Anodic oxidation of tantalum in water and biological solutions using current limiting constant voltage method

A reliable method is developed for preparing tantalum pentoxide film targets in natural water and biological fluids (urine, blood plasma and serum) by the anodization of tantalum metal using a current limiting constant voltage method. Tantalum pentoxide film targets are successfully prepared at a current density of 10 mA cm⁻² at an anodic voltage ranging from 20 V to 100 V without any oxide breakdown. The results show that for the same applied voltage, more ionic concentration in biological solutions leads to a higher rate of oxide growth than in water and a darker interference color. The analysis shows that anodic oxidation is more likely to breakdown in a biological environment than in pure water for the same oxidation time and applied voltage. The oxide film capacitance is found to be only slightly dependent on pH and anodic voltage with higher capacitive films in biological solutions than for water.     

Voltage-dependent fabrication of nanoporous alumina and the applications to nanocapacitors

We investigated the fabrication of anodized aluminum oxide by anodization processes under DC and AC voltage biases. A two-step anodization process was used to fabricate the anodized aluminum oxide dielectrics in order to regulate the ordered nanopore array at the surface. AC samples showed the distorted nanoporous structure instead of a straight nanopore array in DC samples. As increasing the frequency of AC bias the nanovoid or nanocavity structure was formulated with the increased density of nanovoids. Nanoporous alumina was used for the fabrication of Ni-insulator-Al capacitors. The DC sample shows the tunneling process of an increase in leakage current and breakdown. When a negative voltage is applied to the capacitor device, the small current of 2 nA flows at a voltage of 0 V, indicating the existance of residual leakage current. The AC sample had very low leakage currents of the AAO dielectrics and the AAO hexagonal unit cell formed the nanocapacitor with a capacitance of 1-2 aF.     

Fabrication of novel porous anodic alumina membranes by two-step hard anodization

Porous anodic alumina (PAA) membranes with highly ordered hexagonal cells and a novel pore structure have been fabricated by two-step hard anodization in a H(2)SO(4)-Al(2)(SO(4))(3)-H(2)O system at 40 and 50?V, giving average cell diameters of 77 and 96?nm, respectively. There are several tiny pores embedded in each big shallow pore on the top of the membranes, and there is only one pore in one cell at their bottom. The cells on both sides of the membranes present almost the same periodic arrangement. In order to explore the formation of the novel pore structure, PAA membranes fabricated at different current densities (30-200?mA?cm(-2)) are obtained by maintaining a constant voltage at 40?V. The experimental results show that the interpore distance is not only dependent on the anodization voltage, but is also influenced by the current density, which means that the pore structure of PAA membranes fabricated by hard anodization can be accurately designed and controlled by adjusting the anodization voltage and current density simultaneously.     

Self-ordering of anodic nanoporous alumina fabricated by accelerated mild anodization method

A processing method called “accelerated mild anodization” is developed for preparation of high density and uniform nanoporous by anodization of aluminum. The idea is to use two different temperatures for both sides of sample in order to maintain mid level of current density during the anodization process. Here we have used high temperature for the back side of the sample in order to increase the current density while the electrolyte is kept at low temperature in the level of mild anodization. It is shown that not only the film growth is considerably fast, almost ten times faster than mild anodization, but also the anodization voltage is constant and anodization current variation is much less compared to hard anodization technique. Using oxalic acid, interpore distances of 89, 104, 117 and 130 nm were obtained for 35, 40, 45 and 50 V anodization voltages, respectively. It is found that the interpore distances are proportional to the anodization potential, almost same as that for the mild anodization. The porosity obtained tended to obey the same rule as that in mild anodization. This method is promising for industrial application due to short fabrication time as well as high-speed pore ordering.     

阳极氧化电流密度和时间对多孔硅形貌和电子发射特性的影响

为了应用于场发射显示器,采用电化学阳极氧化,快速热氧化和磁控溅射等方法制备出了金属/多孔硅/硅基底/金属结构的多孔硅电子发射体,并运用扫描电子显微镜观察了多孔硅的微观形貌,结果发现多孔硅的孔径随着电化学阳极氧化电流密度的增加而增加,多孔硅层的厚度随着阳极氧化电流密度和时间乘积的增加而增加。在真空系统中测量了多孔硅的电子发射特性,电子发射的阈值电压Vth随着多孔硅层厚度的增加而增加;最大的发射效率η为7.5‰,此效率出现在孔径6~16 nm,多孔硅层厚度为11.06μm的样品中,对应的器件电压Vps为30V。     

Controlled fabrication of patterned lateral porous alumina membranes

Confined lateral alumina templates are fabricated with different pore sizes by changing the acid electrolyte and the anodization voltage. The control of the number of pore rows down to one dimension is also achieved, by controlling the thickness of the starting aluminum film as well as the anodization voltage. We observe that the mechanism of pore formation in the lateral regime is very similar to that in the classical vertical situation.     

纳米多孔阳极氧化铝膜形貌的控制研究

采用38g／L的草酸溶液为电解液,制备了孔洞规则有序,孔径和厚度均一可控的氧化铝模板,并研究了阳极氧化工艺对阳极氧化铝膜形貌的影响。实验结果表明,采用38g／L的草酸溶液作为电解液,经两步法阳极氧化可制得孔径均一,排列规则的多孔阳极氧化铝膜。随着氧化电流密度的升高,氧化膜孔径逐渐增大。电流密度过大,反应放热严重,氧化铝膜孔径均匀性,孔洞形状规则性和有序性都下降。在恒电流密度条件下,氧化膜厚度随着氧化时问的延长呈线性增长,但对氧化铝膜孔径影响较小。     

Vertically Oriented Titania Nanotubes Prepared by Anodic Oxidation on Si Substrates

Vertically oriented titania nanotube arrays were fabricated by anodization of titanium film deposited on silicon substrates under different processing conditions. The anodic formation of nanoporous titania on silicon substrate was investigated in aqueous solutions mixed with highly corrosive Na₂SO₄/NaF/citric acid. In the result of the anodization of titanium film deposited at room temperature, a very thin layer of ~70 nm having a worm-like structure was grown on the top of the porous layer. But, in the case of titanium film deposited at 500deg, vertically oriented TiO₂ nanotube arrays were formed. The average tube outer diameter of the nanotube was 74 nm to 100 nm. The longest nanotube of 681 mum was obtained at 15 V and 30 min. The current density transient curve recorded during anodization under a constant voltage showed a typical behavior for self-organized pore formation.     

电流密度对AZ91D镁合金阳极氧化膜表面形貌及粘接性能的影响

采用恒电流方式在AZ91D镁合金表面制备多孔阳极氧化膜。通过电压-时间曲线研究了电流密度对氧化行为的影响。采用SEM、单板拉剪试验研究了电流密度对氧化膜表面形貌及拉伸强度的影响。结果表明,电流密度不影响击穿电压及临界电压的大小。随着电流密度增加,电压达到击穿电压及临界电压的时间缩短,氧化膜孔隙率及拉伸强度均先增加后减小,当电流密度为10 mA/cm~2时拉伸强度最大,可达到22.40 MPa。     

Fabrication of TiO2 Nanotube Arrays by Rectified Alternating Current Anodization

Anodization is a popular method of preparing TiO₂ nanotube array films（TiNTs） by using direct current（DC）power as the driving voltage.In this study,three driving voltage modes,namely,the sine alternating current（sine） mode,the full-wave rectification of sine waves via four diodes（sine-4D,where D means diode） mode,and the DC mode,were used to prepare TiNTs by anodization.At 20 V,TiNTs were formed under sine-4D mode but only irregular porous TiO₂ films were formed under DC mode.At 50 V,TiNTs formed under both the sine-4D and DC modes.No TiNTs formed in the sine mode anodization at either 20 or 50 V.Compared with the DC mode,the sine-4D mode required a lower oxidation voltage for TiNT formation,which suggests that sine-4D is an economical,convenient,and efficient driving voltage for TiNT preparation by anodization.The morphologies and structures of TiNT samples anodized at 50 V in the sine-4D and DC modes at different oxidation time（1,5,10,30,60,and 120 min） were analyzed.TiNT growth processes were similar between the studied modes.However,the growth rate of the films was faster under the sine-4D mode than the DC mode during the first 30 min of anodization.     

Effects of anodizing conditions on anodic alumina structure

In this paper, two-step anodization was used to obtain porous anodic alumina (PAA) films, which are widely used as the temples to fabricate nanomaterials. Effects of anodizing conditions such as anodizing voltage and the concentration of electrolyte on steady current density (I _s) and anodic alumina structure were investigated for oxalic acid as an electrolyte. The result shows that I _s is dependent on anodizing voltage exponentially. The relationship between the concentration of electrolyte and the pore diameter is almost linear, while there is no effect on inner-pore distance. At different anodizing voltage, the effect degree of the concentration of oxalic acid on the pore diameter is various. In oxalic acid electrolyte with given concentration matching with a specifically anodizing voltage, optimal nano-pores arrangements can be obtained. The higher voltage induces the collapse of thin inner wall and disordered alumina nanowires (ANWs) were formed.     

Effects of temperature and voltage mode on nanoporous anodic aluminum oxide films by one-step anodization

Many conventional anodic aluminum oxide (AAO) templates were performed using two-step direct current anodization (DCA) at low temperature (0–5 °C) to avoid dissolution effects. This process is relatively complex. Pulse anodization (PA) by switching between high and low voltages has been used to improve wear resistance and corrosion resistance in barrier type anodic oxidation of aluminum or hard anodization for current nanotechnology. However, there are only few investigations of AAO by hybrid pulse anodization (HPA) with normal-positive and small-negative voltages, especially for the one-step anodization, to shorten the running time. In this article, the effects of temperature and voltage modes (DCA vs. HPA) on one-step anodization have been investigated. The porous AAO films were fabricated using one-step anodization in 0.5 M oxalic acid in different voltage modes including the HPA and DCA and the environment temperature were varied at 5–15 °C. The morphology, pore size and oxide thickness of AAO films were characterized by high resolution field emission scanning electron microscope. The pore size distribution and circularity of AAO films can be quantitatively analyzed by image processing of SEM. The pore distribution uniformity and circularity of AAO by HPA is much better than DCA due to its effective cooling at relatively high temperatures. On the other hand, increasing environment temperature can increase the growth rate and enlarge the pore size of AAO films. The results of one-step anodization by hybrid pulse could promote the AAO quality and provide a simple and convenient fabrication compared to DCA.     

An investigation into the parameters affecting the breakdown voltage and inter-particle bonding in the electrical discharge compaction of metal powders

The aim of present investigation is to gain deeper understanding of breakdown behavior and inter-particle bonding by conducting experimental tests. This may lead to improve the state of compaction by relative arrangement of initial parameters to maintain uniform distribution of current density and producing compacts with sufficient mechanical strength. Experimental work was carried out using two different set-ups. The first arrangement was employed to provide steady-state alternating voltage. The effect of column geometry and particle size on breakdown voltage was investigated under this condition. The second set-up, capacitor discharge circuit, was used to provide impulse voltage. Under this condition, the influence of column geometry, particle size, application of axial pressure, evacuation of air, energy input, electrode material and configuration on breakdown voltage was studied. Also, scanning electron microscopy was employed to study the effect of different parameters on inter-particle bonding. The results of experiments conducted on the influence of each of the voltage and capacitance on the compaction properties are also discussed.     

工艺条件对镁合金微弧氧化的影响

在含有Na2SiO3、NaF、甘油及KOH的电解液中以恒电流方式对AZ31B镁合金进行微弧氧化处理，研究了电解液组分、浓度、电流密度及氧化处理时间等对微弧氧化过程及膜层性能的影响．研究表明：随着电解质浓度的增加，起火时间、起火电压基本呈下降趋势，氧化膜厚度呈增长趋势；过量的NaF会抑制放电；甘油的存在可稳定电解液，抑制尖端放电，使膜层的厚度降低；电流密度的增加可以降低起火时间，增加氧化膜的厚度，对放电电压没有明显影响；随着氧化处理时间的延长，氧化膜的厚度不断增长．     

Effect of anodization voltage on electron field emission from carbon nanotubes in anodized alumina template

In this work, electron field emission from AAO-CNT structure is studied as a function of anodizing voltage. It is found that the turn-on electric field of AAO-CNTs reduces from 5 V/microm to 4 V/microm as anodization voltage increase from 20 to 30 V. On the other hand, CNTs the turn-on electric field of AAO-CNTs increases from 4 V/microm to 6 V/microm as anodization voltage increase from 30 to 40 V. Thus, anodization voltage of 30 V provides an optimal AAO-CNTs structure for electron field emission. The emission data have been analyzed based on the Fowler Nordhiem (F-N) model. AAO template prepared with 30 V anodization voltage is found to yield CNT nanoarray with optimum alignment and spacing that increase field enhancement factor by the lowering of field screening effect without significant lowering of CNTs density.     

两步阳极氧化法制备多孔阳极氧化铝膜

直流恒压下,在酸性溶液中对铝实施两步阳极氧化制备了多孔氧化铝膜。采用扫描电镜(SEM)、原子力显微镜(AFM)对制备的多孔氧化铝膜进行形貌分析,孔径在纳米级且孔分布具有高度均匀性。采用SEM对试样进行观察,分析了工艺对多孔氧化铝膜形貌的影响。利用阳极氧化初期电流密度的变化并结合阳极氧化过程中的试样的SEM照片,分析了多孔氧化铝膜的形成机理。     

High voltage characterization of tin oxide varistors

High current characterization of SnO₂-based varistors have been carried out and the results obtained have been compared with those ZnO-based commercial surge arresters. It is shown that for a high temperature sintering (1350 °C), the leakage current, breakdown voltage and the saturation at high current density are similar between both types of surge arresters. The paper shows, for the first time, that the high breakdown voltage observed with tin dioxide cannot be easily maintained since the saturation of the ceramics is reached for current densities of several amperes per square centimetre.     

PVC绝缘试样漏电痕迹劣化的混沌方法分析

如何评估固体绝缘材料的耐电痕性,识别和选择合适的绝缘材料非常重要。文中按照IEC 60112国际标准试验法对PVC进行电痕劣化试验,运用混沌方法对其电痕劣化试验的电流波形进行分析,计算其最大Lyapunov指数,作出吸引子相图,以期描述绝缘劣化的变化规律。试验结果表明,同一电压级别具有相似的吸引子,不同的电压级别吸引子不同。随着电痕劣化的发展,吸引子相图随之演变。电流波形的吸引子相图是表征电痕劣化进程的一种有效方法。     

Electrical breakdown in amorphous As2S3 films

We investigated the electrical breakdown under d.c. electric fields of amorphous As₂S₃ films about 100–1000 Å thick sandwiched between two aluminium electrodes. It was found that the breakdown field decreased with increases in either the temperature or the thickness of the sample. Attempts were made to fit the observed electrical breakdown voltages with several previously proposed theories of thermal and electrical breakdown. It was found that a satisfactory fit could only be obtained by a modification of Klein's theory provided that the current was observed to depend on the voltage in an exponential manner in accordance with the Poole mechanism in which the carriers are released from high density localized states within the forbidden energy region. According to this modification, the breakdown field should depend inversely on the temperature and this agrees well with the experiment. The formation of localized conducting channels within the sample because of breakdown was verified by taking a scanning electron micrograph of the junction area of the broken film.     

A Method to Evaluate the Second Breakdown Resistance of Power Transistors

A new evaluation method of forward-biased second breakdown resistance in power transistors employing a characteristic variation of current gain is presented. Predictions for second breakdown resistance in power transistor are made by monitoring the changing point of the base current at the collector-base junction under the constant voltage and constant current conditions. This characteristic phenomenon which is related to the variation of the current gain is dependent on the temperature and current density at a localized spot of the transistor. It is concluded that the time delay required to reach the changing point of the base current can be considered as a measure of the second breakdown resistance. The forward-biased second breakdown resistance can be measured without damage or degradation of the sample transistors, if the supplied power is removed before the base current reaches the changing point.