Fuel additives and heat treatment effects on nanocrystalline zinc ferrite phase composition

Nanocrystalline ZnFe₂O₄ powder was prepared by the auto-combustion method using citric acid, acetic acid, carbamide and acrylic acid as fuel additives. Pure spinel zinc ferrite with the crystallite size of about 15 nm can be obtained by using acrylic acid as fuel additive. Samples prepared using other fuel additives contain ZnO impurities. In order to eliminate ZnO impurities, the sample prepared with citric acid as fuel additive was annealed at different temperatures up to 1000 °C in air and in argon. Annealed powders have pure ZnFe₂O₄ phase when annealing temperature is higher than 650 °C in air. Sample annealed at 650 °C in air is paramagnetic. However, annealed powders become a mixture of Fe₃O₄ and FeO after annealing at 1000 °C in argon atmosphere due to Zn volatility and the reduction reaction.     

Correlation between ferromagnetism and defects in MgO nanocrystals studied by positron annihilation

High purity MgO nanopowders were pressed into pellets and annealed in air from 100 to 1400 °C. Variation of the microstructures was investigated by X-ray diffraction and positron annihilation spectroscopy. Annealing induces an increase in the MgO grain size from 27 to 60 nm with temperature increasing up to 1400 °C. Positron annihilation measurements reveal vacancy defects including Mg vacancies, vacancy clusters, microvoids and large pores in the grain boundary region. Rapid recovery of Mg monovacancies and vacancy clusters was observed after annealing above 1200 °C. Room temperature ferromagnetism was observed for MgO nanocrystals annealed at 100, 700, and 1000 °C. However, after 1400 °C annealing, MgO nanocrystals turn into diamagnetic. Our results suggest that the room temperature ferromagnetism in MgO nanocrystals might originate from the interfacial defects.     

Effects of annealing temperature on structure and magnetic properties of CoAl0.2Fe1.8O4/SiO2 nanocomposites

CoAl_0.2Fe_1.8O₄/SiO₂ nanocomposites were prepared by sol–gel method. The effects of annealing temperature on the structure and magnetic properties of the samples were studied by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer and Mössbauer spectroscopy. The results show that the CoAl_0.2Fe_1.8O₄ in the samples exhibits a spinel structure after being annealed. As annealing temperature increases from 800 to 1200 °C, the average grain size of CoAl_0.2Fe_1.8O₄ in the nanocomposites increases from 5 to 41 nm while the lattice constant decreases from 0.8397 to 0.8391 nm, the saturation magnetization increases from 21.96 to 41.53 emu/g. Coercivity reaches a maximum of 1082 Oe for the sample annealed at 1100 °C, and thereafter decreases with further increasing annealing temperature. Mössbauer spectra show that the isomer shift decreases, hyperfine field increases and the samples transfer from mixed state of superparamagnetic and magnetic order to the completely magnetic order with annealing temperature increasing from 800 to 1200 °C.     

Synthesis and characterization of lithium ferrite by oxalate precursor route

Nanocrystalline lithium ferrite (LiFe₅O₈) powders have been synthesized by oxalate precursor route. The effects of Fe³⁺/Li⁺ mole ratio, and annealing temperature on the formation, crystalline size, morphology and magnetic properties were systematically studied. The Fe³⁺/Li⁺ mole ratio was controlled from 5 to 3.33 while the annealing temperature was controlled from 600 to 1100 °C. The resultant powders were investigated by differential thermal analyzer (DTA), X-ray diffractometer (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometer (VSM). DTA results showed that LiFe₅O₈ phase started to form at around 520 °C. XRD indicated that LiFe₅O₈ phase always contained α-Fe₂O₃ impurity and the hematite phase formation increased by increasing the annealing temperature ?850 °C for different Fe³⁺/Li⁺ mole ratios 5, 4.55 and 3.85. Moreover, lithium ferrite phase was formed with high conversion percentage at critical annealing temperature 750–800 °C. Single well crystalline LiFe₅O₈ phase was obtained at Fe³⁺/Li⁺ mole ratio 3.33 and annealing temperatures from 800 to 1000 °C. Maximum saturation magnetization (68.7 emu/g) was achieved for the formed lithium ferrite phase at Fe³⁺/Li⁺ mole ratio 3.33 and annealing temperature 1000 °C.     

Room temperature magnetism of Fe doped CdS nanocrystals

Undoped and Fe doped CdS nanocrystals with Fe content of 2–5 at% of average crystallite size 1.2–2 nm have been obtained using chemical co-precipitation method with 2-mercaptoethonal as capping agent at 80 °C. X-ray diffraction (XRD) results showed that the undoped CdS nanocrystals were in mixed phase of cubic and hexagonal, where as the doped CdS nanocrystals were in hexagonal phase. Room-temperature ferromagnetism has been observed in Fe-doped CdS nanocrystals. Magnetic studies indicated diamagnetism in undoped, ferromagnetism in lightly doped (2 and 3 at%) and paramagnetism in samples of higher Fe content (4 and 5 at%). The substitutional incorporation of Fe³⁺ ion in Cd²⁺ sites was reflected in structural and electron paramagnetic resonance (EPR) measurements. Isolated as well as interacting Fe³⁺ ions are observed in EPR.     

Effect of high magnetic field annealing on the microstructure and magnetic properties of Co-Fe layered double hydroxide

The effects of high magnetic field (10 T) on the products obtained by calcination of Co-Fe LDH precursors at different temperatures were investigated. The XRD results indicated that Fe^III substituted for Co^III in Co₃O₄ to yield Co^IICo^IIIFe^IIIO₄ under the calcination of Co-Fe LDH precursors at 400 °C. The products obtained by magnetic field annealing at 400 °C had a porous plate-like morphology, whereas the products without magnetic field annealing were composed of nanoparticles. It was seen that CoFe₂O₄ phase could be formed at low temperature (about 500 °C) under the magnetic field annealing. The grain size of products obtained by magnetic field annealing at 800 °C was larger than that of zero magnetic field. It was found that the saturation magnetization was significantly enhanced after magnetic field annealing, especially at lower temperature (≤600 °C). The possible reason for the effects on the microstructure and magnetic properties of products obtained by magnetic field annealing was discussed.     

Room-temperature ferromagnetism in Zn1−xMnxO

In view of recent controversies on above room-temperature ferromagnetism (RTFM) in transition-metal-doped ZnO, the present paper aims to shed some light on the origin of ferromagnetism by investigating annealing effects on structure and magnetism for polycrystalline Zn_1−xMn_xO powder samples prepared by solid-state reaction method and annealed in air at different temperatures. Magnetic measurements indicate that the samples are ferromagnetic at room temperature (RTFM). Room temperature ferromagnetism has been observed in the sample annealed at a low temperature of 500 °C with a saturated magnetization (M_s) of 0.159 emu/g and a coercive force of 89 Oe. A reduction in RTFM is clearly observed in the sample annealed at 600 °C. Furthermore, the saturation magnetic moment decreases with an increase in grain size, suggesting that ferromagnetism is due to defects and/or oxygen vacancy confined to the surface of the grains. The experimental results indicate that the ferromagnetism observed in Zn_1−xMn_xO samples is intrinsic rather than associated with secondary phases.     

Crystallization and electrochemical performance of La0.6Sr0.4Co0.2Fe0.8O3 − δ-Ce0.8Gd0.2O1.9 thin film cathodes processed by single solution spray pyrolysis

La_0.6Sr_0.4Co_0.2Fe_0.8O_3 − δ-Ce_0.8Gd_0.2O_1.9 (LSCF-CGO) thin films obtained by spray pyrolysis of a single precursor solution were investigated by XRD, TEM and impedance spectroscopy at annealing temperatures ranging from 500 to 900 °C. Films annealed at 600 °C contained a mixture of amorphous regions and crystalline regions composed of fine crystallites (< 5 nm). Annealing above 600 °C increased the ratio of crystalline to amorphous material, led to the segregation of the films into distinct LSCF and CGO phases, and promoted grain growth. The electrical behavior of the films depended on annealing temperature. At testing temperatures of 400 °C and below, the polarization resistance of films with lower annealing temperatures was larger than the polarization resistance of films with higher annealing temperatures. However, at testing temperatures of 500 °C and above the polarization resistance of films with lower annealing temperatures was equal to or lower than the polarization resistance of films with higher annealing temperatures. This was reflected by the activation energy that decreased with increasing annealing temperature. The varying electrical behavior may be related to microstructural changes that caused bulk diffusion to be the rate-limiting step in films with lower annealing temperatures and oxygen dissociation to be the rate-limiting step in films with higher annealing temperatures.     

Influence of Fe-doping on the structural,optical and magnetic properties of ZnO nanoparticles

Zn_1–xFe_xO (x=0–0.05) nanoparticles were synthesized without a catalyst by a two-step method. Fe was doped into ZnO by a source of metallic Fe sheets in a solid–liquid system at 80 °C, and the Zn_1−xFe_xO nanoparticles were obtained by annealing at 300 °C. X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy were used to characterize the structural properties of the as-grown Zn_1−xFe_xO. The optical properties were determined by Infrared and Ultraviolet–visible spectroscopy. The results confirm that the crystallinity of the ZnO is deteriorated due to Fe-doping. XPS results show that there is a mixture of Fe⁰⁺ and the Fe³⁺ in the representative Zn_0.95Fe_0.05O sample. The optical band gap of Zn_1−xFe_xO is enhanced with increasing of Fe-doping. Room temperature ferromagnetism was observed in all the Fe-doped ZnO samples.     

Microstructure, magnetic and optical properties of sputtered polycrystalline ZnO films with Fe addition

Microstructure, magnetic and optical properties of polycrystalline Fe-doped ZnO films fabricated by cosputtering with different Fe atomic fractions (x_Fe) have been examined systematically. Fe addition could affect the growth of ZnO grains and surface morphology of the films. As x_Fe is larger than 7.0%, ZnFe₂O₄ grains appear in the films. All the films are ferromagnetic. The ferromagnetism comes from the ferromagnetic interaction activated by defects between the Fe ions that replace Zn ions. The average moment per Fe ion reaches a maximum value of 1.61 μ_B at x_Fe = 4.8%. With further increase in x_Fe, the average moment per Fe ion decreases because the antiferromagnetic energy is lower than the ferromagnetic one due to the reduced distance between the adjacent Fe ions. The optical band gap value decreases from 3.245 to 3.010 eV as x_Fe increases from 0% to 10%. Photoluminescence spectra analyses indicate that many defects that affect the optical and magnetic properties exist in the films.     

Effect of annealing temperature on structure, magnetic properties and optical characteristics in Zn0.97Cr0.03O nanoparticles

The Cr-doped zinc oxide (Zn_0.97Cr_0.03O) nanoparticles were successfully synthesized by sol-gel method. The relationship between the annealing temperature (400 °C, 450 °C, 500 °C and 600 °C) and the structure, magnetic properties and the optical characteristics of the produced samples was studied. The results indicate that Cr (Cr³⁺) ions at least partially substitute Zn (Zn²⁺) ions successfully. Energy dispersive spectroscopy (EDS) measurement showed the existence of Cr ion in the Cr-doped ZnO. The samples sintered in air under the temperature of 450 °C had single wurtzite ZnO structure with prominent ferromagnetism at room temperature, while in samples sintered in air at 500 °C, a second phase-ZnCr₂O₄ was observed and the samples were not saturated in the field of 10000 Oe. This indicated that they were mixtures of ferromagnetic materials and paramagnetic materials. Compared with the results of the photoluminescence (PL) spectra, it was reasonably concluded that the ferromagnetism observed in the studied samples was originated from the doping of Cr in the lattice of ZnO crystallites.     

Photoluminescence of ZnO nanowires dependent on O2 and Ar annealing

High-purity ZnO nanowires have been synthesized on Si substrates without the presence of a catalyst at 600 °C by a simple thermal vapor technique. Photoluminescence (PL) spectra of the annealed samples at 900 °C under oxygen and argon gases have been investigated. After O₂ or Ar annealing, the PL visible-emission intensity that is related to intrinsic defects (oxygen vacancies) is greatly reduced compared with as-grown ZnO nanowires because the oxygen-gas ions or oxygen interstitials diffuse into the oxygen vacancies during annealing process. The blue-band peak of the O₂- or Ar-annealed ZnO naonowires is also smaller than the green-band peak in the visible broadband because of the reduction of oxygen vacancies. Therefore, the main intrinsic defects (oxygen vacancies) of as-grown ZnO nanowires can be reduced by O₂ or Ar annealing, which is an important procedure for the development of advanced optoelectronic ZnO nanowire devices.     

The effects of annealing at 1000 °C for 24 h on the extrinsic properties of Pr-Fe-B and Nd-Fe-B-type sintered magnets

Recent studies have shown the effects of a post sintering heat treatment at 1000 °C for 24 h on the microstructure and magnetic properties of Pr-Fe-B/Nd-Fe-B magnets based on Nd₁₆Fe₇₆B₈ and Pr₁₆Fe₇₆B₈. In an attempt to understand the influence of environmental factors, an investigation into the effects of annealing under different degrees of vacuum for both types of sintered magnets has been carried out. The effect of annealing the Pr-Fe-B magnets at 1000 °C for 24 h resulted in a general increase in the magnetic properties, especially the intrinsic coercivity, although the degree of improvement appeared to be dependent on the initial annealing conditions (ambient pressure). Oxygen analysis of sintered and annealed magnets indicates a change in the nature of the grain boundary phases after the annealing treatment. The effect of annealing the Nd-Fe-B magnets at 1000 °C for 24 h resulted in a general decrease in the magnetic properties, especially the intrinsic coercivity.     

Effect of annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films on sapphire (0 0 0 1) substrates by magnetron sputtering

ZnO thin films were epitaxially grown on sapphire (0 0 0 1) substrates by radio frequency magnetron sputtering. ZnO thin films were then annealed at different temperatures in air and in various atmospheres at 800 °C, respectively. The effect of the annealing temperature and annealing atmosphere on the structure and optical properties of ZnO thin films are investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL). A strong (0 0 2) diffraction peak of all ZnO thin films shows a polycrystalline hexagonal wurtzite structure and high preferential c-axis orientation. XRD and AFM results reveal that the better structural quality, relatively smaller tensile stress, smooth, uniform of ZnO thin films were obtained when annealed at 800 °C in N₂. Room temperature PL spectrum can be divided into the UV emission and the Visible broad band emission. The UV emission can be attributed to the near band edge emission (NBE) and the Visible broad band emission can be ascribed to the deep level emissions (DLE). By analyzing our experimental results, we recommend that the deep-level emission correspond to oxygen vacancy (V_O) and interstitial oxygen (O_i). The biggest ratio of the PL intensity of UV emission to that of visible emission (I_NBE/I_DLE) is observed from ZnO thin films annealed at 800 °C in N₂. Therefore, we suggest that annealing temperature of 800 °C and annealing atmosphere of N₂ are the most suitable annealing conditions for obtaining high quality ZnO thin films with good luminescence performance.     

Magnetoelectric characterization of xNi0.75Co0.25Fe2O4-(1−x)BiFeO3 nanocomposites

Magnetoelectric (ME) nanocomposites containing Ni_0.75Co_0.25Fe₂O₄-BiFeO₃ phases were prepared by citrate sol-gel process. X-ray diffraction (XRD) analysis showed phase formation of xNi_0.75Co_0.25Fe₂O₄-(1−x)BiFeO₃ (x=0.1, 0.2, 0.3 and 0.4) composites on heating at 700 °C. Transmission electron microscopy revealed the formation of powders of nano order size and the crystal size was found to vary from 30 to 85 nm. Dispersion in dielectric constant (ε) and dielectric loss (tan δ) in the low-frequency range have been observed. It is seen that nanocomposites exhibit strong magnetic properties and a large ME effect. On increasing Ni_0.75Co_0.25Fe₂O₄ contents in the nanocomposites, the saturation magnetization (M_S) and coercivity (H_C) increased after annealing at 700 °C. The large ME output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The large value of ME output can be attributed to small grain size of ferrite phase of nanocomposite being prepared by citrate precursor process.     

On the room-temperature ferromagnetism in (ZnO)0.98(MnO2)0.02

Room-temperature ferromagnetism (RTFM) is investigated in the polycrystalline bulk (ZnO)_0.98(MnO₂)_0.02 samples prepared by a modified solid-state sintering route. Successive sintering of a sample was carried out in air at different temperatures in the range of 400-1000 °C. The study of magnetization and phase-investigation in the sample was carried out after each sintering step. The progressive suppression of impurities and the consequent reduction in RTFM is clearly observed in the samples with increase in the sintering temperature up to 800 °C. The subsequent successive sintering of the (ZnO)_0.98(MnO₂)_0.02 sample up to 1000 °C yields fully paramagnetic sample exhibiting wurtzite structure. The studies support the conjecture (Kundaliya et al., Nat. Mater. 3 (2004) 709 [18]) that RTFM in this system has an origin related to a randomly distributed impurity phase produced by local dissolution of ZnO and MnO₂.     

Thermal stability of alumina thin films containing γ-Al2O3 phase prepared by reactive magnetron sputtering

The paper reports on thermal stability of alumina thin films containing γ-Al₂O₃ phase and its conversion to a thermodynamically stable α-Al₂O₃ phase during a post-deposition equilibrium thermal annealing. The films were prepared by reactive magnetron sputtering and subsequently post-deposition annealing was carried out in air at temperatures ranging from 700 °C to 1150 °C and annealing times up to 5 h using a thermogravimetric system. The evolution of the structure was investigated by means of X-ray diffraction after cooling down of the films. It was found that (1) the nanocrystalline γ-Al₂O₃ phase in the films is thermally stable up to 1000 °C even after 5 h of annealing, (2) the nanocrystalline θ-Al₂O₃ phase was observed in a narrow time and temperature region at ≥1050 °C, and (3) annealing at 1100 °C for 2 h resulted in a dominance of the α-Al₂O₃ phase only in the films with a sufficient thickness.     

Effect of ZnO on phase emergence, microstructure and surface modifications of calcium phosphosilicate glass/glass-ceramics having iron oxide

The effect of ZnO on phase emergence and microstructure properties of glass and glass-ceramics with composition 25SiO₂-50CaO-15P₂O₅-(10 − x)Fe₂O₃-xZnO (where x = 0, 2, 5, 7 mol%) has been studied. They have been characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Surface modifications of these glass-ceramics in simulated body fluid have been studied using Fourier transform infrared reflection spectroscopy (FTIR), XPS and SEM. Results have shown a decrease in the fraction of non-bridging oxygen with increase in zinc oxide content. Emergence of crystalline phases in glass-ceramics at different heat treatment temperatures was studied using XRD. When glass is heat treated at 800 °C calcium phosphate, hematite and magnetite are developed as major phases in the glass-ceramics samples with ZnO up to 5 mol%. In addition to these, calcium silicate (Ca₃Si₂O₇) phase is also observed when glass is heat treated at 1000 °C. The microstructure of the glass-ceramics heat treated at 800 °C exhibits the formation of nano-size (40-50 nm) grains. On heat treatment at 1000 °C crystallites grow to above 50 nm size and more than one phase are observed in the microstructure. The formation of thin flake-like structure with coarse particles is observed at high zinc oxide concentration (x = 7 mol%). In vitro studies have shown the surface modifications and formation of Ca-P-rich layer on the glass-ceramics when immersed in simulated body fluids (SBF) for different durations. The bioactive response was found to depend on ZnO content.     

Effect of nanoscale thin films of Dy2Fe14B on the magnetic hysteresis characteristics of melt-spun ribbons of Nd2Fe14B

The effectiveness of nanoscale Dy₂Fe₁₄B thin films on coercivity and energy product of melt-spun ribbons of Nd₂Fe₁₄B at high temperatures was investigated. It is hypothesized that the nanoscale Dy-thin film will act as an obstacle for the nucleation of reverse domains and also maximize the energy of domain walls and thereby improve the magnetic performance at high temperatures. Pulsed laser deposition (PLD) of amorphous Dy₂Fe₁₄B layers on Nd₂Fe₁₄B melt-spun ribbons was performed for a nominal thickness of 40 nm. The coated ribbons were then annealed in environmentally controlled quartz furnace at two different cycles (750 °C for 15 min and 900 °C for 2 h) to cause crystallization. Magnetic hysteresis tests conducted at 300 and 400 K revealed that there is small but consistent improvement in the magnetic properties of the coated ribbons annealed at 750 °C for 15 min. However, higher temperature annealing (900 °C for 2 h) drastically reduced the magnetic properties. The incomplete recrystallization of amorphous structure at 750 °C for 15 min and large grain growth and formation of non-magnetic phases at 900 °C for 2 h are believed to be responsible for not meeting the expected magnetic performance.     

Co掺杂纳米ZnO微结构的正电子湮没研究

利用正电子湮没技术研究了10 at.% Co掺杂的Co₃O₄/ZnO纳米复合物中退火对缺陷的影响. 利用X射线衍射(XRD)测量了Co₃O₄/ZnO纳米复合物的结构和晶粒尺寸. 随着退火温度升高,Co₃O₄相逐步消失,ZnO晶粒尺寸也有显著增加. 经过1000 ℃以上退火后,Co₃O₄相完全消失,并出现了CoO的岩盐结构. 正电子湮没寿命测量显示出Co₃O₄ /ZnO纳米复合物中存在大量的Zn空位和空位团. 这些空位缺陷可能存在于纳米复合物的界面区域. 当退火温度达到700 ℃后Zn空位开始恢复,空位团也开始收缩. 900 ℃以上退火后,所有空位缺陷基本消失,正电子寿命接近ZnO完整晶格中的体态寿命值. 符合多普勒展宽谱测量也显示Co₃O₄ /ZnO纳米复合物经过900 ℃以上退火后电子动量分布与单晶ZnO基本一致,表明界面缺陷经过退火后得到消除. 关键词： ZnO 界面缺陷 正电子湮没