Microstructure, dielectric and ferroelectric properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 (NBTB) and 0.05BiFeO3-0.95NBTB ceramics: Effect of sintering atmosphere

0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ (NBTB) and 0.05BiFeO₃-0.95NBTB (BF-doped NBTB) lead-free ceramics were prepared by solid state reaction method. The ceramics were sintered at 1180 °C for 2 h in O₂ and N₂. All ceramics exhibited a single phase of perovskite structure. Relative amount of tetragonal phase was related to the sintering atmospheres. Both grain size and shape were influenced by the sintering atmospheres. Sintering the ceramics in N₂ weakened their dielectric anomalies corresponding to the transition from ferroelectric phase to the so-called “intermediate phase”. When the NBTB and BF-doped NBTB ceramics were sintered in N₂, their maximum dielectric constant and the degree of diffuseness of the transition from the “intermediate phase” to paraelectric phase increased, but their Curie temperatures decreased. The difference in dielectric properties of the ceramics sintered in different atmospheres was closely related to the difference in oxygen vacancy concentration. The correlation between ferroelectric properties and sintering atmospheres is associated with a competing effect among oxygen vacancy concentration, A-site vacancy concentration and percentage of tetragonal phase.     

Sol-gel derived (Li, Ta, Sb) modified sodium potassium niobate ceramics: Processing and piezoelectric properties

Nano-scaled lead-free perovskite powders (50-100 nm) with a composition of (Na_0.52K_0.4425)(Nb_0.8825Sb_0.08)O₃-0.0375LiTaO₃ (NKNS-3.75LT) have been synthesized via a modified low-cost citrate sol-gel route, in which Nb₂O₅ and Ta₂O₅ were transformed into stable and soluble chelate complexes to replace costly metal alkoxides. The thermal decomposition process and structural transformation behavior of the as-prepared xerogel was investigated by means of thermo-gravitometry and differential scanning calorimetry, X-ray diffractometry and Fourier-transform infrared spectrometry. The results indicated that the ideal crystallization of the NKNS-3.75LT powder with a single perovskite structure occurs at 500-600 °C, significantly lower than that for a conventional mixed-oxide route. Transmission electron microscopy observation showed that the crystalline powder has particle sizes in the range of 30-100 nm and exhibits polyhedron-like morphology, probably resulting in a specific grain growth behavior upon densification. The sol-gel derived NKNS-3.75LT ceramics sintered at 1090 °C exhibit excellent piezoelectric properties of d₃₃ ∼418 pC/N and k_p ∼0.54 for NKN based random ceramics.     

Phase pure synthesis of BiFeO3 nanopowders using diverse precursor via co-precipitation method

Amorphous powder of BiFeO₃ (BFO) was synthesized at low-temperature (80 °C) by co-precipitation method. Optimal synthesis conditions for phase pure BFO were obtained. Powders were calcined in the temperature range from 400 to 600 °C for 1 h. Iso-statically pressed powder was sintered at 500 °C for 2 h. Differential scanning calorimetric thermo-gram guided for phase transition, crystallization and melting temperatures. X-ray diffraction confirmed the amorphous nature of as synthesized powder and phase formation of calcined powders. Calcination at temperature ≥400 °C resulted in nano crystalline powders with perovskite structure. Average crystallite size increased with the increase in calcination temperature. Scanning electron microscopic studies revealed dense granular microstructure of the sintered samples. The sintered samples exhibited high dc resistivity at room temperature which decreased with the increase in temperature. Dielectric constant, dielectric loss tangent and ac conductivity measurements were carried out in the frequency range (10 Hz to 2 MHz). The samples responded weak electric and magnetic polarization at room temperature with unsaturated and hysteresis free loops, respectively.     

Fabrication of Nd:YAG transparent ceramics with both TEOS and MgO additives

Neodymium doped YAG transparent ceramics were fabricated by vacuum reactive sintering method using commercial α-Al₂O₃, Y₂O₃ and Nd₂O₃ powders as the starting materials with both tetraethyl orthosilicate (TEOS) and MgO as sintering aids. The morphologies and microstructure of the powders and Nd:YAG transparent ceramics were investigated. Fully dense Nd:YAG ceramics with average grain size of ∼10 μm were obtained by vacuum sintering at 1780 °C for 8 h. No pores and grain-boundary phases were observed. The in-line transmittance of the ceramic was 83.8% at 1064 nm.     

Microstructure and electrical properties of PLZT ceramics from Pb3O4 as the lead source

In this paper, Pb₃O₄, rather than conventional PbO, was introduced in the solid phase synthesis of (Pb_0.92La_0.08)(Zr_0.35Ti_0.65)_0.98O₃ (PLZT) ceramics. The microstructure and electrical properties of the PLZT ceramics were investigated. Results showed that due to the activity of Pb₃O₄, pyrochlore phase PLZT was depressed effectively. The PLZT ceramics with 5.0 wt.% excess Pb₃O₄ in the original materials had a well-crystallized pure perovskite structure when being sintered at 1100 °C for 6 h. In addition, the amount of Pb₃O₄ had an influence on the electrical properties of the PLZT ceramics. Residual polarization as high as 36.9 μC/cm² was obtained from the samples with 5.0 wt.% excess Pb₃O₄.     

High permittivity and low dielectric loss of the Ca1−xSrxCu3Ti4O12 ceramics

The Ca_1−xSr_xCu₃Ti₄O₁₂ (CSCTO) giant dielectric ceramics were prepared by conventional solid-state method. X-ray diffraction patterns revealed that a small amount of Sr²⁺ (x < 0.2) had no obvious effect on the phase structure of the CSCTO ceramics, while with increasing the Sr²⁺ content, a second phase of SrTiO₃ appeared. Electrical properties of CSCTO ceramics greatly depended on the Sr²⁺ content. The Ca_0.9Sr_0.1Cu₃Ti₄O₁₂ ceramics exhibited a higher permittivity (71,153) and lower dielectric loss (0.022) when measured at 1 kHz at room temperature. The ceramics also performed good temperature stability in the temperature range from −50 °C to 100 °C at 1 kHz. By impedance spectroscopy analysis, all compounds were found to be electrically heterogeneous, showing semiconducting grains and insulating grain boundaries. The grain resistance was 1.28 Ω and the grain boundary resistance was 1.31 × 10⁵ Ω. All the results indicated that the Ca_0.9Sr_0.1Cu₃Ti₄O₁₂ ceramics were very promising materials with higher permittivity for practical applications.     

Effect of A site and B site doping on structural, thermal, and dielectric properties of BiFeO3 ceramics

Polycrystalline Bi_1−xBa_xFe_1−yM_yO₃ (M = Co and Mn; x = 0.1, y = 0.1) were synthesized by solid-state route method to study the compositional driven structural transformations in multiferroics. Room temperature X-ray diffraction patterns confirmed the formation of perovskite structure. Rietveld-refined crystal structure parameters revealed the existence of rhombohedral R3c symmetry for both the samples. The samples were found to be nearly free from any other secondary phases. Raman analysis reveals that Ba atom substitutes Bi site and Mn and Co atom substitutes Fe site into the BiFeO₃ with the shifting of phonon modes. The red shift is attributed to Co or Mn doping where as blue shift occurs from Ba doping. The differential scanning calorimetry reveals the corresponding Neel temperature 370 °C and 326 °C for Co and Mn doped samples. Ba and Co substitution with x = 0.1 and y = 0.1 has not affected the Neel temperature of the parent BiFeO₃ as well of Ba and Mn substitution. The variation of frequency dispersion in permittivity and loss pattern due to A-site and B-site substitution in BiFeO₃ was observed in the dielectric response curve.     

Thermal and electrical properties of TiB2–MoSi2

The present communication reports the effect of MoSi₂ addition on high temperature thermal conductivity and room temperature (RT) electrical properties of TiB₂. The thermal diffusivity and the thermal conductivity of the hot pressed TiB₂–MoSi₂ samples were measured over a range from room temperature to 1000 °C using the laser-flash technique, while electrical resistivity was measured at RT using a four linear probe method. The reciprocal of thermal diffusivity of TiB₂ samples exhibit linear dependence on temperature and the measured thermal conductivity of TiB₂-2.5% MoSi₂ composites correlate well with the theoretical predictions from Hashin’s model and Hasselman and Johnson’s model. A common observation is that the thermal conductivity of all the samples slightly increases with temperature (up to 200 °C) and then decreases with further increasing temperature. It is interesting to note that both the thermal conductivity and electrical conductivity of TiB₂ samples enhanced with the addition of 2.5 wt.% MoSi₂ sinter additive. Among all the samples, TiB₂-2.5 wt.% MoSi₂ ceramics measured with high thermal conductivity (77 W/mK) and low electrical resistivity (12 μΩ-cm) at room temperature. Such an improvement in properties can be attributed to its high density and low volume fraction of porosity. On the other hand, both the thermal and electrical properties of TiB₂ were adversely affected with further increasing the amount of MoSi₂ (10 wt.%).     

Effect of Cr7C3 on the mechanical, thermal, and electrical properties of Cr2AlC

In this work, phase pure Cr₂AlC and impure Cr₂AlC with Cr₇C₃ have been fabricated to investigate the mechanical, thermal, and electrical properties. The thermal expansion coefficient is determined as 1.25 × 10⁻⁵ K⁻¹ in the temperature range of 25-1200 °C. The thermal conductivity of the Cr₂AlC is 15.73 W/m K when it is measured at 200 °C. With increasing temperature from 25 °C to 900 °C, the electrical conductivity of Cr₂AlC decreases from 1.8 × 10⁶ Ω⁻¹ m⁻¹ to 5.6 × 10⁵ Ω⁻¹ m⁻¹. For the impure phase of Cr₇C₃, it has a strengthening and embrittlement effect on the bulk Cr₂AlC. And the Cr₂AlC with Cr₇C₃ would result in a lower high-temperature thermal expansion coefficient, thermal conductivity, specific heat capacity and electrical conductivity.     

Electrical, magnetic, and magnetoelectric characterization of fine-grained Pb(Zr0.53Ti0.47)O3-(Ni0.5Zn0.5)Fe2O4 composite ceramics

Fine-grained Pb(Zr_0.53Ti_0.47)O₃-(Ni_0.5Zn_0.5)Fe₂O₄ (PZT-NZFO) magnetoelectric (ME) composite ceramics were fabricated by a modified hybrid process at a low sintering temperature of 900 °C. Well-controlled crystallized grain size and homogeneous microstructure with a good mixture of two phases were observed in the ceramics. The ceramics show coexistence of ferrimagnetic and ferroelectric phases with well-formed ferromagnetic and ferroelectric hysteresis loops at room temperature. A significant ME effect was observed with a ME coefficient of 0.537 V cm⁻¹ Oe⁻¹ in the vicinity of electromechanical resonance. In addition, high capacitance can be obtained at low frequency, and magnetic properties in the ceramics can be tailored by the grain size of the ferromagnetic particles in a simple and flexible way.     

Effect of addition of Ba(W0.5Cu0.5)O3 in Pb(Mg1/3Nb2/3)O3-Pb(Zn1/3Nb2/3)O3-Pb(Zr0.52Ti0.48)O3 ceramics on the sintering temperature, electrical properties and phase transition

In this work, we report on the Pb(Mg_1/3Nb_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃-Pb(Zr_0.52Ti_0.48)O₃ (PMN-PZN-PZT) ceramics with Ba(W_0.5Cu_0.5)O₃ as the sintering aid that was manufactured in order to develop the low-temperature sintering materials for piezoelectric device applications. The phase transition, microstructure, dielectric, piezoelectric properties, and the temperature stability of the ceramics were investigated. The results showed that the addition of Ba(W_0.5Cu_0.5)O₃ significantly improved the sintering temperature of PMN-PZN-PZT ceramics and could lower the sintering temperature from 1005 to 920 °C. Besides, the obtained Ba(W_0.5Cu_0.5)O₃-doped ceramics sintered at 920 °C have optimized electrical properties, which are listed as follows: (K_p = 0.63, Q_m = 1415 and d₃₃ = 351 pC/N), and high depolarization temperature above 320 °C. These results indicated that this material was a promising candidate for high-power multilayer piezoelectric device applications.     

Effect of annealing on microstructure of CuO-doped (Zr0.8Sn0.2)TiO4

Zirconium tin titanate ceramics, and specifically the (Zr_0.8Sn_0.2)TiO₄ (ZST) are the preferred materials for use in resonators or filters working at microwave frequency. However, the problem with these materials is their high sintering temperature which is as high as 1550 °C. In this research, the influence of different amounts of CuO additions on the densification and microstructure evaluations of ZST ceramics modified with 1 wt% ZnO has been studied in two different states: before and after annealing. Results showed that annealing process at 1250 °C resulted in some changes in density, grain size and shape due to the rearrangement of liquid phase and its reactivation, but no secondary phase was detected. Also, variations in lattice parameters during annealing compared to those of as-sintered specimens suggested the possibility of disorder-order transformation during long time high temperature heat treatment process.     

Electrical and thermal characterization of Sm doped ceria electrolytes synthesized by combustion technique

Nanocrystalline samarium doped ceria electrolyte [Ce_0.9Sm_0.1O_1.95] was synthesized by citrate gel combustion technique involving mixtures of cerium nitrate oxidizer (O) and citric acid fuel (F) taken in the ratio of O/F = 1. The as-combusted precursors were calcined at 700 °C/2 h to obtain fully crystalline ceria nano particles. It was further made into cylindrical pellets by compaction and sintered at 1200 °C with different soaking periods of 2, 4 and 6 h. The sintered ceria was characterized for the microstructures, electrical conductivity, thermal conductivity and thermal diffusivity properties. In addition, the combustion derived ceria powder was also analysed for the crystallinity, BET surface area, particle size and powder morphology. Sintered ceria samples attained nearly 98% of the theoretical density at 1200 °C/6 h. The sintered microstructures exhibit dense ceria grains of size less than 500 nm. The electrical conductivity measurements showed the conductivity value of the order of 10⁻² S cm⁻¹ at 600 °C with activation energy of 0.84 eV between the temperatures 100 and 650 °C for ceria samples sintered at 1200 °C for 6 h. The room temperature thermal diffusivity and thermal conductivity values were determined as 0.5 × 10⁻⁶ m² s⁻¹ and 1.2 W m⁻¹ K⁻¹, respectively.     

Thermal properties of oxides with magnetoplumbite structure for advanced thermal barrier coatings

Oxides having magnetoplumbite structure are promising candidate materials for applications as high temperature thermal barrier coatings because of their high thermal stability, high thermal expansion, and low thermal conductivity. In this study, powders of LaMgAl₁₁O₁₉, GdMgAl₁₁O₁₉, SmMgAl₁₁O₁₉, and Gd_0.7Yb_0.3MgAl₁₁O₁₉ magnetoplumbite oxides were synthesized by citric acid sol-gel method and hot-pressed into disk specimens. The thermal expansion coefficients (CTE) of these oxide materials were measured from room temperature to 1500 °C. The average CTE value was found to be ∼ 9.6 × 10^− 6/C. Thermal conductivity of these magnetoplumbite-based oxide materials was also evaluated using steady-state laser heat flux test method. The effects of doping on thermal properties were also examined. Thermal conductivity of the doped Gd_0.7Yb_0.3MgAl₁₁O₁₉ composition was found to be lower than that of the undoped GdMgAl₁₁O₁₉. In contrast, thermal expansion coefficient was found to be independent of the oxide composition and appears to be controlled by the magnetoplumbite crystal structure. Preliminary results of thermal conductivity testing at 1600 °C for LaMgAl₁₁O₁₉ and LaMnAl₁₁O₁₉ magnetoplumbite oxide coatings plasma-sprayed on NiCrAlY/Rene N5 superalloy substrates are also presented. The plasma-sprayed coatings did not sinter even at temperatures as high as 1600 °C.     

Microwave properties of Ba(Zn1/3Ta2/3)O3 dielectric resonators

Ba(Zn_1/3Ta_2/3)O₃ (BZT) dielectric resonators were prepared by solid-state reaction. The starting materials were BaCO₃, ZnO, and Ta₂O₅ powders with high purity. The double calcined BZT pellets were sintered in air at temperatures of 1575, 1600, 1625, and 1650 °C for 4 h. The X-ray diffraction data allowed the study of the unit cell distortion degree and the presence of the secondary phases. A long-range order with a 2:1 ratio of Ta and Zn cations on the octahedral positions of the perovskite structure was observed with the increase of the sintering temperature. The dielectric constant of BZT resonators measured around 6 GHz was between 26 and 28. High values of Q × f product (120 THz) were obtained for BZT resonators sintered at 1650 °C/4 h. The temperature coefficient of the resonance frequency exhibits positive values less than 6 ppm/°C. The achieved dielectric parameters recommend BZT dielectric resonators for microwave and millimeter wave applications.     

Complex impedance analysis of (Y2O3 + CeO2)-YCr0.5Mn0.5O3 composite NTC ceramics

Ceramic compositions based on (aY₂O₃ + bCeO₂)-0.4YCr_0.5Mn_0.5O₃ (a + b = 0.6) were prepared by conventional solid state reaction at 1200 °C, and sintered under air atmosphere at 1600 °C. For 0 ≤ a < 0.6, XRD patterns have shown that the major phases presented in the calcined powders are Y₂O₃, CeO₂ and orthorhombic perovskite YCr_0.5Mn_0.5O₃ phase, respectively. SEM and EDAX observations confirm the YCr_0.5Mn_0.5O₃ phases mostly exist at the grain, whereas the Y₂O₃ and CeO₂ phases mainly exist at the grain boundaries. Complex impedance analysis shows that, for 0 < a ≤ 0.6, single semicircular arc whose shape does not show any change with temperature. Nevertheless, for a = 0, two overlapping semicircular arcs are observed at and above 300 °C. The grain boundary properties exhibit thermistor parameters with a negative temperature coefficient characteristic. The relaxation behavior and conduction for the grain boundary could be due to a space-charge relaxation mechanism and oxygen vacancies, respectively.     

Microstructural evolution during high-temperature oxidation of spark plasma sintered Ti2AlN ceramics

Microstructures of Ti₂AlN ceramics synthesized and simultaneously consolidated from starting mixtures of Ti/Al/TiN powders by spark plasma sintering (SPS) were characterized using X-ray diffraction, scanning electron microscopy, focused ion beam (FIB) and transmission electron microscopy (TEM). When sintered for 10 min at 1300 °C, nearly single-phase Ti₂AlN ceramics with elongated (∼22 × 6 × 6 μm) grains were obtained. After sintering for 10 min at 1200 °C and chemical etching, Ti₂AlN nanowhiskers (150-200 nm dia., 1-5 μm long) were exposed in pores coexisting with TiAl, TiN and Ti₂AlN grains. FIB-TEM studies revealed single-crystal Ti₂AlN nanowhiskers in a TiAl matrix with orientation relationship [1 1 −2 0]_H//[−1 0 1]_γ, (0 0 0 1)_H//(1 1 1)_γ, γ = TiAl, H = Ti₂AlN. The nanowhiskers are believed to form by diffusion of TiN into TiAl during SPS and to be exposed during the chemical etch. Microstructural development during high-temperature oxidation of dense Ti₂AlN ceramics for 1 h at <1200 °C involves gradual formation on the surface of layered microstructures containing anatase, rutile and α-Al₂O₃. After 1 h at >1200 °C, more complex layered microstructures containing Al₂TiO₅, rutile, α-Al₂O₃ and continuous voids layers form. After heating to 1100 °C for 1 h and cooling to room temperature, planar defects are observed in surface TiO₂ grains identified as stacking faults bounded by partial dislocations. After heating for 1 h at 1400 °C and cooling to room temperature, cracks propagate in TiO₂ grains. It is believed that planar defects and cracks arise from stress generation in the oxide scale. Thermal stresses formed on cooling may arise from thermal expansion mismatch of phases (TiO₂, Al₂O₃ and Al₂TiO₅) in the oxide scale, the high anisotropy of thermal expansion in Al₂TiO₅ and thermal expansion mismatch between the oxide scale and Ti₂AlN substrate. Growth stresses formed during the isothermal oxidation treatment may arise from the volume changes associated with oxidation reactions of Ti₂AlN. An oxidation mechanism for Ti₂AlN ceramics is proposed, which involves initial reaction with atmospheric oxygen to form oxide phases, demixing of the mixed oxide phases, void formation due to the Kirkendall effect and gaseous NO_x release. Oxidation of Ti₂AlN <1200 °C with 1 h hold times is limited, while above this temperature the oxide scale grows rapidly, and Ti₂AlN ceramics undergo heavy oxidation.     

Transport mechanism in a ceramic system: LiCo3/5Fe1/5Cu1/5VO4

The electrical impedance and modulus properties of a LiCo_3/5Fe_1/5Cu_1/5VO₄ ceramic system were measured by impedance spectroscopy method in the frequency range 10²-10⁶ Hz and temperature range 22-250 °C. X-ray diffraction study reveals formation of the compound in a cubic crystal system with lattice parameters a = 8.2756 (3) Å. Field emission scanning electron microscopy is used to investigate the grain morphology of the material. Nyquist plots confirm the existence of bulk and grain boundary effects at 22 °C ≤ T ≤ 200 °C, and bulk, grain boundary and polarization effects at T ≥ 225 °C. Electrical modulus study indicates a non-Debye behavior of the material. A detailed study of bulk conductivity shows electric conduction in the material as a thermally activated process.     

Effect of Dy on the properties of Sm-doped ceria electrolyte for IT-SOFCs

Co-doped ceria-based electrolytes of Ce_0.8Sm_0.2−xDy_xO_2−δ (x = 0, 0.02, 0.06, 0.10, 0.14) were sintered from powders obtained by solid state reaction method. The phase identification, thermal expansion, ionic conductivities and microstructures of samples were studied by X-ray diffraction (XRD), dilatometry, AC impedance spectroscopy (IS) and scanning electron microscopy (SEM). The results showed that the addition of Dy led to higher ionic conductivity and lower activation energy in comparison with Sm singly doped ceria Ce_0.8Sm_0.2O_2−δ (SDC) in the temperature range of 300-800 °C. As the addition amount of Dy increased up to 2 mol% (Ce_0.8Sm_0.18Dy_0.02O_2−δ), the sample attained the highest ionic conductivity, about 50% higher than that of SDC at 500 °C. The effect of Dy on the grain boundary conductivity was more apparent than that of the bulk conductivity. XRD measurements indicated that all the samples were single phase. The thermal expansion was linear for all the samples. The addition of Dy did not change the thermal expansion coefficient (TEC) significantly.