Modification on PTCR behavior of (Sr0.2Ba0.8)TiO3 materials by post-heat treatment after microwave sintering

Abstract

Effect of post-sintering treatment on PTCR behavior of (Sr_0.2Ba_0.8)TiO₃ materials prepared by microwave-sintering (ms) process was compared to that prepared by rapid thermal sintering (RTS) process. The microwave-sintering process needed only 1130°C-40 min to effectively densify (Sr_0.2Ba_0.8)TiO₃ materials. The grain size was around 6 μm and PTCR characteristics was around ρ_max/ρ_min≈ 10^1.75, with T_c = 50°C. Lowering the cooling rate after sintering substantially increases the resistivity jump (ρ_max/ρ_min) from 10² to 10^3.4, without altering the microstructure. The annealing at 1250°C for 2 h markedly increased the resistivity jump to (ρ_max/ρ_min)≈10⁶. On the other hand, the rapid thermal sintering (RTS) process required 1320°C-30 min to fully develop the good microstructure (～15 μm) and PTCR property (ρ_max/ρ_min ～ 10^3.0). Post-sintering process, including cooling rate control and annealing, did not improve the electrical properties of these samples, that is ascribed to the slow-cooling rate characteristics of RTS-process for a temperature lower than 800°C.     

MOCVD of (Ba,Sr)TiO3: Nucleation and Growth

(Ba_0.7Sr_0.3)TiO₃ and SrTiO₃ thin films were deposited on Pt electrodes in a planetary multi-wafer MOCVD reactor. The nucleation behavior and the size of the stable nuclei were investigated by different SPM techniques. Characteristic differences were observed for different deposition temperatures, i.e. a homogeneous nucleation of small BST grains on the larger Pt grains at 565°C and a dominating nucleation at the grain boundaries at 655°C. The micro structural evolution after further film growth was investigated by HRTEM and revealed randomly oriented grains (typical inplane size 10–20 nm) with a high density of twins at 565°C and (100)-oriented defect free grains of only slightly increased size at 655°C. For SrTiO₃ the inplane grain size was increased, however, the (100) texture was less perfect. As the electrical properties like permittivity and also leakage current depend on film thickness the final discussions of the electrical properties are based on thickness series (5 nm–100 nm films) and evaluated within the phenomenological dead layer model.     

Dielectric Properties of Acceptor-Doped (Ba,Ca)(Ti,Zr)O3 Ceramics

The dielectric properties of (Ba,Ca)(Ti,Zr)O₃ ceramics containing various acceptor and donor dopants on the B sites have been studied. Formation of charge compensating complexes between acceptors, donors and oxygen vacancies has been observed. All acceptors cause a reduction of the Curie point. The maximum dielectric constant increases with the average grain size almost independent of the acceptor incorporated. Ceramics fired in reducive atmospheres reveal pronounced changes of the dielectric properties after annealing in oxidizing atmosphere. In particular the losses and the maximum dielectric constants are significantly affected by the annealing time, temperature and partial pressure of oxygen.     

Interface Structures and Epitaxial Behavior of Ferroelectric (Ba,Sr)TiO 3 Thin Films

Perovskite Ba 0.6 Sr 0.4 TiO 3 thin films were epitaxially grown on (001) LaAlO 3 and (001) MgO by using pulsed laser ablation. The films are single crystal cubic structure with interface relationship of (001) BST // (001) MgO and <100> BST // <100> MgO for the films on (001) MgO and (001) BST // (001) LAO and <100> BST // <100> LAO . High-resolution transmission electron microscopy studies indicated that the initially grown layer of the film on both (001) MgO and (001) LAO is TiO 2 monolayer. Equally spaced misfit dislocations were found to form at the interface to release the lattice mismatch energy. Physical property measurements show that the films exhibit excellent room temperature dielectric behavior with very low dielectric loss of 0.007 and very large tenability of 45% at 1 MHz. The figure of merit of 60°/dB has been achieved for the microwave phase shifter operated at 24 GHz and room temperature.     

The resistance degradation of (Ba0.5Sr0.5)TiO3 thin films

Abstract

The leakage current and dielectric properties of (Ba_0.5Sr_0.5)TiO₃(BST) thin films prepared by pulsed laser deposition (PLD) were investigated. It was found that leakage currents for positive bias voltage were higher than that for negative bias voltage, which was attributed to the lattice mismatch between bottom Pt electrode and BST thin film. The time-dependent breakdown process under positive voltage was observed, which was interpreted as the increase of the internal electric field in the film near the bottom electrode. However, the internal electric field can be decreased and eventually recovered by applying negative bias voltage. It was found that internal electric field near the interface can influence the capacitance of the BST thin film capacitor. An explanation for the thickness effect of BST thin films was given.     

Ceramics Materials Based on (Ba, Sr)TiO3 Solid Solutions for Tunable Microwave Devices

Structure and electrical properties at radio frequencies as well as within the 3.5–35 GHz frequency range have been investigated for ceramic samples of the (1–y)(Ba_xSr_{1 – x})TiO₃ · yMgO (BSM) system where x = 0.4–0.6; y = 0.15–0.30. For the compositions studied the bulk ferroelectrics were synthesized with the dielectric constant of 400–600 and high tunability coefficient. We indicated that the quality factor of the samples was in the range of 100–1000 within the frequency band of 3.5–35 GHz. The phase correlations and unit cell constants of the perovskite phase of the BSM samples were studied. The low loss factor and high tunability of the bulk material allowed us using the BSM ferroelectric ceramic layer for tunable accelerating structures of the Argonne Dielectric Wakefield Accelerator and for high power switches design and development for the future linear colliders.     

Structure,Ferroelectric, Dielectric and Energy Storage Studies of Ba0.70Ca0.30TiO3, Ba(Zr0.20Ti0.80)O3 Ceramic Capacitors

Ba_0.70Ca_0.30TiO₃-(BCT),Ba(Zr_0.2Ti_0.8)O₃-(BZT) ceramics were fabricated by conventional mixed oxide route to develop inorganic dielectric materials suitable for use as an insulator with high dielectric constant and low energy loss for capacitor applications. The structural phase transition, ferroelectric, dielectric and energy storage properties of BCT, BZT ceramic capacitors were investigated. Room temperature X-ray diffraction (XRD) patterns revealed prominent peaks corresponding to tetragonal perovskite crystal structure for both BCT, BZT solid solutions. Slim ferroelectric hysteresis (P-E) loops were observed for BCT, BZT solid solutions. Temperature dependent dielectric property measurements of BCT, BZT solid solutions have shown a high dielectric constant and low dielectric loss. Room temperature (300K) breakdown field strength and energy densities were obtained from the integral area of P-E loops. For the BCT ceramics, the largest recoverable energy (unreleased energy) density is 1.41 J/cm³ with dielectric breakdown strength as high as 150 kV/cm. For the BZT ceramics, the largest recoverable energy (unreleased energy) density is 0.71 J/cm³ with dielectric breakdown strength as high as 150 kV/cm. Bulk BCT, BZT ceramics have shown interesting energy densities; these might be the strong candidate materials for capacitor applications.     

Interface potential barrier height and leakage current behavior of Pt/(Ba,Sr)TiO3/Pt capacitors fabricated by sputtering process

Abstract

Variations of the leakage current behaviors and interface potential barrier height (φ _B ) of rf-sputter deposited (Ba, Sr)TiO₃ (BST) thin films, with thickness ranging from 20nm to 150 nm are investigated as a function of the thickness and bias voltages. The top and bottom electrodes are dc-sputter-deposited Pt films. φ _B critically depends on the BST film deposition temperature, postannealing atmosphere and time after the annealing. The postannealing under N₂ atmosphere results in a high interface potential barrier height and low leakage current. Maintaining the BST capacitor in air for a long time reduces the φ _B from about 2.4 eV to 1.6eV due to the oxidation. φ _B is not so dependent on the film thickness in this experimental range. The leakage conduction mechanism is very dependent on the BST film thickness; the 20nm thick film shows tunneling current, 30 and 40 nm thick films show Schottky emission current and the thicker films show a mixed characteristics and bulk and interface limited currents although the mechanism is not clearly understood at this moment.     

Preparation and electrical properties of Ni/(Ba,Sr)TiO3 PTC composite with low resistivity

Ni/ (Ba,Sr)TiO₃ PTC composite of low resistivity was fabricated by a solid state route. A mildly reducing sintering atmosphere was employed to avoid the oxidation of nickel. Metallic nickel is the main chemical state after sintering in the mildly reducing sintering atmosphere. With the increase in nickel amount, the room-temperature resistivity declines and the PTC effect worsens. The quantum mechanical tunneling effect at the Ni–(Ba_,Sr)TiO₃ interface is presumably the prime factor in the deterioration of the PTC effect. PbO–B₂O₃–ZnO–SiO₂ glass was added to modify the interface between nickel and (Ba,Sr)TiO₃ ceramics. The intergranular phase introduced by the glass has an amorphous structure and exists at the interfaces and triple junctions of (Ba,Sr)TiO₃ grains and nickel grains. No obvious diffusion occurs at the interface between crystalline (Ba,Sr)TiO₃ grain and the intergranular phase. Also the added-glass improves the distribution of metal phase. The proper glass addition screens interfacial electron tunneling effect and improves the composite electrical properties. An abundance of the intergranular phase due to excess glass will, however, result in high room-temperature resistivity. The influences of nickel amount and glass amount on the microstructure evolution and electrical properties were analyzed.     

High piezoelectric properties of (Ba,Ca)TiO3-0.04LiF ceramics sintered at a low temperature

Lead-free piezoelectric ceramics are strongly needed to replace the lead-based piezoelectric ceramics with increasing environmental concerns. Barium titanate (BaTiO₃) systems are one of the most promising candidates due to excellent electrical properties. However, the sintering temperature for traditionally sintered BaTiO₃ ceramics are about 1300°C, which restricts the applications of BaTiO₃ ceramics. It is necessary to develop high piezoelectric properties of BaTiO₃ based ceramics which are able to sinter at low temperature. The (Ba_0.94Ca_x)Ti_0.94O_δ-0.04LiF (x?=?0.00?~?0.05 mol) ceramics were synthesized by a conventional sintering method at 1050°C. All the samples show high relative densities over 90%. X-Ray Diffraction pattern indicated that the crystallographic structure of the samples (x?=?0.00 and 0.01 mol) are orthorhombic phase and changes to pseudocubic one with increasing Ca content to x?=?0.03 mol. Two-phases with orthorhombic and pseudocubic symmetries coexisted at x?=?0.02 mol, which contributes the excellent properties, in which the piezoelectric constant d ₃₃?=?361 pC/N, the planar electromechanical coupling coefficient k_p?=?41.2%, the Curie temperature Tc?=?70°C, the temperature of phase transition T _O-PC?=?34°C near the room temperature, the relative permittivity ε _r?=?4028 and the remanent polarization P _r?=?9.39 μC/cm².     

Multilayer Ceramic Capacitors with Thin (Ba,Sr)TiO3 Layers by MOCVD

Barium strontium titanate ((Ba,Sr)TiO₃; BST) thin films were prepared on platinum-coated MgO substrates at 650°C by metalorganic chemical vapor deposition (MOCVD). Perovskite single phased BST thin films were obtained. Dielectric constant at 1 kHz–100 mV was 1000. Multilayer ceramic capacitor with twelve BST dielectric layers of 0.26 m thick was formed on the MgO substrate. Capacitance and dissipation factor (tan) at 1 kHz–100 mV were 32 nF and 1.5% respectively. Capacitance per unit volume of 33 F/mm³ provided 10 to 20 times larger volumetric efficiency than the conventional multilayer ceramic capacitors. Temperature coefficient of capacitance was –4000 ppm/°C. The leakage current at 1 V was 2.3×10^-9 A that yielded an acceptable CR product of 12.8 M-F. MOCVD was proposed as one of the promising manufacturing technologies for multilayer ceramic capacitors of high performance with sub-micron thick dielectric layers.     

Hysteresis analysis in capacitance-voltage characteristics of Pt/(Ba,Sr)TiO3/Pt structures

Abstract

Capacitance-Voltage (C-V) characteristics of Pt/(Ba, Sr)TiO₃/Pt MIM capacitor were investigated. Hysteresis observed in the C-V characteristics of BST films was analyzed. The dependence of the C-V characteristics on the sweeping direction of applied voltage indicates that the hysteresis is caused by the interface trap charge between the BST film and the Pt electrode. A new method was proposed to characterize the interface traps from the hysteresis of C-V characteristics of MIM capacitor. The trapped electron density near the lower interface of the BST thin films was constant (～ 3 × 10¹²cm^?2) for all the film thickness ranging from 500 Å to 2000 Å, which suggests that the hysteresis is not caused by the bulk property of the BST film but caused by the interfacial property.     

Tunability and leakage currents of (Ba,Sr)TiO3 ferroelectric ceramics with various additives

The Ba_xSr_{1 − x} TiO₃ ferroelectric ceramics with magnesium (BSM) and neodymium (BSN) additives were studied. Measurements were made of tunability, dielectric losses (tan δ), leakage currents, the correlations between current-voltage I(U) and capacitance-voltage C(U) characteristics. I(U) characteristics of high quality BSM ceramics have four regions: Ohmic, where the conductivity is linear; the horizontal region (or negative differential resistivity); the exponential dependence; and the vertical current enhancement. These BSM samples (∼20% Mg additives) were distinguished by highest breakdown strength (more than 1000 V), low tan δ (less than 10^{− 3} at 1 MHz) and high tunability (up to 10% at E _max∼2 V/μm).     

Growth of (Ba,Sr)TiO 3 Thin Films by MOCVD: Stoichiometry Effects

(Ba x Sr 1 m x )TiO 3 thin films were deposited in a planetary multi-wafer MOCVD reactor combined with a liquid delivery system using 0.35 molar solutions of Ba(thd) 2 and Sr(thd) 2 and a 0.4 molar solution of Ti(O-i-Pr) 2 (thd) 2 . The film growth on Pt-(111) was investigated within a wide parameter field, e.g., the deposition temperature was varied between 560C and 650C, which yields films with microstructures ranging from randomly oriented polycrystalline to perfectly (100)-textured columnar structures. Special emphasis is given to film stoichiometry: starting with (Ba 0.7 Sr 0.3 )TiO 3 the Group-II/Ti content was varied from 0.9 to 1.1 and the Ba content was reduced to the limit of pure SrTiO 3 films. The electrical properties of MIM structures were investigated after deposition of Pt top electrodes. The nominal thickness of the films was varied between 5 and 100 nm and permittivity and leakage current both are shown to depend strongly on the film thickness. These dependencies on the film thickness are analyzed within the phenomenological dead layer model. The dependence of the electrical properties on stoichiometry are discussed in detail.     

Effect of Nb Doping on (Sr,Ba)TiO3 (BST) Ceramic Samples

The effect of doping the Sr_0.3Ba_0.7Ti_(1–5y/4)Nb _y O₃ ceramic with different concentration of Nb is studied by scanning electron microscopy (SEM), X-ray diffraction and thermoelectric analysis. It is observed that the grain size decreases as the Nb concentration increases. The critical temperature T _c has a linear decrease at a rate of 19°C/mol% of Nb. The temperature dependence of the dielectric permittivity presents strongly broadened curves, which suggest a non Curie-Weiss behavior near the transition temperature. The diffuse phase transition coefficient () was also determined and its value leads to the conclusion that the degree of disorder in the system increases with the presence of the Nb cation.     

Phase Formation,Dielectric, Ferroelectric and Magnetic Properties of Cr2O3 Doped (Ba0.85Ca0.15)(Ti0.90Zr0.10)O3 Ceramics

Abstract

In this paper, we studied the effect of Cr₂O₃ doping (0–0.8?wt%) on the phase formation, dielectric, ferroelectric and magnetic properties of (Ba_0.85Ca_0.15)(Ti_0.90Zr_0.10)O₃ (BCTZ) ceramics prepared by the solid state combustion method. All samples were prepared with a calcined and sintered temperature of 1050?°C and 1450?°C, respectively, for 2?h. The results of the XRD patterns showed the coexistence between the tetragonal (T) and orthorhombic (O) phases for all samples, and the tetragonal phase increased with increasing amounts of Cr₂O₃. The dielectric constant decreased when Cr₂O₃ increased. The P-E hysteresis loops of the BCTZ ceramics doped with Cr₂O₃ between 0 and 0.2?wt% showed slim and saturated loops. By increasing Cr₂O₃ doping from 0.4 to 0.8?wt%, the P-E loops were unsaturated and a leakage current was produced. The undoped BCTZ ceramics exhibited diamagnetic behavior. The sample with 0.2?wt% Cr₂O₃ showed ferromagnetic behavior. Increasing Cr₂O₃ doping from 0.4 to 0.8?wt%, caused the ceramics to exhibit paramagnetic behavior. Doping with Cr₂O₃ led to improper ratios between the tetragonal and orthorhombic phases, decreased density and increased porosity which caused a decrease in the electric properties.     

Liquid source chemical vapor deposition of high-dielectric-constant (Ba,Sr)TiO3 films

High-dielectric-constant (Ba, Sr)TiO₃ [BST] films were deposited by the liquid source chemical vapor deposition (CVD) method. The system consisted of a single-wafer, low-pressure thermal CVD reactor, a vaporizer for liquid source materials, and a shower-type gas nozzle head, giving stable BST film deposition on a 6-in. diam. substrate with uniform thickness and uniform chemical composition ratio. The source materials employed were Ba(DPM)₂, Sr(DPM)₂, and TiO(DPM)₂ dissolved in tetrahydrofuran (THF), resulting in conformal step coverage of BST films at lowered substrate temperatures, where DPM denotes dipivaloylmethanate. Moreover, the two-step deposition technique was developed to restart protrusions formed on BST film surfaces at low temperatures, where the BST films consisted of a buffer layer and a main layer; the buffer layer was a layer about 60 Å thick of CVD-BST film annealed in N₂. Thus, the two-step CVD deposition of BST films on Pt and Ru electrodes achieved an equivalent SiO₂ thickness of t_eq ∼ 0.5 nm, a leakage current of J_L ∼ 1.0 × 10⁻⁸ A/cm² (at +1.1 V), and a dielectric loss of tan δ ∼ 0.01 at a total film thickness of 250 Å, along with conformal coverage of 80% for a trench with an aspect ratio of 0.65. Then, for BST films deposited on patterned electrodes 0.24 μm wide, 0.60 μm deep, and 0.15 μm high (each spaced by 0.14 μm), the capacitance was demonstrated to be increased without significant deterioration of the leakage current: the capacitance was increased in comparison with that for films on flat electrodes, by a factor corresponding to the increase in surface area due to sidewalls of storage-node-like pattern features. This capacitance increase reflects the most characteristic advantage of CVD, an excellent step coverage on microscopic pattern features. These electrical properties satisfy the specifications for capacitors for Gb-scale dynamic random access memories (DRAMs), giving a storage capacitance of more than 25 fF/cell for a stacked capacitor having a storage node 0.2 to 0.3 μm high. © 1998 Scripta Technica, Electr Eng Jpn, 125(1): 47–54, 1998     

Effect of excess TiO2 on the phase evolution and densification of sol-gel derived (Ba,Sr)TiO3 powders

Nanoscaled (Ba_2/3Sr_1/3)Ti_{1 + x} O₃ powders have been prepared by sol-gel technique. Their phase evolution and densification behaviors have been studied by differential thermal analysis (DTA) and high temperature dilatometer, respectively. It is found the addition of 2 mol%-excess amount of TiO₂ lowers the activation energy required for the formation of the perovskite phase by about 130 kJ/mol and thus lowers the crystallization temperature of BST powders. However, the excess amount of TiO₂ makes the nano-powder difficult to sinter. Transmission electron microscopy reveals that a metastable nano-porous layer has formed on the surface of TiO₂-excess nanopowder and this may account for the slow densification rate.     

MSnO3(M=Ca,Sr,Ba)的溶胶-凝胶法合成及性能

采用溶胶-凝胶法制备了钙钛矿结构的MSnO3(M=Ca,Sr,Ba)晶体.将其作为锂离子电池负极材料的活性物质,利用恒电流电池测试仪研究其电化学性能.结果表明:CaSnO3、SrSnO3和BaSnO3的首次放电容量分别为894.3 mAh/g、703.8 mAh/g和673.6 mAh/g,首次充电容量分别为418.3 mAh/g、257.8 mAh/g和224.8 mAh/g,不可逆容量损失分别达53.2%、63.3%和66.6%.     

Slow Dielectric Relaxation in Pb(Zr,Ti)O3 and (Ba,Sr)TiO3 Ferroelectric Thin Films

An impedance analysis of Ni/Pb(Zr,Ti)O₃/Pt thin-film structures based on measurements at the frequencies from 100 Hz to 100 MHz, along with the data of Grazing Angles XRD, TEM and photo-electric study, is used to obtain electronic structure of the PZT thin films deposited by sol-gel method on silicone substrates. Both slow capacitance relaxation and charging/discharging currents versus time under step-voltage excitation have been studied in (Ba,Sr)TiO₃ thin films between SrRuO₃ electrodes.