Depletion-layer Dielectric Properties of Positive Temperature Coefficient of Resistance Barium Titanate

For pure and impurity-added positive temperature coefficient of resistance (PTCR) barium titanate ceramic samples, a −11°C shift of the Curie point at the grain-boundary/depletion-layer region was observed. This result is obtained by fitting the PTCR grain-boundary resistance and capacitance data to a theory which combines a double-depletion-layer model with the Devonshire thermodynamic theory of barium titanate. The parameters used in the fitting are obtained from independent experiments. The shift of the Curie point is believed to result from the grain-boundary clamp ing effect near the cubic-tetragonal phase transition point.     

Origin of the electrical activity of iron atoms in vitreous arsenic selenide

Iron atoms in vitreous arsenic selenide films form single electron donor centers, while the Fermi level shifts from the middle of the band gap to the bottom of the conduction band with an increase in the iron concentration due to the filling of single electron states of the acceptor type located below the Fermi level.     

Formation of Potential Barrier Related to Grain-Boundary Character in Semiconducting Barium Titanate

Resistance–temperature ( R – T ) characteristics were measured directly at single-grain boundaries in 0.1-mol%-niobium-doped barium titanate bicrystals that had been fabricated from polycrystalline sinters, to determine a geometrical grain-boundary character dependence of the positive temperature coefficient of resistivity (PTCR) effect. Both random boundaries and low-Σ boundaries exhibit a similar grain-boundary character dependence of the PTCR effect through a simple geometrical analysis, using the coincidence of reciprocal lattice points. Differences of the R – T characteristics in individual boundaries have been explained in terms of the formation of a potential barrier that is associated with the oxidation of grain boundaries during cooling, after sintering or annealing. The grain-boundary character is likely to affect the diffusivity of O²⁻ ions and, hence, is crucial to the formation of the potential barrier.     

Effect of Microstructure on the PTCR Effect in Semiconducting Barium Titanate Ceramics

The microstructural influence on the PTCR effect in semiconducting barium titanate ceramics was studied and a method for preparing the ceramic bodies exhibiting a PTCR effect of more than seven orders of magnitude was established. Commercial barium titanyl oxalate was used as a starting material and Sb₂O₃ was added as a doping substance. The average grain sizes of the ceramic bodies prepared were 2 to 5 μm over a sintering range of 60 to 92%, to examine in detail the microstructural influence on the PTCR effect. No extra element, such as Mn or Cr, was added to develop the PTCR effect in the present PTCR materials.     

铁酸镧B位掺Mn/Co/Cr试样的红外辐射性能与机理

采用高温固相烧结法制备了粉体材料LaFeO₃、LaFe_0.75Co_0.25O₃、LaFe_0.75Cr_0.25O₃和LaFe_0.75Mn_0.25O₃,通过XRD、FTIR、SEM、XPS等检测手段对材料进行表征,同时运用CASTEP模块模拟计算了材料的电子能带结构和光学性质。实验结果表明：掺入Mn/Cr/Co离子后晶格发生畸变,晶格对称性降低。Mn/Cr/Co掺杂后的粉体材料在近中红外波段发射率排序为：掺Mn>掺Co>掺Cr>纯LaFeO₃,其中掺Mn在0.2～2.5μm波段为0.8722,2.5～5μm波段为0.6755,远大于纯LaFeO₃的发射率（近中红外波段发射率均为0.5左右）。发射率提升的机理在于：Mn掺杂后,引入了Mn³⁺杂质能级,产生了激活能小的Mn³⁺?Mn⁴⁺跳跃小极化子,电子-氧空位载流子吸收亦增强,同时体系的晶格畸变导致振动吸收加剧。第一性原理计算结果表明掺杂Mn/Cr/Co材料的禁带宽度分别0.793eV、2.406eV、1.722eV均小于纯LaFeO₃的3.817eV,结合态密度计算结果分析其原因,主要Mn3d、Cr3d、Co3d轨道与O2p轨道杂化形成杂质能级,同时Mn3d、Cr3d、Co3d在导带也存在态密度峰,且峰的位置都比LaFeO₃峰更靠近费米能级,作为新的导带底相当于缩短了价带顶到导带底之间的间隙宽度。LaFe_0.75Mn_0.25O₃材料在近中红外波段的优异辐射性能表现,可作为耐高温抗氧化高发射率材料在高温热工炉窑具有潜在应用前景。     

Interfacial Segregation in Perovskites: IV, Internal Boundary Layer Devices

A proposed model for interfacial segregation in perovskites, with induced heterogeneous defect distributions, is extended here to account for the formation of internal boundary layer devices, such as positive temperature coefficient of resistance (PTCR) thermistors and internal boundary layer capacitors (IBLC). Boundary layer effects in doped BaTiO₃ are attributed to factors which contribute to the formation of highly resistive boundary layers by a segregation-induced shift in donor incorporation and/or acceptor segregation, and the inhibiting action of segregated donors on boundary mobility and grain growth. The distribution of space charges, formed by electron transfer from conductive grains to resistive boundary layers, leads to the formation of impedance barriers in the grain-boundary vicinity. Depending on the grain size, and on relative size and spatial distribution of the space charge layer and the resistive layer, a transition from semiconducting properties to insulating properties may take place. This model accounts for the observed PTCR and IBLC phenomena.     

Reliability characteristics and conduction mechanisms in resistive switching memory devices using ZnO thin films

ABSTRACT: In this work, bipolar resistive switching characteristics were demonstrated in the Pt/ZnO/Pt structure. Reliability tests show that ac cycling endurance level above 106 can be achieved. However, significant window closure takes place after about 102 dc cycles. Data retention characteristic exhibits no observed degradation after 168 h. Read durability shows stable resistance states after 106 read times. The current transportation in ZnO films is dominated by the hopping conduction and the ohmic conduction in high-resistance and low-resistance states, respectively. Therefore, the electrical parameters of trap energy level, trap spacing, Fermi level, electron mobility, and effective density of states in conduction band in ZnO were identified.     

Photoelectric emission and work function of semiconducting diamonds

Photoemission of electrons was observed in p-type semiconducting diamonds. The emitted electrons responding to photons which have the necessary threshold energy are believed to have originated from filled surface states. The surface charge densities are sufficiently large so that the Fermi level is greatly affected by the surface states. The work function of the semiconducting diamonds investigated is approximately 6.0 eV.     

Electrical Properties of PTCR Barium Titanate

When properly doped, barium titanate ceramics display positive temperature coefficient resistance (PTCR) behavior. This has been proved to be a Schottky barrier type of grain-boundary effect. However there has not yet been a complete point-to-point comparison between the experimental data and theory for the entire set of the material nonlinear dielectric properties. In this study, a methodology has been developed which allows the study of the depletion layer dielectric properties while the PTCR effect is being investigated. An equivalent dielectric constant, the value of which is to be determined from this experiment, is treated as an average of the dielectric properties of the depletion layer and is used to analyze the grain-boundary resistance and capacitance data based on a simple double-depletion-layer model. The theoretical relationship between this equivalent dielectric constant and the material dielectric properties is also explored in this study.     

Effect of PO2 on Bulk and Grain Boundary Resistance of n-Type BaTiO3 at Cryogenic Temperatures

Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO₃ polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P _O2 as low as 10⁻¹⁵ atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO₃ sintered in reducing atmosphere and then annealed at P _O2= 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000°C. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200–1250°C. This result suggests that the PTCR effect in polycrystalline BaTiO₃ is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries.     

Gd2O2S电子结构及光学性质的第一原理研究

利用基于密度泛函理论的第一原理方法研究了Gd2O2S的电子结构及光学性质.能带结构分析表明:Gd2O2S是一种间接带隙的半导体材料,其间接带隙值为3.22 eV,且价带顶和导带底分别位于G点和M点.态密度计算表明价带顶部由O2p和S 3p态杂化而成,导带底部主要由Gd 5d态构成.在对带隙进行1.15 eV的剪刀修正后,通过第一原理方法研究了Gd2O2S的光学性质.计算并分析了Gd2O2S的介电函数、复折射率、吸收系数和透光率.其中,静态折射率的计算值与实验值吻合得很好,Gd2O2S在可见到红外区的理论透光率的计算值为76.5％.     

N掺杂锐钛矿二氧化钛光响应红移的模拟计算

利用密度泛函理论(DFT),采用平面波超软赝势方法,模拟计算了掺N锐钛矿相TiO2对能隙、Fermi能级、态密度的影响,以探讨其可见光活性的机理。计算结果表明:表面HOMO和LUMO分别主要由Ti的3d轨道和O、N的2p轨道构成,掺杂出现的N2S态增加了N掺杂的稳定性。并发现N掺杂从两个方面使TiO2材料的带隙变窄:一为N掺杂使Fermi能级移入价带,使带隙变窄;二为在价带的上方引入了独立的能级,在导带下方引入一个宽为0.9 eV左右的能带,这两个掺杂能级使其禁带减小。     

Evidence of band bending and surface Fermi level pinning in graphite oxide

We present the electronic structure of graphite oxide in the vicinity of the Fermi level measured using ultraviolet photoemission and inverse photoemission spectroscopies and compare it with X-ray absorption spectra. The expected p-type behavior of graphite oxide is not observed at the surface and the presence of band bending is invoked. The observed electronic structure of graphite oxide exhibited an n-type semiconducting band structure with a band gap of 2.3 ± 0.4 eV. An oxygen related state, at 0.8 eV above Fermi level, and the suppression of the unoccupied carbon weighted states at the conduction band minimum suggests that the oxygen vacancies at the surface of graphite oxide contribute to the n-type semiconducting electronic structure of the surface.     

A potential electrolyte (Ce1-x CaxO2-δ) for fuel cells:Theoretical and experimental study

First-principles calculations are performed using density function theory to explore the effects of dopant Ca in ceria (Ce_1-xCa_xO_2-δ). The impact of oxygen vacancy on band gap and density of states is examined in doped ceria using generalized gradient approximations. Vacancy association and vacancy formation energies of the doped ceria are calculated to reveal the effect of dopant on ion conduction. The experimental study of the sample (Ce_0.875Ca_0.125O_2-δ) was performed to compare with the theoretical results. The obtained results from theoretical calculation and experimental techniques show that oxygen vacancy increases the volume, lattice constant (5.47315?Å) but decrease the band gap (1.72?eV) and bulk modulus. The dopant radius (1.173?Å) and lattice constant (5.4718?Å) are also calculated by equations which is close to the DFT lattice parameter. The result shows that oxygen vacancy shifts the density of states to lower energy region. Band gap is decreased due to shifting of valence states to conduction band. Vacancy formation shows a significance increase in density of states near the Fermi level. Density of states at Fermi level is proportional to the conductivity, so an increase in density of states near the Fermi level increases the conductivity. The experimental measured ionic conductivity is found to 0.095?S?cm^?1 at 600?°C. The microstructural studies is also reported in this work.     

Grain Boundary Oxidation in PTCR Barium Titanate Thermistors

The phenomenon of grain boundary oxidation in PTCR BaTiO₃ thermistors is discussed. In particular, the energy spectra of the surface states were calculated for different samples, and these were related to the nominal composition, the impurity content of the base BaTiO₃ powder used, and the prevalent atmospheric conditions during cooling and/or annealing. It is proposed that the interaction of manganese with oxygen creates deep-lying traps, and, in general, some proof is offered that the majority of the surface states are due to different oxidizing chemisorbed gases. It is believed that the ability of a particular sample to adsorb such gases in adequate amounts, and thus exhibit an appreciable PTCR effect, is related to the presence of acceptor-type dopants perferentially segregated onto the grain surfaces. Notably, the role of 3 d transition metal cations in this process is discussed in some detail.     

Deep Levels Near the Grain Boundary in a Zinc Oxide Varistor: Energy Change Due to Electrical Degradation

Localized deep levels in a ZnO varistor were investigated by isothermal capacitance transient spectroscopy, in relation to a degradation of nonlinear electrical characteristics caused by thermal treatment in a slightly reduced atmosphere. Two discrete deep levels were observed near the grain boundary. Before the degradation, they were located at 0.36 and 0.19 eV below a conduction band. After the degradation, they shifted toward the conduction band by 0.16 and 0.09 eV, respectively: the energy shift was not identical for the two levels. It was suggested that the energy shift of the localized levels reflected lattice disorders originally existing near the grain boundary or proper changes of chemical states induced by the reduction.     

Effect of sintering aids on the densification and electrical properties of SiO2 –containing Ce0.8Sm0.2O1.9 ceramic

In this work, five different metal-oxide additives (metal?=?Ba, Co, Fe, Li, and Mn) were examined as sintering aids and SiO₂ impurity scavengers for Ce_0.8Sm_0.2O_1.9 (SDC). 2?mol% additives were loaded into the SDC with ～150?ppm (moderately impure) and ～2000?ppm (highly impure) SiO₂. Ba-, Co-, Fe- and Mn-oxides showed comparative sintering-aid effect on both moderately- and highly-impure SDC specimens, but the sintering-assisting effect of Li-oxide was completely neutralized in highly impure SDC. Regarding electrical property, the deleterious effect of 2000?ppm SiO₂ impurity on the grain-boundary conduction of SDC can be effectively alleviated by adding Ba-, Co-, Fe-, or Mn-oxides. Microstructure analysis revealed that Ba-oxide reacted directly with SiO₂ and consequently enhanced grain-boundary conduction. By contrast, with the addition of Co-, Fe-, and Mn-oxides, the improved grain-boundary conductions of impure SDC were related to the scavenging reactions between Si, Ca (another original impurity) and Sm components.     

Electronic Structure of a Near Σ11 a-axis Tilt Grain Boundary in α-A12O3

The electronic structure and interatomic bonding of a near Σ11 α-axis tilt grain boundary in α-A1₂O₃ are determined using first-principles calculations based on a model structure constructed by Kenway. This Σ11 boundary, which has a low grain boundary energy, has been the focus of various previous studies, including high-resolution transmission microscopy (HRTEM) and pair potential based structural studies. The relaxed structure model, which contains 72 Al atoms and 108 O atoms, is periodic in two dimensions and reproduces the grain boundary structure obtained by HRTEM. To accurately identify the changes arising in the boundary, parallel calculations on the bulk supercell models containing the same number of Al and O atoms with and without surfaces were also carried out. This grain boundary does not have deep levels in the band gap, and its defect states appear mainly near the O-2p derived valence band maximum and the Al-3s derived conduction band minimum. The calculated partial densities of states for Al in the conduction band region are in good agreement with recent measurements on the near-edge energy loss spectrum and the valence electron energy loss spectrum. By means of Mulliken population analysis, the effective charges, the bond order, and the partial density of states of the grain boundary atoms are identified and elucidated. In the region of the grain boundary, there is increased charge transfer from Al to O, arising mainly from the lower coordination number of the grain boundary atoms and therefore the reduction in the covalent bond formation. In addition, it is found that as the Al-O bond lengths decrease, the Al—O charge transfer increases. For this Σ11 boundary, the structural features most associated with grain boundary electronic structure changes are undercoordination and shortened bond lengths.     

Differential Negative Resistance and Piezoresistivity in Thin Semiconducting BaTiO3 Ceramic Bars

Thin, semiconducting barium titanate (BaTiO₃) ceramic bars, with a diameter of 10 to 20 μm, consisting of single grains joined together in series have been prepared to investigate the piezoresistivity in the materials, which was evaluated from their current ( I )-voltage ( V ) characteristics under the loading condition of various bending stresses. I-V characteristics of single grain boundaries in some of the materials were found to exhibit distinct differential negative resistance (DNR) at room temperature with its feature changing with stress. The DNR appeared on the I-V curves at an electric field of several volts per one grain, and has been confirmed to be connected with the transition of current between two conduction states in the grain boundary region. The obtained results indicate that this phenomenon cannot be interpreted by a rise in the temperature of the materials up to their positive temperature coefficient of resistivity (PTCR) region above the Curie point by Joule heating due to current flow, that is their self-heating effect. This newly observed DNR phenomenon has thus been tentatively interpreted by the morphological change in the ferroelectric domain structure in the vicinity of grain boundaries under mechanical and electric stresses, on an assumption that different configurations of ferroelectric domains yield different conduction states in the grain boundary due to a difference in the degree of surface acceptor charge compensation or the anisotropic carrier mobilities in the crystal.     

Additive manufacturing of barium titanate based ceramic heaters with positive temperature coefficient of resistance (PTCR)

Lanthanum/manganese doped barium titanate (BT) based PTCR functional heater elements/structures were fabricated with desirable electrical properties for the first time using Additive Manufacturing (AM). 3D printed components of varying size and shape and prototype honeycomb lattices with high density were achieved through AM. Aqueous, less organic containing (2.5 wt% additives versus 10–30 wt% added typically), eco-friendly ink formulations were developed with suitable rheological properties for 3D printing. For BT prints, the sintered densities of the 3D ceramic parts were found to be >99% TD, highest reported value so far. The microstructure, electrical properties and heating characteristics of the printed PTCR components were studied in detail and their thermal stability evaluated using infrared imaging and benchmarked against commercial PTCR heating element. The heating behaviour of the solid and porous 3D printed components was demonstrated to be similar, paving the way for light weight (?47% reduction in weight) heaters suitable for automotive/aerospace applications and less materials wastage during device fabrication.