Extrinsic behavior in La0.67Ca0.33MnO3–BaTiO3 composites

A manganite matrix based nano-composite series, (1 ? x)La_0.67Ca_0.33MnO₃(LCMO)–(x)BaTiO₃(BTO), has been prepared by the pyrophoric method. Influence of BTO phase on structural and magneto-transport properties of LCP phase has been studied using structural and transport investigations. The series exhibits a conduction threshold at x_m ～ 0.30. Overall pattern of temperature dependence of resistivity for this series has been fitted with a percolation model. Almost 200% improvement has been observed by the formation of composite when compared to the parent sample.     

Giant permittivity phenomena in layered BaTiO3–Ni composites

Layered BaTiO₃–Ni cermet composites with a constant composition but diversified microstructures were produced by a rolling-and-folding processing method. These composites differ from conventional laminates in that their interface has a tendency to be wavy, with a globular or elongated second phase within a continuous matrix phase. Based on an analysis of the (di)electric properties and Monte Carlo simulations we confirmed the critical influence of the composite's microstructural characteristics on the percolation threshold. We found that the dielectric properties of the composite, when it is in the insulation regime, were controlled by the insulating BaTiO₃ phase. A giant effective permittivity of around 200 000, with modest losses of tan δ < 0.04, was measured when the percolation threshold approached the composition of the cermet. Partial decomposition and deformation of the layered structure resulted in the creation of conducting paths, whereas further homogenization again shifted the percolation threshold above the actual composition of the cermet.     

Energy-storage properties of Bi0.5Na0.5TiO3-BaTiO3-KNbO3 ceramics fabricated by wet-chemical method

0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃ (BNTBT) and KNbO₃ (KN) powders with average particle size of ∼50 nm and ∼300 nm were synthesized by sol-gel method and hydrothermal method, respectively. Then, (1 − x)(BNTBT)-xKN (BNTBT-KN, x = 0, 0.01, 0.03, 0.05, 0.07) ceramic samples were prepared using these two powder precursors. The structure, dielectric and energy-storage properties of BNTBT-KN ceramics were comprehensively investigated. All the ceramic samples were in single perovskite structure, indicating that KN can completely dissolve into BNTBT within the studied composition range. BNTBT-KN ceramics exhibited a high dielectric constant at room temperature, being in the order of 1430–1550. Ferroelectric hysteresis loops at room temperature became more slim with the increase of KN content, which largely improved energy-storage density and efficiency. For the composition of x = 0.05, the maximum recoverable energy-storage density reached 1.72 J/cm³ under 16.8 kV/mm, which is superior to linear dielectrics and even some Pb-based systems. All these results demonstrate that 0.95BNTBT-0.05KN fabricated by wet-chemical method is a promising lead-free dielectric material for energy-storage capacitors.     

Cold sintering process for 8 mol%Y2O3-stabilized ZrO2 ceramics

In this communication, the cold sintering process was applied to benefit the green body compaction of 8 mol%Y₂O₃-stablized ZrO₂ ceramics (8Y-YSZ). Compared to conventionally processed ceramics, an enhanced densification behavior was demonstrated in cold sintering related ones following a second step conventional sintering process. Dense ceramics up to ∼96% of theoretical density were achieved after sintering at 1200 °C. The resulted ceramics demonstrated a fine microstructure with a grain size ∼200 nm. A mechanical performance with a Vickers hardness of 13.6 GPa and a fracture toughness of 2.85 MPa m^1/2 was also reported.     

New Bi(Mg0.75W0.25)O3–PbTiO3 ferroelectric ceramics

An investigation was made into the Bi(Mg_0.75W_0.25)O₃–PbTiO₃ (BMW-PT) system. The aim was to find the optimum composition with respect to piezoelectric and dielectric properties and link this to the presence of a morphotropic phase boundary (MPB). The optimum composition was found to be 62 mol% PT. The maximum values of the piezoelectric constant d₃₃ = 150 pm/V and the maximum relative dielectric constant ɛ₃₃/ɛ₀ = 1400. The dielectric loss is high with tan δ typically ∼5%. Results from capacity versus the temperature measurements indicate a Curie temperature of 220 °C, which is significantly lower than the predicted value.     

Structural,chemical and dielectric properties of ceramic injection moulded Ba(Zn1/3Ta2/3)O3 microwave dielectric ceramics

We report the development of a ceramic injection moulding (CIM) process to produce complex-shaped structures using high-performance microwave ceramic materials. In particular, we describe the synthesis methods and the structural, chemical and dielectric properties of Ba(Zn_1/3Ta_2/3)O₃ (BZT) doped with Ni and Zr ceramics produced using ceramic injection moulding. Sintering the ceramic injection moulded Ba(Zn_1/3Ta_2/3)O₃ to a relative density of ∼94% was possible at a temperature of 1680 °C and a time of 48 h. The best samples to date exhibit a dielectric constant, ɛ_r, of ∼30, a Q value, of ∼31,250 (i.e. tan δ < 3.2 × 10⁻⁵) at 2 GHz, and a temperature coefficient of resonance frequency, τ_f, of 0.1 ppm/°C.     

Tb3+/Yb3+ co-doped Y2O3 upconversion transparent ceramics: Fabrication and characterization for IR excited green emission

Tb³⁺/Yb³⁺ co-doped Y₂O₃ transparent ceramics were fabricated by vacuum sintering of the pellets (prepared from nanopowders by uniaxial pressing) at 1750 °C for 5 h. Zr⁴⁺ and La³⁺ ions were incorporated in Tb³⁺/Yb³⁺ co-doped Y₂O₃ nanoparticle to reduce the formation of pores which limits the transparency of ceramic. An optical transmittance of ∼80% was achieved in ∼450 to 2000 nm range for 1 mm thick pellet which is very close to the theoretical value by taking account of Fresnel’s correction. High intensity luminescence peak at 543 nm (green) was observed in these transparent ceramics under 976 and 929 nm excitations due to Yb–Tb energy transfer upconversion.     

High quality microwave dielectric ceramic sintered at extreme-low temperature below 200° and co-firing with base metal

Ultra-low temperature co-fired ceramics technology (ULTCC) requires the microwave dielectric ceramics with lower intrinsic sintering temperature than the melting point of inner electrodes. In the present work, a novel HBO₂ ceramic was found to be densified at extreme-low temperature below 200 °C, with pores, residual H₃BO₃, amorphous B₂O₃ inside, with a relative permittivity ∼2.12 ± 0.02, a Qf value ∼32,700 ± 300 GHz and a temperature coefficient of resonant frequency value ∼ − 43 ± 3 ppm/°C. This material can be easily obtained by dehydration from H₃BO₃ by sintering at low temperature below 200 °C. Its extreme-low sintering temperature and water solubility also provides the possibility to achieve some novel multi-functional inorganic-organic composite in the future.     

Mechanical properties up to 1900 K of Al2O3/Er3Al5O12/ZrO2 eutectic ceramics grown by the laser floating zone method

Directionally solidified Al₂O₃–Er₃Al₅O₁₂–ZrO₂ eutectic rods were processed using the laser floating zone method at growth rates of 25, 350 and 750 mm/h to obtain microstructures with different domain size. The mechanical properties were investigated as a function of the processing rate. The hardness, ∼15.6 GPa, and the fracture toughness, ∼4 MPa m^1/2, obtained from Vickers indentation at room temperature were practically independent of the size of the eutectic phases. However, the flexural strength increased as the domain size decreased, reaching outstanding strength values close to 3 GPa in the samples grown at 750 mm/h. A high retention of the flexural strength was observed up to 1500 K in the materials processed at 25 and 350 mm/h, while superplastic behaviour was observed at 1700 K in the eutectic rods solidified at the highest rate of 750 mm/h.     

Polypropylene/carbon nanotube nano/microcellular structures with high dielectric permittivity,low dielectric loss,and low percolation threshold

Nano/microcellular polypropylene/multiwalled carbon nanotube (MWCNT) composites exhibiting higher electrical conductivity, lower electrical percolation, higher dielectric permittivity, and lower dielectric loss are reported. Nanocomposite foams with relative densities (ρ_R) of 1.0–0.1, cell sizes of 70 nm–70 μm, and cell densities of 3 × 10⁷–2 × 10¹⁴ cells cm⁻³ are achieved, providing a platform to assess the evolution of electrical properties with foaming degree. The electrical percolation threshold decreases more than fivefold, from 0.50 down to 0.09 vol.%, as the volume expansion increases through foaming. The electrical conductivity increases up to two orders of magnitude in the nanocellular nanocomposites (1.0 > ρ_R > ∼0.6). In the proper microcellular range (ρ_R ≈ 0.45), the introduction of cellular structure decreases the dielectric loss up to five orders of magnitude, while the decrease in dielectric permittivity is only 2–4 times. Thus, microcellular composites containing only ∼0.34 vol.% MWCNT present a frequency-independent high dielectric permittivity (∼30) and very low dielectric loss (∼0.06). The improvements in such properties are correlated to the microstructural evolution caused by foaming action (biaxial stretching) and volume exclusion. High conductivity foams have applications in electromagnetic shielding and high dielectric foams can be developed for charge storage applications.     

Microstructure and mechanical properties of titanium aluminum carbides neutron irradiated at 400–700 °C

This work reports the first mechanical properties of Ti₃AlC₂-Ti₅Al₂C₃ materials neutron irradiated at ∼400, 630 and 700 °C at a fluence of 2 × 10²⁵ n m⁻² (E > 0.1 MeV) or a displacement dose of ∼2 dpa. After irradiation at ∼400 °C, anisotropic swelling and loss of 90% flexural strength was observed. After irradiation at ∼630–700 °C, properties were unchanged. Microcracking and kinking-delamination had occurred during irradiation at ∼630–700 °C. Further examination showed no cavities in Ti₃AlC₂ after irradiation at ∼630 °C, and MX and A lamellae were preserved. However, disturbance of (0004) reflections corresponding to M-A layers was observed, and the number density of line/planar defects was ∼10²³ m⁻³ of size 5–10 nm. HAADF identified these defects as antisite Ti_Al atoms. Ti₃AlC₂-Ti₅Al₂C₃ shows abrupt dynamic recovery of A-layers from ∼630 °C, but a higher temperature appears necessary for full recovery.     

High temperature properties of multiferroic BiFeO3–DyFeO3–BaTiO3 solid solutions

Dielectric and magnetic properties of the xBiFeO₃–yDyFeO₃–zBaTiO₃ solid solution ceramics at high temperature range of RT ∼600 °C have been characterized. For the more detailed understandings of the multiferroic property, the relation between the crystal structure transition, magnetic transition, dielectric transition with increasing temperature have been analyzed. Residual magnetization M_r under the low and high applied magnetic fields (H = 20 Oe, 8 kOe) and the dielectric properties, ɛ_r and tan δ, with varying measuring frequency and temperature have been characterized using the vibrating sample magnetometer and LCR meter, respectively. The neutron diffraction data has been collected at the temperature range of RT ∼800 °C. The low DyFeO₃ concentration samples (y = 0, 0.025) show the magnetic transitions at temperature range of 410–430 °C, while the high DyFeO₃ samples (y ≥ 0.05) show the additional transition at 250–290 °C. The magnetic transition at 410–430 °C corresponds to the crystal structural transition to the tetragonal P4mm from the rhombohedral R-3c, at which the BiFeO₃ and the DyFeO₃ samples lose their antiferromagnetic ordering.     

Synthesis,characterization and sinterablity of 10 mol% Sm2O3-doped CeO2 nanopowders via carbonate precipitation

A carbonate coprecipitation method has been used for the facile synthesis of highly reactive 10 mol% Sm₂O₃-doped CeO₂ (20SDC) nanopowders, employing nitrates as the starting salts and ammonium hydrogen carbonate (AHC) as the precipitant. The AHC/RE³⁺ (RE = Ce + Sm) molar ratio (R) and the reaction temperature (T) affect significantly the final yield and precursor properties, including chemical composition and particle morphology. Suitable processing conditions are T = 60 °C and R = 5.0–10, under which precipitation is complete and the resultant precursors show ultrafine particle size, spherical particle shape, and good dispersion. Thus, the processed precursors are rare-earth carbonates with an approximate formula of Ce_0.8Sm_0.2(CO₃)_1.5·1.8H₂O, which directly yield oxide solid-solutions upon thermal decomposition at a low temperature of ∼440 °C. The 20SDC solid solution powders calcined at 700 °C show excellent reactivity and have been densified to ∼99% of the theoretical via pressureless sintering at a very low temperature of 1200 °C for 4 h.     

Ba0.7Sr0.3TiO3–glass–silver percolative composite

Ba_0.7Sr_0.3TiO₃–glass–silver (BST–glass–Ag) composites were prepared by the solid state ceramic route. Their percolation behavior and dielectric properties were examined. The pure BST had a percolation limit of 24 vol% of silver whereas an addition of 8 wt% of 50PbO–30B₂O₃–20SiO₂ (PBS) glass lowered the percolation limit to 14 vol% of Ag. Glass addition lowered the sintering temperature of BST from 1300 to 975 °C and addition of Ag further lowered the sintering temperature to 925 °C, minimizing the Ag loss during sintering. The relative permittivity increased from 2700 for pure BST to about five orders of magnitude in the BST–glass–Ag composites near the percolation threshold.     

Dice-and-fill processing and characterization of microscale and high-aspect-ratio (K,Na)NbO3-based 1–3 lead-free piezoelectric composites

Piezoelectric composites with high-aspect-ratio lead-free piezoelectric microrods embedded in a polymer matrix were fabricated by applying the modified dice-and-fill method to a (K,Na)NbO₃-based piezoelectric ceramic, 0.9475[Li_0.02(K_0.48Na_0.53)_0.98](Nb_0.80Ta_0.20)O₃–0.0525AgSbO₃–0.5 wt% MnO₂ (abbreviated as LKNNT–AS–M) with enhanced piezoelectricity but low mechanical strength. The Pb-free LKNNT–AS–M/polymer composite specimens containing ∼10–23 vol% ceramic pillars with sectional width ∼50 μm, separation ∼55–110 μm and high aspect ratio ∼11, were prepared and investigated with an emphasis on their electrical properties. The developed composites possess a higher thickness factor k_t ∼63–67%, lower planar factor k_p ∼27–34% and enhanced k_t/k_p ratio ∼2.3. The composites also possess favorable properties which includes low electrical (tanδ ∼0.029) and mechanical (Q_m ∼6) losses, low acoustic impedance Z ∼4.2–7.8 Mrayl and ∼5 times higher voltage coefficient g₃₃ compared to LKNNT–AS–M piezoceramic. The promising results indicate that the LKNNT–AS–M/polymer composites have the potential to be used as active elements in high performance ultrasonic transducers.     

Preparation of Li2TiO3 ceramic with nano-sized pores by ultrasonic-assisted solution combustion

Li₂TiO₃ is a candidate material for tritium breeding in the future nuclear fusion reactor. In this study, Li₂TiO₃ powder was synthesized by ultrasonic-assisted solution combustion synthesis (USCS) in a single step. The ultrasonic transducer with the power of 1000 W was introduced in the synthesis process. The crystallite size of Li₂TiO₃ powder prepared by utilization of ultrasonic power is significantly decreased to ∼5.0 nm, while the one obtained without ultrasonic power is 20.0 nm. Li₂TiO₃ ceramic sintered from USCS powder at 800 °C exhibits the small grain size of 330 nm and the open pores size of 140 nm. The crush load of the ceramic reaches 37.2 N although the structure is porous. Compared with the ceramic prepared by solid-state reaction and conventional solution combustion synthesis, USCS sample has a higher conductivity of 2.0 × 10⁻⁶ S m⁻¹ at room temperature, indicating the lower tritium diffusion barrier in the ceramic.     

Low-temperature sintered Bi0.5Na0.5TiO3-SrTiO3 incipient piezoceramics and the co-fired multilayer piezoactuator thereof

Lead-free Bi_0.5Na_0.5TiO₃-SrTiO₃ incipient piezoceramics with Li₂CO₃ and MnO₂ additives were successfully fabricated at low firing temperature for applications in co-fired multilayer piezoactuators. The addition of Li₂CO₃ effectively shifted the sintering temperature from 1230 °C down to 1075 °C, where the ceramics were co-fired with a Ag/Pd (75/25) inner electrode. The prototype actuators were prepared by tape-casting method using ceramics with the composition of 0.74Bi_0.5Na_0.5TiO₃-0.26 SrTiO₃ + 0.15 wt%MnO₂ + 0.45 wt%Li₂CO₃. The total number of active layers was 13, and each ceramic layer had a thickness of 60 μm. The actuator output a large strain up to ∼0.20% at a driving field of 4 kV/mm, due to the field-induced phase transition between the ergodic relaxor and ferroelectric phases. The excellent voltage-displacement performance of the prototype actuator demonstrates the potential for industrial applications.     

Tribological study of amorphous BC4N coatings

Amorphous BC₄N thin films with a thickness of ∼ 2 μm have been deposited by Ion Beam Assisted Deposition (IBAD) on hard steels substrates, in order to study the wear behavior under high loads and the applicability as protective coatings. The bonding structure of the a-BC₄N film was assessed by X-ray Absorption Near Edge Spectroscopy (XANES) and Infrared Spectroscopy, indicating atomic mixing of B–C–N atoms, with a proportion of ∼ 70% sp² hybrids and ∼ 30% sp³ hybrids. Nanoindentation shows a hardness of ∼ 18 GPa and an elastic modulus of ∼ 170 GPa. A detailed tribological study is performed by pin-on-disk tests, combined with spectromicroscopy of the wear track at the coating and wear scar at pin. The tests were performed at ambient conditions, against WC/Co counterface balls under loads up to 30 N, with the sample rotating at 375 rpm. The coatings suffer a continuous wear, at a constant rate of 2 × 10^− 7 mm³/Nm, without catastrophic failure due to film spallation, and show a coefficient of friction of ∼ 0.2.     

Dielectric properties of carbon nanofibre/alumina composites

Carbon nanofibre (CNF)/Al₂O₃ composites with concentrations between 1 and 9 vol.% of CNF were prepared by the traditional ceramic processing route followed by spark plasma sintering. The dielectric properties of these composites have been studied in a broad frequency range from mHz to the infrared range. Unlike conventional composites, the percolation threshold in this system is more complex depending on the particles topology. Positive and negative variations by several orders of magnitude in the low frequency AC conductivity have been detected for concentrations near the threshold at ∼2 vol.% of CNF. To explain these results, a modified percolation model has been proposed which takes into consideration the effect of the concentration of the filler on the microstructure of the composite.     

Enhanced thermal conductivity of low-temperature sintered borosilicate glass–AlN composites with β-Si3N4 whiskers

The effects of β-Si₃N₄ whiskers on the thermal conductivity of low-temperature sintered borosilicate glass–AlN composites were systematically investigated. The thermal conductivity of borosilicate glass–AlN ceramic composite was increased from 11.9 to 18.8 W/m K by incorporating 14 vol% β-Si₃N₄ whiskers, and high flexural strength up to 226 MPa were achieved along with low relative dielectric constant of 6.5 and dielectric loss of 0.16% at 1 MHz. Microstructure characterization and percolation model analysis indicated that thermal percolation network formation in the ceramic composites led to the high thermal conductivity. The crystallization of the borosilicate microcrystal glass also contributed to the enhancement of thermal conductivity. Such ceramic composites with low sintering temperature and high thermal conductivity might be a promising material for electronic packaging applications.