Enhanced electrical properties in lead-free NBT–BT ceramics by series ST substitution

Comprehensive electrical properties of 0.94(Na_1/2Bi_1/2)TiO₃–0.06BaTiO₃ lead-free ceramics by doping series SrTiO₃ were investigated. High piezoelectric constant of 205 pC/N and electromechanical coupling factor of 0.34 were obtained due to the forming of the rhombohedral–tetragonal morphotropic phase boundary at x=0.02–0.06. Very large recoverable strain of 0.34% was obtained at x=0.10 due to the coexistence of ferroelectric and relaxor pseudocubic phases. A large electrocaloric effect (ΔT_max=1.71 K and ΔT/ΔE=0.34 K mm kV⁻¹ at 50 kV cm⁻¹) which determined by indirect measurements method was obtained at 120 °C at x=0.02, which is significantly higher than that of lead-free ferroelectric ceramics reported so far. Moreover, lower operating temperatures of 50 °C and 30 °C were proposed when x=0.10 and 0.20 with ΔT_max=0.79 K and 0.6 K, respectively. These properties added together indicate a promising material for applications in cooling systems and actuators.     

Microwave dielectric properties of low-fired Li2MnO3 ceramics co-doped with LiF–TiO2

Li₂MnO₃ ceramics co-doped with 2 wt% LiF and x wt% TiO₂ (x=0, 3, 5, 7, 10) were prepared by solid-state reaction for low-temperature co-fired ceramics (LTCC) applications. The sintering temperatures of Li₂MnO₃ ceramics were successfully lowered to 925°C due to the formation of a LiF liquid phase. Their temperature stability was improved by doping with TiO₂. A typical Li₂MnO₃-2 wt% LiF-5 wt% TiO₂ sample with well-densified microstructures displayed optimum dielectric properties (ε_r=13.8, Q×f= 23,270 GHz, τ_f=1.2 ppm/°C). Such sample was compatible with Ag electrodes, which suggests suitability of the developed material for LTCC applications in wireless communication systems.     

Effects of LiF addition on sintering behavior and microwave dielectric properties of (Mg0.95Zn0.05)2(Ti0.8Sn0.2)O4 ceramics

The effects of LiF addition on the sinterability and microwave dielectric properties of (Mg_0.95Zn_0.05)₂(Ti_0.8Sn_0.2)O₄ (MZTS) ceramics were investigated. A small amount of LiF addition can effectively lower the sintering temperature of MZTS from 1325 °C to 1150 °C due to the liquid phase effect and induce no apparent degradation of the microwave dielectric properties. With increasing LiF content, the apparent density and dielectric constant decreased gradually, the quality factor increased firstly and then decreased. In particular, MZTS–3.0 wt% LiF ceramics sintered at 1150 °C for 5 h exhibited good microwave dielectric properties of ?_r = 13.05, Q · f = 119,310 GHz (at 10 GHz) and τ_f = ?59.2 ppm/°C.     

Low temperature cofirable Ca[(Li1/3Nb2/3)0.95Zr0.15]O3+δ microwave dielectric ceramic with ZnO–B2O3–SiO2 frit

The sintering properties and microwave dielectric properties of Ca[(Li_1/3Nb_2/3)_1?xZr_3x]O_3+δ (x = 0.05, abbreviated as CLNZ) ceramic doped with ZBS frit are investigated for LTCC applications. XRD patterns and SEM photographs show that dense and single perovskite phase ceramics can be obtained with ZBS doping content of less than 10 wt%, before the Ca₂Nb₂O₇ pyrochlore phase begins to segregates. The results show that ZBS vitreous phase stays at the grain boundary in the final sintered ceramics, suggesting it acts as liquid phase lubrication during sintering, and has effectively lowered the sintering temperature of CLNZ ceramics from 1170 °C to 940 °C. The preferred orientation of CLNZ solid solution varies from (1 2 1) plane to (1 0 1) plane as ZBS content and sintering temperature increase. The optimal microwave dielectric properties of ?_r = 32.0, Q_f = 6.64 THz and τ_f = ?27.1 ppm/°C can be obtained in 15 wt% ZBS doped CLNZ ceramic when sintered at 940 °C for 4 h. The Ag-cofiring experiment clearly shows that no chemical reaction takes place between Ag and the ZBS-doped CLNZ ceramic, indicating its great potential applications in LTCC field.     

Effects of glass additions on the microstructure and dielectric properties of barium strontium titanate (BST) ceramics

Commercial glass frits (lead borosilicate glasses) were employed as the sintering aids to reduce the sintering temperatures of BST ceramics. The effects of the glass content and the sintering temperature on the microstructures, dielectric properties and tunabilities of BST ceramics have been investigated. Densification of BST ceramics of 5 wt% glass content becomes significant from sintering temperature of 1000 °C. The glass content shows a strong influence on the Curie temperature T_c, permittivity and the diffuse transition. X-ray results show all BST ceramics exhibit a perovskite structure and also the formation of a secondary phase, Ba₂TiSi₂O₈. The shift of BST diffraction peaks towards higher angle with increasing the glass content indicates the substitution of Pb²⁺ in Ba²⁺ site, which mainly accounts for the diffuse transition observed in these BST ceramics. BST ceramics with 10 wt% glass additives possess the highest tunability at all four sintering temperatures. A tunability of 12.2% at a bias field of 1 kV/mm was achieved for BST ceramics with 10 wt% glass content sintered at 900 °C.     

Effects of sintering parameters and Nd doping on the microwave dielectric properties of Y2O3 ceramics

Y₂O₃ ceramics with good dielectric properties were prepared via co-precipitation reaction and subsequent sintering in a muffle furnace. The effects of Nd doping and sintering temperature on microwave dielectric properties were studied. With the increase in sintering temperature, the density, quality factor (Q×f), and dielectric constant (ε_r) values of pure Y₂O₃ ceramics increased to the maximum and then gradually decreased. The Y₂O₃ ceramics sintered at 1500 °C for 4 h showed optimal dielectric properties: ε_r=10.76, Q×f=82, 188 GHz, and τ_f=−54.4 ppm/°C. With the addition of Nd dopant, the Q×f values, ε_r, and τ_f of the Nd: Y₂O₃ ceramics apparently increased, but excessive amount degraded the quality factor. The Y₂O₃ ceramics with 2 at% Nd₂O₃ sintered at 1460 °C displayed good microwave dielectric properties: ε_r=10.4, Q×f=94, 149 GHz and τ_f=−46.2 ppm/°C.     

Fabrication and high-temperature thermoelectric properties of Ti-doped Sr0.9La0.1TiO3 ceramics

Ti-doped Sr_0.9La_0.1TiO₃ ceramics with high density were successfully prepared in argon atmosphere by conventional solid state reaction. The influences of titanium doping content on the microstructure and thermoelectric properties were investigated. The results showed that titanium was oxidized during the calcination procedure. TiO₂ phase survived and coexisted with Sr_0.9La_0.1TiO₃ phase in the sintered ceramics. The Seebeck coefficients were increased from −163 to −259 μV/K as the temperature increased from 350 K to 1073 K. The thermal conductivity can be significantly reduced by doping Ti. Thermoelectric figure of merit (ZT) first decreased and then increased with increasing Ti doping content. Ceramics showed the best thermoelectric properties when Ti doping amount was 5 wt%, the maximum PF was 7.13 μW/K²/cm, and ZT value was 0.144 at 1073 K.     

Structural,dielectric and ferroelectric properties of lead-free Ba0.85Ca0.15Zr0.10Ti0.90O3 ceramics prepared by Plasma Activated Sintering

Lead-free Ba_0.85Ca_0.15Zr_0.10Ti_0.90O₃ (BCZT) ceramics were prepared by Plasma Activated Sintering (PAS). The influence of PAS sintering temperature on the crystalline phase, microstructure, and, dielectric and ferroelectric properties of BCZT ceramics were studied. The phase structure of BCZT ceramics first changed from rhombohedral phase to the coexistence of rhombohedral and tetragonal phases and then to tetragonal phase as the sintering temperature increased. Microstructural characterization of BCZT ceramics indicated that PAS can obtain a compact microstructure at lower temperatures of 1150–1300 °C compared with that from common pressureless sintering. The BCZT ceramics showed different degrees of diffuseness with increased temperature, and the diffuseness exponents C are all approximately on the order of 10⁵ °C. The dielectric and ferroelectric properties of BCZT ceramics were enhanced with increased sintering temperature. BCZT ceramics sintered at 1250 °C exhibited optimum properties of room-temperature ε_r=2863, ε_m=6650, and 2P_r=25.24 μC/cm², resulting from the relatively higher tetragonal phase content of the MPB between tetragonal and rhombohedral phases together with a compact microstructure.     

High performance Aurivillius-type bismuth titanate niobate (Bi3TiNbO9) piezoelectric ceramics for high temperature applications

This paper reports the significant improved piezoelectric properties of high temperature bismuth titanate niobate (Bi₃TiNbO₉, BTN) polycrystalline ceramics. The piezoelectric performance of BTN ceramics is significantly enhanced by cerium modifications. The dielectric measurements indicate that the Curie temperature T_c gradually decreases over the temperature range of 907–889 °C with cerium contents increasing up to 0.7 wt%. The BTN-5Ce (BTN+0.5 wt% CeO₂) exhibits optimized piezoelectric properties with a piezoelectric constant d₃₃ of 16 pC/N, which is five times the value of unmodified BTN (d₃₃~3 pC/N), while BTN-5Ce maintains a high Curie temperature T_c of 894 °C. The temperature-dependent electrical impedance and electromechanical coupling factors (k_p, and k_t) reveal that the BTN-5Ce exhibits thermally stable electromechanical coupling characteristics up to 500 °C but significantly deteriorates at 600 °C due to high conductivity at a higher temperature. The thermally stable electromechanical properties in combination with the ceramics׳ high electrical resistivity (10⁶ Ω cm at 500 °C) and high Curie temperature (~900 °C) demonstrate that cerium-modified BTN ceramics are good materials for high temperature sensing applications.     

Microwave dielectric properties and low-temperature sintering of Ba3Ti4Nb4O21 ceramics with B2O3 and CuO additions

The low sintering temperature and the good dielectric properties such as high dielectric constant (ɛ_r), high quality factor (Q × f) and small temperature coefficient of resonant frequency (τ_f) are required for the application of chip passive components in the wireless communication technologies. In the present study, the sintering behaviors and dielectric properties of Ba₃Ti₄Nb₄O₂₁ ceramics were investigated as a function of B₂O₃–CuO content. Ba₃Ti₄Nb₄O₂₁ ceramics with B₂O₃ or CuO addition could be sintered above 1100 °C. However, the additions of both B₂O₃ and CuO successfully reduced the sintering temperature of Ba₃Ti₄Nb₄O₂₁ ceramics from 1350 to 900 °C without detriment to the microwave dielectric properties. From the X-ray diffraction (XRD) studies, the sintering behaviors and the microwave dielectric properties of low-fired Ba₃Ti₄Nb₄O₂₁ ceramics were examined and discussed in the formation of the secondary phases. The Ba₃Ti₄Nb₄O₂₁ sample with 1 wt% B₂O₃ and 3 wt% CuO addition, sintered at 900 °C for 2 h, had the good dielectric properties: ɛ_r = 65, Q × f = 16,000 GHz and τ_f = 101 ppm/°C.     

Effects of Li2CO3 and Sm2O3 additives on low-temperature sintering and piezoelectric properties of PZN-PZT ceramics

To assist the development of applications for multilayer piezoelectric devices, the low-temperature sintering piezoelectric ceramics of 0.3Pb(Zn_1/3Nb_2/3)O₃-0.7Pb(Zr_0.49Ti_0.51)O₃ with Li₂CO₃ and Sm₂O₃ additives were fabricated by a conventional solid-state reaction, and their structural and piezoelectric properties were studied. With the addition of Li₂CO₃, the minimum sintering temperature of 0.3Pb(Zn_1/3Nb_2/3)O₃-0.7Pb(Zr_0.49Ti_0.51)O₃ piezoelectric ceramics was reduced from 1125 °C to 950 °C through the formation of a liquid phase and its piezoelectric properties showed almost no degradation. When the sintering temperature was below 950 °C, however, the piezoelectric properties degraded obviously. The additional Sm₂O₃ resulted in a significant improvement in the piezoelectric properties of 0.3Pb(Zn_1/3Nb_2/3)O₃-0.7Pb(Zr_0.49Ti_0.51)O₃ ceramic with added Li₂CO₃. When sintered at 900 °C, the optimized properties of the 0.3Pb(Zn_1/3Nb_2/3)O₃-0.7Pb(Zr_0.49Ti_0.51)O₃ piezoelectric ceramic with 0.3 wt% Li₂CO₃ and 0.3 wt% Sm₂O₃ were obtained as d₃₃ = 483 pC/N, k₃₁ = 0.376, Q_m = 73, ɛ_r = 2524, tan δ = 0.0178.     

Co-doping effects of A-site Y3+ and B-site Al3+ on the microstructures and dielectric properties of CaCu3Ti4O12 ceramics

Different doping elements have been used to reduce the dielectric losses of CaCu₃Ti₄O₁₂ ceramics, but their dielectric constants usually are undesirably decreased. This work intends to reduce their dielectric losses and simultaneously enhance their dielectric constants by co-doping Y³⁺ as a donor at A site and Al³⁺ as an acceptor at B site for substituting Ca²⁺ and Ti⁴⁺, respectively. Samples with different doping concentrations x = 0, 0.01, 0.02, 0.03, 0.05 and 0.07 have been prepared. It has been shown that their dielectric losses are generally reduced and their dielectric constants are simultaneously enhanced across the frequency range up to 1 MHz. The doped sample with x = 0.05 exhibits the highest dielectric constant, which is well over 10⁴ for frequency up to 1 MHz and is about 20% higher than the undoped sample. Impedance spectra indicate that the doped samples have much higher grain boundary resistance than the undoped one.     

“Dark hole” cure in Ba4.2Nd9.2Ti18O54 microwave dielectric ceramics

Large size Ba_4.2Nd_9.2Ti₁₈O₅₄ (BNT) ceramics doped with MnCO₃, CuO and CoO were prepared by the conventional solid-state method. Only a single BaNd₂Ti₄O₁₂ phase was formed in all samples. No second phase was found in the XRD patterns. The bulk density increases slightly because of the dopants. The SEM results showed that the grain size of Mn²⁺and Cu²⁺-doped BNT ceramics became larger with the increasing amount of dopants. The permittivity of all samples stays the same. However, the Q×f value of BNT ceramics increases by doping, especially with Mn²⁺ ions. The conductivity of BNT ceramic doped with Mn²⁺(0.5 mol‰) under high temperature is lower than that without doping. There are fewer defects in Mn²⁺-doped BNT ceramics. The XPS results indicated that Ti reduction was suppressed in BNT ceramics doped with 0.5 mol‰ Mn²⁺. BNT ceramics doped with 0.5 mol‰ Mn²⁺ ions sintered at 1320 °C for 2 h exhibited good microwave dielectric properties, with ε_r=88.67, Q×f=7408 GHz and τf = 82.98 ppm/°C.     

Silver cofirable (Ca0.9, Mg0.1)SiO3 microwave dielectric ceramics with Li2CO3–Bi2O3 additive

The effects of Li₂CO₃–Bi₂O₃ (LB) additive on the microstructure, phase formation, microwave dielectric properties and applicability for low-temperature co-fired ceramics (LTCC) technology of (Ca_0.9Mg_0.1)SiO₃ (CMS) ceramics were investigated. The sintering temperature of the CMS ceramics was reduced from 1290 °C to 890 °C by the addition of LB. Secondary phases SiO₂ and Bi₄(SiO₄)₃ were detected when LB content was less than 9 wt%. Low melting point liquid phases were formed when LB content was 11–14 wt%. The Q_f value initially increased with the addition of LB and attained the maximum value for the 9 wt% LB-doped CMS ceramic. When the LB content exceeded 9 wt%, the Q_f value decreased because of the presence of liquid phase and abnormal growth of grains. ?_r of 6.92, Q_f of 27,600 GHz and τ_f of ?43.6 ppm/°C were obtained for 9 wt% LB-doped CMS ceramics sintered at 890 °C for 2 h. Also the ceramics can be well co-fired with Ag electrode.     

Anti-reduction of Ti4+ in Ba4.2Sm9.2Ti18O54 ceramics by doping with MgO,Al2O3 and MnO2

The anti-reduction of Ti⁴⁺ ions in Ba_4.2Sm_9.2Ti₁₈O₅₄ (BST) ceramics at high sintering temperature over 1300 °C was investigated. MgO, Al₂O₃ and MnO₂ were added separately to suppress the reduction of Ti⁴⁺ ions so as to improve the microwave dielectric properties of BST ceramics. The microstructure of BST ceramics was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was used to study the electroconductivity of BST ceramics and valency changes of Ti ions. The results showed that MgO or Al₂O₃, when acting as an acceptor, could effectively suppress the reduction of Ti⁴⁺ ions and significantly improve the Q × f values of BST ceramics at the cost of dielectric constant. Meanwhile, MnO₂ as an oxidant had also improved the Q × f values but with no decrease in dielectric constant. Excellent microwave dielectric properties were achieved in Ba_4.2Sm_9.2Ti₁₈O₅₄ ceramics doped with 0.2 wt.% Al₂O₃ sintered at 1340 °C for 3 h: ?_r = 76.9, Q × f = 10,120 GHz and τ_f  = ?22.7 ppm/°C.     

The structure and properties of 0.95MgTiO3–0.05CaTiO3 ceramics co-doped with ZnO–ZrO2

The (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were prepared by conventional solid-state route. The dielectric properties and structure of (Mg_0.93Ca_0.05Zn_0.02)(Ti_1?xZr_x)O₃ ceramics were investigated. It has been found that MgTiO₃ and CaTiO₃ are the main phases and a second phase CaZrTi₂O₇ appeared in 95MCT ceramics co-doped with Zn–Zr. With Zn–Zr additive, the sintering temperature of 95MCT ceramics can be reduced to 1300 °C, and adjust the temperature coefficient of dielectric constant. With the increasing of Zr content, dielectric constant ?_r decrease from 22.6 to 19.91 and the temperature coefficient of dielectric constant α_c from 5.93 to 2.52 ppm/°C when x = 0.01, 0.02, 0.03 and 0.04 mol respectively. The 95MCT ceramics with x = 0.02 has a dielectric constant ?_r of 22.02, a dielectric loss of 2.78 × 10^?4 and a temperature coefficient of dielectric constant α_c value of 2.98 ppm/°C.     

Lead-free (Ba0.85Ca0.15)(Ti0.9Zr0.1)O3–CeO2 ceramics with high piezoelectric coefficient obtained by low-temperature sintering

Lead-free (Ba_0.85Ca_0.15)(Ti_0.9Zr_0.1)O₃–x%CeO₂(BCZT–xCe) piezoelectric ceramics have been prepared by the traditional ceramic process and the effects of CeO₂ addition on their phase structure and piezoelectric properties have been studied. The addition of CeO₂ significantly improves the sinterability of BCZT ceramics which results in a reduction of sintering temperature from 1540 °C to 1350 °C without sacrificing the high piezoelectric properties. X-ray diffraction data show that CeO₂ diffuses into the lattice of BCZT and a pure perovskite phase is formed. SEM images indicate that a small addition of CeO₂ greatly affects the microstructure. Main piezoelectric parameters are optimized at around x = 0.04 wt% with a high piezoelectric coefficient (d₃₃ = 600 pC/N), a planar electromechanical coefficient (k_p = 51%), a high dielectric constant (?_r = 4843) and a low dissipation factor (tan δ = 0.012) at 1 kHz, which indicates that the BCZT–xCe ceramics are promising for lead-free practical applications.     

Heteroepitaxial growth of Cu2O films on Nb-SrTiO3 substrates and their photovoltaic properties

In this paper, p-type Cu₂O thin films have been epitaxially grown on n-type semiconducting (001) oriented Nb-SrTiO₃ (NSTO) substrates with different Nb doping concentration by pulsed laser deposition technique. X-ray diffraction and high resolution transmission electron microscopy reveal a cube-on-cube epitaxial relationship between Cu₂O and NSTO. It is found that the deposition temperature, the thickness of Cu₂O films and the Nb doping concentration of NSTO substrates have critical impact on the photovoltaic (PV) properties of the Cu₂O/NSTO heterojunction devices. A maximum PV performance is observed in ITO/Cu₂O/NSTO device when the deposition temperature, film thickness and Nb doping concentration of NSTO are 550 °C, 76 nm, and 0.7 wt% NSTO, respectively. The optimized PV output corresponds to the open circuit voltage, short-circuit current density, fill factor and photovoltaic conversion efficiency about 0.45 V, 1.1 mA/cm², 46% and 0.23%,respectively. This work offers an insight into the strategy for developing and designing novel optoelectronics of NSTO-based oxide heterostructures.     

Densification behavior and electrical properties of CuO-doped Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 ternary ceramics

CuO modified Pb(In_1/2Nb_1/2)O₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PIN–PMN–PT) ternary relaxor based ferroelectrics with the composition near the morphotropic phase boundary were synthesized by two-step columbite precursor method. The introduction of CuO significantly improved the sinterability of PIN–PMN–PT ceramics, resulting in the full densification of samples at lower sintering temperatures. It also profoundly modified the crystal structure and fracture mode of the ceramics. Properly increasing CuO content led to the disappearance of rhombohedral-tetragonal phase transition, remarkably improved the Curie temperature (T_c), and made the ceramics more relaxorlike. The ternary ceramics doped with 0.25 wt% CuO possessed optimum piezoelectric properties (d₃₃=584 pC/N, d₃₃^*=948 pC/N, and k_p=0.68), high ferroelectric properties (E_c=9.9 kV/cm, and P_r=33.1 μC/cm²), low dielectric loss (tan δ=0.9%), and wider temperature usage range (T_c=225 °C). The obtained properties are much higher than those of previously reported PIN–PMN–PT based ceramics, indicating that CuO doped PIN–PMN–PT is a promising candidate for electromechanical applications with high performance and wide temperature/electric field usage ranges.     

Temperature stability and high-Qf of low temperature firing Mg2SiO4–Li2TiO3 microwave dielectric ceramics

In this work, a series of low-temperature-firing (1−x)Mg₂SiO₄–xLi₂TiO₃–8 wt% LiF (x = 35–85 wt%) microwave dielectric ceramics was prepared through conventional solid state reaction. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses showed that the Li₂TiO₃ phase was transformed into cubic phase LiTiO₂ phase and secondary phase Li₂TiSiO₅. Partial substitution of Mg²⁺ ions for Ti³⁺ ions or Li⁺Ti³⁺ ions increased the cell volume of the LiTiO₂ phase. The dense microstructures were obtained in low Li₂TiO₃ content (x ≤ 65 wt%) samples sintered at 900 °C, whereas the small quantity of pores presented in high Li₂TiO₃ content (x ≥ 75 wt%) samples sintered at 900 °C and low Li₂TiO₃ content (x = 45 wt%) sintered at 850 and 950 °C. Samples at x = 45 wt% under sintering at 900 °C for 4 h showed excellent microwave dielectric properties of ε_r = 10.7, high Q × f = 237,400 GHz and near-zero τ_f = − 3.0 ppm/°C. The ceramic also exhibited excellent chemical compatibility with Ag. Thus, the fabricated material could be a possible candidate for low temperature co-fired ceramic (LTCC) applications.