Synthesis,structural and ferroelectric properties of barium titanate based glass-ceramic nano-composites

Novel glass-ceramic nano-composites based on BaTiO₃–V₂O₅–Bi₂O₃ glasses were prepared via heat-treatment at crystallization temperature (T_cr). The nanostructural and ferroelectric properties were investigated by differential scanning calorimeter (DSC), transmission electron microscope (TEM), X-ray diffraction (XRD) and dielectric measurements, respectively. It was shown by XRD and TEM studies that appropriate heat-treatment glasses can turn into glass-ceramic nano-composites consisting of crystallites smaller than 30 nm embedded in the glassy matrix. The observed dielectric constant of heat‐treated samples, showed average broad peak around 324 K. The average Curie temperature (T_c) observed around 324 K in the present system. This observation of these glass-ceramic nano-composites can be utilized to control T_c by proper adjustment of BaTiO₃ content. This result, therefore, supports dipolar orientation and phase transition in the glass-ceramic samples dispersed with nanocrystalline BaTiO₃. Such a feature can be used for industrial applications where the shape of the application article is a necessary requirement such as integrated capacitor.     

Effects of sintering temperature on microstructure and property evolution of Fe81Cu2Nb3Si14 soft magnetic materials fabricated from amorphous melt-spun ribbons by spark plasma sintering technique

Bulk Fe₈₁Cu₂Nb₃Si₁₄ soft magnetic materials were fabricated by spark plasma sintering (SPS) of amorphous powder, which was prepared by ball milling of melt-spun ribbons. Effects of sintering temperature on the evolution of microstructure and related properties were systematically investigated. Results show that the as-milled powders exhibit similar microstructure and thermal property in comparison with the original melt-spun ribbons. With an increase in the sintering temperature, the relative density, microhardness and saturation magnetization of the sintered samples improved obviously, but the coercive force decreased at the beginning and then increased with the increase of sintering temperature. The sintered samples exhibit typical soft magnetic characteristic. The desirable soft magnetic property of the sintered samples was achieved by SPS at 630 °C. Meanwhile, microstructure and densification behaviors of the sintered samples were also investigated.     

Relationship between the imperfection of Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> garnet samples of different crystallographic orientations and their physical properties

The kinetic, optical, and dielectric properties of Y₃Fe₅O₁₂ garnet single crystals of different structural quality have been investigated. It is established that the ratio of the intensities of coherent and incoherent scattering characterizes the conduction electron concentration in inhomogeneous magnetic samples. A correlation between the physical properties of the samples and the character of their structural inhomogeneity is revealed.     

Dielectric Properties of Beta Barium Borate and Potassium Pentaborate Single Crystals

Single crystals of beta barium borate and potassium pentaborate (commonly known as β-BaB₂O₄ or BBO and KB₅O₈. 4 H₂O or KB5) were grown by high and low temperature solution growth methods, respectively. The dielectric constant and loss tangent were determined as a function of temperature. Frequency response of the dielectric constant and tan δ have been studied over a frequency range of 0.1 −100 kHz; it is found that both the dielectric constant and tan δ decrease with increase in frequency for both crystals. The dielectric constant and tan δ are found to increase with increase in temperature for both the crystals. Further, the dielectric constant and tan δ measured on different orientations of the grown crystals reveal the anisotropic nature of the crystals.     

Effect of frit size on sintering, crystallization and electrical properties of wollastonite glass-ceramics

Wollastonite glass-ceramics were prepared through pressureless sintering. The sinterability of the prepared samples of the glassy powder in the system (SiO₂-CaO-Na₂O-Fe₂O₃-WO₃) was investigated in the temperature range 720-900 °C and soaking time of 180 min. The influence of the increase in the glass powder particle size on the sinterability and dielectric properties of the glass-ceramic samples was studied.The sintered specimens were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) methods. XRD analysis revealed that wollastonite was the main crystalline phase in the sintered glass-ceramics. Dielectric properties such as dielectric constant and dielectric loss were measured via a network analyzer at 10 GHz.It was observed that the increase of the glassy powder particle size improved the sinterability and dielectric properties of the glass-ceramic specimens. Wollastonite glass-ceramics with 16 μm particle size had maximum constant and minimum loss (ε_r = 10.10 and tan δ = 0.005) compared with the other glass-ceramics.     

Synthesis and characterization of TM-doped CuO (TM = Fe,Ni)

Polycrystalline Cu_1?xTM_xO samples (x = 0 and 0.06; TM = Ni²⁺ and Fe³⁺) were grown using a co-precipitation method. The structural and magnetic properties were investigated by means of temperature dependent magnetic susceptibility and room temperature X-ray powder diffraction (XRPD). The XRPD analyses of the samples reveal the formation of single phase with structure isomorphous to the CuO. Interestingly, T-dependent magnetization shows the reduction of Néel temperature, T_N, from 213 K in the copper oxide to 70 K in the Fe-doped sample (x = 0.06). Because in the Ni-doped samples T_N seems to be unaffected, this decrease in T_N is believed to be due to the different electronic structure of the dopant. The ferromagnetic behavior observed at room temperature in all samples can be related to both the level of oxygen (excess or vacancy) of our samples and to the difference in the magnetic structure of the dopant.     

Structural,dielectric and optical properties of transparent glasses and glass-ceramics of SrBi2B2O7

Optically clear glasses of SrBi₂B₂O₇ (SBBO) were fabricated via the conventional melt-quenching technique. The amorphous nature of the as-quenched samples of this compound was confirmed by X-ray powder diffraction (XRD) studies. Its glassy nature was established by differential scanning calorimetry (DSC). However, the optical microscopy revealed the presence of isolated hexagonal shaped crystallites especially at the edges of the as-quenched glasses. The glass plates that were heat-treated around the onset of the glass transition temperature (670 K) for 12 h yielded transparent (～60% transmission) glass-ceramics of SrBi₂B₂O₇ (SBBO) with well defined microstructure. These were found to be textured associated with an orientation factor of about 0.77 (77%). The optical transmission studies carried out in the 100–1200 nm wavelength range confirmed both the as-quenched and heat-treated samples to be transparent from 400 to 1200 nm. The dielectric properties of the as-quenched as well as the heat-treated (670 K/12 h) samples were studied as a function of frequency (100 Hz–10 MHz) at various temperatures (303–873 K). The dielectric dispersion at higher temperatures in the as-quenched glass was rationalized using Jonscher’s dielectric dispersion relations. The prefactor A(T) and the exponent n(T) in the Jonscher’s expression were found to be maximum and minimum respectively around the crystallization temperature (T_cr) of the as-quenched SBBO glasses.     

Decrease in dielectric loss of CaCu3Ti4O12 by the addition of TeO2

CaCu₃Ti₄O₁₂ (CCTO) has challenged for the last few years the scientific community due to its large dielectric constant, which is almost temperature and frequency independent, from 100 K to 400 K and from 1 kHz to 1 MHz, respectively. This makes the material desirable for many electronic applications. However, the dissipation factor is very large, with tan δ values, at room temperature and 1 kHz, higher than 0.1.In our work we report how the addition of TeO₂ lowers the dielectric loss and, although there is a decrease of dielectric constant of doped samples relatively to the undoped one, high dielectric constant values are still being reached. The sample of doped CCTO with 1.5% of TeO₂ by weight, presents, at room temperature and 60 kHz, a large dielectric constant, over 3000, and a dissipation factor around 0.09, which represents a decrease on tan δ over 30% relatively to the CCTO undoped sample. Two relaxation processes were identified for all the samples, one at MHz region and the other one at low frequency region (< 1 kHz). DC bias voltage was applied up to 40 V and a strong dc bias influence on the low frequency region was observed both at dielectric and impedance responses of the undoped sample, which was much weaker than the dc bias effects on the 4% Te doped sample.Dielectric measurements will be discussed and correlated with the samples' microstructure, supported on internal barrier layer capacitance (IBLC) and surface barrier layer capacitance (SBLC) models.     

Growth,composition, ferroelectric and magnetic properties of new multiferroic Pb3.3Mn4.8Ni1.1Ti0.56O15.3 single crystals

Multiferroic single crystals in the novel system Pb‐Mn‐Ni‐Ti‐O have been grown by the high temperature solution growth method. At room temperature the crystals are indexed in the hexagonal space group P6₃cm. The dielectric and magnetic properties along with the temperature dependence of the c‐lattice parameter have been studied in the temperature range 2 K ‐ 500 К. The magnetic measurements reveal a paramagnetic to antiferromagnetic phase transition around 48 K. The dielectric permittivity exhibits a maximum at 430 K, indicating ferroelectric to paraelectric phase transition. The temperature dependent Raman and XRD measurements around 430 K reveal an anomaly and abrupt change of the lattice parameter along the z‐axis respectively, thus confirming the ferroelectric‐to‐paraelectric phase transition.     

The role of coordination and valance states of tungsten ions on some physical properties of Li2O–Al2O3–ZrO2–SiO2 glass system

A series of Li₂O–Al₂O₃–ZrO₂–SiO₂ glasses doped with different concentrations of WO₃ (0 to 5.0 mol.%) have been synthesized. Differential thermal analysis of the samples indicated increasing glass forming ability with the increasing concentration of WO₃ in the glass matrix. A variety of spectroscopic (optical absorption, IR, Raman and ESR) and dielectric properties (over a range of frequency and temperature) of these glasses have been investigated. The optical absorption and ESR spectral studies have pointed out that a part of tungsten ions do exist in W⁵⁺ state in addition to W⁶⁺ state especially in the samples containing low concentration of WO₃. The IR and Raman spectral studies have suggested that there is a decreasing degree of disorder in the glass network with increase in the concentration of WO₃. The values of dielectric parameters viz., dielectric constant, loss and ac conductivity at any frequency and temperature are observed to decrease as the concentration of WO₃ is increased. Such changes have been attributed to decrease of redox ratio or decreasing proportions of W⁵⁺ ions that act as modifiers in the glass network. The quantitative analysis of the results of ac conductivity and dielectric properties have indicated an increase in the insulating character of the glasses with the concentration of WO₃; this is attributed to the presence of tungsten ions largely in W⁶⁺ ions that participate in the glass network forming with WO₄ structural units.     

Ni1‐x Crx alloy for self controlled magnetic hyperthermia

It is known now that an increase in tumor temperature decreases the tumor resistance to chemo‐ and radiation therapies. Hyperthermia treatment of the tumor cells where damage to the healthy cells can be avoided is viable by using magnetic nanoparticles with controlled Curie temperatures. Nickel‐Chromium (Ni_1‐x Cr_x) particles with varying compositions have been investigated as thermoseeds for use in localized self controlled hyperthermia treatment of cancer. A series of Ni_1‐x Cr_x alloys, have been prepared to find the specific composition which has Curie temperature around 316‐317 K. The samples were cast by arc melting technique, and were annealed at 850 ^oC for 5 hours in sealed quartz tubes. Magnetic properties of the samples were investigated, including Curie temperature, saturation magnetization and hysterisis using Superconducting Quantum Interference Device (SQUID) and Vibrating Sample Magnetometer (VSM). The Curie temperatures of the alloys were found to decrease almost linearly from 401 K to 289 K as the Cr concentration was increased from x = 4.54 wt% to x = 5.90 wt%. The results showed that Ni_1‐x Cr_x alloys might be good candidates for self regulating magnetic hyperthermia applications. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Conductivity and dielectric properties of polymer electrolytes PEO:LiN(CF3SO2)2 near glass transition

Results of measurements of PEO:LiN(CF₃SO₂)₂ polymer electrolytes of composition between 12:1 and 1.5:1 EO:Li, performed by impedance spectroscopy, impedance spectroscopy simultaneous with microscope observation, differential scanning calorimetry and X-ray diffraction were analyzed focusing on electrical properties of semicrystalline and rapidly cooled amorphous samples. In the loss spectra measured at low temperature, the occurrence of two dielectric relaxations was evidenced: local (β) and segmental (α). For each of the investigated electrolytes, the temperature dependence of the ionic conductivity and the frequency of α relaxation could be described by the VTF function with the same value of parameter T₀, which indicated close coupling of both phenomena. The local (β) relaxation exhibited an Arrhenius type temperature dependence. With increasing amount of salt, two effects were observed for amorphous samples: an increase of the glass transition temperature T_g affecting the α relaxation and changes of structure of PEO:LiN(CF₃SO₂)₂ complexes reflected in shift of the β relaxation frequency. Crystallization caused decrease of both the ionic conductivity and the strength of dielectric relaxations. The presence of crystalline phase was also reflected in a shift of the T_g of amorphous phase remaining in the system with respect to the T_g of amorphous electrolyte obtained by rapid cooling.     

Dielectric relaxation and morphologic properties of CaCu3Ti4O12 doped with GeO2

CaCu₃Ti₄O₁₂ (CCTO) is a material with giant dielectric constant, presenting good stability over a wide temperature and frequency ranges. The preparation method and doping has a great influence on the microstructure and dielectric properties of this material. In this work, doping CCTO with 2–10 wt% GeO₂ has been shown to increase the dielectric constant. We studied the prepared samples by X-ray diffraction (XRD), scanning electron microscopy (SEM) and impedance spectroscopy. X-ray diffraction shows the presence of nanocristals. Grains and grain boundaries compositions have been observed by scanning electron microscopy with energy dispersive X-ray spectrometry mapping. Impedance spectroscopy measurements, in the frequency range from 75 kHz to 30 MHz, and temperature from 250 to 325 K, have been performed. The data were analyzed using the Cole–Cole model of dielectric relaxation.     

Thermal and electrical properties of BaO-B2O3-ZnO glasses

Glasses in the BaO-ZnO-B₂O₃ system were examined as potential replacement for PbO glass frits with low firing temperature (500-600 °C) for the dielectric layer of a plasma display panel (PDP). The glasses were evaluated for glass transition temperature (T_g), thermal expansion coefficient (α) and dielectric constant ε. The electrical and the thermal properties were also compared with theoretical data calculated by a known empirical equation. T_g of the glasses varied between 480 and 560 °C, and α was in the range of 7-9×10⁻⁶ K⁻¹. The dielectric constant ranges from 14 to 19 and the theoretical data showed lower α and ε than the experimental data. The results suggest that BaO-ZnO-B₂O₃ glasses would be suitable as an alternative to Pb-based dielectric layer in PDPs.     

Effect of doping method on the formation of charge-compensating defects in PbMoO4:Nd3+ crystals

PbMoO₄:Nd³⁺ single crystals have been grown using different doping schemes. Their dielectric properties have been studied in the temperature range of 20?C550°C at frequencies from 25 to 10⁶ Hz. The activation energies of dielectric relaxation are determined for all samples, and the Nd³⁺ luminescence decay kinetics is studied. The most realistic models of activator centers in PbMoO₄:Nd³⁺ crystals are proposed based on the optical and dielectric spectroscopy data.     

Flux growth and low temperature dielectric relaxation in piezoelectric Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals

Single crystals of Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT 91/9) have been grown by flux method after modifications in temperature profile, flux ratio and addition of excess ZnO/B₂O₃ which resulted in enhanced perovskite yield (more than 95%). Only a few crystals showed the presence of pyrochlore phase/variation in composition. A comparative characterization of these crystals were carried out in respect of piezoelectric charge coefficient d₃₃, dielectric constant, ac conductivity and hysteresis loop after cutting and poling the crystals along [001] direction. The total activation energy for conduction has been found to increase with Ti‐content in the sample. The effect of ZnO on growth behavior has been analyzed. A detailed analysis of PZNT (91:9) has been carried out at low temperature in respect of the various thermodynamic parameters related to the dielectric relaxation mechanism, like optical dielectric constant, static dielectric constant, free energy of activation for dipole relaxation, enthalpy of activation and relaxation time, have been calculated in the vicinity of transition temperature in the lower temperature region. The activation energy for relaxation at ‐10 and ‐49 °C have been found to be 0.09 and 0.02 eV respectively. The results were analyzed and a detailed dielectric analysis and low temperature relaxation behavior of PZNT crystals were interpreted. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Magnetic and electric field characterization of La2CoMnO6 crystals doped with Pb

Crystals of La₂CoMnO₆ doped with Pb were successfully grown by the high temperature solution growth method and their magnetic and transport properties were studied. The examined crystal is found to have predominantly ordered Co²⁺/Mn⁴⁺ structure with randomly distributed Mn³⁺ substituting Co²⁺. A relaxor‐like temperature dependence of the dielectric constant with relaxation maximum is established in the temperature interval 180–210 K. On the base of the dc‐conductivity data, it is assumed that the charge transport in the interval 180–350 K is governed by small‐polaron hopping, whose onset coincides with the Curie temperature.     

Spectroscopic and dielectric properties of a lithia-containing glass

Glass of the composition 65SiO₂-20CaO-15Li₂O (mol %) was prepared and subjected to heat treatment. The obtained samples were characterized before and after heat treatment by DTA, TG, XRD, SEM, IR and dielectric spectroscopy. DTA showed an endothermic peak at 954 °C, accompanied by a pronounced change in the microstructure, as revealed by SEM. XRD showed that metasilicate predominates on heat treatment at 726 °C, while on heat treatment at 726 °C, then at 954 °C, disilicate crystallizes as the main phase. The IR spectra of the heat-treated glasses revealed that the vibrations of O-H groups are drastically decreased, while those due to non-bridging oxygens Si-O⁻ are increased. The dielectric constant (ε′), the loss tangent (tan δ) and the ac conductivity (σ_ac) for the prepared glasses were investigated before and after heat treatment over a moderately wide range of frequency and temperature. The activation energy of the dielectric relaxation process was found to depend on the techniques of sample preparation. A drop of dielectric constant values was observed for the heat-treated sample, which can be attributed to the ordering of the induced crystalline phases. The conductivity behavior suggests a hopping mechanism responsible for conduction.     

Magnetic properties of quasicrystalline Al65Cu22Fe13 powders synthesized by solid-phase diffusion

The magnetic properties of quasicrystalline Al₆₅Cu₂₂Fe₁₃ powders synthesized by solid-phase diffusion as a result of thermal treatment in vacuum and in a hydrogen atmosphere have been studied. The magnetic properties of the samples synthesized in hydrogen were found to be much better than those of the samples synthesized in vacuum. It is shown that an increase in the treatment time in vacuum decreases the number of magnetic nanoclusters and elimninates the time instability of their magnetic properties (magnetic susceptibility and magnetization).     

Dispersion of the polarization processes in Ca<Subscript>0.32</Subscript>Ba<Subscript>0.68</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> crystals

The switching processes in calcium barium niobate crystals CaxBa_1–x Nb₂O₆ with x = 0.32 have been investigated. The dielectric hysteresis loops observed in the samples exposed to alternating fields of different frequencies and constant amplitude have been analyzed. It is shown that the loop formation is caused by the sample heating. The sample temperature increases due to the dielectric loss energy release only up to a certain frequency, above which complete switching becomes impossible.