Effects of annealing temperature on structure and electrical properties of sol–gel derived 0.65PMN-0.35PT thin film

0.65Pb(Mg_1/3Nb_2/3)O₃-0.35PbTiO₃ (0.65PMN-0.35PT) thin films were deposited on Pt/Ti/SiO₂/Si substrates annealed from 550 to 700 °C using sol-gel process. The effects of annealing temperature on microstructure, insulating, ferroelectric and dielectric properties were characterized. The result reveals that 0.65PMN-0.35PT thin films possess a polycrystalline structure, matching well with the perovskite phase despite the existence of a slight pyrochlore phase. The film samples annealed at all temperatures exhibit relatively dense surfaces without any large voids and the grain size increases generally with the increase of the annealing temperature. Meanwhile, pyrochlore phase is considerably generated because of the deformation of perovskite phase caused by volatilization of Pb at an excessive high-temperature. The film annealed at 650 °C exhibits superior ferroelectricity with a remanent polarization (P_r) value of 13.31 μC/cm², dielectric constant (ε_r) of 1692 and relatively low dielectric loss (tanδ) of 0.122 at 10⁴ Hz due to the relatively homogeneous large grain size of 130 nm and low leakage current of approximately 10^-6 A/cm².     

Synthesis of NiMn2O4 thin films via a simple solid-state reaction route

Herein, NiMn₂O₄ (MNO) spinel oxide thermistor films were synthesized on a SiO₂/Si substrate via annealing the electron beam evaporated Mn–Ni–Mn metal trilayers in air at different temperatures. The X-ray diffraction (XRD) results indicate that polycrystalline spinel-structured MNO thermistor films were formed. The surface particle size of the series MNO films quickly reduced from ~300 to ~120 nm with a temperature increase from 650 to 750 °C, and then, slowly reduced to 80 nm or even smaller with a temperature increase from 750 to 950 °C. Specifically, 750 °C anneal formed the spinel MNO film with largest B value of 5067 and Ea value of 0.4366. The proposed synthesis route for MNO spinel oxide film has been proven to be feasible.     

Chemical and interfacial design in the visible-light-absorbing ferroelectric thin films

Ferroelectric thin films with switchable polarization and anomalous photoelectric effects have received extensive attention recently. However, the improvement of photoelectric performance is accompanied by the weakening of ferroelectricity. Here, both chemical and interlayer design are used to regulate the polarization and optical properties of BiFeO₃-based ferroelectric films. We achieved an improvement in both ferroelectricity and bandgap by chemical composition. The remanent polarization has been enhanced to 73.8 μC/cm² from 0.2 μC/cm², ascribed to the structural transition. The band gap of Eu-BiFeO₃ films has been reduced to 2.23 eV from 2.42 eV due to the unique energy level from Eu 4f, indicating the enhanced visible-light-absorbing capability. We have designed a "sandwich" interfacial structure of homogeneous Eu-BiFeO₃ films. A clever combination between optimal ferroelectricity and narrow band gap with near Eu contents of BFO films would generate an interfacial layer with a homogeneous gradient component, which should favor the switching of ferroelectric domains. The results show that the remanent polarization improved by 17 % to 86.2 μC/cm² while the band gap has also improved. Intriguingly, the short-circuit current density (J_sc) and open circuit (V_oc) of the photovoltaic signal of the optimal films are 89.0 nA and 0.412 V, respectively. This provides a simple and intelligent way to design the ferroelectric-photoelectric thin films and lays the foundation for optical information storage devices.     

Development and analysis of low resistance ohmic contact to n-AlGaN/GaN HEMT

High quality ohmic contacts were realized in order to obtain an AlGaN/GaN high electron mobility transistor (HEMT) on different substrates (Si, Al₂O₃, SiC). A metallization scheme based on Ti/Al/Ni/Au was used. The ohmic contacts were obtained using an optimized rapid thermal annealing (RTA) at temperature as high as 900 °C for 30 s in a N₂ atmosphere up to the two dimensional electron gas (2DEG). The piezoelectric stress, measured by microRaman spectroscopy, reveals to be dependent on the distance between the transmission line model (TLM) patterns on Si substrate grown by molecular beam epitaxy (MBE). This should be related to the non-linearity of the total resistance (R_t) measured on the largest spacing between two electrodes. After a long thermal treatment at 500 °C for 2000 h, the ohmic contact shows a very stable behaviour.     

Bifunctional mesoporous Cu–Al–MCM-41 materials for the simultaneous catalytic abatement of NOx and VOCs

This study explored the possibility of using waste organic solvent as the source of volatile organic compound (VOC) and it served as a reducing agent of selective catalytic reduction (SCR) deNO_x process, in which the VOC itself can be catalytically oxidized on the mesoporous Cu and/or Al substituted MCM-41 catalysts. The synthesized Cu–Al–MCM-41 catalysts were extensively characterized by powder low-angle X-ray diffraction (XRD), N₂ adsorption–desorption measurements, transmission electron microscopy (TEM), UV–Visible diffuse reflectance spectroscopy (UV–Vis DRS), ²⁷Al magic angle spinning-nuclear magnetic resonance spectroscopy (MAS-NMR), electron paramagnetic resonance spectroscopy (EPR) and inductively coupled plasma–mass spectrometer (ICP–MS) analysis. The XRD, TEM and N₂ adsorption–desorption studies clearly demonstrated the presence of a well ordered long range hexagonal array with uniform mesostructures. The Cu–Al–MCM-41 materials showed a better long-term-stability than that of copper ion-exchanged H–ZSM-5 (Cu–ZSM-5) zeolite. The Cu–Al–MCM-41 material was found to be an efficient catalyst than that of Cu–MCM-41 without aluminum for the simultaneous catalytic abatement of NO_x and VOCs, which was attributed to the presence of well dispersed and isolated Cu²⁺ ions on the Cu–Al–MCM-41 catalyst as observed by UV–Vis DRS and EPR spectroscopic studies. And the presence of aluminum (Al³⁺ ions) within the framework of Cu–Al–MCM-41 stabilized the isolated Cu²⁺ ions thus it led to higher and stabilized activity in terms of NO_x reduction.     

Annealing-temperature-modulated optical,electrical properties,and leakage current transport mechanism of sol–gel-processed high-k HfAlOx gate dielectrics

Deposition of HfAlO_x gate dielectric films on n-type Si and quartz substrates by sol-gel technique has been performed and the optical, electrical characteristics of the as-deposited HfAlO_x thin films as a function of annealing temperature have been investigated. The optical properties of HfAlO_x thin films related to annealing temperature are investigated by ultraviolet-visible spectroscopy (UV–vis) and spectroscopy ellipsometry (SE). By measurement of UV–vis, average transmission of all the HfAlO_x samples are about 85% owing to their uniform composition. And the increase in band gap has been observed with the increase of annealing temperature. Moreover, the increase of refractive index (n) and density with the increase of annealing temperature are obtained by SE measurements. Additionally, the electrical properties based on Al/Si/HfAlO_x/Al capacitor are analyzed by means of the high frequency capacitance-voltage (C-V) and the leakage current density-voltage (J-V) characteristics. Results have shown that 400 °C-annealed sample demonstrates good electrical performance, including larger dielectric constant of 12.93 and lower leakage current density of 3.75×10⁻⁷ A/cm² at the gate voltage of 1 V. Additionally, the leakage current conduction mechanisms as functions of annealing temperatures are also discussed systematically.     

Atomic order-disorder engineering in the La2Zr2O7 pyrochlore under low energy ion irradiation

Cation and anion disordering affect the structural and electronic properties of the isometric A₂B₂O₇ pyrochlore materials. Here, we report a study on the structural response of La₂Zr₂O₇ at two different temperatures (300 K and ~88 K) as a function of ion fluence (1 × 10¹³, 5 × 10¹³, and 1 × 10¹⁴ ions/cm²). The effect of ion fluence and irradiation temperature on the structural properties have been investigated using the grazing angle x-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy. GIXRD results confirmed that the weakening/broadening of the diffraction peaks and lattice volume expansion increases monotonically as a function of ion fluence at both the temperatures and are more pronounced at ~88 K. The cation and anion disordering appear to be ion fluence and irradiation temperature-dependent. Raman spectroscopy shows that the atomic disordering is more pronounced with enhanced ion fluence and revealed the involvement of the X_48f parameter in the enhancement of disordering in the system. The HRTEM analysis revealed that the deterioration in the atomic ordering (amorphization) is significantly more pronounced at ~88 K. The qualitative analysis of cation/anion disordering and structural deformation revealed that irradiation parameters play a crucial role in developing and altering the properties of the pyrochlore materials for the technological applications.     

Neodymium substituted CaBi4Ti4O15 bismuth layered compound

In the present work, CaBi_4−xNd_xTi₄O₁₅ (0 ≤ x ≤ 1) ceramics were prepared and characterized. Although the substitution of Nd³⁺ for Bi³⁺ led to a decrease of tolerance factor t, the ferroelectricity in CaBi_4−xNd_xTi₄O₁₅ was not enhanced but weakened. With increasing x, the Curie temperature of CaBi_4−xNd_xTi₄O₁₅ was lowered and the peak of dielectric constant was significantly suppressed. Besides, the variation of lattice parameters of CaBi_4−xNd_xTi₄O₁₅ showed an approach of a and b, which revealed a development trend from ferroelectric orthorhombic structure to the prototype tetragonal structure with the substitution. This strongly suggested that Bi³⁺ is crucial for the ferroelectricity in four-layered CaBi₄Ti₄O₁₅, just like found in two-layered Bi₃TiNbO₉ and three-layered Bi₄Ti₃O₁₂ compounds. The importance of Bi³⁺ to the ferroelectricity should be due to the special electron configuration of Bi³⁺ but not its relatively small size, since Nd³⁺ is even smaller than Bi³⁺. The size mismatch between A-site cations and the provskite cuboctahedral cavity, indicated by t, seemed to play a secondary role in the contribution to the ferroelectricity of CaBi₄Ti₄O₁₅ and its analogues. The dielectric properties of CaBi_4−xNd_xTi₄O₁₅ ceramics were also investigated at microwave resonant frequency.     

Induction of ferroelectricity in nanoscale ZrO2 thin films on Pt electrode without post-annealing

Large stable ferroelectricity in nanoscale undoped zirconia (ZrO₂) thin films prepared without post-annealing has been demonstrated for the first time. Remanent polarizations up to 12 μC cm⁻² were obtained in the as-deposited ZrO₂ thin films prepared by remote plasma atomic layer deposition at 300 °C substrate temperature on the Pt electrode. Ferroelectric crystallization of the films was achieved without post-annealing, which is highly beneficial to the application of the films in non-volatile memories and ultralow-power nanoelectronics. The existence of the ferroelectric orthorhombic phase with noncentrosymmetric space group Pbc2₁ in the as-deposited ZrO₂ thin films was confirmed by high-resolution transmission electron microscopy.     

Improving electrical conductivity and wear resistance of hafnium nitride films via tantalum incorporation

Transition metal nitrides are being widely applied, as durable sensors, semiconductor and superconductor devices, their electrical conductivity and wear resistance having a significant influence on these applications. However, there are few reports about how to improve above properties. In this paper, tantalum was incorporated into hafnium nitride films through Hf_1-xTa_xN_y [x=Ta/(Hf+Ta), y=N/(Hf+Ta)] solid solution. The electrical conductivity and wear resistance of the films were significantly improved, due to the increase of the electron concentration (tantalum has one more valence electron than hafnium) and the increase in H/E and H³/E² ratios caused by the effect of solid solution hardening, respectively. The highest electrical conductivity of Hf_1-xTa_xN_y films is 8.3×10⁵ S m⁻¹, which is 1.7 times and 5.2 times of that of hafnium nitride and tantalum nitride films, respectively. In addition, the lowest wear rate of films is 1.2×10⁻⁶ mm³/N m, which is only 10% and 48% of that of hafnium nitride and tantalum nitride films, respectively. These results indicate that alloying with another transition metal is an effective method to improve electrical conductivity and wear resistance of transition metal nitrides.     

Significantly improved dielectric properties of TiO2 ceramics through acceptor-doping and Ar/H2 annealing

The acceptor-doped rutile TiO₂ ceramics, x mol% M₂O₃-(1-x) mol% TiO₂ (M = Al³⁺, Ga³⁺, and In³⁺), were prepared by solid state reaction method. The influence of Ar/H₂ annealing on the structural and dielectric properties of the ceramics were systematically investigated. Our results reveal that the dielectric properties of the ceramics can be significantly improved by the Ar/H₂ annealing. Ga³⁺ is found to be the most suitable dopant with the best doping level of 5 mol%. Excellent dielectric properties of colossal and flat dielectric permittivity (~1.2 × 10⁵ (@1 kHz and 25 °C), low dielectric loss (~0.1), and good frequency stability were achieved over the temperature range of -70–150 °C in the Ar/H₂-annealed 5 mol% Ga₂O₃-95 mol% TiO₂ ceramic. This approach of acceptor-doping and Ar/H₂ annealing leads to two thermally activated relaxations in the sample. The low-temperature relaxation is argued to be a Maxwell-Wagner relaxation caused by frozen electrons, while the high-temperature relaxation is a glass-transition-like relaxation associated with the freezing process of the electrons. This work highlights that engineering low-temperature Maxwell-Wagner relaxation paves a new way other than the frequently used acceptor-donor dual doping to design superior dielectric properties in the TiO₂ system.     

A low temperature approach for photo/cathodoluminescent Gd2O2S:Tb (GOS:Tb) nanophosphors

Titrating the aqueous solution of equimolar RE(NO₃)₃ and (NH₄)₂SO₄ with NH₄OH to pH~9 at ~4°C produced an amorphous precursor that yielded phase-pure and well-dispersed RE₂O₂S nanopowder (RE = Gd_0.99Tb_0.01; GOS:Tb) via a RE₂O₂SO₄ intermediate upon annealing in H₂. The powders calcined at the typical temperatures of 700/1200°C exhibited unimodal size distributions and have the average crystallize sizes of ~17/55 nm, average particle sizes of ~284/420 nm, and specific surface areas of ~14.62/4.53 m²/g (equivalent particle sizes: ~56/180 nm). The 1200°C product exhibited sharp green luminescence at ~544 nm (FWHM = 2.3 nm; λ_ex = 275 nm), with an absolute quantum yield of ~24.8% and a fluorescence lifetime of ~1.34 ms at room temperature. It was also shown that the powder possesses favorable thermal stability (the activation energy for thermal quenching of luminescence ~0.305 eV) and is stable under electron beam irradiation up to 7 kV and 50 μA. The synthetic technique has the advantages of scalability and favorable dispersion and high chemical/phase purity for GOS powder, which may allow the sintering of scintillation ceramics at lower temperatures.     

Autowave chemical transformations of highly exothermic mixtures based on niobium oxide with aluminum

Composite materials based on Nb with functional (Si, C, B) and alloying dopants (Hf, Ti, Al, etc.) are promising materials for aircraft engine applications. Our previous studies have shown that such composites can be synthesized in the autowave mode (combustion mode) using highly exothermic mixtures of Nb₂O₅ with Al, Si, Hf, and Ti. It has been found that in the combustion wave, Hf actively participates in the reduction of Nb₂O₅, which complicates its introduction into the composite material and leads to an excess of Al in the alloy. In the present study, we investigated the possibility of replacing Hf in the starting mixture by less active HfAl₃ and also determined the effect of the grain size of HfAl₃ on the Hf content in the composite material.     

Preparation and investigation of structure,magnetic and dielectric properties of (BaFe11.9Al0.1O19)1-x - (BaTiO3)x bicomponent ceramics

(BaFe_11.9Al_0.1O₁₉)_1-x - (BaTiO₃)_x with x?=?0, 0.25, 0.5, 0.75 and 1 bicomponent ceramics has been prepared from single-phase compounds of BaFe_11.9Al_0.1O₁₉ (x?=?0) (BFO) and BaTiO₃ (x?=?1) (BTO) by a standard ceramic technique. The constituent materials have been chosen considering their perspective ferrimagnetic and ferroelectric properties, respectively for BFO and BTO. Moreover, Ba-hexaferrites are reported to exhibit ferroelectricity at room temperature as well, and the combination of two ferroelectric phases is of interest. Systematic investigations of the structural, magnetic and dielectrical properties versus chemical composition (x) have been performed. The ferrimagnetic phase transition temperature is almost independent of the BTO content, which is determined by intensity of the Fe³⁺-O^2--Fe³⁺ indirect superexchange interactions in the BFO hexaferrite phase. However, the coercivity of composite samples is lower due to the contribution of the microstructure-dependent shape anisotropy to the total magnetic anisotropy energy. The permittivity vs. temperature behavior confirmed the existence of two ferroelectric phase transitions corresponding to structural phase transitions in BTO at ~ 400?K and BFO at ~ 700?K. It has been observed that the dielectrical properties of composite samples, including the temperatures of the phase transitions, critically depended on concentration x which affects the composite microstructure. This behavior has been discussed in terms of microstructure analysis and such parameters as the grain size, porosity and density.     

Crystal structure,impedance, and multiferroic property of SrZrO3 and MnO2 modified 0.725BiFeO3−0.275BaTiO3 ceramics

SrZrO₃ modified and SrZrO₃+MnO₂ co-modified 0.725BiFeO₃−0.275BaTiO₃:xSrZrO₃+yMnO₂ (BF-BT:xSZ+yMnO₂, x, y = 0–0.075) multiferroic ceramics were prepared and the structure, resistance and multiferroic properties were comparatively investigated. SZ plays a more dominant role than MnO₂ in making a phase transition from rhombohedral to pseudocubic around x = 0.02, y = 0, which implies that SZ has formed solid solution with BF-BT whereas most Mn cations may have not entered into lattice of host but exist as second phase isolated particles. The modifications have complex effects on impedance spectra but the SZ-induced MPB has significantly increased resistance. As a result, such modifications show great effects on multiferroics. Briefly, the rhombohedral-pseudocubic morphotropic phase boundary composition shows peak ferroelectricity with remanent polarization of 8.2 μC/cm² and switchable remanent magnetization of 0.22 emu/g. Then the ferroelectricity and magnetism weaken slightly with increasing SZ contents. However, the introduction of MnO₂ results in monotonously suppressed ferroelectricity but enhanced magnetization. These results not only provide promising multiferroic material with switchable polarization and magnetization at room temperature, but also may stimulate further work on co-modified BF-BT solid solutions with optimized multiferroic properties.     

Synthesis and photoluminescent properties of Sm3+-doped SnO2 nanoparticles

SnO₂ ceramic nanoparticles homogeneously doped with Sm³⁺ ions were synthesized via a sol-gel method, followed by drying and annealing in air. X-ray diffractometry, FT-IR spectrometry and transmission electron microscopy were used to characterize the nanoparticulate samples. After annealing, both doped and undoped SnO₂ nanopowders were shown to adopt the rutile tetragonal crystal form and to exhibit characteristic Sn–O–Sn vibrations. The average particle size was found to be in the region of 23–28 nm. Photoluminescence analysis at room temperature demonstrated strong enhancement of the visible emission from Sm³⁺, via energy transfer from the SnO₂ host matrix to the dopant. The maximum emission efficiency was observed for a concentration of 1.5 atom% Sm³⁺.     

High-temperature fracture toughness of SiC–Mo5(Si,Al)3C composites

The fracture toughness (K_1C) of melt-infiltrated SiC–Mo₅(Si,Al)₃C composites was measured from room temperature to 1400°C using the indentation strength method at 1 atm argon atmosphere. The fracture toughness was found to increase from 3.6 MPa·m^1/2 at room temperature to 7.7 MPa·m^1/2 at 1400°C. This increase was mainly attributed to the plastic deformation of the infiltrated Mo₅(Si,Al)₃C phases at high temperatures, which act as ductile toughening inclusions. The influence of annealing temperatures and atmospheres on K_1C was studied. The effect of indentation load on K_1C was also analyzed.     

DFT Predictions of Crystal Structure,Electronic Structure,Compressibility, and Elastic Properties of Hf–Al–C Carbides

To understand the potential for use of the Hf–Al–C ternary compounds, (HfC)_nAl₃C₂ (Hf₂Al₃C₄ and Hf₃Al₃C₅) and (HfC)_nAl₄C₃ (Hf₂Al₄C₅ and Hf₃Al₄C₆) were investigated using density functional theory, including crystal structure, electronic structure, compressibility, and elastic properties. The theoretical density of (HfC)_nAl₃C₂ (4.10–4.16 g/cm³) is higher than that of (HfC)_nAl₄C₃ (3.92–3.98 g/cm³), due to the smaller number of lighter Al–C layers. With increasing numbers of Hf–C layers, the Hf–C and Al–C bond lengths remain almost unchanged. In none of the compounds is there a gap around the Fermi energy (E_f), which implies they are metal‐like conductors. With increasing pressure, there is greater shrinkage along the c axis than the a axis. The bond stiffness increases with increasing pressure. In general, (HfC)_nAl₃C₂ has higher elastic stiffness than (HfC)_nAl₄C₃, with the moduli increasing with the number of Hf–C layers. The Hf–Al–C compounds as well as the brittle Zr–Al–C compounds all have low shear moduli/bulk moduli ratio (G/B) from 0.71 to 0.78, suggesting that the G/B ratio is not always a suitable measure of ductility.     

Effect of Sr(Zn1/3Nb2/3)O3 modification on the energy storage performance of BaTiO3 ceramics

Lead-free (1-x)BaTiO₃-xSr(Zn_1/3Nb_2/3)O₃ (abbreviated as BT-xSZN, x = 0–0.08) relaxor ferroelectric ceramics were prepared using the traditional solid phase technology, and the effects of SZN modification on their phase structures, microstructures, dielectric performance, ferroelectricity and energy storage performance were studied in detail. A pure perovskite phase was observed in the BT-xSZN ceramics. The BT-based ceramics modified by SZN exhibited refined grain size. As the SZN content was increased, the breakdown strength initially increased and then decreased, and the ferroelectric loops gradually became ‘slim’. The BT-xSZN (x = 0.07) ceramics demonstrated a favourable energy storage performance with high recoverable energy density (W_rec = ~1.45 J/cm³) and energy storage efficiency (η = ~83.12%) at 260 kV/cm. Results indicate that the energy storage performance of BaTiO₃ ceramics modified by SZN can be remarkably improved, widening their applications in energy storage at low temperatures.     

Microstructure,optical, electrical properties,and leakage current transport mechanism of sol–gel-processed high-k HfO2 gate dielectrics

Deposition of high-k HfO₂ gate dielectric films on n-type Si and quartz substrates by sol–gel spin-on coating technique has been performed and the structural, optical and electrical characteristics as a function of annealing temperature have been investigated. The structural and optical properties of HfO₂ thin films related to annealing temperature are investigated by X-ray diffraction (XRD), ultraviolet–visible spectroscopy (UV–vis), and spectroscopic ellipsometry (SE). Results indicate that the monoclinic form of HfO₂ appears when temperature rises through and above 500 °C. The reduction in band gap is observed with the increase of annealing temperature. Moreover, the increase of refractive index (n) and density and the decrease of the extinction coefficient with the increase of annealing temperature are obtained by SE measurements. Additionally, the electrical properties based on Al/Si/HfO₂/Al capacitor are analyzed by means of the high frequency capacitance–voltage (C–V) and the leakage current density–voltage (J–V) characteristics. And the leakage current conduction mechanisms as functions of annealing temperatures are also discussed.