Realistic dielectric response of high temperature sintered ZnO ceramic: a microscopic and spectroscopic approach

High temperature sintering (1200–1400 °C) has been performed on ZnO ceramics. An X-ray Absorption Fine Structure (XAFS) study shows that high sintering temperature introduces a constant amount of V_O and V_Zn defects without any significant effect on the crystal or electronic structure of Wurtzite ZnO. The combined effects of grain boundaries and voids are considered responsible for the apparent colossal dielectric constant (ε′) > 10⁴ at low frequency (∼10² Hz) for all the sintered ZnO ceramics. The superior contact among grains of the ZnO-1200 sample enhances both the interfacial and orientational polarization of the Zn²⁺–V_O dipoles, which results in the increase of low and high frequency dielectric constants (ε′) and the corresponding dielectric loss (tan δ) also increases. On the other hand, high temperature sintering of ZnO at 1300 °C and 1400 °C introduces voids at the expense of reduced grain and grain boundary contact areas, thus affecting both the interfacial and orientational polarization with corresponding reduction of dielectric constant (ε′) and dielectric loss. Orientational polarizations due to Zn²⁺–V_O dipoles are suggested to remain fixed and it is the microstructure which controls the dielectric properties of high temperature sintered ZnO ceramics.

Superior grain contacts of ZnO-1200 samples enhance low and high frequency dielectric constants (ε′) and dielectric loss (tan δ).     

Microwave dielectric properties of low-temperature-fired MgNb2O6 ceramics for LTCC applications

MgNb₂O₆ ceramics doped with (Li₂O–MgO–ZnO–B₂O₃–SiO₂) glass were synthesized by the traditional solid phase reaction route. The effects of LMZBS addition on microwave dielectric properties, grain growth, phase composition and morphology of MgNb₂O₆ ceramics were studied. The SEM results show dense and homogeneous microstructure with grain size of 1.72 μm. Raman spectra and XRD patterns indicate the pure phase MgNb₂O₆ ceramic. The experimental results show that LMZBS glass can markedly decrease the sintering temperature from 1300 °C to 925 °C. Higher density and lower porosity make ceramics have better dielectric properties. The MgNb₂O₆ ceramic doped with 1 wt% LMZBS glass sintered at 925 °C for 5 h, possessed excellent dielectric properties: ε_r = 19.7, Q·f = 67 839 GHz, τ_f = −41.01 ppm °C⁻¹. Moreover, the favorable chemical compatibility of the MgNb₂O₆ ceramic with silver electrodes makes it as promising material for low temperature co-fired ceramic (LTCC) applications.

MgNb₂O₆ ceramics doped with (Li₂O–MgO–ZnO–B₂O₃–SiO₂) glass were synthesized by the traditional solid phase reaction route.     

Cold sintering of YBa2Cu3O7−δ

Cold sintering is a sintering technique which enables ceramic powders to be densified at greatly reduced temperatures compared to traditional solid state techniques, which often require temperatures in excess of 1000 °C. These temperatures often preclude the exploitation of size or orientational effects in ceramics as these are lost during heating. One such effect is the orientation of the crystallographic c axis in YBa₂Cu₃O_7−δ (YBCO) which can be controlled through applied pressure. This effect is of interest for increasing critical current density which is highly dependent on the orientation of the a–b (CuO₂) planes within the ceramic. Using cold sintering, we demonstrate that dense YBCO can be created at 180 °C (vs. 1000 °C using solid state) and demonstrate that the likely sintering mechanism is mediated by the cracking which occurs in YBCO when exposed to water. In addition, the ceramics produced show and retain the orientational effect, representing a unique opportunity to study the effect on critical current density. We show that the intergranular critical current when the a–b planes are parallel to the applied field is around 15% higher than when perpendicular.

Cold sintered superconducting YBa₂Cu₃O_7−δ densified at 180 °C shows enhanced critical current densities by exploiting grain alignment created during pressing.     

Structure of bismuth tellurite and bismuth niobium tellurite glasses and Bi2Te4O11 anti-glass by high energy X-ray diffraction

Glass and anti-glass samples of bismuth tellurite (xBi₂O₃–(100 − x)TeO₂) and bismuth niobium tellurite (xBi₂O₃–xNb₂O₅–(100 − 2x)TeO₂) systems were prepared by melt-quenching. The bismuth tellurite system forms glasses at low Bi₂O₃ concentration of 3 to 7 mol%. At 20 mol% Bi₂O₃, the glass forming ability of the Bi₂O₃–TeO₂ system decreases drastically and the anti-glass phase of monoclinic Bi₂Te₄O₁₁ is produced. Structures of glass and the anti-glass Bi₂Te₄O₁₁ samples were studied by high-energy X-ray diffraction, reverse Monte Carlo simulations and Rietveld Fullprof refinement. All glasses have short short-range disorder due to the existence of at least three types of Te–O bonds of lengths: 1.90, 2.25 and 2.59 Å, besides a variety of Bi–O and Nb–O bond-lengths. The medium-range order in glasses is also disturbed due to the distribution of Te–Te pair distances. The average Te–O co-ordination (N_Te–O) in the glass network decreases with an increase in Bi₂O₃ and Nb₂O₅ mol% and is in the range: 4.17 to 3.56. The anti-glass Bi₂Te₄O₁₁ has a long-range order of cations but it has vibrational disorder and it exhibits sharp X-ray reflections but broad vibrational bands similar to that in glasses. Anti-glass Bi₂Te₄O₁₁ has an N_Te–O of 2.96 and is significantly lower than in glass samples.

Te–O, Bi–O and Nb–O bond lengths, co-ordinations in bismuth tellurite, bismuth niobium tellurite glasses and Bi₂Te₄O₁₁ anti-glass by HEXRD, RMC and Riteveld analysis.     

Influence of Al2O3 nanoparticle doping on depolarization temperature,and electrical and energy harvesting properties of lead-free 0.94(Bi0.5Na0.5)TiO3–0.06BaTiO3 ceramics

In this research article, the effects of Al₂O₃ nanoparticles (0–1.0 mol%) on the phase formation, microstructure, dielectric, ferroelectric, piezoelectric, electric field-induced strain and energy harvesting properties of the 0.94(Bi_0.5Na_0.5)TiO₃–0.06BaTiO₃ (BNT–6BT) ceramic were investigated. All ceramics have been synthesized by a conventional mixed oxide method. The XRD and Raman spectra showed coexisting rhombohedral and tetragonal phases throughout the entire compositional range. An increase of the grain size, T_F–R, T_m, ε_max and δ_A values was noticeable when Al₂O₃ was added. Depolarization temperature (T_d), which was determined by the thermally stimulated depolarization current (TSDC), tended to increase with Al₂O₃ content. The good ferroelectric properties (P_r = 32.64 μC cm⁻², E_c = 30.59 kV cm⁻¹) and large low-field d₃₃ (205 pC N⁻¹) values were observed for the 0.1 mol% Al₂O₃ ceramic. The small Al₂O₃ additive improved the electric field-induced strain (S_max and ). The 1.0 mol% Al₂O₃ ceramic had a large piezoelectric voltage coefficient (g₃₃ = 32.6 × 10⁻³ Vm N⁻¹) and good dielectric properties (ε_r,max = 6542, T_d = 93 °C, T_F–R = 108 °C, T_m = 324 °C and δ_A = 164 K). The highest off-resonance figure of merit (FoM) for energy harvesting of 6.36 pm² N⁻¹ was also observed for the 1.0 mol% Al₂O₃ ceramic, which is suggesting that this ceramic has potential to be one of the promising lead-free piezoelectric candidates for further use in energy harvesting applications.

In this research article, the effects of Al₂O₃ nanoparticles (0–1.0 mol%) on the phase, microstructure, dielectric, ferroelectric, piezoelectric, electric field-induced strain and energy harvesting of the BNT–6BT ceramic were investigated.     

Dielectric analysis and electrical conduction mechanism of La1−xBixFeO3 ceramics

Bulk-phase polycrystalline La_1−xBi_xFeO₃ (x = 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were prepared by citric sol–gel and sintering methods. The structural, morphological, and electrical properties of the resulting sol–gel solutions were investigated using various techniques. In an X-ray diffraction analysis, all samples crystallized in the orthorhombic structure with the Pbnm space group and showed an increase in lattice constant with increasing Bi content which was also confirmed by vibrational analysis. The sample surfaces and average grain sizes were examined by scanning electron microscopy. The grain distribution was non-uniform and the grain size increased with the increasing Bi content. The complex electrical conductivities and dielectric analyses of these materials were investigated as functions of frequency by impedance spectroscopy at various temperatures (75–200 °C). The frequency-dependent dielectric constant at each temperature increased with increasing Bi content. A Jonscher''s power law analysis revealed that the AC and DC conductivities arose by completely different mechanisms. The temperature dependence and dielectric relaxation of the DC conductivity satisfied the Arrhenius law and decreased with increasing Bi content. The activation energy ranged from 0.20 to 0.45 eV and was similar in the conduction and relaxation mechanisms, indicating that both transport mechanisms were based on hopping phenomena. We believe that lowering the activation energy will help with the optimization of constituents as promising candidates in novel materials for future electrocatalysts.

Bulk-phase polycrystalline La_1−xBi_xFeO₃ (x = 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were prepared by citric sol–gel and sintering methods.     

Reaction induced robust PdxBiy/SiC catalyst for the gas phase oxidation of monopolistic alcohols

Reaction induced Pd_xBi_y/SiC catalysts exhibit excellent catalytic activity (92% conversion of benzyl alcohol and 98% selectivity of benzyl aldehyde) and stability (time on stream of 200 h) in the gas phase oxidation of alcohols at a low temperature of 240 °C due to the formation of Pd⁰–Bi₂O₃ species. TEM indicates that the agglomeration of the 5.8 nm nanoparticles is inhibited under the reaction conditions. The transformation from inactive PdO–Bi₂O₃ to active Pd⁰–Bi₂O₃ under the reaction conditions is confirmed elaborately by XRD and XPS.

Reaction induced Pd_xBi_y/SiC catalysts exhibit excellent catalytic activity and stability in the gas phase oxidation of monopolistic alcohols at a low temperature of 240 °C due to the formation of Pd⁰–Bi₂O₃ species.     

High electrostrictive properties and energy storage performances with excellent thermal stability in Nb-doped Bi0.5Na0.5TiO3-based ceramics

As a promising candidate material replacing Pb(ZrTi)O₃ (PZT), the lead-free Bi_0.5Na_0.5TiO₃ (BNT) system exhibits outstanding piezoelectric and ferroelectric properties. However, the weak thermal stability of these electric properties hampers its practical applications. In this work, we designed and prepared novel Nb-doped 0.76Bi_0.5Na_0.5TiO₃–0.24Bi_0.5K_0.5TiO₃ (BNT–BKT) ceramics with superior temperature stability of electric properties. Both strain as well as discharging properties of 5% Nb-doped BNT–BKT ceramics varied less than 3% and 12.5% respectively from room temperature to 160 °C, ascribed to the enlarged gap between the depolarized temperature (T_d or T_F–R) and the maximum dielectric temperature (T_m). In addition, we investigated the impacts of Nb doping on the phase transition, dielectric, piezoelectric and ferroelectric behaviors of BNT–BKT ceramics in detail. Temperature dependent dielectric spectrums indicated that T_d decreased below room temperature with Nb modifying, revealing that the phase structure transformed from ferroelectric into ergodic relaxor. Accordingly, the maximum strain value of 0.21% and recoverable energy storage of 1.2 J cm⁻³ were simultaneously acquired at the critical composition of 5% Nb incorporation. Our results provide an effective means of obtaining BNT-based ceramics with simultaneously thermally stable strain and discharge properties for wide temperature actuator and capacitor applications.

Lowering T_d to enlarge the temperature range between T_d and the maximum dielectric temperature (T_m) proved to be an effective strategy to enhance the thermal stability strain and energy storage properties in BNT-based ceramics.     

A novel microwave dielectric ceramic Li2NiZrO4 with rock salt structure

Low loss Li₂NiZrO₄ ceramics with rock salt structure were successfully prepared by the solid-phase reaction method. The relationship between sintering temperature, phase composition and dielectric properties of Li₂NiZrO₄ ceramics was reported for the first time. The grain size gradually increased and the porosity decreased with the sintering temperature increasing. When the sintering temperature exceeds 1300 °C, the grains grow abnormally and some grains begin to melt. The XRD patterns indicated the second phase ZrO₂ appeared due to the volatilization of lithium. The grains grow abnormally and a second phase of ZrO₂ increased the loss of Li₂NiZrO₄ ceramics. The samples sintered at 1300 °C possessed the best dielectric properties: ε_r = 12.3, Q_f = 20000 GHz, τ_f = −23.4 ppm °C⁻¹, which would make the ceramic a possible candidate for millimeter-wave applications.

Low loss Li₂NiZrO₄ ceramics with rock salt structure were successfully prepared by the solid-phase reaction method.     

Effect of Bi2O3 content on the microstructure and electrical properties of SrBi2Nb2O9 piezoelectric ceramics

Lead-free ceramics, SrBi₂Nb₂O₉–xBi₂O₃ (SBN–xBi), with different Bi contents of which the molar ratio, n(Sr) : n(Bi) : n(Nb), is 1 : 2(1 + x/2) : 2 (x = −0.05, 0.0, 0.05, 0.10), were prepared by conventional solid-state reaction method. The effect of excess bismuth on the crystal structure, microstructure and electrical properties of the ceramics were investigated. A layered perovskite structure without any detectable secondary phase and plate-like morphologies of the grains were clearly observed in all samples. The value of the activation energy suggested that the defects in samples could be related to oxygen vacancies. Excellent electrical properties (e.g., d₃₃ = 18 pC N⁻¹, 2P_r = 17.8 μC cm⁻², ρ_rd = 96.4% and T_c = 420 °C) were simultaneously obtained in the ceramic where x = 0.05. Thermal annealing studies indicated the SBN–xBi ceramics system possessed stable piezoelectric properties, demonstrating that the samples could be promising candidates for high-temperature applications.

Lead-free ceramics, SrBi₂Nb₂O₉–xBi₂O₃ (SBN–xBi), with different Bi contents of which the molar ratio, n(Sr) : n(Bi) : n(Nb), is 1 : 2(1 + x/2) : 2 (x = −0.05, 0.0, 0.05, 0.10), were prepared by conventional solid-state reaction method.     

Composition design,electrical properties,and temperature stability in (1 − x)K0.44Na0.56Nb0.96Sb0.04O3-xBi0.45La0.05Na0.5ZrO3 lead-free ceramics

In this work, we designed a new system of (1 − x)K_0.44Na_0.56Nb_0.96Sb_0.04O₃-xBi_0.45La_0.05Na_0.5ZrO₃ (KNNS-xBLNZ, 0 ≤ x ≤ 0.06) ceramics, and systemically investigated both their electrical performance and temperature stability. Through optimizing the composition, a relatively good comprehensive performance (e.g., d₃₃ ∼ 455 ± 10 pC N⁻¹, k_p ∼ 0.47 ± 0.02, T_C ∼ 266 °C, strain ∼ 0.148%, and ) is obtained in the ceramics with x = 0.040, which is attributed to the construction of a rhombohedral–orthorhombic–tetragonal (R–O–T) phase boundary. Moreover, a good temperature stability of remnant polarization (P_r) as well as strain value (P_{r100 °C}/P_rRT ∼ 89.6%, P_{r180 °C}/P_rRT ∼ 73.2%, S_{100 °C}/S_RT ∼ 92.6%, S_{180 °C}/S_RT ∼ 74.1%) is gained in KNNS-0.040BLNZ ceramics with a broad temperature range from room temperature to 180 °C. Hence, we believe that KNNS-xBLNZ ceramics opens a window for the practical application of lead-free ceramics.

A new system of (1 – x)K_0.44Na_0.56Nb_0.96Sb_0.04O₃-xBi_0.45La_0.05Na_0.5ZrO₃ (KNNS-xBLNZ, 0 ≤ x ≤ 0.06) ceramics was designed, and systemically investigated both their electrical performance and temperature stability.     

Improvement of electric field-induced strain and energy storage density properties in lead-free BNKT-based ceramics modified by BFT doping

In this research, the effects of Ba(Fe_0.5Ta_0.5)O₃ (BFT) additive on the phase evolution, the dielectric, ferroelectric, piezoelectric, electric field-induced strain responses, and energy storage density of the Bi_0.5(Na_0.80K_0.20)_0.5TiO₃–0.03(Ba_0.70Sr_0.03)TiO₃ (BNKT–0.03BSrT) ceramics have been systematically investigated. The ceramics have been prepared by a solid-state reaction method accompanied by two calcination steps. X-ray diffraction indicates that all ceramics coexist between rhombohedral and tetragonal phases, where the tetragonal phase becomes dominant at higher BFT contents. The addition of BFT also promotes the diffuse phase transition in this system. A significant enhancement of electric field-induced strain response (S_max = 0.42% and = 840 pm V⁻¹) is noted for the x = 0.01 ceramic. Furthermore, the giant electrostrictive coefficient (Q₃₃ = 0.0404 m⁴ C⁻²) with a giant normalized electrostrictive coefficient (Q₃₃/E = 8.08 × 10⁻⁹ m⁵ C⁻² V⁻¹) are also observed for this composition (x = 0.01). In addition, the x = 0.03 ceramic shows good energy storage properties, i.e. it has a high energy storage density (W = 0.65 J cm⁻³ @ 120 °C) with very high normalized storage energy density (W/E = 0.13 μC mm⁻²), and good energy storage efficiency (η = 90.4% @ 120 °C). Overall, these results indicate that these ceramics are one of the promising candidate piezoelectric materials for further development for actuator and high electric power pulse energy storage applications.

The effects of Ba(Fe_0.5Ta_0.5)O₃ additive on phase, dielectric, ferroelectric, piezoelectric, electric field-induced strain, and energy storage density of the Bi_0.5(Na_0.80K_0.20)_0.5TiO₃–0.03(Ba_0.70Sr_0.03)TiO₃ ceramics have been investigated.     

Microstructural analysis,magnetic properties,magnetocaloric effect,and critical behaviors of Ni0.6Cd0.2Cu0.2Fe2O4 ferrites prepared using the sol–gel method under different sintering temperatures

This work focuses on the microstructural analysis, magnetic properties, magnetocaloric effect, and critical exponents of Ni_0.6Cd_0.2Cu_0.2Fe₂O₄ ferrites. These samples, denoted as S1000 and S1200, were prepared using the sol–gel method and sintered separately at 1000 °C and 1200 °C, respectively. XRD patterns confirmed the formation of cubic spinel structures and the Rietveld method was used to estimate the different structural parameters. The higher sintering temperature led to an increased lattice constant (a), crystallite size (D), magnetization (M), Curie temperature (T_C), and magnetic entropy change (−ΔS_M) for samples that exhibited second-order ferromagnetic–paramagnetic (FM–PM) phase transitions. The magnetic entropy changed at an applied magnetic field (μ₀H) of 5 T, reaching maximum values of about 1.57–2.12 J kg⁻¹ K⁻¹, corresponding to relative cooling powers (RCPs) of 115 and 125 J kg⁻¹ for S1000 and S1200, respectively. Critical exponents (β, γ, and δ) for samples around their T_C values were studied by analyzing the M(μ₀H, T) isothermal magnetizations using different techniques and checked by analyzing the −ΔS_Mvs. μ₀H curves. The estimated values of β and γ exponents (using the Kouvel–Fisher method) and δ exponent (from M(T_C, μ₀H) critical isotherms) were β = 0.443 ± 0.003, γ = 1.032 ± 0.001, and δ = 3.311 ± 0.006 for S1000, and β = 0.403 ± 0.008, γ = 1.073 ± 0.016, and δ = 3.650 ± 0.005 for S1200. Obviously, these critical exponents were affected by an increased sintering temperature and their values were different to those predicted by standard theoretical models.

This work focuses on the microstructural analysis, magnetic properties, magnetocaloric effect, and critical exponents of Ni_0.6Cd_0.2Cu_0.2Fe₂O₄ ferrites.     

Investigation into defect chemistry and relaxation processes in niobium doped and undoped SrBi4Ti4O15 using impedance spectroscopy

The aurivillius family of compounds SrBi₄Ti₄O₁₅ (SBTi) and SrBi₄Ti_3.8Nb_0.2O₁₅ has been prepared using solid state reaction techniques. The niobium doping enhances the value of the dielectric constant, but decreases the phase transition temperature and grain size of SBTi. Grain conductivity evaluated from the impedance data reveals that Nb doping increases the resistance of grains which indicates the decrease in oxygen vacancies. The negative temperature coefficient of resistance shown by the grain boundary conductivity is explained using the Heywang–Jonker model. The variation of ac conductivity with frequency is found to obey Jonscher’s universal power law. The frequency exponent (n), pre-exponential factor (A), and bulk dc conductivity (σ_dc) are determined from the fitting curves of Jonscher’s universal power law. From the frequency exponent (n) versus temperature curve, we conclude that the conduction mechanism of SBTi changes from large-polaron tunneling (300–475 °C) to small-polaron tunneling (475–550 °C), and in that of the niobium doped it is small-polaron tunneling (300–375 °C) to correlated band hopping (375–550 °C). Activation energies have been calculated from different functions such as loss tangent, relaxation time, grain and grain boundary conductivities, and ac and dc conductivity. The activation energies reveal that conductivity in the sample has contributions from migrations of oxygen vacancies, bismuth ion vacancies, electrons ionized from strontium vacancies, strontium ion vacancies and valence fluctuations of Ti⁴⁺/Ti³⁺ ions.

From the analysis of impedance, ac conductivity we conclude that the donor ion Nb⁵⁺ doping in SrBi₄Ti₄O₁₅, reduces the oxygen vacancy and improves the dielectric and ferroelectric properties.     

Electrical properties of Sb2O3-modified BiScO3–PbTiO3-based piezoelectric ceramics

Compared with pure Pb-based perovskite ferroelectric materials, BiMeO₃–PbTiO₃ (Me = Sc³⁺, In³⁺, and Yb³⁺) systems have remarkable advantages in their Curie temperatures. As a member of this group, the BiScO₃–PbTiO₃ (BS–PT) solid solution has drawn considerable attention from scientists for its high Curie temperature and excellent piezoelectric coefficient. However, BS–PT ceramics still have some shortcomings, such as high dielectric loss and low mechanical quality factor, which make them unsuitable for high-temperature applications. Herein, we report the effect of the addition of complex ions on the electrical properties of BS–PT ceramics. Sb₂O₃-doped 0.36BiScO₃–0.64PbTi_0.97Fe_0.03O₃ + 1 mol% MnO₂ (BS–PTFMn + x% Sb₂O₃) ceramics were fabricated and their electrical properties were studied. BS–PTFMn + 0.75% Sb₂O₃ had an optimal piezoelectric coefficient, exhibiting which indicates that Sb₂O₃ doping can improve the piezoelectric properties of the BS–PT ceramics, exhibiting a “soft” effect of Sb₂O₃ doping. In addition, the thermal depolarization temperature (T_d) of BS–PTFMn + 0.75% Sb₂O₃ ceramics remained above 300 °C, such as 325 °C for BS–PTFMn + 0.75% Sb₂O₃. It was concluded that the piezoelectric properties of BS–PT ceramics were enhanced by the addition of Sb₂O₃.

Compared with pure Pb-based perovskite ferroelectric materials, BiMeO₃–PbTiO₃ (Me = Sc³⁺, In³⁺, and Yb³⁺) systems have remarkable advantages in their Curie temperatures.     

Crystal growth control of rod-shaped ε-Fe2O3 nanocrystals

Herein we report crystal growth control of rod-shaped ε-Fe₂O₃ nanocrystals by developing a synthesis based on the sol–gel technique using β-FeO(OH) as a seed in the presence of a barium cation. ε-Fe₂O₃ nanocrystals are obtained over a wide calcination temperature range between 800 °C and 1000 °C. A low calcination temperature (800 °C) provides an almost cubic rectangular-shaped ε-Fe₂O₃ nanocrystal with an aspect ratio of 1.4, whereas a high calcination temperature (1000 °C) provides an elongated rod-shaped ε-Fe₂O₃ nanocrystal with an aspect ratio of 3.3. Such systematic anisotropic growth of ε-Fe₂O₃ is achieved due to the wide calcination temperature in the presence of barium cations. The surface energy and the anisotropic adsorption of barium on the surface of ε-Fe₂O₃ can explain the anisotropic crystal growth of rod-shaped ε-Fe₂O₃ along the crystallographic a-axis. The present work may provide important knowledge about how to control the anisotropic crystal shape of nanomaterials.

Crystal growth control of rod-shaped ε-Fe₂O₃ nanocrystals is achieved by a synthesis based on the sol–gel technique.     

Dielectric ceramics/TiO2/single-crystalline silicon nanomembrane heterostructure for high performance flexible thin-film transistors on plastic substrates

A dielectric ceramics/TiO₂/single-crystalline silicon nanomembrane (SiNM) heterostructure is designed and fabricated for high performance flexible thin-film transistors (TFTs). Both the dielectric ceramics (Nb₂O₃–Bi₂O₃–MgO) and TiO₂ are deposited by radio frequency (RF) magnetron sputtering at room temperature, which is compatible with flexible plastic substrates. And the single-crystalline SiNM is transferred and attached to the dielectric ceramics/TiO₂ layers to form the heterostructure. The experimental results demonstrate that the room temperature processed heterostructure has high quality because: (1) the Nb₂O₃–Bi₂O₃–MgO/TiO₂ heterostructure has a high dielectric constant (∼76.6) and low leakage current. (2) The TiO₂/single-crystalline SiNM structure has a relatively low interface trap density. (3) The band gap of the Nb₂O₃–Bi₂O₃–MgO/TiO₂ heterostructure is wider than TiO₂, which increases the conduction band offset between Si and TiO₂, lowering the leakage current. Flexible TFTs have been fabricated with the Nb₂O₃–Bi₂O₃–MgO/TiO₂/SiNM heterostructure on plastic substrates and show a current on/off ratio over 10⁴, threshold voltage of ∼1.2 V, subthreshold swing (SS) as low as ∼0.2 V dec⁻¹, and interface trap density of ∼10¹² eV⁻¹ cm⁻². The results indicate that the dielectric ceramics/TiO₂/SiNM heterostructure has great potential for high performance TFTs.

Dielectric ceramics/TiO2/single-crystalline silicon nanomembrane heterostructure for high performance flexible thin-film transistors.     

Enhancing the thermoelectric power factor of nanostructured ZnCo2O4 by Bi substitution

Bi_xZnCo_2−xO₄ (0 ≤ x ≤ 0.2) nanoparticles with different x values have been prepared by the sol–gel method; the structural, morphological, thermal and thermoelectric properties of the prepared nanomaterials are investigated. XRD analysis confirms that Bi is completely dissolved in the ZnCo₂O₄ lattice till the x values of ≤0.1 and the secondary phase of Bi₂O₃ is formed at higher x value (x > 0.1). The synthesized nanomaterials are densified and the thermoelectric properties are studied as a function of temperature. The electrical resistivity of the Bi_xZnCo_2−xO₄ decreased with x value and it fell to 4 × 10⁻² Ω m for the sample with x value ≤ 0.1. The Seebeck coefficient value increased with the increase of Bi substitution till the x value of 0.1 and decreased for the sample with higher Bi content (x ≤ 0.2) as the resistivity of the sample increased due to secondary phase formation. With the optimum Seebeck coefficient and electrical resistivity, Bi_0.1ZnCo_1.9O₄ shows the high-power factor (α²σ_{550 K}) of 2.3 μW K⁻² m⁻¹ and figure of merit of 9.5 × 10⁻⁴ at 668 K respectively, compared with other samples. The experimental results reveal that Bi substitution at the Co site is a promising approach to improve the thermoelectric properties of ZnCo₂O₄.

Nanostructuring and Bi substitution have considerably increased the thermoelectric power factor and ZT of Bi_xZnCo_2−xO₄; Bi_1.9ZnCo_1.9O₄ shows a higher power factor than that of other Bi substituted samples.     

Annealing temperature effects on photoelectrochemical performance of bismuth vanadate thin film photoelectrodes

The effects of annealing treatment between 400 °C and 540 °C on crystallization behavior, grain size, electrochemical (EC) and photoelectrochemical (PEC) oxygen evolution reaction (OER) performances of bismuth vanadate (BiVO₄) thin films are investigated in this work. The results show that higher temperature leads to larger grain size, improved crystallinity, and better crystal orientation for the BiVO₄ thin film electrodes. Under air-mass 1.5 global (AM 1.5) solar light illumination, the BiVO₄ thin film prepared at a higher annealing temperature (500–540 °C) shows better PEC OER performance. Also, the OER photocurrent density increased from 0.25 mA cm⁻² to 1.27 mA cm⁻² and that of the oxidation of sulfite, a hole scavenger, increased from 1.39 to 2.53 mA cm⁻² for the samples prepared from 400 °C to 540 °C. Open-circuit photovoltage decay (OCPVD) measurement indicates that BiVO₄ samples prepared at the higher annealing temperature have less charge recombination and longer electron lifetime. However, the BiVO₄ samples prepared at lower annealing temperature have better EC performance in the absence of light illumination and more electrochemically active surface sites, which are negatively related to electrochemical double-layer capacitance (C_dl). C_dl was 0.0074 mF cm⁻² at 400 °C and it decreased to 0.0006 mF cm⁻² at 540 °C. The OER and sulfide oxidation are carefully compared and these show that the efficiency of charge transport in the bulk (η_bulk) and on the surface (η_surface) of the BiVO₄ thin film electrode are improved with the increase in the annealing temperature. The mechanism behind the light-condition-dependent role of the annealing treatment is also discussed.

The effects of annealing treatment on crystallization behavior, grain size, electrochemical (EC) and photoelectrochemical (PEC) oxygen evolution reaction (OER) performances of bismuth vanadate (BiVO₄) thin films are investigated in this work.     

Reactive intermediate phase cold sintering in strontium titanate

Dense (>96% theoretical) strontium titanate ceramics were fabricated at 950 °C (conventional sintering temperature > 1400 °C) using a reactive intermediate phase cold sintering process. An aqueous solution of SrCl₂ mixed with TiO₂ nanoparticles was added to SrTiO₃ powders and pressed at 180 °C to obtain a highly compacted green body. During the post-press heating step at 950 °C, the TiO₂ and SrCl₂ create in-filling micro-reactions around each grain resulting in dense (>96%) SrTiO₃ ceramics. Nano- and micron-sized starting powders were used, demonstrating that this reactive intermediate phase cold sintering route can densify a wide range of starting powder sizes, as it not reliant on an amorphous-to-crystalline precipitation through the terrace ledge kink mechanism, as has been identified repeatedly in previous cold sintering mechanisms. Moreover, this process has the potential to densify a wide variety of functional oxides, as a range of different low-temperature chemical synthesis routes could be used.

Strontium titanate ceramics (>96% theoretical) have been fabricated using a reactive intermediate phase cold sintering process with a range of starting-powder sizes.