W. T. Chang

Hybrid pulse anodization for the fabrication of porous anodic alumina films from commercial purity (99%) aluminum at room temperature.

Most porous anodic alumina (PAA) or anodic aluminum oxide (AAO) films are fabricated using the potentiostatic method from high-purity (99.999%) aluminum films at a low temperature of approximately 0-10 degrees C to avoid dissolution effects at room temperature (RT). In this study, we have demonstrated the fabrication of PAA film from commercial purity (99%) aluminum at RT using a hybrid pulse technique which combines pulse reverse and pulse voltages for the two-step anodization. The reaction mechanism is investigated by the real-time monitoring of current. A possible mechanism of hybrid pulse anodization is proposed for the formation of pronounced nanoporous film at RT. The structure and morphology of the anodic films were greatly influenced by the duration of anodization and the type of voltage. The best result was obtained by first applying pulse reverse voltage and then pulse voltage. The first pulse reverse anodization step was used to form new small cells and pre-texture concave aluminum as a self-assembled mask while the second pulse anodization step was for the resulting PAA film. The diameter of the nanopores in the arrays could reach 30-60 nm.

Fabrication of a novel micro liquid flow sensor using a TaN thin film

A micro flow sensor with high temperature coefficient of resistance (TCR) is very important because of its high sensitivity of flow sensors. Different from the conventional positive-TCR Pt film used in micro flow sensor with self-heating, higher negative-TCR micro flow sensor without self-heating is developed and fabricated in this research. The novel TaN-material micro liquid flow sensors are fabricated by reactive magnetron co-sputtering system at different nitrogen flow ratios. The magnitude of negative-TCR TaN films increases with increasing nitrogen flow ratios, so the high-TCR micro flow sensor can be prepared successfully at higher N2 flow ratio. In order to understand the sensing ability, micro flow sensor is fabricated by MEMS technology. Measurement results show that this TaN micro flow sensor has high sensitivity of 407 ohm/(ml/min) at Re ≪ 5, corresponding to volume flow rate of 0.167 ml/min. This device with high-negative-TCR materials can be beneficial for the measurement of low-velocity flow of liquid because of its high sensitivity at low Reynolds number.

Nano-mechanical behavior of low temperature electroplated nano-crystalline Ni films

Conventional electroplated Ni films were performed at high temperatures of 45–50 °C. In this paper, we have studied the low temperature electroplating i.e. low temperature electroplating of nano-crystalline Ni films and their nano-mechanical property. The deposits were electroplated in potentiostatic mode and the low electrolytic temperature varied from 0 to 20 °C. Grazing incidence X-ray diffractometer was used to examine the polycrystalline phase characteristics and grain size. MTS Nano Indenter with continuous stiffness measurement (CSM) module and Berkovich indenter were employed to characterize the nano-mechanical property of deposits. The experimental results showed that hardness increased with decreasing electrolytic temperature. It might be attributed to the residual compressive stress. Therefore, low temperature electroplating is favorable for the enhancement of Ni deposits hardness.

Application of pulse voltage for enhancing uniform nano-structured anodic aluminum oxide

Conventional anodic aluminum oxide (AAO) process was performed in low electrolytic temperature and high purity aluminum (Al) foil. In this article, pulse reverse voltage was applied to commercial purity (99 %) and high purity (99.997%) Al foils at room temperature to form AAO. DC method was also used as a reference. The pulse AAO method provides advantages not only the reduced process complexity and at low cost but also better uniformity than that performed by DC one. The results show that pore uniformity can be improved by pulse reverse voltage in the 3h duration anodization compared to DC method. The pulse reverse voltage presents the high uniformity (77 ~ 78 %) in both low and high purity Al. The effects on different Al purity and process durations will further discussed in this study.

Comparison of methods for sensing contact potential difference of nanocrystalline alloy films using a kelvin actuating capacitor

In this study, contact potential difference (CPD) of the electrochemical co-deposited nanocrystalline nickel-cobalt (Ni-Co) alloy films were measured by a home-made Kelvin probe. We present not only the detailed sensing mechanism but also discuss two different methods of CPD determinations namely null and off null method. It is also compared with the work function (WF) measured by ultraviolet photoemission spectroscopy (UPS). In electrochemical co-deposited nanocrystalline Ni-Co films, the Co content decreases with increasing current density from 22.53% at 1 ampere per square decimeter (ASD), 20.8% at 5ASD to 15.26% at 10 ASD. It is particularly suitable for Kelvin probe calibration due to the composition difference. The diameter of Kelvin probe is about 4 mm, and electrodeposited by nickel. The WF of the probe head was evaluated via UPS, and the value is 4.1 eV. The traditional determination of CPDs is null method at low scan rate and noise sensitive while the off null method provides higher signal-to-noise ratio, effective measurement and also shorter process time. Both UPS and Kelvin probe method show the identical results that the WF increases with increasing current density because of low Co content with low WF. The results of null and off null measurements for CPDs are in good agreement with that by UPS.

Sensing and determination of contact potential difference between two metals using an actuating capacitor

In this article, measurement of contact potential difference (CPD) between two dissimilar metals or alloys has been investigated by an actuating capacitor attached to a low cost home-made Kelvin probe system. The detailed sensing mechanism and analysis of the output signals for the CPD determination have been proposed. The resolution can reach 0.01 eV with three scientific digitals which is potentially used for the high-resolution measurement of non-contact mechanical, electrical properties and material science in a micro-domain phenomenon at low cost. A pure Ni probe was fabricated to measure the CPD of the electrodeposited Ni film and used as calibration data. The effect of Co atomic concentration on CPD of the electrodeposited Ni-Co alloys was studied for extending future application.