Synthesis and characterization of multiferroic BiFeO3 powders fabricated by hydrothermal method

The influence of processing parameters on phase formation and particle size of hydrothermally synthesized BiFeO₃ powders was investigated. BiFeO₃ powder was synthesized by dissolving bismuth nitrate and iron nitrate in KOH solution at temperatures ranging from 150 to 225 °C. X-ray diffraction patterns and scanning electron microscopy observation indicated that rod-like α-Bi₂O₃ phase was formed at initial stage of reaction and dissolved into ions to form thermodynamically stable BiFeO₃ phase. Single-phase perovskite BiFeO₃ has been formed using a KOH concentration of 8 M at a temperature of ≥175 °C in a 6 h reaction period. BiFeO₃ particle growth was promoted by lowering the KOH concentration, or increasing the duration time or reaction temperature. The effects of processing conditions on the formation of crystalline BiFeO₃ powders were discussed in terms of a dissolution–precipitation mechanism. The magnetization of the BiFeO₃ powders at room temperature showed a weak a ferromagnetic nature.     

Elastic–plastic analysis of ultrafine-grained Si2N2O–Si3N4 composites by nanoindentation and finite element simulation

Ultrafine-grained Si₂N₂O–Si₃N₄ composites are fabricated by hot-press sintering of amorphous nano-sized silicon nitride powders at 1600, 1650, and 1700 °C with nano-sized Al₂O₃ and Y₂O₃ as additives. Sintered materials of increasing average grain sizes of 280, 360, and 480 nm were obtained with increasing sintering temperature. Hardness and elastic modulus are tested by nanoindentation. Finite element simulations of the nanoindentation are performed to study the elastic and plastic mechanical properties based on the elastic modulus and P–h curve that were obtained through the nanoindentation tests. A theoretical method is proposed for calculating the stress–strain relationship of brittle ceramic materials based on the experimental nanoindentation data. Several relevant coefficients in the theoretical calculation formula are determined by comparing the calculation and simulation results.     

Room temperature magnetoelectric coupling study in multiferroic Bi4NdTi3Fe0.7Ni0.3O15 prepared by a multicalcination procedure

Single-phase Bi₄NdTi₃Fe_0.7Ni_0.3O₁₅ polycrystalline samples were synthesized following a multicalcination procedure. The sample exhibited multiferroic property at room temperature, which was demonstrated by the ferroelectric (2P_r=8.52 μC/cm², 2E_c=89 kV/cm at applied electric field 110 kV/cm) and magnetic (2M_r=388 m emu/g, 2H_c=689 Oe at applied magnetic field 1.04 T) hysteresis loops. More importantly, magnetoelectric coupling effect is observed from measurements of electrical properties not only under small but also under large electric signal when an external magnetic field is applied. The present results suggest a new candidate for a room temperature multiferroic material with magnetoelectric coupling effect.     

Effect of sintering temperature on the microstructure and mechanical properties of ZrO2-3 mol%Y2O3 sol–gel films

The microstructural evolution and mechanical properties of ZrO₂-3 mol%Y₂O₃ films were investigated as a function of the sintering temperature in the range from 100 °C to 1500 °C, using a battery of characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and nanoindentation. It was found that the crystallization occurs at temperatures close to 300 °C. A gradual increase in the grain and crystallite sizes is observed as the sintering temperature increases up to 1000 °C, and above this sintering temperature the tendency changes abruptly with a rapid increase in these values. Although Young's modulus of the coatings did not change with sintering temperature, a slight decrease was observed in the hardness values above 1000 °C which is attributed to microstructure coarsening. Finally, a slight degradation of the films occurs above 1300 °C, which is due to the occurrence of a process of grain spheroidization.     

Diffusion bonding of alumina using interlayer of mixed hydride nano powders

In this study, mixed hydride/alanate nano powders in the Al–Mg system were used as the interlayer for low temperature diffusion bonding of dense alumina parts. Decomposition of hydride nanopowders at bonding temperatures in-situ formed metals and alloys nano particles with oxide free surfaces and high sinter-ability in the interlayer. Nano powders sintering behavior in the interlayer and formation of compounds in the reaction layer during diffusion bonding were studied. Mixture of 50–50 M ratio of AlH₃ and Mg(AlH₄)₂, as the interlayer improved bond strength of the joints. Diffusion bonding products were formed in the MgO–Al₂O₃ spinel system with different stoichiometries. Bond strength improved up to 202 MPa by induction hot pressing alumina parts at low bonding temperature of 400 °C under pressure of 20 MPa during 30 min bonding period.     

Surface modification of 3Y-TZP with cerium oxide

Surface modification with cerium oxide of tetragonal zirconia polycrystals stabilized with 3 mol.% yttria (3Y-TZP) was carried out in order to improve the resistance to low temperature degradation. Specimens were coated with pressed CeO₂ powder and then annealed at 1400 °C and 1500 °C for periods of up to 10 h. Similar treatments were performed in specimens coated with a sub-micron CeO₂ layer by means of magnetron sputtering. Cerium penetration in the surface modified specimens is about 10 μm into the bulk and the grain size increases in the surface layer affected by cerium diffusion. The indentation bulk fracture toughness and Vickers hardness are not affected by the surface modification treatments. Berkovich nanoindentation was performed to observe the contact hardness and elastic modulus at the surface, showing no significant difference after surface modification. Surface modification with ceria induces a large increase in the resistance to hydrothermal ageing without impairing mechanical properties.     

Influence of precursor Bi/Fe ion concentration on hydrothermal synthesis of BiFeO3 crystallites

In this study we investigated the effect of precursor Bi³⁺/Fe³⁺ ion concentration on the hydrothermal synthesis of BiFeO₃ crystallites. It is demonstrated that the phase-purity and morphology of the products is highly dependent on the metal ion concentration. Phase-pure BiFeO₃ crystals can be prepared at the Bi³⁺/Fe³⁺ ion concentration ranging from 0.025 to 0.0625 M. The samples prepared at n(Bi³⁺/Fe³⁺)=0.025, 0.0375, 0.05, and 0.0625 M, are composed, respectively, of cuboid-like particles (100–200 nm), regular spherical agglomerates (30–40 μm) made up of irregular grains with size about several hundred nanometers, irregular flower-like clusters formed from irregular grains of several hundred nanometers in size, and octahedron-shaped particles (500–600 nm). These samples have a similar bandgap energy of 2.20 eV and exhibit a typical antiferromagnetic behavior at room temperature.     

Effect of Al2O3 particle size on preparation and properties of ZTA ceramics formed by gelcasting

Zirconia-toughened alumina (ZTA) ceramics were prepared using three different kinds of Al₂O₃ powders (marked PW-A average particle size: 7.53 μm, marked PW-B average particle size: 1.76 μm, marked PW-C average particle size: 0.61 μm) by gelcasting. Effect of Al₂O₃ particle size on zeta potential, dispersant dosage and solid volume fractions of ZTA suspensions as well as the mechanical properties of ZTA green bodies and ceramics were investigated. The optimum dosages of dispersant for ZTA suspensions prepared by PW-A, PW-B and PW-C are 0.4 wt%, 0.5 wt% and 0.7 wt%, respectively. The highest solid volume fractions of ZTA suspensions can reach 62 vol% (SP-A), 60 vol% (SP-B) and 52 vol% (SP-C), respectively. The green bodies show a bending strength as high as 20 MPa, which can meet the requirement of machining. The Al₂O₃ powder with fine particle size is beneficial to the improvement of mechanical properties. The ZTA ceramics prepared by PW-B Al₂O₃ powder show the highest bending strength (680 MPa) and toughness (7.49 MPa m^1/2).     

Piezoresponse force microscopy characterization of rare-earth doped BiFeO3 thin films grown by the soft chemical method

The multiferroic behavior with ion modification using rare-earth cations on crystal structures, along with the insulating properties of BiFeO₃ (BFO) thin films was investigated using piezoresponse force microscopy. Rare-earth-substituted BFO films with chemical compositions of (Bi_1.00−xRE_xFe_1.00O₃ (x=0; 0.15), RE=La and Nd were fabricated on Pt (111)/Ti/SiO₂/Si substrates using a chemical solution deposition technique. A crystalline phase of tetragonal BFO was obtained by heat treatment in ambient atmosphere at 500 °C for 2 h. Ion modification using La³⁺ and Nd³⁺ cations lowered the leakage current density of the BFO films at room temperature from approximately 10⁻⁶ down to 10⁻⁸ A/cm². The observed improved magnetism of the Nd³⁺ substituted BFO thin films can be related to the plate-like morphology in a nanometer scale. We observed that various types of domain behavior such as 71° and 180° domain switching, and pinned domain formation occurred. The maximum magnetoelectric coefficient in the longitudinal direction was close to 12 V/cm Oe.     

Transparent yttria produced by spark plasma sintering at moderate temperature and pressure profiles

Transparent yttria (Y₂O₃) bodies were fabricated by spark plasma sintering, and the effects of the sintering temperature on relative density, microstructure, and the optical and mechanical properties of Y₂O₃ bodies were investigated. Fully dense Y₂O₃ bodies were obtained at sintering temperatures 1473-1873 K. The average grain size was 0.24-0.32 μm at 1473-1573 K, and steadily increased to 1.97 μm with an increase in temperature to 1823 K. The highest transmittance was obtained in the Y₂O₃ body sintered at 1573 K and annealed at 1323 K, showing 81.7% (99% of the theoretical value) at a wavelength of 2000 nm.     

Sintering of Al2O3-SiC composite from sol-gel method with MgO, TiO2 and Y2O3 addition

Al₂O₃-SiC composite ceramics were prepared by pressureless sintering with and without the addition of MgO, TiO₂ and Y₂O₃ as sintering aids. The effects of these compositional variables on final density and hardness were investigated. In the present article at first α-Al₂O₃ and β-SiC nano powders have been synthesized by sol-gel method separately by using AlCl₃, TEOS and saccharose as precursors. Pressureless sintering was carried out in nitrogen atmosphere at 1600 °C and 1630 °C. The addition of 5 vol.% SiC to Al₂O₃ hindered densification. In contrast, the addition of nano MgO and nano TiO₂ to Al₂O₃-5 vol.% SiC composites improved densification but Y₂O₃ did not have positive effect on sintering. Maximum density (97%) was achieved at 1630 °C. Vickers hardness was 17.7 GPa after sintering at 1630 °C. SEM revealed that the SiC particles were well distributed throughout the composite microstructures. The precursors and the resultant powders were characterized by XRD, STA and SEM.     

Sintering of glass-added BaZn1/3Nb2/3O3 ceramics

The complex perovskite oxide Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been studied for its attractive dielectric properties which place this material interesting for applications as multilayer ceramics capacitors or hyperfrequency resonators. This material is sinterable at low temperature with combined glass phase–lithium salt additions, and exhibits, at 1 MHz very low dielectric losses combined with relatively high dielectric constant and a good stability of this later versus temperature. The 2 wt.% of ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of LiF-added BZN sample sintered at 900 °C exhibits a relative density higher than 95% and attractive dielectric properties: a dielectric constant ?_r of 39, low dielectrics losses (tan(δ) < 10⁻³) and a temperature coefficient of permittivity τ_? of 45 ppm/°C⁻¹. The 2 wt.% ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of B₂O₃-added BZN sintered at 930 °C exhibits also attractive dielectric properties (?_r = 38, tan(δ) < 10⁻³) and it is more interesting in terms of temperature coefficient of the permittivity (τ_? = −5 ppm/°C). Their good dielectric properties and their compatibility with Ag electrodes, make these ceramics suitable for L.T.C.C applications.     

Mechanochemical synthesis of MgTa2O6 ceramic

MgTa₂O₆ powders were prepared by mechanochemical synthesis from MgO and Ta₂O₅ in a planetary ball mill in air atmosphere using steel vial and steel balls. High-energy ball milling gave nearly single-phase MgTa₂O₆ after 8 h of milling time. Annealing of high-energy milled powder at various temperatures (700–1200 °C) indicated that high-energy milling speed up the formation and crystallization of MgTa₂O₆ from the amorphous mixture. The powder derived from 8 h of mechanical activation gave a particle size of around 28 nm. Although at low-annealing temperatures the grain size was almost the same as-milled powder, the grain size increased with annealing temperature reaching to around 1–2 μm after annealing at 1200 °C for 8 h.     

Nano-twinned structure and photocatalytic properties under visible light for undoped nano-titania synthesised by hydrothermal reaction in water-ethanol mixture

Nano-TiO₂ crystals showing visible light driven photocatalytic activity were synthesized by hydrothermal reaction in an ethanol-water mixture. The experiments were conducted to optimise the synthesis conditions for nano titania, in the range of temperature from 200 to 400 °C. X-ray diffraction depicted that the products obtained were anatase at 250 °C and above. For the products obtained at 250 °C, detailed analysis was conducted since it depicted high crystallinity with smallest particle sizes. Shape of the crystal was rounded rectangular with the size of 4 ± 1 nm to 7 ± 1 nm. The high-resolution transmission electron microscopy (HRTEM) revealed the existence of novel nano-twin structure in anatase grains and surface defects around the nanocrystals. Photocatalytic property was investigated for these undoped titania samples under UV and visible light. The nano twin structure, surface defects, and nano-meter size of the synthesized titania are believed to play a crucial role for the high catalytic activity.     

Investigation of nano particle additives on lithium doped KNN lead free piezoelectric ceramics

Lead free potassium sodium niobate modified piezoelectric ceramics were synthesized through conventional mixed oxide method. Crystal structure and microstructure were analyzed by X-ray diffraction and scanning electron microscopy (SEM). The effects of nano ZnO, CuO and SnO₂ additives as the nano scale sintering aids, on microstructure and electrical properties of (K₀₅₀Na_0.50)_0.94Li_0.06NbO₃ (KNNL-6) ceramics were investigated. The optimum dielectric and piezoelectric properties of ?_r = 560, d₃₃ = 215 pC/N and tan δ = 0.008 were obtained for pure KNNL-6 that sintered at 1000 °C for 2 h. The results show that with addition of nano particle sintering aids, the piezoelectric coefficient d₃₃ of (K₀₅₀Na_0.50)_0.94Li_0.06NbO₃ ceramics was decreased. The decrease in piezoelectric charge coefficient could be due to the hardening effect, which lowers the piezoelectric charge.     

Effect of synthesis condition on the structural and electrochemical properties of Li[Ni1/3Mn1/3Co1/3]O2 prepared by the metal acetates decomposition method

In order to get homogeneous layered oxide Li[Ni_1/3Mn_1/3Co_1/3]O₂ as a lithium insertion positive electrode material, we applied the metal acetates decomposition method. The oxide compounds were calcined at various temperatures, which results in greater difference in morphological (shape, particle size and specific surface area) and the electrochemical (first charge profile, reversible capacity and rate capability) differences. The Li[Ni_1/3Mn_1/3Co_1/3]O₂ powders were characterized by means of X-ray diffraction (XRD), charge/discharge cycling, cyclic voltammetry and SEM. XRD experiment revealed that the layered Li[Ni_1/3Mn_1/3Co_1/3]O₂ material can be best synthesized at temperature of 800 °C. In that synthesized temperature, the sample showed high discharge capacity of 190 mAh g⁻¹ as well as stable cycling performance at a current density of 0.2 mA cm⁻² in the voltage range 2.3-4.6 V. The reversible capacity after 100 cycles is more than 190 mAh g⁻¹ at room temperature.     

Synthesis and mechanical properties characterization of multiferroic BiFeO3-CoFe2O4 composite nanofibers

Multiferroic nanofibers with excellent mechanical properties have great potential applications in multifunctional nanodevices. BiFeO₃-CoFe₂O₄ (BFO-CFO) composite nanofibers with different molar ratios were successfully synthesized by sol-gel-based electrospinning method. The mechanical properties of BFO-CFO composite nanofibers were examined by nanoindentation technique, and further investigated by amplitude modulation-frequency modulation (AM-FM) method based on atomic force microscopy (AFM). The results of AM-FM showed that the elastic moduli of BFO-CFO composite nanofibers increased with the increase of CFO ratio, which was consistent with the results of nanoindentation. These results indicated that AFM-based AM-FM is a powerful method for nondestructively investigating the mechanical properties of materials at nanoscale, and that the results of BFO-CFO composite nanofibers are also of practical importance for the future applications of multifunctional nanodevices.     

Tailoring the multiferroic properties of BiFeO3 by co-doping Er at Bi site with aliovalent Nb,Mn and Mo at Fe site

Single phase multiferroic undoped BiFeO₃ notoriously suffers due to the poor spin–charge coupling resulting in limitations to device applications. The present work focuses on the tailoring of its multiferroic and magnetoelectric coupling properties by synthesizing multiferroic Bi_0.95Er_0.05Fe_0.98TM_0.02O₃ (TM = Nb, Mn and Mo) ceramics. The ferroelectric, magnetic, current leakage measurements and magnetoelectric effect were investigated. XRD along with the Reitveld refinement results confirms that all the samples possess perovskite based rhombohedral structure and reveals that doping of (Er, Nb), (Er, Mn) and (Er, Mo) induced the crystallographic distortion in the BFO lattice and hence induced a variation in the bond lengths and bond angle. Dual doping significantly enhanced the electrical, magnetic properties and magnetoelectric coupling as compared to BiFeO₃. Doping has lowered the leakage current by three to four orders compared to BFO. The lattice distortion, reduced leakage current and destruction of spin–cycloidal structure could be the origin for these improved features. The (Er, Nb) doped BiFeO₃ yields enhanced ferroelectric character with the maximum polarization value of 0.46 µC/cm², maximum ME coupling of 0.22 mV/cm at a magnetic field of 130 G, an improved magnetization with a remanance value of 0.0903 emu/g and the lowest leakage current density.     

Low temperature sintering and microwave dielectric properties of CaSiO3–Al2O3 ceramics for LTCC applications

The effects of CuO, Li₂CO₃ and CaTiO₃ additives on the densification, microstructure and microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics for low-temperature co-fired applications were investigated. With a single addition of 1 wt% Li₂CO₃, the CaSiO₃–1 wt% Al₂O₃ ceramic required a temperature of at least 975 °C to be dense enough. Large amount addition of Li₂CO₃ into the CaSiO₃–1 wt% Al₂O₃ ceramics led to the visible presence of Li₂Ca₃Si₆O₁₆ and Li₂Ca₄Si₄O₁₃ second phases. Fixing the Li₂CO₃ content at 1 wt%, a small amount of CuO addition significantly promoted the sintering process and lowered the densification temperature to 900 °C whereas its addition deteriorated the microwave dielectric properties of CaSiO₃–1 wt% Al₂O₃ ceramics. Based on 10 wt% CaTiO₃ compensation in temperature coefficient, good microwave dielectric properties of ε_r=8.92, Q×f=19,763 GHz and τ_f=−1.22 ppm/°C could be obtained for the 0.2 wt% CuO and 1.5 wt% Li₂CO₃ doped CaSiO₃–1 wt% Al₂O₃ ceramics sintered at 900 °C. The chemical compatibility of the above ceramics with silver during the cofiring process has also been investigated, and the result showed that there was no chemical reaction between silver and ceramics, indicating that the as-prepared composite ceramics were suitable for low-temperature co-fired ceramics applications.     

Low-temperature solution combustion synthesis of high performance LiNi0.5Mn1.5O4

High-voltage LiNi_0.5Mn_1.5O₄ spinels were synthesized by a low temperature solution combustion method at 400 °C, 600 °C and 800 °C for 3 h. The phase composition, structural disordering, micro-morphologies and electrochemical properties of the products were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and constant current charge–discharge test. XRD analysis indicated that single phase LiNi_0.5Mn_1.5O₄ powders with disordered Fd-3m structures were obtained by the method at 400 °C, 600 °C and 800 °C. The crystallinity increased with increasing preparation temperatures. XRD and FTIR data indicated that the degree of structural disordering in the product prepared at 800 °C was the largest and in the product prepared at 600 °C was the least. SEM investigation demonstrated that the particle size and the crystal perfection of the products were increased with increasing temperatures. The particles of the product prepared at 600 °C with ~200 nm in size are well developed and homogeneously distributed. Charge/discharge curves and cycling performance tests at different current density indicated that the product prepared at 600 °C had the largest specific capacity and the best cycling performance, due to its high purity, high crystallinity, small particle size as well as moderate amount of Mn³⁺ ions.