PTCR properties of Sb2O3-doped (Ba,Sr)TiO3

Perovskite barium–strontium titanate, (Ba,Sr)TiO₃ was prepared and effects of Sb₂O₃ additives on its PTCR properties were investigated. The (Ba,Sr)TiO₃ with 0.05∼0.25 mol% Sb₂O₃ showed semiconducting PTCR behavior and anomalous grain growth was also observed when sintered at 1360 °C. It was considered that charge compensation by doping Sb₂O₃ as well as anomalous grain growth by sintering leads to resistivity reduction from insulating to semiconducting transition.     

Y和YF3掺杂钛酸钡系PTCR材料的结构及性能

在不同烧结气氛下制备了Ｙ和掺杂钛酸钡材料,借助于ＸＲＤ、ＳＥＭ、ＸＲＦ和阻温测试分析仪,研究了烧结气氛对Ｙ和ＹＦ３掺杂钛杂钛钡材料结构和性能的影响,研究结果表明,你舂压气氛可促进Ｙ和ＹＦ３掺杂钛酸钡材料的烧结,晶粒长大,而且这二种掺杂钛酸钡材料都是ｎ型半导体,经过氩气氛烧结的Ｙ掺杂钛酸钡材料ＰＴＣＲ效应较弱,而对在氩气氛中烧结的０．３ｍｏｌ％ＹＦ３掺杂钛酸钡材料却观察到了较好的ＰＴＣＲ效应,这种效     

Redox processes at grain boundaries in barium titanate-based polycrystalline ferroelectrics semiconductors

Barium titanate, which is characterized by a positive temperature coefficient of resistance (PTCR), is widely used in practice. At the same time, it is unknown why only a small percentage of the introduced donor dopant takes part in the formation of PTCR effect, which phases appear at grain boundaries, how the introduced acceptor dopants affect the properties of grains. Elucidation of the above questions is of considerable scientific and practical interest. It has been shown that the phases Bа₆Ti₁₇O₄₀ and Y₂Ti₂O₇ precipitate on grains of barium titanate doped with donor dopant (yttrium). We identified paramagnetic impurities (iron, manganese, chromium) in starting reagents. These impurities can occupy titanium sites. Therefore, the part of the donor dopant that is spent on the charge exchange of acceptor dopants does not participate in the charge exchange of titanium Ti⁴⁺ → Ti³⁺, which is responsible for the appearance of PTCR effect in barium titanate. It has been found that an extra acceptor dopant (manganese) is distributed mainly at grain boundaries and in the grain outer layer. It has been shown that manganese ions introduced additionally (as acceptor dopants) increase the potential barrier at grain boundaries and form a high-resistance outer layer in PTCR ceramics. The resistance of grains, outer layers, and grain boundaries as a function of the manganese content has been investigated.

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Studies on semiconductive (Ba0.8Sr0.2) (Ti0.9Zr0.1)O3 ceramics

In order to produce semiconductive (Ba_0.8Sr_0.2) (Ti_0.9Zr0.1)O₃ ceramics (BSZT), providing low resistivity for boundary-layer capacitor applications, a controlled valency method and a controlled-atmosphere method were applied and studied. In the controlled-valency method, trivalent ions (La³⁺ Sb³⁺) and pentavalent ions (Nb⁵⁺, Sb⁵⁺, Ta⁵⁺) were doped into BSZT ceramics, while in the controlled-atmosphere method, samples were sintered in air and a reducing atmosphere. The doped BSZT ceramics sintered in the reducing atmosphere showed much lower resistivities and smaller temperature coefficient of resistivity (TCR) than those sintered in air, indicating that low partial pressure of oxygen will increase the solubility of the donor dopant and enhance the grain growth. In addition, a small negative TCR at low temperature, as well as a small positive TCR at higher temperature, are also observed for specimens fired in a reducing atmosphere. The former is attributed to the semiconductive grain and the latter to the small barrier layer formed at the grain boundary.     

The abnormal grain growth and dielectric properties of (Nb,Ba) doped TiO2 ceramics

The effect of consolidation pressure and crystallite size of powders crystal phases of TiO₂ on sintered microstructure of TiO₂ ceramics doped with 0.25 mol % Nb and 1.0 mol % Ba were investigated. Also, the development sequence of abnormal grain growth of (niobium, barium) doped TiO₂ ceramics was proposed. The second phases of as-sintered surface were determined. The dielectric properties of Ag-electroded samples were correlated with the resistivity of the bulk (Nb, Ba) doped TiO₂ ceramics. Abnormal grain growth lowered the resistivity of bulk material of (Nb, Ba) doped TiO₂ ceramics, and moved the relaxation frequency of fan δ to high frequency region over 10⁵ Hz. Controlling the sintered microstructures can obtain reasonably good dielectric properties.     

The effect of CuO on the grain growth of ZnO

The grain growth kinetics in the 1, 2, 3 and 4 wt.% CuO doped ZnO was studied using the simplified phenomenological grain growth kinetics equation Gⁿ – G₀ⁿ = K₀ t exp (– Q/RT) together with microstructure properties of the sintered samples. The grain growth exponent value (n) was found to be 3 for 1, 2 and 3 wt.% CuO doped ZnO and 5.5 for 4 wt.% CuO doped ZnO. The apparent activation energy was decreased with CuO doping up to 3 wt.% from 250 kJ/mol to 150 kJ/mol but it was not changed significantly (155 kJ/mol) by 4 wt.% CuO doping. CuO doping up to 3 wt.% promoted the grain growth of ZnO whereas 4 wt.% CuO doping inhibited the grain growth of ZnO because of formation of Cu-rich secondary phase in the grain boundaries.     

Influence of sintering conditions on the electrical properties and the PTCR effect of the multilayer Ba1.005(Ti1?xNbx)O3 ceramics with Ni internal electrode

The effects of the Nb-dopant content and the sintering conditions on the electrical properties and the positive temperature coefficient of resistance (PTCR) effect of Ba_1.005(Ti_1?xNb_x)O₃ (BTN) ceramics were investigated, which were sintered at 1,070–1,220?°C for 0.5–6?h in a reducing atmosphere and then re-oxidized at 600–750?°C for 1?h. The results indicated that both the sintering temperature and sintering time affected the electrical properties and the PTCR effect of the multilayer BTN samples, whose room-temperature (RT) resistance first reduced and then increased as a function of the donor–doped concentration at all sintering temperature; moreover, the higher the sintering temperature was, the lower the critical dopant concentration. The BTN ceramics showed a remarkable PTCR effect, with a resistance jump greater by 3.3 orders of magnitude, along with a low RT resistance of 0.3?Ω at a low reoxidated temperature of 600?°C after sintering in a reducing atmosphere.     

Y和YF3掺杂钛酸钡系PTCR材料的结构及性能

在不同烧结气氛下制备了Ｙ和ＹＦ_３掺杂钛酸钡材料,借助于ＸＲＤ、ＳＥＭ、ＸＲＦ和阻温测试分析仪,研究了烧结气氛对Ｙ和ＹＦ_３掺杂钛酸钡材料结构和性能的影响．研究结果表明,低氧分压气氛可促进Ｙ和ＹＦ_３掺杂钛酸钡材料的烧结,晶粒长大,而且这二种掺杂钛酸钡材料都是ｎ型半导体．经过氩气气氛烧结的Ｙ掺杂钛酸钡材料ＰＴＣＲ效应较弱;而对在氩气气氛中烧结的０．３ｍｏｌ％ＹＦ_３掺杂钛酸钡材料却观察到了较好的ＰＴＣＲ效应,这种效应的产生可能与Ｆ元素取代Ｏ位而导致材料的价控半导有关．     

Properties of barium-strontium titanate PTCR ceramics sintered on different powder beds

Barium-strontium titanate (BST) ceramics, co-doped with Sb and Mn oxides, were sintered on different powder beds: Sb-doped BST; Al₂O₃; or Sb,Mn-codoped BST powder. Phase formation, microstructure and the electrical properties of the samples were analysed. The PTCR behavior depended significantly on the type of powder bed used. The BST ceramic pellet sintered on the Sb-BST powder displayed the largest PTCR effect, with a ρ_max/ρ_RT ratio of ∼10⁶. This was an order of magnitude greater than for samples sintered on the other two powders. Complex impedance analysis confirmed that this was due to a large increase in grain boundary resistance at 250 °C.     

Influence of Bi3+ ions in enhancing the magnitude of positive temperature coefficients of resistance in n-BaTiO3 ceramics

Bi³⁺ ions substituting at Ba-sites in a limited concentration range with another donor dopant occupying the Ti-sites in polycrystalline BaTiO₃ enhanced the positive temperature coefficient of resistance (PTCR) by over seven orders of magnitude. These ceramics did not require normal post sinter annealing or a change to an oxygen atmosphere during annealing. These ceramics had low porosities coupled with better stabilities to large applied electric fields and chemically reducing atmospheres. Bi³⁺ ions limited the grain growth to less than 8 m in size, they enhanced the concentration of acceptor-type trap centres at the grain-boundary-layer regions and maintained complete tetragonality at low grain sizes in BaTiO₃ ceramics.     

Effects of reducing and oxidizing atmospheres on the PTCR characteristics of porous n-BaTiO3 ceramics by adding polyethylene glycol

Donor doped BaTiO₃ (n-BaTiO₃) ceramics were fabricated by adding polyethylene glycol (PEG) at 20 wt %. The effects of reducing and oxidizing atmospheres on the PTCR characteristics of the porous n-BaTiO₃ ceramics were investigated. The PTCR characteristics of the porous n-BaTiO₃ ceramics is strongly affected by chemisorbed oxygen at the grain boundaries and are recovered as the atmosphere is changed from the reducing gas to oxidizing gas. The low room-temperature resistivity of the porous n-BaTiO₃ ceramics in reducing atmospheres may be caused by the decrease in potential barrier height, which originates from an increase in the number of electrons owing to the desorption of chemisorbed oxygen atoms at the grain boundaries. In addition, the high room-temperature resistivity of the porous n-BaTiO₃ ceramics in oxidizing atmospheres may be caused by the increase in potential barrier height, which results from the adsorption of chemisorbed oxygen atoms at the grain boundaries.     

Subsolidus grain growth in donor doped barium titanate

Grain growth in donor doped BaTiO₃ was studied below the temperature of liquid phase formation. It was found that a high donor concentration and high oxygen partial pressure effectively inhibit grain growth rate during sintering. Defect chemistry, derived from a model consistent with electrical measurements, well describes the grain growth properties of donor doped BaTiO₃ in the subsolidus grain growth region.     

Effect of Isovalent Ba-Site Substitutions on the Properties of (Ba1 – x – y M y Y x )TiO3(M = Ca, Sr, Pb) PTCR Ceramics

An approach is proposed for fabricating fine-grained, low-resistivity BaTiO₃-based PTCR ceramics via partial isovalent substitutions on the Ba site. The grain size of the ceramics thus prepared is shown to decrease as the ratio of ionic radii r(Ba²⁺)/r(M²⁺) (M = Ca, Sr, Pb) increases. Isovalent substitutions on the Ba site narrow down the range of donor dopant (yttrium) concentrations in which PTCR materials can be prepared. The experimental results agree well with thermodynamic calculations under the assumption that the materials contain the Y³⁺Ti³⁺O₃ phase, as suggested by ESR data, which point to the presence of Y³⁺–Ti³⁺ associates. Partial calcium, strontium, and lead substitutions on the Ba site reduce the average grain size of PTCR ceramics, which is probably due to the lattice strain arising from the isovalent substitution. Partial replacement of Ba²⁺ with mixtures of different isovalent elements (e.g., Sr²⁺ and Pb²⁺) offers the possibility of obtaining fine-grained, low-resistivity PTCR ceramics, without changing the phase transition temperature.     

Liquid phase sintering of Re2O3 YSZ ceramics: Part II Grain boundary electrical properties

Grain boundary electrical properties of Y₂O₃ stabilised zirconia with small additions of Er₂O₃ and Pr₂O₃ sintered via silicate liquid phase were studied by the impedance spectroscopy technique. Grain boundary specific conductivity of the praseodymium doped samples was found to be independent of sintering time, while the erbium doped sample showed high anomalous conductivity for the 1.0 h sintered samples. The electrical behaviour is explained considering the grain boundary to be a series association of the glass film and the space charge region. Specific conductivity and Debye length of the space charge region of erbium doped samples were found to be 6.7 × 10^–8 S/cm and 0.25 nm, respectively.     

Effect of Bi2O3 and Y2O3 doping methods on electrical properties and PTCR behavior of Ba0.95Ca0.05TiO3 ceramics

Lead-free PTCR ceramics based on Bi₂O₃ and Y₂O₃ doped Ba_0.95Ca_0.05TiO₃ were fabricated by the conventional mixed oxide method, while Bi₂O₃ and Y₂O₃ were doped directly or after pre-calcining, in the molar ratio of Bi₂O₃:Y₂O₃ = 1:1. There were two synthesizing route, i.e. the materials were pre-calcined at 900 °C to obtain BiYO₃ firstly and then doped into the basic materials, and the materials were directly doped into the starting materials, both of which could obtain samples with different electrical properties and PTCR behavior. The samples were characterized by using X-ray diffraction, scanning electron microscope, dielectric constant-temperature and resistivity-temperature measurement instrument. It was revealed that the perovskite lattice, the microstructure and the PTCR behavior of Ba_0.95Ca_0.05TiO₃ varied with different doping contents and methods. A further research was conducted so as to study the electrical properties of ceramics by impedance spectroscopy.     

TiCl3-doped Ba0.92Ca0.08TiO3 PTCR ceramics with low r.t. resistivity

The influence of TiCl₃ solution on the room temperature (r.t.) resistivity and electrical properties of Ba_0.92Ca_0.08TiO₃ PTCR ceramics was studied. The results indicate that the PTC effect can be improved significantly when an appropriate amount of TiCl₃ in solution is added to the original materials. Some of the doped Ti³⁺ ions segregate at grain boundaries behaving as acceptors by substituting for Ti site or valence varying (from Ti³⁺ to Ti⁴⁺). As a result, the surface charge density N _s) increases and the barrier height at grain boundaries () is enhanced.     

Improvement in dielectric properties of Al2O3-doped Li0.30Cr0.02Ni0.68O ceramics

Li_0.30Cr_0.02Ni_0.68O giant dielectric ceramics doped with Al₂O₃ were prepared by solid-state reaction via sol-gel process. The sintered samples were characterized using X-ray powder diffraction and scanning electron microscopy, and dielectric properties were also investigated. All doped samples showed the single phase of cubic rock-salt structure NiO. With increasing Al₂O₃ content, the crystallite size and grain size decreased, possibly due to an occurrence of the secondary phases at grain boundaries which inhibit the grain growth. The sample with 0.2 wt.% Al₂O₃ showed nearly 7 times lower tanδ (2.37) and higher ε_r (7.25 × 10⁶) measured at 1 kHz and room temperature when compared to the pure sample.     

Segregation and Local Structure at Grain Boundaries in SiO2-Doped Tetragonal ZrO2 Polycrystalline Materials

SiO₂ doping in Y₂O₃ stabilized tetragonal ZrO₂ (TZP) materials introduced significant change in mechanical properties around 0.3 wt.% doping level [1]. In order to understand the influence of grain boundary structure and chemistry by doping, high purity undoped and SiO₂-doped 3Y-TZP samples were studied using high-resolution and analytical TEM. Typical grain boundary structures are different for the two types of samples, while amorphous film was not observed at most grain boundaries. A new EDS analysis method was introduced to detect the weak Si and Y signals which overlap with the predominent Zr peaks. It revealed that Si segregation to the grain boundary saturates at 12 at./nm² (or 1.5 monolayer of SiO₂) when the SiO₂ doping level reached and surpassed 0.3 wt.%. It is the segregated atoms which enhanced the grain boundary diffusivity and therefore altered the deformation mechanism.     

BaTiO3基PTCR陶瓷中B2O3蒸汽掺杂的异常行为

钛酸钡基半导化陶瓷中的PTCR效应通常与材料中的施受主掺杂密切相关。在高温下B2O3具有较高的蒸汽压,通过B2O3蒸汽掺杂的研究表明,含主族元素B的氧化物蒸汽掺杂,钛酸钡基半导化陶瓷样品的升阻比同样得到了大幅度提高,同时室温电阻率也有所增加。B2O3蒸汽掺杂BaTiO3基材料的PTCR效应的提升可能得益于硼填隙和钡缺位相关的复合缺陷在晶界上的形成。     

Effect of MnCO3 doping on the dielectric and tunable properties of BSTO/MgO composite for phased array antennas

Undoped and MnCO₃ doped (0.3, 0.5, 1, 2mol%) Ba_0.55Sr_0.45TiO₃/MgO composites were prepared by conventional ceramic processing method and their structural, surface morphological, tunable properties and their dielectric properties at low and microwave frequencies were extensively investigated. The results indicate that the addition of MnCO₃ slightly decreases the average grain size. The proper doping of MnCO₃ can dramatically improve the properties of Ba_0.55Sr_0.45TiO₃/MgO composite with lower dielectric loss and higher tunability. For 0.5mol% MnCO₃ doped samples, the microwave loss at 4.19 GHz is 2.91 × 10⁻³ and the value of tunability is 7.4% with the external DC field 2 kV mm⁻¹, which presented the best properties and can basically meet the requirements of phased array applications.