The origin of varistor property of SrTiO3-based ceramics

The role of oxygen in the heat-treatment process of SrTiO₃ varistor ceramics has been investigated in this paper. The varistor voltage of SrTiO₃ ceramics has been found to be independent of the sample thickness and it increases with the heat-treatment temperature. It has been further revealed that the dielectric property is mainly governed by a highly resistive surface layer. The XPS results of Mn 2p and O 1s suggest that the surface layer is formed by oxygen diffusion and surface chemisorption at grain boundaries during the heat treatment in air. The chemisorption of oxygen in grain boundaries, which leads to the highly resistive surface layer, is the origin of the varistor property of SrTiO₃ ceramics.     

On the behaviour of minor active elements during oxidation of selected Ni-base high-temperature alloys

We have examined the distribution of active minor elements in the oxide scales developed by selected Ni-base alloys with commercial grades. Emphasis is placed upon Mn, La and Si in a chromia-forming alloy and Y in an alumina-forming alloy. Initially, La and Y have been segregated at free surfaces and then become constituents of the oxides in contact with the substrates. A continuous layer of MnCr₂O₄ is formed above La- and Si-modified inner chromia layer. Silicon has been homogenously distributed throughout the grain structure, however, some La is present as LaCr₂O₃ particles and most of the remainder has been segregated at grain boundaries. The results indicate that the collective effect of Mn, Si and La is to extend protection by chromia to temperatures in excess of 1000 °C. Yttrium in the alumina- forming alloy is found to predominantly segregate at grain boundaries of nanostructured oxide with improved mechanical strength.     

Evolution of microstructure in laser clad Fe–Cr–Mn–C alloy

Abstract

The laser surface cladding technique was used to form in situ Fe–Cr–Mn–C alloys on AISI 1016 steel substrate. In this process, mixed powders containing Cr, Mn, and C in the weight ratio 10: 1 : 1 were delivered using a screw feed, gravity flow, carrier gas aided system into the melt pool generated by a 10 kW CO₂ laser. This technique produced an ultrafine microstructure in the clad alloy layer. The microstructure of the laser surface clad region was investigated by optical, scanning and transmission electron microscopy, and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic transformation or retained austenite phase.

MST/356     

疲劳裂纹萌生的细观效应

Ｘ射线衍射分析表明，在疲劳极限载荷下循环加载１０＾７次事，退火态１６Ｍｎ在厚度约３个铁素体晶粒大小的表层内，３００Ｍ钢在厚度经１５个原奥氏体晶粒大小的表层内，喷丸强化３００Ｍ钢在距表面２００－３２０μｍ的亚表层内发生ＸＲＤ效应的明显变化。     

Investigations of the electrical property, diffuse reflectance and ESR spectra of the La-(Fe,Mn)-codoped PTCR BaTiO3 annealed in reducing atmosphere

Electrical properties of La-(Fe,Mn)-codoped positive temperature coefficient of resistivity (PTCR) BaTiO₃ ceramics were studied by combining their diffuse reflectance measurements and electron spin resonance (ESR) spectroscopy. La-(Fe,Mn)-codoped samples showed high durability to reducing atmosphere. It is assumed that Fe and Mn ions segregated in the grain boundary contribute to the density of surface acceptor states, meanwhile localizing electrons in a form of Ti³⁺ and stabilizing the chemisorbed oxygens through La³⁺-Mn^3+,4+ or La³⁺-Fe³⁺ pairs. In addition, ESR signals of Fe³⁺ in annealed samples was intensified above Curie temperature (T_c), indicating that Fe ions still maintained its high valence states (Fe³⁺) in the grain boundary even after annealing in reducing atmosphere.     

Abnormal dielectric behaviors in Mn-doped CaCu3Ti4O12 ceramics and their response mechanism

Mn-doped CaCu₃Ti₄O₁₂ (CCTO) polycrystalline ceramics have been prepared by the conventional solid state sintering. Our results indicate that 10% Mn doping can decrease the dielectric permittivity in CaCu₃Ti₄O₁₂ by about 2 orders of magnitude (from 10⁴ to 10²). The grain and grain boundary activation energies show an obvious increase from 0.054 eV to 0.256 eV, and decrease from 0.724 eV to 0.258 eV with increasing the Mn doping concentration, respectively, which may be caused by the variation of Cu and Ti valence states in the CCTO samples evidenced by the X-ray absorption spectra. The similar grain and grain boundary activation energies result in invalidation of the internal boundary layer capacitance effect for the 10% Mn-doped CCTO sample, and thus result in the dramatic decrease of dielectric permittivity.     

Effect of strain on formation of ultrafine ferrite in surface of hot rolled microalloyed steel

Abstract

The phenomenon of ultra grain refinement of ferrite in surface layers of hot rolled strip has been studied in a low carbon, niobium microalloyed steel. Wedge specimens were used, to vary the nominal equivalent strain applied during rolling from zero to approximately unity, and the cooling rate after rolling was varied from ~ 20 to 1 K s ^-1. In contrast with previous work, which contended that a very coarse austenite grain size and a low rolling temperature near the Ar ₃ were essential to obtain ultrafine ferrite in surface layers, such ultrafine layers were observed after rolling coarse austenite at up to 150 K above the Ar ₃ and after rolling fine grained austenite near the Ar ₃. In the case of coarse grained austenite, a critical nominal rolling strain needed to be exceeded to trigger the surface layer phenomenon, upon which cooling rate had little effect on the surface layer's grain size. Refining the prior austenite grain size had the further beneficial effect of refining the grain size at the centre of the rolled product, for example to 2·6 μm, while the surface layer was refined to 0·7 μm.     

Composite Nanostructure Construction on the Grain Surface of Li-Rich Layered Oxides

Li-rich layered oxides (LLOs) are fascinating high-energy cathodes for lithium-ion batteries (LIBs), but still suffer from critical drawbacks that retard their practical applications. Although surface modification is effective to protect LLOs from structural deterioration, the delicate design of structures on a grain surface with promising scalability for industrial application is still challenging. Herein, using the atomic layer deposition (ALD) technique, a composite nanostructure comprising a uniform LiTaO₃ coating layer (≈3 nm) and a spinel interlayer structure (≈1 nm) is constructed on the grain surface of industrial LLO (Li_1.13Mn_0.517Ni_0.256Co_0.097O₂) agglomerated spheres. The surface composite nanostructure can not only enhance the structural/interfacial stability of the LLO, but also facilitates Li⁺ diffusion, thereby significantly improving its cycle stability, rate performance, thermal stability, and voltage maintenance. Specifically, the LLO coated with 10 ALD cycles exhibits a small voltage decay rate of 0.9 mV per cycle, a reversible capacity of 272.8 mAh g⁻¹ at 0.1 C, and a capacity retention of 85% after 200 cycles at 1 C, suggesting the important role of surface composite nanostructure for improving the electrochemical performance. This work provides new insights into the composite nanostructure design on the grain surface of cathode materials for high-performance LIBs.     

Influence of the energy density on the structure and morphology of polycrystalline silicon films treated with electron beam

Microstructures of electron beam remelted polycrystalline silicon films (20 μm thickness) deposited on a borosilicate glass solar cell absorber with a tungsten inter layer were investigated. It was found that WSi₂ compound was formed in both the tungsten/silicon interface and the grain boundaries of the silicon. The wetting and adhesion between the silicon melt and substrate were enhanced by the formation of WSi₂ compound. The electrical properties of the solar absorber were deteriorated by the tungstendisilicide interlayer. Because of the fast melting and cooling of the silicon film, the silicon film was solidified in a non-equilibrium manner due to the fast cooling rate during the EB remelting. Microstructural analysis indicated that the surface morphology of the film was affected by the EB energy density used in the remelting process. The capping layer became smooth and continuous and the number of pinholes was reduced when the EB energy density was increased. The deposited films exhibited large voids in the outmost surface layer and more WSi₂/Si eutectic crystallites in silicon layer and the WSi₂ layer became thicker if the EB energy density was too high.     

Effect of Mn on the Surface Morphological Properties of (Bi, Pb)2 Sr2Ca2Cu3?x Mn x O10+δ (Bi-2223) Superconductor

Samples of (Bi, Pb)₂Sr₂Ca₂Cu_3?x Mn _x O_10+?? (Bi-2223, x=0.0 to 0.30) were synthesized by a solid-state reaction route. The surface morphology investigated through scanning electron microscopy and atomic force microscopy (SEM and AFM) as a result shows that voids and grain sizes increase as the Mn concentration increases, and besides, nanosphere-like structures occur on the surface of the Mn-doped Bi-2223 sample. For x=0, compact granular structures of variously shaped thin grains and larger pores are observed in some local region. In the three-dimensional (3D) AFM view of the same surface the formation of the humps and roughness in some places can also be clearly seen, which is due to the formation of an oxide layer with different thicknesses, depending on the chemical composition of the phases. Besides the said features, two types of inhomogeneity have been observed in our investigations such as, first type, planar nanogranules of various sizes and, second type, closely packed planar rounded nanogranules.     

Research on the microstructure,fatigue and corrosion behavior of permanent mold and die cast aluminum alloy

Permanent mold (PM) and high pressure die cast (HPDC) AlMg5Si2Mn are employed to investigate the microstructure, fatigue strength and corrosion resistance. Results indicated that the mechanical properties (R_m, R_0.2 and δ) of HPDC specimens (314 MPa, 189 MPa and 7.3%) are significantly better than those of PM specimens (160 MPa, 111 MPa and 2.5%) due to the finer grain size and less cast defects. Fatigue cracks of PM samples dominantly initiated from shrinkage pores and obscure fatigue striations are observed in crack growth region. Corrosion and pitting potentials of PM and HPDC AlMg5Si2Mn alloy are around −1250 mV, −760 mV and −1220 mV, −690 mV respectively. Numerous pits are observed around the grain boundaries because the corrosion potential of Mg₂Si is more anodic than that of α-Al matrix. In addition, the superior corrosion resistance of HPDC samples can be attributed to the fine grain size and the high boundary density which improved the formation of oxide layer on the surface and prevented further corrosion.     

First principles investigation of the structure and stability of LiNiO2 doped with Co and Mn

The structures of LiNiO₂ doped with Co and Mn were investigated. The distributions of the transition metals (TMs) throughout the crystal were examined using density functional theory calculations, and the underlying mechanism was analyzed based on molecular orbital methods. The positions of the Li ions in de-lithiated material states were also investigated, and the mechanism underlying the structural stabilization introduced by the Co and/or Mn atoms is discussed. The crystal was found to be most stable if Co and Mn were present in the same layer. This TM distribution eased stress arising from differences in the TM–O bond length caused by the different effective charges of the transition metals. Li ions were preferentially located in the Li layer closest to the Co and Mn layers. Thus, a cluster containing Co, Mn, and Li ions is expected to persist, even under highly de-lithiated states. These results explain the improved stability of LiNiO₂ upon the introduction of Co and/or Mn.     

Luminescence mechanisms of organic/inorganic hybrid organic light-emitting devices fabricated utilizing a Zn2SiO4:Mn color-conversion layer

Zn₂SiO₄:Mn phosphor layers used in this study were synthesized by using the sol-gel method and printed on the glass substrates by using a vehicle solution and a heating process. Organic/inorganic hybrid organic light-emitting devices (OLEDs) utilizing a Zn₂SiO₄:Mn color-conversion layer were fabricated. X-ray diffraction data for the synthesized Zn₂SiO₄:Mn phosphor films showed that the Zn ions in the phosphor were substituted into Mn ions. The electroluminescence (EL) spectrum of the deep blue OLEDs showed that a dominant peak at 461 nm appeared. The photoluminescence spectrum for the Zn₂SiO₄:Mn phosphor layer by using a 470 nm excitation source showed that a dominant peak at 527 nm appeared, which originated from the ⁴T₁-⁶A₁ transitions of Mn ions. The appearance of the peak around 527 nm of the EL spectra for the OLEDs fabricated utilizing a Zn₂SiO₄:Mn phosphor layer demonstrated that the emitted blue color from the deep blue OLEDs was converted into a green color due to the existence of the color-conversion layer. The luminescence mechanisms of organic/inorganic hybrid OLEDs fabricated utilizing a Zn₂SiO₄:Mn color-conversion layer are described on the basis of the EL and PL spectra.     

Selective oxidation and segregation during annealing of steels at low oxygen partial pressures

Abstract

Industrial annealing of automotive steel grades is carried out in order to improve the mechanical properties and to adjust the grain size. However, the surface chemistry changes drastically due to selective oxidation and segregation phenomena, which significantly influence the further surface treatment, i.e. hot dip galvanizing.

The paper discusses results of laboratory experiments on selective oxidation and segregation of minor alloying elements Mn, Al, Cr and Si and non-metallic elements B, P and S during annealing of steel sheets in an N₂–5%H₂ atmosphere as a function of dew point. It will be shown that at lower dew points mostly external oxidation of Al and Mn occurs, where B shows a high tendency to segregate to the free surface and to form a BN film, especially observed on ferritic/austenitic steel. With increasing dew point, the oxidation of Al becomes internal and Mn, Si and Cr are oxidized externally. Boron also segregates to the surface by forming mixed Mn–B oxides and in the case of ferritic steels suppresses Si segregation and oxidation. The formation of phosphates by segregation of P becomes important at a D.P. of 0°C.     

Effects of B+ and Cr+ ion implantation on the oxidation of Ni3Al

Ni₃Al samples were implanted with different doses of 150keV B⁺ and Cr⁺ ions to modify the surface region and the high-temperature oxidation behaviour was tested. The surface layer structure was investigated by Auger electron spectroscopy, and transmission electron microscopy, secondary ion mass spectroscopy and optical microscopy before and after testing. The experimental results show that boron atoms exist in the form of interstitial atoms. No evidence was found that any new phase existed in boron implanted Ni₃Al. Implanted Ni₃Al alloy has better oxidation resistance than the unimplanted ones at 900°C. For B⁺-implanted Ni₃Al, the oxide layer is basically composed of fine-grained NiO inner layer and an a-Al₂O₃ outer layer. Boron is oxidized into B₂0₃ of comparatively larger grain size. B₂0₃ particles are enriched at grain boundaries and defects. This curtails the short-circuit transportation of oxygen and improves the oxidation resistance of Ni₃Al. Implantation with Cr⁺ and B⁺ combines the good effects of both elements and produces a remarkable improvement on the oxidation resistance. The effects of implanted elements and the possible reaction mechanisms are discussed.     

Effects of A/B cation ratio on the microstructure and lifetime of (Ba1-xCax)z(Ti0.99-yZryMn0.01)O3 (BCTZM) sintered in reducing atmosphere

Microstructure and dielectric properties of (Ba_1-xCa_x)_z(Ti_0.99-yZr_yMn_0.01)O₃ (BCTZM) with various cation ratios (A/B) sintered in reducing atmosphere and then annealed to reoxidize the ceramic bodies were investigated. With decreasing A/B cation ratio, concurrent with the grain size reduction, the insulation resistance and lifetime of the BCTZM are both increased. The degradation of insulation resistance is closely related to oxygen partial pressure used during annealing and the resulting microstructure of the ceramics. Within the same volume, the specimen with fine grain size provides more grain boundaries, in comparison to large grain size specimens, which helps in providing an efficient diffusion pathway for oxygen during annealing. The result of oxidation of the Mn, that is, a change in the valence state of Mn from Mn⁺² to Mn⁺³ is confirmed from thermogravimetric analysis and electron paramagnetic resonance analysis.     

Rapid thermal annealing effects on atomic layer epitaxially grown Zns:Mn thin films

ZnS:Mn thin films were coated on transparent conducting layer indium tin oxide (ITO) 2×2 in. glass substrates by an atomic layer epitaxy process. Grazing-angle X-ray diffraction on thin films shows a phase-pure ZnS with a wurtzite structure oriented along the 002 direction. Photoluminescence and CL measurements were carried out on the films. The emission from ZnS:Mn thin films consists of two strong bands at 515 and 452 nm with an excitation band at 329 nm. For improved brightness, the samples were annealed in a rapid thermal annealing furnace under different gas atmospheres (N₂, O₂ and forming gas) so that the green emission was increased. The green emission is due to donar-acceptor combination. Promising results were obtained when the thin films were annealed in forming gas at 600°C for one minute. Scanning electron microscope micrographs showed that the particles are well crystallized, with a grain size of 0.3–0.5 μm. This paper reports the particle size and morphology on the luminescent characteristics of a Mn²⁺ center in ZnS thin films. Received: 16 April 1999 / Reviewed and accepted: 21 April 1999     

Microstructural characterization of porous manganese thin films for electrochemical supercapacitor applications

An in-depth microstructural characterization was performed on manganese oxide materials that have been produced for electrochemical supercapacitor applications using a novel physical vapor deposition process. Manganese was e-beam evaporated and deposits as a combination of the cubic forms of Mn and MnO with a porous zigzag structure. The electrochemically oxidized sample that is used as the supercapacitor base material is tetragonal Mn₃O₄. An apparent active layer with increased sodium levels was imaged by STEM, lending some credence to the argument that the pseudocapacitance effect is based entirely on a surface layer of adsorbed sodium. Upon furnace annealing the zigzag structure near the free surface is destroyed and replaced with a columnar oxide layer of cubic MnO and tetragonal Mn₃O₄. This capping effect ultimately reduces the usable surface area and is thought to account for the reduction in capacitance seen on annealing.     

Influence of Mn addition on the ordering of DO3 Fe-28%Al intermetallics

The DO₃-type ordering in Fe-28Al and Fe-28Al-1.5Mn alloys are investigated by TEM and XRD. The results show that Mn addition into DO₃-ordered Fe₃Al alloy could decrease degree of ordering. Two major factors are considered to have effect of Mn on ordering behaviour of the alloy: reducing grain size and reducing antiphase domain size. The further investigation of deformed antiphase boundaries and slip lines in the alloy with Mn addition suggests that Mn could promote slip and cross slip in DO₃ Fe₃Al alloy during deformation.     

Barrier layer and grain boundary effects in Nd/Zr doped BaTiO3 ceramics

Gradient structures with barrier layer characteristics and core-shell morphology have been developed in BaTiO₃ ceramics with Nd₂O₃ and ZrO₂ as co-dopants. Features include reduced Curie temperatures and anisotropic stress gradients, resulting from an oxidized surface layer and reduced interior, developed during air sintering. Co-doping was typically carried out through solution milling of the BaTiO₃ powders with nitrate precursors of the dopant oxides, spray drying and sintering of the pressed pellets in air ambient at 1300-1320 °C/60-90 min with furnace cooling. Structural characterization, as well as dielectric and d.c. resistance measurements of the pellets, as-sintered and after removing equal amounts of material from both surfaces, revealed the existence of an oxidized surface layer and barrier layer microstructures consisting of graded regions of oxidized insulating surfaces over partially oxidized or conducting grain interiors. In this complex structure, the ZrO₂ segregates to the grain boundary region, forming a core-shell structure, with Nd₂O₃ partitioning between the BaTiO₃ and ZrO₂ phases. The overall system was modeled in terms of an equivalent circuit and the analysis indicates that the dielectric constant and the loss behavior are strongly impacted by both the surface and grain boundary barrier characteristics, with the surface barrier effects having the more dominant effect on the dielectric properties of the doped compositions. Indications are that fine-tuning of the system to optimize the grain boundary effect could lead to extraordinary dielectric constant effects which could potentially be utilized in high energy storage devices.