Effects of Ta2O5 addition on the microstructure and temperature dependence of magnetic properties of MnZn ferrites

MnZn ferrites with the chemical formula Mn_0.68Zn_0.25Fe_2.07O₄ have been prepared by the conventional ceramic technique. Toroidal cores were sintered at 1350 °C for 4 h in N₂/O₂ atmosphere with 4% oxygen. Then the influence of Ta₂O₅ addition on the microstructure and temperature dependence of magnetic properties of MnZn ferrites was investigated by characterizing the fracture surface micrograph and measuring the magnetic properties over a temperature ranging from 25 to 120 °C. The results show that, when the Ta₂O₅ concentration is not more than 0.04wt%, the grain size has a slight increase with the increase of Ta₂O₅ concentration, the temperature of secondary maximum peak in the curve of initial permeability versus temperature and the lowest power loss shift to lower temperature. However, excessive Ta₂O₅ concentration (>0.04wt%) results in the exaggerated grain growth and porosity increase, which make the initial permeability and saturation magnetic flux density decrease and the power loss increase at room temperature. Furthermore, the temperature of secondary maximum peak in the curve of initial permeability versus temperature and the lowest power loss shift to about 100 °C.     

Effects of V2O5 addition on NiZn ferrite synthesized using two-step sintering process

The combined influence of a two-step sintering (TSS) process and addition of V₂O₅ on the microstructure and magnetic properties of NiZn ferrite was investigated. As comparison, samples prepared by the conventional single-step sintering (SSS) procedure were also studied. It was found that with 0.3 wt% V₂O₅ additive, the sample sintered by the two-step sintering process at a high temperature of 1250 °C for 30 min and a lower temperature of 1180 °C for 3 h exhibited more homogeneous microstructure and higher permeability with a high Q-factor. The results showed that the TSS method with suitable additive brought positive improvement of the microstructure and magnetic properties of NiZn ferrite.     

Effect of CaO-B2O3-SiO2 glass on the magnetic and dielectric properties of NiCuZn ferrites

NiCuZn ferrites with different contents of CaO-B₂O₃-SiO₂ glasses were synthesized by a conventional ceramic technology and sintered at 1050 °C. It was found that the addition of CaO-B₂O₃-SiO₂ influenced the magnetic and dielectric properties of the ferrites. The saturation magnetization increased at first and reached its maximum with the sample of 2 wt% CaO-B₂O₃-SiO₂, and then decreased. The initial permeability decreased with the content of CaO-B₂O₃-SiO₂ but the cut-off frequency increased. The quality factor decreased first and then increased; the maximum quality factor was obtained in the sample with 3 wt% CaO-B₂O₃-SiO₂. With increasing content of CaO-B₂O₃-SiO₂, the permittivity increased sharply. The possible reasons responsible for these changes are explained.     

Effects of sintering temperature and Bi2O3 content on microstructure and magnetic properties of LiZn ferrites

The effects of sintering temperature and Bi₂O₃ content on the microstructure and magnetic properties of lithium–zinc (LiZn) ferrites prepared by a conventional ceramic method were investigated. The results show that the densification behavior and grain growth rate were greatly improved by the addition of Bi₂O₃, because a liquid phase sintering occurred during the sintering process at high temperature due to the low-melting point of Bi₂O₃ (825 °C). X-ray diffraction (XRD) patterns of the slightly doped samples did not reveal the appearance of any phase other than spinel LiZn ferrite. However, the secondary phase of perovskite BiFeO₃ was detected for Bi₂O₃ content of more than 0.25 wt%. The studies further show that Bi oxide was present at grain boundary, and promoted the grain growth as reaction center at lower temperature. A high saturation magnetization, squareness ratio, minimum ferromagnetic resonance linewidth and low coercive force were obtained for the sample with 1.00 wt% Bi₂O additive at lower sintering temperature (1100 °C).     

钴掺杂二氧化锡纳米粉的光致发光和磁学性质

研究了钴掺杂对二氧化锡纳米粉的光致发光性质和磁学性质的影响,发现钴掺杂对发光带的位置影响很小,但紫外发光带与蓝色发光带的强度之比随掺杂含量的增加而下降.当钴掺杂含量达到0.02时,样品中的铁磁性被完全破坏.讨论了样品中的磁相互作用的机理,认为掺杂离子的不均匀分布、自旋极化子与掺杂离子之间的耦合都可能导致反铁磁性的相互作用,这种反铁磁性的作用破坏了铁磁性. 关键词： 钴掺杂二氧化锡 光致发光 磁学性质     

氧氩比对SnO2/Ag/SnO2透明导电膜光电性能的影响

在室温及不同的氧氩比条件下,采用射频磁控溅射Ag层和直流磁控溅射SnO₂层,在载玻片衬底上制备出了SnO₂/Ag/SnO₂多层薄膜.用霍尔效应测试仪、四探针电阻测试仪和紫外-可见-近红外光谱仪等表征了薄膜的电学性质和光学性质.实验结果表明:当氧氩比为1:14时,所制得的薄膜的光电性质优良指数最大,为1.69×10^-2 Ω^-1;此时,薄膜的电阻率为9.8×10^-5 Ω·cm,方电阻为9.68 Ω/sq,在400~800 nm可见光区的平均光学透射率达85%;并且,在氧氩比为1:14时,利用射频磁控溅射Ag层和直流磁控溅射SnO₂层在PET柔性衬底上制备出了光电性质优良的柔性透明导电膜,其在可见光区的平均光学透过率达85%以上,电阻率为1.22×10^-4 Ωcm,方电阻为12.05 Ω/sq.     

Study of the synthesis and Cl2 gas-sensing properties of nanometer HNO3-doped SnO2

We focused on the effects of the inorganic acid HNO₃ on the gas-sensing properties of nanometer SnO₂ and prepared the powders via a dissolution-pyrolysis method. Furthermore, the powders were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectra (EDS). Several aspects were surveyed, including the calcining temperature, concentration of nitric acid and the working temperature. The results showed that the gas response of 3 wt% HNO₃-doped SnO₂ powders (calcined at 500 °C) to 10 ppm Cl₂ reached 316.5, at the working temperature 175 °C. Compared with pure SnO₂, appropriate HNO₃ could increase the gas sensitivity to Cl₂ gas more significantly.     

Influence of V2O5 on the magnetic properties of nickel–zinc–copper ferrites

Additions of V₂O₅ were used to decrease the sintering temperature and to enhance the densification of NiZnCu ferrite. With a small amount of V₂O₅ (0.6–1.2 wt%) as a sintering aid, densification at low sintering temperature was observed. EDS analyses showed the presence of vanadium inside the ferrite grains. Microstructure investigation revealed heterogeneous grain size. Magnetic properties were deteriorated when increasing the V₂O₅ content. As a consequence, the permeability decreased and the core losses increased, which can be explained by the pinning of the domain walls to the grain defects.     

The influence of nickel dopant on the microstructure and optical properties of SnO2 nano-powders

The microstructure and optical properties of Ni-doped SnO2 nano-powders are studied in detail. By Ni-doping, not only the grain size reduces, but also the grain shape changes from nano-rods to spherical particles. The crystallization becomes better with annealing temperature increasing. The band gap energy decreases as nickel doping level increases. The sp-d hybridization and alloying effect due to amorphous SnO2-x phase should be responsible for the band gap narrowing effect. Nickel dopant does not change the photoluminescence （PL） peak positions.     

Influences of Bi2O3 on microstructure and magnetic properties of MnZn ferrite

MnZn ferrites were prepared by conventional oxide ceramic process. The effects of Bi₂O₃ on microstructure and magnetic properties of MnZn ferrites were investigated by means of characterizing the fracture surface micrograph, composition of grain boundary, magnetic properties and density by scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDS), B-H analyzer and Archimedes method, respectively. The results indicate that Bi₂O₃ mainly segregates and concentrates in the grain boundary regions, promotes solid-state reaction and grain growth, reduces porosity and enhances density. Optimum addition of Bi₂O₃ increases the permeability and saturation magnetic induction, meanwhile ensures the well frequency stability of permeability.     

Substrate effects on the oxygen gas sensing properties of SnO2/TiO2 thin films

In this study, SnO₂/TiO₂ thin films are fabricated on SiO₂/Si and Corning glass 1737 substrates using a R.F. magnetron sputtering process. The gas sensing properties of these films under an oxygen atmosphere with and without UV irradiation are carefully examined. The surface structure, morphology, optical transmission characteristics, and chemical compositions of the films are analyzed by atomic force microscopy, scanning electron microscopy and PL spectrometry. It is found that the oxygen sensitivity of the films deposited on Corning glass 1737 substrates is significantly lower than that of the films grown on SiO₂/Si substrates. Therefore, the results suggest that SiO₂/Si is an appropriate substrate material for oxygen gas sensors fabricated using thin SnO₂/TiO₂ films.     

Effects of applied magnetic field and pressures on the magnetic properties of nanocrystalline CoFe2O4 ferrite

Nanocrystalline CoFe₂O₄ ferrite with crystallite sizes of 30 nm have been successfully prepared by an emulsion method. X-ray diffractometer (XRD) shows that nanocrystalline CoFe₂O₄ ferrite possesses face center cubic structure. Crystal structure of the CoFe₂O₄ nanocrystals will not be changed by the applied magnetic field and pressures. The obtained CoFe₂O₄ nanocrystalline powders were pressed into thin columns with different pressures. Meanwhile, the dependences of the applied pressures and the direction of applied magnetic field on the magnetic properties of the CoFe₂O₄ nanocrystals were investigated in detail using vibrating sample magnetometer (VSM). The pressed CoFe₂O₄ nanocrystal gains the most excellent magnetisms in a parallel applied magnetic field.     

The effect of surfactants on the magnetic and optical properties of Co-doped SnO2 nanoparticles

A series of Co-doped SnO₂ nanoparticles have been synthesized by the co-precipitation route. Different amounts of surfactant have been used in order to study the effect of surfactants (CTAB) on the magnetic and optical properties. Structural analyses reveal that Co dopants are substituted into rutile SnO₂ nanoparticles without forming any secondary phase. The increase of the surfactant promotes the adsorption of organic molecules on the surfaces of nanoparticles. Meanwhile, both the ferromagnetism and the orange emission drop progressively. The dependence of ferromagnetic properties on the surfactant concentration could be explained based on the bound magnetic polaron, where the carriers are provided by oxygen vacancies. XANES spectra reveal that the electrons transfer from Co 3d bands to the surfactant ions. Such electron-transfer process suppresses the formation of oxygen vacancies and leads to the decline of the ferromagnetism and optical emission.     

Effect of Y2O3 additive on the microstructure and high-frequency properties of Z-type hexaferrites

In order to improve the high-frequency electromagnetic properties of Z-type hexaferrites such as high cut-off frequency and low dielectric constant, Y₂O₃ was introduced. The influence of Y₂O₃ additive on the phase composition, densification, microstructural and electromagnetic properties of the ceramics with composition of Ba₃(Co_0.4Zn_0.6)₂Y_xFe_24−xO₄₁ (x=0−1) was investigated. The results show that as the amount of Y₂O₃ additive increased, the major phase changed to Z-phase; simultaneously, a small amount of garnet phase appeared. With increasing Y₂O₃ content, the garnet phase separated out on grain boundaries as a secondary phase restraining grain growth. When x was varied from 0 to 1, the dielectric constant decreased and the ferroelectric resonance peak shifted toward a higher frequency. Meanwhile, the initial permeability increased at first, and then decreased with further increasing x, which could be mainly attributed to the change of phase composition and sintering density. With increasing Y₂O₃ content, the minimum reflection point of the samples shifted toward a higher frequency.     

Glass additive influence on the sintering behavior, microstructure and microwave magnetic properties of Cu-Bi-Zn co-doped Co2Z ferrites

The Bi₂O₃-B₂O₃-ZnO-SiO₂ (BB35SZ) glass effects on the sintering behavior and microwave magnetic properties of Cu-Bi-Zn co-doped Co₂Z ferrites were investigated to develop low-temperature-fired ferrites. The glass wetting characteristics on the Co₂Z ferrite surface, X-ray diffractometer, scanning electron microscopy and a dilatometer were used to examine the BB35SZ glass effect on Co₂Z ferrite densification and the chemical reaction between the glass and Co₂Z ferrites. The results indicate that BB35SZ glass can be used as a sintering aid to reduce the densification temperature of Co₂Z ferrites from 1300 to 900 °C. 3(Ba_0.9Bi_0.1O)·2(Co_0.8Cu_0.2O)·12(Fe_1.975Zn_0.025O₃) ferrite with 2 wt% BB35SZ glass can be densified below 900 °C, exhibiting an initial permeability of 3.4. This process provides a promising candidate for multilayer chip magnetic devices for microwave applications.     

Synthesis and optical properties of SnO2-CuO nanocomposite

SnO₂-CuO nanocomposite was synthesized by impregnating SnO₂ nanowires with CuCl₂ solution and subsequent calcination. SEM and XRD were used to characterize the morphology and structure of the product. The optical properties were analyzed by Raman and photoluminescence (PL) spectra at room temperature. Except the strong orange emission of SnO₂, the PL spectrum showed a red shoulder at 678 nm which originated from the interface between SnO₂ and CuO.     

Effect of Mg doping on the electrical properties of SnO2 nanoparticles

Sol-gel derived Mg doped tin oxide (Sn_1−xMg_xO₂) nanocrystals were synthesized with x ranging between 0.5 and 7 at. %. Characteristic single phase tetragonal structure of pure and doped samples was obtained and doping saturation was inferred by X-ray diffraction analysis. Structural, morphological and phase informations were obtained by high resolution transmission electron microscope, field emission scanning electron microscope and X-ray photoelectron spectroscopy respectively whereas bonding information was obtained from Fourier transformed infrared spectroscopy. Measurement of different electrical parameters with frequency (200 Hz-10⁵ Hz) has been carried out at room temperature. Ultrahigh dielectric constant and metallic AC conductivity were observed for undoped tin oxide and the profiles reflected highly sensitive changes in the atomic and interfacial polarizability generated by doping concentrations. Relaxation spectra of tangent loss of any sample did not show any loss peak within the frequency range. Both the grain and grain boundary contributions are observed to increase as the doping concentration increased. Results of first principle calculation based on density functional theory indicated effective Fermi level (E_F) suppression due to Mg doping which is responsible for the experimentally observed conductivity variation. AC conductivity was found to depend strongly on the doping concentration and the defect chemistry of the compound. Mg doped SnO₂ may find applications as a low loss dielectric and high density energy storage material.     

First-principles study of ferromagnetism in Zn- and Cd-doped SnO2

The electronic structures and magnetic properties of Zn- and Cd-doped SnO₂ are investigated using first-principles calculations within the generalized gradient approximation (GGA) and GGA+U scheme. The substitutional Zn and Cd atoms introduce holes in the 2p orbitals of the O atoms and the introduced holes are mostly confined to the minority-spin states. The magnetic moment induced by doping mainly comes from the 2p orbitals of the O atoms, among which the moment of the first neighboring O atoms around the dopant are the biggest. The U correction for the anion-2p states obviously increases the moment of the first neighboring O atoms and transforms the ground states of the doped SnO₂ from half-metallic to insulating. The magnetic coupling between the moments induced by two dopants is ferromagnetic and the origin of ferromagnetic coupling can be attributed to the p–d hybridization interaction involving holes.     

In-situ bridging of SnO2 nanowires between the electrodes and their NO2 gas sensing characteristics

A simple and efficient way of making highly sensitive SnO₂ nanowire-based gas sensors without an individual lithography process was studied. The SnO₂ nanowires network was floated upon the Si substrate by separating the Au catalyst layer from the substrate. As the electric current is transported along the networks of the nanowires, not along the surface layer on the substrate, the gas sensitivities could be maximized in this networked and floated structures. The sensitivity was 5-30 when the NO₂ concentration was 1-10 ppm. The response time was ca. 20-60 s.     

Gd-doped SnO2 nanoparticles: Structure and magnetism

Gd-doped SnO₂ nanoparticles were chemically prepared doping 0-12.5% Gd into SnO₂ and calcined at 600 °C. X-ray diffraction and Fourier transformed infrared spectroscopy measurements show the formation of single phase of Sn_1−xGd_xO₂ up to x=0.0625 while at x=0.125, an additional secondary phase of tetragonal GdO₂ (not cubic Gd₂O₃) is detected. The transmission electron microscopy studies show that the individual particles are single crystalline with an average size in the range of 10-12 nm. Magnetization measurements show the absence of ferromagnetic and antiferromagnetic ordering in all samples; however surface spin effects and enhanced Gd-O-Gd interactions are proposed to account for the observed magnetic properties of the samples.