Ashish A. Shah

An electrolytically actuated micropump

An electrolytically actuated bubble micropump capable of precise dosing at high pumping speed is discussed. The fluid displacement is achieved by sequentially generating electrolytic gas bubbles directly inside the microfluidic channels. The operation of the pump thus requires no moving mechanical parts. A series of phase-shifted dc voltage pulses (ranging from 3.3 to 4.5 V) were applied to inflate the bubbles. Results show that the liquid displacement and pumping rate can be easily and accurately controlled by adjusting the applied voltage, pulse width, and pulse interval. An optimized pump rate of 24 nl/min, corresponding to a flow velocity of 640 μm/s was achieved for a channel size of 25×25 μm in cross section. The measured volume displacements and pump rates were found to be in close agreement with the Faraday bubble growth mechanism. Results also show that the pump can reliably operate at high backpressure (up to 110.1 kPa), and a decrease in pump rate due to elevated backpressure can be offset simp...

In situ study of temperature dependent magnetothermoelastic correlated behavior in ferromagnetic shape memory alloys

This study reports the first in situ observation of temperature-dependent micromagnetic and twin structure in oriented single crystals of Ni–Mn–Ga Heusler alloys. Micromagnetic measurements were made over a temperature interval of 50 to −35 °C covering both forward and reverse martensitic transformation. Magnetic domains in the martensite phase were found to be uniformly spaced (25–30 μm); the direction of the domain walls conforms to the changing direction of the magnetic easy axis as they traverse from one twin to another. The martensite twins could be reoriented in applied fields as low as 1300 Oe.

rf plasma‐generated superconducting Y1Ba2Cu3O7−x films

Using an inductively coupled rf plasma operated at atmospheric pressure, superconducting thin films of Y1Ba2Cu3O7−x have been grown in situ on single‐crystal yttria‐stabilized zirconia [100] from an aerosol precursor. X‐ray diffraction and rocking curve results indicate that the films are highly c‐axis oriented normal to the substrate with a lattice constant of 11.67–11.68 A. The full width at half maximum of these films ranges from 0.55° to 1.0°. The films have typical Tc’s of 86 K and Jc’s in excess of 105 A/cm2 at 77 K and zero field; magnetic measurements reveal an anisotropy of 3:1 at 69 K. The dependence of the processing parameters on the transport properties of the superconducting films is discussed.

In situ, rf plasma deposition of Bi2Sr2Ca2Cu3Ox thin films at atmospheric pressure

Superconducting Bi2Sr2Ca2Cu3Ox thin films have been prepared in situ at atmospheric pressure by an inductively coupled, argon‐oxygen rf plasma. This high Tc phase has been achieved by exposing the grown film, in situ, to the same argon‐oxygen plasma used for the deposition of the film. No post‐annealing steps are required. Bi2Sr2Ca2Cu3Ox films with critical temperatures of 100 K and critical current densities of 1×105 A/cm2 at 77 K and zero field have been obtained at a deposition rate of 100–200 A/min. The films are mirror smooth with very few particulates and pinholes. Control of the substrate heater temperature is critical as high‐quality films grow only in a narrow temperature range. The effect of plasma treatment and substrate heater temperature is discussed.

Bubble Based Microfluidic Sensors

A microfluidic sensor using an electrolytic bubble sensing element has been developed and characterized. Impedance measurements in a microfluidic channel are used to monitor the volume changes of an electrolytic bubble. Parameters affecting bubble volume are identified and provide the basis for detection of properties such as pressure, flow rate and chemical content in a microfluidic channel. A finite element simulation illustrates the sensitivity of impedance measurements to bubble volume in a channel. Furthermore, prototype chips of various configurations were fabricated and are presented to illustrate the design considerations for different sensor applications. The sensor was tested and experimental results are presented characterizing its use as a pressure sensor. Results on surface modification experiments give evidence to support the extension of the bubble based sensor to use in chemical detection in microfluidic channels.Copyright

Fundamental Investigation of Ferromagnetic Shape Memory Alloys: A New Perspective

In the present study, the evolution of micromagnetic structure and microstructure is studied in-situ both as a function of temperature and applied magnetic field, using single crystal Fe-Pd and Ni-Mn-Ga Heusler alloys. Through the development of a novel technique called ‘Magnetic Transition Spectrum’ to study temperature dependent domain dynamics, the relative sequence of micromagnetic reconfiguration with respect to the martensitic transformation has been determined for the first time. Results show that the FSMAs may be viewed as magnetic mosaics , a new perspective, which is also more amenable to modeling the physical properties of these alloys. Finally, the concept of magnetic mosaics has been used to synthesize a novel class of materials with engineered magnetic anisotropies , and is briefly discussed.

Sequential Electrolytic Bubble-Based Micro-Pump Dosing System

An electrolytic bubble actuated micropump has been fabricated and characterized for dosing application. The micropump consists of a series of bubbles formed directly inside a microfluidic channel, and the volume displacement characteristics of inflating/deflating electrolytic bubbles are utilized to move the liquid along the channel. Prototype chips with five sequential bubbles were built using standard photolithography techniques. The pump performance was characterized as a function of the voltage required to generate electrolytic bubbles of varying size (3.5–4.5 V), pulse-width (50–300 ms), time interval between pulses (50–300 ms), and backpressure; each parameter being varied independently. It was found that the size of the bubble, and hence the quantity of liquid displaced, increases with voltage. Also, the pump rate increases with a reduction in pulse-width and the time interval between pulses. The optimum pump rate of 24 nl/min, corresponding to a flow velocity of 640 μm/s, was obtained for the current channel size of 25×25μm in cross-section. The pump performed successfully against backpressures up to 107 kPa.Copyright

Development of bismuth‐2212 conductors

The influence of processing parameters on the fabrication of conductors of the Bi‐2212 oxide superconductor are discussed. Critical currents and critical current densities of short samples have reached acceptable levels for 4.2–20 K applications. However, problems related to reproducibility and uniformity of transport properties along the conductor length still exist. Possible methods to circumvent these and other associated problems are presented.

Effect of RF plasma deposition parameters on Y1Ba2Cu3O7−x thin films

Yttrium, barium and copper nitrates are dissolved in de‐ionized water to form a 0.038M: 0.078M: 0.1o5M solution, which is used to generate, in‐situ Y1Ba2Cu3O7−x superconducting films in an argon‐oxygen rf plasma. rf power is operated at 4.5 kW, 13.56 MHz and 760 Torr pressure. The best zero resistance temperature, onset critical temperature and current density measured by the four probe transport method are 86 K, 92 K and 4×105 A/cm2 at 77 K and zero field. X‐ray diffraction shows the films to be oriented with c‐axis perpendicular to the substrate surface. Variations in the critical temperatures of the films and their microstructure depend on rf power, solution composition, solution concentration, plasma gas and aerosol carrier gas. It has been seen that the concentration of barium in the solution is more critical than that of copper. The concentration of the solution is related to the rf power, to prepare the best film, the concentration and the power levels have to be matched. While the variation of bot...

Degradation mechanisms of high deposition rate Y1Ba2Cu3O7−x thin film properties

Superconducting Y1Ba2Cu3O7−x thin films have been grown at a high deposition rate of 10,000A/min, in‐situ, at atmospheric pressure. The rf plasma deposition technique used has been scaled up by more than 100 times when compared to a technique such as laser ablation. After a 30 second deposition, a 0.8 μm thick, high deposition rate film has the same critical temperature of 87K as the lower rate, 100A/min, film. On the other hand, the critical current is an order of magnitude lower than the critical current density of 6×105 A/cm2, at 77K and zero field, of the lower rate film. Degradation of the high rate film properties, such as crystal structure, homogeneity and current carrying capacity have been investigated with respect to the low deposition rate films. Oxygen deficiency, presence of BaCuO2 impurities and incomplete orientation of the film have been specifically identified as some of the reasons responsible for the degradation. Scanning Auger Electron Spectroscopy (AES), Auger depth profile, EDS mappi...