Effect of Na-doping on thermoelectric and magnetic performances of textured Bi2Sr2Co2Oy ceramics

Bi₂Sr_2-xNa_xCo₂O_x (x = 0.0, 0.025, 0.050, 0.075, 0.100, and 0.125) samples were prepared through the solid-state route and textured using the laser floating zone technique. Microstructural analysis of as-grown samples showed well oriented grains and a relatively high amount of secondary phases due to their incongruent melting. Annealing procedure has drastically decreased the number and amount of secondary phases. Moreover, Na-doping has further decreased the secondary phases content and improved grain alignment. These modifications have been reflected in a large decrease of electrical resistivity with the annealing procedure. The maximum power factor values have been obtained in 0.075 Na-doped annealed samples, 0.20 mW/K²m, which are much higher than the best values obtained in textured materials through hot uniaxial pressing. Magnetic properties were very similar for all samples, with paramagnetic Curie temperature and effective magnetic moment values of ?48.6 K and ≈2 μ_B, respectively.     

Low thermal conductivity and thermoelectric properties of Si80Ge20 dispersed Bi2Sr2Co2Oy ceramics

Bi₂Sr₂Co₂O_y is a thermoelectric material with low thermal conductivity. The Bi₂Sr₂Co₂O_y/Si₈₀Ge₂₀ composite samples were prepared by solid phase sintering at high temperature to investigate the effects of Si₈₀Ge₂₀ alloys as the second phase on the microstructure and thermoelectric properties of the fabricated composites. An appropriate amount of the dispersed Si₈₀Ge₂₀ in the Bi₂Sr₂Co₂O_y matrix can reduce the resistivity of the composite successfully. In particular, the increase in phonon scattering caused by the second phase leads to a significant decrease in thermal conductivity, which improves the thermoelectric properties of the material significantly. At 923 K, the thermal conductivity of the Bi₂Sr₂Co₂O_y + 0.2 wt% Si₈₀Ge₂₀ sample achieves an ultralow value of 0.58 W/K·m. Its corresponding optimal dimensionless thermoelectric figure of merit value is 0.36, which is 56% higher than that of the pure Bi₂Sr₂Co₂O_y sample.     

Low temperature thermoelectric properties of K-substituted Bi2Sr2Co2Oy ceramics prepared via laser floating zone technique

In the present study, Bi₂Sr_2-xK_xCo₂O_y (x = 0.0, 0.050, 0.075, 0.100, and 0.15) ceramic precursors have been produced using the classical ceramic route, followed by texturing through the laser floating zone technique. XRD results show that the thermoelectric phase is the major one in all cases. Moreover, K-substitution decreases the secondary phases content, when compared to the undoped sample. SEM observations indicate that grain orientation is significantly enhanced when K-content is increased. K-doping decreases electrical resistivity from 32 10⁻⁵ Ω m (in undoped samples) to 20–22 10⁻⁵ Ω m at 300 K, while increasing Seebeck coefficient from 55 μV/K to 100–117 μV/K at 300 K. On the other hand, thermal conductivity is slightly lower in undoped samples (0.93 W/K m, compared to 1.10–1.28 W/K m for doped ones at 300 K), due to their lower electrical conductivity. Finally, ZT values are higher when the K-content increases up to x = 0.10, reaching 0.029 at around 400 K, and slightly decreasing for higher doping levels.     

Co2+掺杂对Bi2WO6结构与光催化活性的影响

采用一步水热合成法制备Co2+掺杂Bi2WO6纳米粉末,利用XRD、EDS、SEM、TEM、BET、UV-Vis等表征手段对Co2+/Bi2WO6形貌和结构进行表征,并以罗丹明B为目标污染物研究了Co2 +/Bi2WO6的催化活性.结果表明Co2+的添加有助于增加催化剂比表面积,抑制了晶粒生长,优化后催化剂的光谱吸收范围向可见光区拓展.Co2+/Bi2WO6催化活性明显高于未优化的Bi2WO6,可见光照射下,150 min内,Co2 +/Bi2WO6对罗丹明B的降解率可达98％.     

Enhanced thermoelectric properties of CNT dispersed and Na-doped Bi2Ba2Co2Oy composites

The thermoelectric properties of Bi₂Ba₂Co₂O_y and Bi_1.975Na_0.025Ba₂Co₂O_y+x wt% carbon nanotubes (CNT; x=0.00, 0.05, 0.10, 0.15, 0.5, and 1.0) ceramic samples synthesised by the solid-state reaction method were investigated from 300K to 950K. Na doping with a small amount played an important role in reducing resistivity and slightly reduced the Seebeck coefficients and the thermal conductivity. The CNT dispersant increased resistivity, but the thermal conductivity was reduced remarkably. In particular, the Bi_1.975Na_0.025Ba₂Co₂O_y+1.0wt% CNT sample exhibited an ultralow thermal conductivity of 0.39 W K⁻¹ m⁻¹ at 923K. This was attributed to the point defects caused by Na doping and the interface scattering caused by the CNT dispersant. The combination of Na doping and CNT dispersion had better effects on thermoelectric properties. The Bi_1.975Na_0.025Ba₂Co₂O_y+0.5wt% CNT sample exhibited a better dimensionless figure of merit (ZT) value of 0.2 at 923K, which was improved by 78.2%, compared with the undoped Bi₂Ba₂Co₂O_y sample.     

Preventing Sodium Poisoning of Photocatalytic TiO2 Films on Glass by Metal Doping

Titanium dioxide (TiO₂) has received much attention as a photocatalyst, specifically in applications that require a mechanically robust thin film. TiO₂ is particularly useful because it does not absorb visible light, making it well suited for coatings on glass. Photocatalytic activity of TiO₂ films is strongly dependent on the substrate and it has been well established that sodium diffusion from glass has a negative effect on this activity. While the prevention of sodium poisoning is possible through the use of a precoating, this requires an additional coating and/or calcination step. Other remedies, such as acid treatment of the glass surface, are also time consuming. Therefore, it is a more attractive option to negate the effects of sodium diffusion without a separate processing step. In this paper, we examined the effects of silver, cobalt, copper, gallium, molybdenum, and tantalum doping on the prevention of sodium poisoning of sol–gel TiO₂ films by comparing the photocatalytic activities on glass and SiO₂ precoated glass. While sodium poisoning degraded the photocatalytic activity of undoped TiO₂ films by 70%, it was only 10% for Mo- and Ta-doped TiO₂ films. Molybdenum was superior to other dopants in terms of photocatalytic activity, both in the presence of sodium and in a sodium-free environment.     

Improved thermoelectric performances in textured Bi1.6Pb0.4Ba2Co2Oy/Ag composites

Bi_1.6Pb_0.4Ba₂Co₂O_y thermoelectric ceramics with small Ag additions (0, 1, 3, and 5 wt%) have been textured using the laser floating zone method. Microstructure has shown a slight decrease on the secondary phases content and a better grain alignment in Ag added samples. These microstructural features are reflected in the thermoelectric properties, which have shown a significant decrease of electrical resistivity, when the Ag content is raised. In spite of a corresponding decrease of Seebeck coefficient, all the Ag-containing samples possess higher Power Factor values than the Bi_1.6Pb_0.4Ba₂Co₂O_y ones. Moreover, the maximum Power Factor values (about 0.36 mW/K².m at 650 °C) have been measured in Bi_1.6Pb_0.4Ba₂Co₂O_y+3 wt% Ag samples, which are the best results reported in this family of materials.     

Nd掺杂对ZnSe薄膜微结构的影响

利用真空蒸发的方法制备ZnSe多晶薄膜,并采用双源法对薄膜进行了稀土元素Nd的掺杂。对薄膜进行了XRD测试,并计算了薄膜的晶粒尺寸、晶格常数以及内应力。结果表明,当原子配比Zn∶Se=0.9∶1时可制备较理想的ZnSe多晶薄膜,稀土Nd掺杂并未改变样品的物相结构,掺杂使得薄膜的晶粒尺寸减小,晶胞体积增加,内应力和晶格常数改变。实验还发现,适度的轻掺杂Nd可增加ZnSe薄膜的光透射性。     

Bi掺杂Ba0.6Sr0.4TiO3陶瓷的研究

本文采用传统的陶瓷工艺合成了Ba0.6Sr0.4TiO3陶瓷,研究了Bi掺杂对材料参数ε、tanδ和E的影响,并探讨了相关的掺杂改性机理。结果表明：适量的Bi能够改善陶瓷的介电性能,同时细化陶瓷的晶粒尺寸。     

Effects of Ho and Ti Doping on Structural and Electrical Properties of BiFeO3 Thin Films

Effects of Ho and Ti ions individual doping and co‐doping on the structural, electrical, and ferroelectric properties of the BiFeO₃ thin films are reported. Pure BiFeO₃, (Bi_0.9Ho_0.1)FeO₃, Bi(Fe_0.98Ti_0.02)O_3+δ, and (Bi_0.9Ho_0.1)(Fe_0.98Ti_0.02)O_3+δ thin films were prepared on Pt(111)/Ti/SiO₂/Si(100) substrates by using a chemical solution deposition method. All thin films were crystallized in distorted rhombohedral structure containing no secondary or impurity phases confirmed by using an X‐ray diffraction study. Changes in microstructural features, such as grain morphology and grain size distribution, for the doped samples were analyzed by a scanning electron microscopy. From the experimental results, a low electrical leakage (1.2 × 10^?5 A/cm² at 100 kV) and improved ferroelectric properties, such as a large remnant polarization (2P_r) of 52 μC/cm² and a low coercive field (2E_c) of 886 kV/cm, were observed for the (Bi_0.9Ho_0.1)(Fe_0.98Ti_0.02)O_3+δ thin film. Fast current relaxation and stabilization observed in the (Bi_0.9Ho_0.1)(Fe_0.98Ti_0.02)O_3+δ imply effective reduction and neutralization of charged free carriers.     

A位和B位Mn掺杂对Sr2Bi4Ti5O18电磁性能的影响

采用固相烧结工艺制备MnxSr2-xBi4Ti5O18(A-SBTi-x,x=0～0.08)和Sr2Bi4MnyTi5-yO18(B-SBTi-y,y=0～0.08)陶瓷样品,对比研究它们的结构和电磁性能.X射线衍射谱显示A位Mn掺杂对材料晶格常数a影响较大:零掺杂时,a=0.3868nm;当Mn摩尔掺量为0.02时,...     

Al-Sc共掺杂ZnO透明导电薄膜的掺杂机制

采用溶胶-凝胶法制备Al-Sc共掺杂ZnO透明导电薄膜,考察了结晶性、晶界状态、紫外-可见光透射光谱及Hall效应,并讨论其掺杂机制,结果表明:Sc单独掺杂时,晶粒尺寸变小,结晶性变差,晶粒表面包裹一层非晶相,透射光谱存在较大的吸收谷,无导电性;Al-Sc共掺杂时,晶粒表面无非晶相,c轴择优取向性和晶界状态变优,透射光...     

Organic molecule intercalation sites in Sr2CaCu2Oy superconductor

The organic molecule adsorption sites in a Sr₂CaCu₂O_y (0^(Sr)212) half unit cell upon water molecule intercalation were estimated by simulation using Molecular Orbital PACkage (MOPAC) program. For water molecules, the unit cell was elongated with increasing the number of molecules. Upon four water molecules intercalated between the two SrO layers, the upper Sr atom moved along the c-axis by 1.49 Å. This value was smaller than but comparable to the experimentally obtained elongation of half of the c-parameter. The primary reason of the elongation is the repulsive force between Sr–H atoms in the two SrO layers. For ethanol and acetone molecules, stable intercalation sites were not found in the 0^(Sr)212 crystal. These results coincided to the experimental results, which did not show intercalations. It was concluded that MOPAC can reproduce the organic molecule intercalation phenomena in oxide crystals.     

铋及钴掺杂TiO2陶瓷负载膜的制备及光催化性能

以钛酸四丁酯为钛源,Bi(NO₃)₃·5H₂O为铋源,Co(NO₃)₂·6H₂O为钴源,采用溶胶-凝胶法在陶瓷基片上分别制备了应用于固定式光催化反应器的Bi-TiO₂复合膜及Co-TiO₂复合膜。研究了涂膜方式及煅烧升温速率对膜表面形态的影响,并以甲基橙溶液为目标降解物,对比了旋涂法及浸渍提拉法制备的薄膜在不同升温速率下的光催化活性。通过热分析仪(TG-DSC)、X射线衍射仪(XRD)和扫描电镜(SEM)等测试手段分析了掺杂薄膜的结构。结果表明,采用旋涂法制备的薄膜与陶瓷基体结合较为紧密,而浸渍提拉法制备的薄膜催化活性较高。当煅烧温度为500 ℃,升温速率为10 ℃/min时,其对甲基橙的降解率较高。当摩尔比为0.010时Bi-TiO₂复合膜的催化活性较佳,100 min光降解甲基橙比率达到8.10%。钴或铋掺杂TiO₂均可提高TiO₂催化剂的光催化活性,Bi-TiO₂膜的催化活性优于Co-TiO₂膜。     

Ferroelectric Thin Films of Bismuth-Containing Layered Perovskites: Part III, SrBi2Nb2O9 and c-Oriented Bi4Ti3O12 Template

Randomly and c-oriented SrBi₂Nb₂O₉ thin films have been obtained by a metalorganic decomposition (MOD) method using a highly c-oriented Bi₄Ti₃O₁₂ layer as a template. This templating procedure is generally applicable to other layered perovskites, and it has the advantage of lowering the temperature for crystallization and texture selection for these thin films. Moreover, the c-oriented Bi₄Ti₃O₁₂ template can serve as a bottom electrode at the same time. Ferroelectric and dielectric properties measured for SrBi₂Nb₂O₉ films of different orientations reveal strong anisotropy following the general trend known for other layered perovskites. Systematic differences in polarization and coercive field can be consistently explained in terms of the different m values and the lack of lone-pair electrons. The latter contribute to a higher polarization and a higher coercive field for SrBi₂Nb₂O₉ thin films.     

Preparation of high performance Bi2Sr2Co1.8Ox thermoelectric materials from nanosized precursors

Bi₂Sr₂Co_1.8O_x thermoelectric (TE) materials were prepared by three different synthesis methods producing nanosized precursors: coprecipitation (with ammonium carbonate or oxalic acid) and attrition milling, which were compared with those obtained by the classical solid state method. Microstructure has shown that precursors produced by coprecipitation and attrition milling methods produced nanometric precursors much smaller than the typical sizes produced by the solid state route. The TE properties are in agreement with the microstructural features, leading to lower resistivity in all the samples, compared with the solid state ones, while Seebeck coefficient is practically unchanged in all cases. As a consequence, maximum power factor values of around four times higher than those obtained in the classical solid state method have been determined. Moreover, the highest power factor value at 650°C is higher than the best results obtained in as-grown textured materials produced by the laser floating zone technique.     

Effect of annealing on the thermoelectric properties of directionally grown Bi2Sr2Co1.8Ox ceramics

The effect of annealing on directionally solidified Bi₂Sr₂Co_1.8O_x ceramic rods has been studied for different times up to 1008 h. Microstructure has shown five different phases in the as-grown materials which have been reduced to two major ones after 1008 h thermal treatment, accompanied by an important grain growth. These microstructural changes are reflected on the mechanical properties which are higher than those measured in the as-grown materials in all cases. Moreover, they also produce an important decrease on the resistivity and increase of thermopower, leading to a raise on the power factor on thermally treated samples, about two times, compared to the as-grown samples.     

Influences of Cr Doping on the Electrical Properties in BiFeO3 Thin Films

Cr-doped BiFeO₃ (BFO) thin films were deposited on Pt(200)/TiO₂/SiO₂/Si(100) substrates by a chemical solution deposition method. The dielectric constant and dissipation factor of the BFO thin films decrease from 165 and 0.054 (undoped) to 100 and 0.02 (3 mol% Cr-doped) at a frequency of 10 kHz as the Cr content increases. The leakage current and ferroelectric properties were improved significantly by means of Cr doping. The leakage current density of 4.1×10^-6 A/cm² for the 3 mol% Cr-doped BFO thin film is about four orders of magnitude lower than that of undoped BFO thin film at an external electric field of 100 kV/cm. The 3 mol% Cr-doped BFO thin film exhibited a well-saturated hysteresis loop with a large remanent polarization (P_r) of 61 μC/cm² at room temperature. The reason for the improved leakage current and ferroelectric properties in Cr-doped BFO thin films can be attributed to the reduced oxygen vacancies in the films by Cr doping.

Communicated by Dr. George W. Taylor