Composition and poling condition-induced electrical behavior of (Ba0.85Ca0.15)(Ti1−xZrx)O3 lead-free piezoelectric ceramics

Lead-free (Ba_0.85Ca_0.15)(Ti_1−xZr_x)O₃ (BCTZ) piezoelectric ceramics were fabricated by normal sintering in air atmosphere. BCTZ ceramics with x = 0.10 possess a coexistence of tetragonal and rhombohedral phases at ∼40 °C. The Curie temperature of BCTZ ceramics decreases with increasing the Zr content. Piezoelectric properties of BCTZ ceramics are dependent on the poling conditions (i.e., the poling temperature and the poling electric field), and the underlying physical mechanism is illuminated by the phase angle. The BCTZ (x = 0.10) ceramic, which locates at the existence of two phases and is poled at E ∼ 4.0 kV/mm and T_p ∼ 40 °C, exhibits an optimum electrical behavior at a room temperature of ∼20 °C: d₃₃ ∼ 423 pC/N, k_p ∼ 51.2%, 2P_r ∼ 18.86 μC/cm², 2E_c ∼ 0.47 kV/mm, ?_r ∼ 2892, and tan δ ∼ 1.53%.     

Low temperature synthesis and dielectric, ferroelectric and piezoelectric study of microwave sintered BaTiO3 ceramics

Barium titanate (BaTiO₃/BT) ferroelectric system was synthesized in single perovskite phase at low temperature by using powders derived from modified solid state reaction (MSSR) and sintered by microwave (MW) processing routes. Conventional calcination temperature was optimized at 900 °C for 4 h. MW sintering of BT samples was carried out at 1100 °C for 30 min to get dense (98% density) ceramics. Room temperature (RT) dielectric constant (?_r) and dielectric loss (tan δ) at 1 kHz frequency of MW sintered BT samples was found to be ∼2500 and 0.03, respectively. Saturated polarization vs. electric field (P-E) loops with remnant polarization (P_r) ∼6 μC/cm² and coercive field (E_c) ∼1.45 kV/cm confirmed the ferroelectric nature of MW sintered BT samples. Piezoelectric coefficient from strain vs. electric field (S-E) loops study was found to be 335 pm/V.     

Preparation and characterization of tetragonal-ZrO2 nanopowders by a molten hydroxides method

Pure tetragonal-ZrO₂ nanopowders are prepared by a molten hydroxides method, using hydrated zirconium nitrate as the starting material at 200 °C. X-ray diffraction analysis and transmission electron microscopy observation reveal that the nanopowders exhibit poor crystalline quality. After heat treated at 400 °C for 10 h in air, the nanopowders are crystalline with size range of ∼10–12 nm and most of them are agglomerated. The formation mechanism of the ZrO₂ nanopowders has been proposed. The heat treated nanopowders have a BET surface area of ∼27.3 m²/g due to agglomeration. The photoluminescence of the heat treated nanopowders has been investigated at room temperature.     

Enhanced mechanical and dielectric behavior of BaTiO3/Cu composites

BaTiO₃/xCu composite ceramics with x = 0-30 wt.% were fabricated by the traditional mixing method in nitrogen gas. The mechanical properties and electric properties of the obtained composites were investigated as a function of the Cu mass fraction using a three bending test and impedance spectroscopy. The results indicated that the relative density of the sintered composites reached above 91%, the Cu-dispersed BaTiO₃ composites enhanced the mechanical properties, particularly the high fracture toughness (∼3.9 MPa m^1/2) and bending strength (∼134 MPa), compared to the monolithic BaTiO₃. Furthermore, the percolation threshold of BaTiO₃/Cu composites was x = 25 wt.%. The permittivity (?_r) markedly increased from ∼2000 for monolithic BaTiO₃ to ∼9000 with increasing Cu up to 30 wt.%. Additionally, the temperature coefficient of this system was less than 5% in the temperature range of 25-115.     

Optical properties of ZnS nanoparticles produced by mechanochemical method

Nanosized zinc sulfide (ZnS) has been synthesized by the mechanochemical route using zinc acetate and sodium sulfide as source materials in a high energy planetary ball mill (HEPBM) with 300 rpm for 2 h. The mechanochemically synthesized powders have been analyzed by X-ray diffraction (XRD) for phase analysis, Field Emission Scanning Electron Microscope (FESEM) for the morphological characterization, UV–vis–NIR spectrophotometer for determining band gap energy and Fluorescence spectroscopy for determining the emission wavelength. The crystallite size of the synthesized ZnS nanoparticles calculated by the Debye–Scherer's formula is in the range 7–9 nm. FESEM morphology shows the fibrous structure of ZnS samples. The value of optical band gap has been found to be in the range 5.2–5.3 eV. Room temperature photoluminescence (PL) spectrum of the samples exhibits a blue light emission using UV excitation wavelength of 280 nm.     

Structural and magnetic properties of NiFe2−xBixO4 (x = 0, 0.1, 0.15) nanoparticles prepared via sol-gel method

NiFe_2−xBi_xO₄ (x = 0, 0.1, 0.15) nanopowders were synthesized via sol-gel method. The precursor gels were calcined at 773 K in air for 1 h to obtain the pure nanostructured NiFe_2−xBi_xO₄ spinel phase. The crystal structure and magnetic properties of the substituted spinel series of NiFe_2−xBi_xO₄ have been investigated by means of ⁵⁷Fe Mössbauer spectroscopy, transmission electron microscopy and alternating gradient force magnetometry. Mössbauer spectroscopic measurements revealed that Bi³⁺ cations tend to occupy octahedral positions in the structure of the substituted ferrite, i.e., the crystal-chemical formula of the as-prepared nanoparticles may be written as: (Fe)[NiFe_1−xBi_x]O₄ (x = 0, 0.1, 0.15), where parentheses and square brackets enclose cations on sites of tetrahedral and octahedral coordination, respectively. Selective area electron diffraction studies provided evidence that the samples of the NiFe_2−xBi_xO₄ series, independently of x, exhibit the cubic spinel structure. The values of the saturation magnetization and the coercive field of NiFe_2−xBi_xO₄ nanoparticles were found to decrease with increasing degree of bismuth substitution.     

Sintering behavior and electromagnetic properties of Fe-deficient NiZn ferrites

Sintering behavior and electromagnetic properties of Ni_0.5Zn_0.5Fe_2−xO_4−3/2x ferrite (x = 0, 0.4, 0.8) by the sol–gel method are investigated. Fe deficiency in the composition enhances sintering and retards grain growth. The near fully dense Fe-deficient samples could be obtained at a sintering temperature as low as 1120 °C and the highest relative density appears in the x = 0.8 sample sintered at 1150 °C. Second phase zincite ZnO resulting from Fe deficiency plays an important role in spinel NiZn ferrites by acting as a grain growth inhibitor and the grain growth of NiZn ferrite is effectively suppressed. When the sintering temperature is above 1200 °C, extensive grain growth occurs due to the probability of serious volatilization of zinc at high temperatures. The ratio of Ni to Zn of NiZn ferrites increases with increasing Fe deficiency due to the separation of zinc from spinel lattice, which results in the decrease in initial permeability and the increase in Curie temperature and resonant frequency.     

An approach to Y2O3:Eu optical nanostructured ceramics

Y₂O₃:Eu³⁺ (1 at.%) translucent nanostructured ceramics with total forward transmission achieving ∼70% of the theoretical limit has been obtained by the transformation-assisted consolidation of custom-made cubic Y₂O₃:Eu³⁺ nanopowders under high pressure (HP). Sintering under the pressure of 7.7 GPa and temperatures in the 100-500 °C range leads to the partial cubic-to-monoclinic phase transition that results in two-phase Y₂O₃:Eu³⁺ nanoceramics. The average grain size of ceramics d ≤ 50 nm for both Y₂O₃:Eu³⁺ polymorph is comparable with crystallite size of initial nanopowders (d ∼ 40 nm), indicating that the grain growth factor is near unity. The phase compositions, morphology, densities, preliminary optical and luminescent properties of synthesized nanostructured ceramics have been studied.     

Effects of the oxygen pressure on the crystalline orientation and strains of YSZ thin films prepared by E-beam PVD

Yttria−stabilized zirconia, YSZ, thin films were prepared by E-beam physical vapor deposition (PVD) at 200 °C under oxygen pressure of 1 × 10⁻³∼1 × 10⁻⁵ Torr. Observations by Field Emission Scanning Electron Microscope (FESEM) proved that different oxygen pressures influenced the thickness of interfacial SiO_x layer formed between the YSZ thin films and Si(100)-substrate. X-ray diffraction (XRD) patterns were used to determine the crystalline structure and calculate the surface grain size of deposited YSZ thin films. XRD patterns also showed that the peaks corresponding to planes (111), (200), (220), and (311) were found and the YSZ thin films revealed the fluorite structure. At lower oxygen pressure (1 × 10⁻⁵∼1 × 10⁻⁴ Torr) YSZ thin films revealed the (111) preferred orientation and at higher oxygen pressure (5 × 10⁻⁴∼1 × 10⁻³ Torr) YSZ thin films revealed the (200) preferred orientation. The effects of oxygen pressure on the lattice constants and the internal strains of YSZ thin films were also investigated.     

Magnetic and catalytic properties of copper ferrite nanopowders prepared by a microwave-induced combustion process

Copper ferrite nanopowders were successfully synthesized by a microwave-induced combustion process using copper nitrate, iron nitrate, and urea. The process only took a few minutes to obtain CuFe₂O₄ nanopowders. The resultant powders were investigated by XRD, SEM, VSM, and surface area measurement. The results revealed that the CuFe₂O₄ powders showed that the average particle size ranged from 300 to 600 nm. Also, it possessed a saturation magnetization of 21.16 emu/g, and an intrinsic coercive force of 600.84 Oe, whereas, upon annealing at 800 °C for 1 h. The CuFe₂O₄ powders specific surface area was 5.60 m²/g. Moreover, these copper ferrite magnetic nanopowders also acted as a catalyst for the oxidation of 2,3,6-trimethylphenol to synthesize 2,3,5-trimethylhydrogenquinone and 2,3,5-trimethyl-1,4-benzoquinone for the first time. On the basis of experimental evidence, a rational reaction mechanism is proposed to explain the results satisfactorily.     

Transesterification of palm oil on KyMg1 − xZn1 + xO3 catalyst: Effect of Mg-Zn interaction

The Mg-Zn interaction effect of K_yMg_1 − xZn_1 + xO₃ heterogeneous type catalyst and its performance on transesterification of palm oil have been studied using the response surface methodology and the factorial design of experiments. The catalyst was synthesized using the co-precipitation method and the activity was assessed by transesterification of palm oil into fatty acid methyl esters. The ratio of the Mg/Zn metal interaction, temperature and time of calcination were found to have positive influence on the conversion of palm oil to fatty acid methyl ester (FAME) with the effect of metal to metal ratio and temperature of calcination being more significant. The catalytic activity was found to decrease at higher calcination temperature and the catalyst type K₂Mg_0.34Zn_1.66O₃ with Mg/Zn ratio of 4.81 gave FAME content of 73% at a catalyst loading of 1.404 wt.% of oil with molar ratio of methanol to oil being 6:1 at temperature of 150 °C in 6 h. A regression model was obtained to predict conversions to methyl esters as a function of metal interaction ratio, temperature of calcination and time. The observed activity of the synthesized catalyst was due to its synergetic structure and composition.     

High piezoelectric coefficient of Pr2O3-doped Ba0.85Ca0.15Ti0.90Zr0.10O3 ceramics

Pr₂O₃-doped Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃ (BCTZ-xPr) ceramics were prepared by the conventional solid-state method. A tetragonal phase is only observed in these ceramics, and the introduction of Pr₂O₃ decreases their sintering temperature without affecting negatively the piezoelectric constant. Enhanced ferroelectric properties were obtained in these BCTZ-xPr ceramics. The ceramic with x=0.06 wt% exhibits a good electrical behavior of d₃₃∼460 pC/N, k_p∼47.6%, ε_r∼4638, and tan δ∼0.015 when sintered at a low temperature of ∼1400 °C. As a result, the BCTZ-xPr ceramic is a promising candidate for lead-free piezoelectric ceramics.     

A comparative study of electric properties of nano-structured and bulk Mn-Mg spinel ferrite

A series of nano-sized particles of Mn_1−xMg_xFe₂O₄ (x = 0.0, 0.1, 0.2 and 0.25) have been synthesized by co-precipitation method. The effect of Mg-substitution on structural and dielectric properties is reported in this paper. X-ray diffraction analysis for both nanosize and bulk samples revealed the nanocrystalline nature in the prepared ferrite samples. The crystallite size increases from 3-6 nm in nanosamples to 63.9-85.5 nm in bulk samples. The dielectric properties for all the samples have been studied as a function of frequency in the range 10-10⁵ Hz at different temperatures. Dielectric properties such as dielectric constant (?′) dielectric loss (?″), dielectric loss tangent (tan δ) and ac conductivity have been studied for the investigated samples as a function of frequency. The data indicated that, the dielectric constant and the loss factor values of our former are ten orders of magnitude than those of the later. The low dielectric behavior makes ferrite materials useful in high frequency applications.     

High P-T phase transformations and metastability in the Zr0.5Hf0.5O2 solid-solution ceramic

High pressure-temperature (P-T) phases of the Zr_xHf_1−xO₂ (x = 0.5) solid-solution have been stabilised in a CO₂ laser heated diamond anvil cell. At room-temperature the monoclinic to orthorhombic-I structural transformation is initiated at 5-8 GPa. The X-ray diffraction (XRD) studies show these two phases coexist to above ∼15 GPa. A progressive increase in the orthorhombic-I phase abundance occurs, to culminate in full conversion at ∼20 GPa. At this lower threshold of ∼20 GPa transformation to the orthorhombic-II (cotunnite) structure can be initiated by heating in the range of 600-1200 °C. Substantial conversion to the cotunnite phase occurs in the same temperature range at 25-30 GPa. Raman signatures have been assigned to the two orthorhombic high-pressure phases, aided by the qualitative assessment of the complementary XRD data. Decompression experiments show that phase mixture composites of these high pressure structures, possibly with enhanced tribological properties, can be recovered to ambient conditions.     

Structural development and magnetic phenomenon in Zn–Cr–Fe multi oxide nano-crystals

The Cr³⁺ ions doped multi-oxide ZnFe_2−xCr_xO₄ ferrite nanoparticles have been synthesized by chemical co-precipitation method. Site occupancies of Zn²⁺, Cr³⁺ and Fe³⁺ ions were analyzed using X-ray diffraction data and Buerger's method. The effect of the constituent phase variation on the magnetic hysteresis behavior was examined by saturation magnetization which decreases with the increase in Cr³⁺ content in place of Fe³⁺ ions at octahedral B-site. Typical blocking temperature (T_B) around 90 K was observed by zero field cooling and field cooling magnetization study. Room temperature Mössbauer spectra show two paramagnetic doublets (tetrahedral and octahedral sites). The isomer shifts of both doublets decrease whereas quadrupole splitting and relative area of tetrahedral A-site increases with increasing Cr³⁺ substitution. The dielectric constant (measured on compositions x=0, 0.4, 0.8 and 1.0) increases when the temperature increases as in the semiconductor. This behavior is attributed to the hopping of electrons between Fe²⁺ and Fe³⁺ ions with a thermal activation.     

Synthesis and electrochemical characterizations of nano-scaled Zn doped LiMn2O4 cathode materials for rechargeable lithium batteries

The LiZn_xMn_2−xO₄ (x = 0.00-0.15) cathode materials for rechargeable lithium-ion batteries were synthesized by simple sol-gel technique using aqueous solutions of metal nitrates and succinic acid as the chelating agent. The gel precursors of metal succinates were dried in vacuum oven for 10 h at 120 °C. After drying, the gel precursors were ground and heated at 900 °C. The structural characterization was carried out by X-ray powder diffraction and X-ray photoelectron spectroscopy to identify the valance state of Mn in the synthesized materials. The sample exhibited a well-defined spinel structure and the lattice parameter was linearly increased with increasing the Zn contents in LiZn_xMn_2−xO₄. Surface morphology and particle size of the synthesized materials were determined by scanning electron microscopy and transmission electron microscopy, respectively. Electrochemical properties were characterized for the assembled Li/LiZn_xMn_2−xO₄ coin type cells using galvanostatic charge/discharge studies at 0.5 C rate and cyclic voltammetry technique in the potential range between 2.75 and 4.5 V at a scan rate of 0.1 mV s⁻¹. Among them Zn doped spinel LiZn_0.10Mn_1.90O₄ has improved the structural stability, high reversible capacity and excellent electrochemical performance of rechargeable lithium batteries.     

Separation of NaCl, glycin from collagen solution by the thermal sensitive polyurethane membrane

A novel thermal sensitive polyurethane (TSPU) membrane with a thermal switch was prepared via wet phase inversion technique and used for separation of NaCl, glycin from collagen solution. From Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD), it was found that the TSPU showed segmented structure (i.e. the hard segment and soft segment) and independent phase transition temperatures (the phase transition temperature of the soft segment defined as switch temperature, T_s). Scanning Electron Microscope (SEM) and Atom Force Microscope (AFM) were employed to study the morphology structures of membranes, the results indicated that the membrane showed relative dense surfaces (or skin) and porous cross sections, which controlled their selectivity and permeation. The porosity of membrane measured by the mass loss of wet membrane after drying revealed that when the temperature varied ± 10 °C around the T_s, the porosity of membranes increased from 51.7% to 73.3%, showing a significant improvement to thermal stimuli. When this TSPU membrane was used for separation of NaCl, glycin from collagen solution, we found the ions of Na⁺ and Cl⁻ could permeate the TSPU membrane at any temperature, and higher temperature resulted in higher penetration rate. Whilst the penetration of glycin relied on the temperature variation, that is, a barrier effect at lower temperature (T < T_s) and higher permeation fluxes at higher temperature were observed. Typically, when the temperature exceeded the T_s, the permeation flux of glycin increased markedly, showing sensitivity to thermal stimuli. Collagen, due to its large molecule size, could not permeate the TSPU membrane in all temperature range. As a result, molecules of NaCl, glycin and collagen with different size could be selectively separated by TSPU membrane driven by the temperature.     

Growth of multioxide planar film with the nanoscale inner structure via anodizing Al/Ta layers on Si

An Al/Ta bilayer specimen prepared by a successive sputter-deposition of a 150-nm tantalum layer and a 180-nm aluminium layer onto a silicon wafer is anodically processed in a sequence of steps in oxalic acid electrolytes, at voltages of up to 53 V, which generates a 260-nm alumina film with well-ordered nanoporous structure. Further potentiodynamic reanodizing the specimen to 220 V causes the simultaneous growth of a 65-nm tantalum oxide layer beneath the alumina film and an array of oxide ‘nanocolumns’ (∼50 mn wide, ∼80 nm apart, ∼7 × 10⁹ cm⁻² population density) penetrating the alumina pores and reaching precisely to the top of the alumina film. The complete filling of the alumina pores is assisted by the high Pilling-Bedworth ratio for Ta/Ta₂O₅ and a substantially increased transport number for tantalum species (0.4), which is an average value of all migrating tantalum ions with different oxidation states. The nanocolumns are shown to be composed of a unique, regular mixture of Ta₂O₅ (dominating amount), suboxides TaO₂ and TaO_x (0.5 < x < 1), Al₂O₃, metallic Ta and Al aggregates, tantalum diboride (TaB₂) and oxidized boron from the electrolyte. The ionic transport processes determining the self-organized growth of these planar oxide nanostructures are considered and described conceptually.     

Electrochemical properties of La1−xSrxFeO3 (x = 0.2, 0.4) as negative electrode of Ni-MH batteries

La_(1−x)Sr_xFeO₃ (x = 0.2,0.4) powders were prepared by a stearic acid combustion method, and their phase structure and electrochemical properties were investigated systematically. X-ray diffraction (XRD) analysis shows that La_(1−x)Sr_xFeO₃ perovskite-type oxides consist of single-phase orthorhombic structure (x = 0.2) and rhombohedral one (x = 0.4), respectively. The electrochemical test shows that the reaction at La_(1−x)Sr_xFeO₃ oxide electrodes are reversible. The discharge capacities of La_(1−x)Sr_xFeO₃ oxide electrodes increase as the temperature rises. With the increase of the temperature from 298 K to 333 K, their initial discharge capacity mounts up from 324.4 mA h g⁻¹ to 543.0 mA h g⁻¹ (when x = 0.2) and from 147.0 mA h g⁻¹ to 501.5 mA h g⁻¹ (when x = 0.4) at the current density of 31.25 mA g⁻¹, respectively. After 20 charge-discharge cycles, they still remain perovskite-type structure. Being similar to the relationship between the discharge capacity and the temperature, the electrochemical kinetic analysis indicates that the exchange current density and proton diffusion coefficient of La_(1−x)Sr_xFeO₃ oxide electrodes increase with the increase of the temperature. Compared with La_0.8Sr_0.2FeO₃, La_0.6Sr_0.4FeO₃ electrode is a more promising candidate for electrochemical hydrogen storage because of its higher cycle capacity at various temperatures.     

Synthesis of nanocrystalline Mn-Zn ferrite powders through thermolysis of mixed oxalates

Nanocrystalline Mn-Zn ferrite powders were synthesized by thermal decomposition of an oxalate precursor. Two polymorphs of a mixed Mn-Zn-Fe oxalate dihydrate were obtained by precipitation of metal ions with oxalic acid: monoclinic α-(Mn, Zn, Fe)₃(C₂O₄)₃·6H₂O is obtained after precipitation and ageing at 90 °C, whereas the orthorhombic β-type is formed after precipitation at room temperature. The morphology of the oxalate crystals can be controlled by the precipitation conditions. The α-polymorph of the mixed oxalate consists of prismatic and agglomerated particles. The β-oxalate forms non-agglomerated crystallites of submicron size. Thermal decomposition of the oxalate at 350 °C in air results in an amorphous product. Nanosize Mn-Zn ferrite powders are formed at 500 °C and a mixture of haematite and spinel is observed at 750 °C. The thermal decomposition of the mixed oxalate is monitored by thermal analysis, XRD and IR-spectroscopy. The morphology of the oxalate particles is preserved during thermal decomposition; the oxide particle aggregates display similar size and shape as the oxalates. The primary particles are much smaller; their size increases from 3 nm to 50 nm after decomposition of the oxalates at 350 and 500 °C, respectively. The powder synthesized by decomposition at 500 °C was sintered at 1150 °C to dense and fine-grained Mn-Zn ferrites.