The heterophase states concerned with the presence of the intermediate monoclinic phase are interpreted using the domain state–interface diagrams calculated for x = 0.28, 0.32 and 0.34. It is shown that optimum volume fraction parameters of the domains in the monoclinic phase of the B type are varied in relatively wide ranges and promote complete stress relief with cubic–monoclinic phase coexistence. Two scenarios of stress relief at x = 0.32 are considered in connection with different heterophase states (either tetragonal–monoclinic of the B type or tetragonal–monoclinic of the C type) in a wide temperature range. Possibilities of elastic matching of two polydomain phases (tetragonal–monoclinic of the B type) with almost equal relative widths of the domains in these phases are shown for x = 0.34. The active role of domains of the monoclinic phases in stress relief and forming the planar unstrained interfaces is discussed.     

Optical properties of amorphous Ge28-xSe72Sbx thin films

Bulk glasses of formal composition Ge_28−xSe₇₂Sb_x with 0≤x≤28 were prepared by applying the quench technique. The optical transmission spectra—using a melt were measured in the range from 200 to 1200 nm for Ge_28−xSe₇₂Sb_x films which are prepared by thermal evaporation technique. A simple, straightforward procedure suggested by Swanepoel, which is based on the use of interference fringes, has been applied to calculate the film thickness. On other hand the driving absorption coefficient (α), consequently the band tail width E_e and the optical band gap have been estimated. The real (ε′) and imaginary parts (ε″) of the dielectric constant have been determined and the optical band gap can also be calculated as a function of imaginary part (ε″). The dispersion parameters such as E₀(single-oscillator energy), E_d (dispersive energy) and M₋₁, M₋₃ (moments) were discussed in terms of the single-oscillator Wemple–DiDomenico model.     

Phase transition-induced high electromechanical activity in [(K0.5Na0.5)1−xLix](Nb0.8Ta0.2)O3 lead-free ceramic system

A high electromechanical activity is observed in the [(K_0.5Na_0.5)_1−xLi_x](Nb_0.8Ta_0.2)O₃ (x = 0, 0.02, and 0.03) lead-free ceramic system at and around the orthorhombic (O)–tetragonal (T) phase transition temperature (T_O–T). This activity is found to originate from an O–T phase transition region at ambient temperature rather than a classical morphotropic phase boundary (MPB) region intrinsic in the Pb(Zr_1−xTi_x)O₃ (PZT) lead-based ceramic system. Li modification enables a large decrease in T_O–T instead of constituting a classical MPB. In contrast to the nearly temperature-independent classical MPB behavior in the PZT system, the strong temperature-dependent phase transition behavior in the system may impose a challenge to temperature demanding applications.     

Synthesis and crystal structure of Ba3Si4C2

SiC powder prepared by the Na flux method at 1023 K for 24 h and Ba were used as starting materials for synthesis of tribarium tetrasilicide acetylenide, Ba₃Si₄C₂. Single crystals of the compound were obtained by heating the starting materials with Na at 1123 K for 1 h and by cooling to 573 K at a cooling rate of −5.5 K/h. The single crystal X-ray diffraction peaks were indexed with tetragonal cell dimensions of a = 8.7693(4) and c = 12.3885(6) Å, space group I4/mcm (No.140). Ba₃Si₄C₂ has the Ba₃Ge₄C₂ type structure which can be described as a cluster-replacement derivative of perovskite (CaTiO₃), and contains isolated anion groups of slightly compressed [Si₄]⁴⁻ tetrahedra and [C₂]²⁻ dumbbells. The electrical conductivity measured for a not well-sintered polycrystalline sample was 2.6 × 10⁻²–7 × 10⁻³ S cm⁻¹ in the temperature range of 370–600 K and slightly increased with increasing temperature. The Seebeck coefficient showed negative values of around −200 to −300 μV K⁻¹.     

The influence of gallium on the magnetocaloric properties of Gd5Si2Ge2

Gd₅Si₂Ge₂ was alloyed with varying amounts of Ga to study its influence on the giant magnetocaloric effect. Investigations on Gd₅(Si_2−xGe_2−x)Ga_2x with 2x = 0.03, 0.05 and 0.13 were carried out using X-ray powder diffraction, temperature and magnetic field dependent magnetization measurements, and differential scanning calorimetry. We observe that as the Ga content increases, the temperature stability range of the monoclinic phase narrows, and the orthorhombic structure gains stability. This is expected to be related to the decrease in the (Si/Ge)(Si/Ge) bond distance in the monoclinic phase. The maximum entropy change for the parent compound at 270 K was found to be 9.8 J kg⁻¹ K⁻¹ in an applied field of 5 T. For 2x = 0.03, this value reduces to 8.5 J kg⁻¹ K⁻¹, and the temperature corresponding to the maximum entropy change shifts marginally to 278 K. For other 2x values, the maximum entropy change further decreases.     

Effect of potassium doping on the structural, magnetic and magnetocaloric properties of La0.7Sr0.3−xKxMnO3 perovskite manganites

We investigate the effect of potassium doping on the structural, magnetic and magnetocaloric properties of La_0.7Sr_0.3−xK_xMnO₃ (x = 0.05, 0.1, 0.15 and 0.2) powder samples. Our polycrystalline compounds were synthesized using the solid-state reaction at high temperature. X-ray diffraction characterizations showed that all our studied samples crystallize in the distorted rhombohedral system with space group. With increasing potassium content, the unit cell volume exhibits a broad maximum around x = 0.15. Magnetization measurements versus temperature showed that all our samples exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature T_C is found to decrease from 365 K for x = 0 to 328 K for x = 0.2 as well as the saturated magnetization M_sp which shifts from 3.68 μ_B/Mn for x = 0 to 3.05 μ_B/Mn for x = 0.2. The critical exponent γ defined as M_sp (T) = M_sp(0)[1−(T/T_C)]^γ is found to remain almost constant and equal to 0.33 for all our samples. The maximum of magnetic entropy changes |ΔS_max| of La_0.7Sr_0.3−xK_xMnO₃ for x = 0.05 and 0.15 is found to be respectively, 1.37 and 1.2 J kg⁻¹ K⁻¹ under a magnetic field change of 1 T.     

Self-propagating high-temperature synthesis with post-heat treatment of La1−xSrxFeO3 (x = 0–1) perovskite as catalyst for soot combustion

La_1−xSr_xFeO₃ (x = 0–1) perovskite, Sr-substituted LaFeO₃, was prepared by Self-propagating high-temperature synthesis (SHS) and its catalytic activity for soot combustion was experimentally examined in comparison with that of a conventional Pt/Al₂O₃ catalyst. The products were also characterized by XRD, FE-SEM, and BET specific surface area. The XRD analysis revealed that all the products had a perovskite phase as the major compound, together with intermediate phases with higher x values (x = 0.7–1). The BET specific surface area of the products increased with x. Moreover, the catalytic activity for soot combustion also increased with x, wherein the BET specific surface area appeared an appropriate index for explaining the observed activity. The sample with x = 0.8 exhibited the highest activity for soot combustion among all the SHS products. The soot combustion temperature of this product was as much as 100 °C lower than that of non-catalytic soot combustion. In other words, it had the same activity as that at only 20 °C higher, in comparison to conventional Pt/Al₂O₃ catalyst. More significantly, average apparent activation energy of sample with x = 0.8 calculated by Friedman method using TG/DTA was approximately 15 kJ/mol lower than that of Pt/Al₂O₃ catalyst. This result suggested that La_1−xSr_xFeO₃ has the possibility to be an alternative catalyst to Pt/Al₂O₃ catalyst.     

Synthesis and characterization of new pyrochlore solid solution Bi1.5Sb1.5Cu1−xMnxO7

A new pyrochlore solid solution with formula Bi_1.5Sb_1.5Cu_1−xMn_xO₇ has been synthesized using ceramic method at 1000 °C. The cell parameter decreases linearly with increasing manganese concentration. Rietveld refinements for (B_1.5Mn_0.5)(Sb_1.5Mn_0.5)O₇ compound using X-ray powder diffraction data confirmed an overall A₂B₂O₇ cubic pyrochlore structure with a = 10.42749 (4) Å and Fd-3m symmetry. The reliability factors are R_wp = 3.48%; R_p = 2.37%; R_exp = 1.65% and R_Bragg = 1.58%. The magnetic susceptibility measurements achieved between 4 and 300 K indicate a paramagnetic behaviour with an oxidation state “2+” of the manganese ion. The electric resistance measured using complex impedance spectroscopy method put in evidence a decrease of the electric resistance with the temperature, which reached 5 × 10² Ω at 675 K. Dielectric properties depend on the variation of frequency and temperature, results indicate a conductive compound.     

Structural, magnetic and electrical properties of (La0.70−xNdx)Sr0.30Mn0.70Cr0.30O3 with 0 ≤ x ≤ 0.30

The structural, magnetic and electrical properties of (La_0.70−xNd_x)Sr_0.30Mn_0.70Cr_0.30O₃ perovskites (0 ≤ x ≤ 0.30) prepared by the usual ceramic procedure were investigated. Structural Rietveld refinement revealed that these compounds crystallize in a rhombohedral perovskite structure when x = 0, 0.10 and 0.20, while for x = 0.30 the structure becomes orthorhombic (Pbnm). It was found that the substitution of La by Nd reduces the Curie temperature (T_C). The FC, ZFC, M(H) and AC susceptibility measurements show typical canted-antiferromagnetism for the Nd-doped samples, in which a ferromagnetic component coexists with predominant antiferromagnetic interactions. The values of the magnetization (M(H)) decrease very slightly when increasing the Nd content, compared to the undoped sample (M_S values at 5 T and 2 K are, respectively, 47.9, 47.3 and 47.5 emu/g for x = 0.10, 0.20 and 0.30, compared to 48.2 emu/g for x = 0), indicating that the Nd³⁺ contribution is negligible compared to the total moment of the ferromagnetic (Mn/Cr) network. The resistivity increases by several orders of magnitude with Nd-doping and the semi-conducting behaviour persists in the whole temperature range. The interaction between Mn⁴⁺–O–Cr³⁺and Cr³⁺–O–Cr³⁺ is responsible for the semi-conducting state.