Shankar Radhakrishnan

Scalable microbeam flowsensors with electronic readout

In this paper, we present a scalable microchannel-embedded cantilever flowsensor with electronic readout. The scalable nature of the sensor addresses the need to realize arrays of flowsensors to characterize localized flow patterns. The electronic readout addresses the need to integrate flowsensors in microfluidic channels for closed-loop flow control. The in-channel nature of the flowsensor provides a wide choice for the cap material (e.g., PDMS, glass, silicon, etc.) enabling division of labor between integrated circuit (IC) and microfluidic foundries. This also holds promise for large-scale integrated microfluidic systems. The cantilever beam is placed inside a microfluidic channel in such a way that it utilizes drag forces to cause a flow-induced stress underneath the beam anchor. A Wheatstone bridge of piezoresistors placed directly under the anchor converts the flow-induced stress into a differential output voltage. We present an analytical model and 3-D simulations for the proposed flowsensor. Flowsensors fabricated by standard photolithography were tested to experimentally verify the validity of the model and simulation results. A flow sensitivity (unamplified) of 0.5 ppm/(/spl mu/L/s) was measured with first generation devices with water flow from 10 /spl mu/L/min to 100 /spl mu/L/min. Through experimental results, it is also shown that scaling down the size of the flowsensor results in higher sensitivity. We present a prediction on the noise-floor of flow measurement based on the results obtained using these prototype flowsensors.

Radioisotope thin-film powered microsystems

Radioactivity and Radioisotopes.- Radioisotope Thin Films for Microsystems.- Radioisotope Micropower Generation: Microfabricated Reciprocating Electro-Mechanical Power Generators.- Radioisotope Micropower Generation: Integrated Radioisotope Actuated Electro-Mechanical Power Generators.- Radioisotope Micropower Generation: 3D Silicon Electronvoltaics.- Radioisotope Direct Charging: Autonomous Wireless Sensors.- Radioisotope Decay Rate Based Counting Clock.

Radioactive Counting Clocks

We report on a radioactive counting clock (RCC) based on radioactive beta emissions from nickel-63 thin films. We present a theoretical analysis of the clock that uses the radioactive source (physics package) to lock and stabilize the frequency of a voltage-to-frequency converter (local oscillator). We present frequency stability measurements of the RCC over 10 days of clock operation. We analyze the limitations on the short-term and long-term frequency stabilities of the RCCs for use in design of clocks that require good frequency stabilities over long-term operations and consume low power, thus holding promise for use in timing and frequency applications in portable systems

Radioisotope Micropower Generation: Integrated Radioisotope Actuated Electro-Mechanical Power Generators

The radioisotope actuated electro-mechanical power generators (REMPG) presented in the previous chapter convert radioisotope emitted kinetic energy to stored electromechanical energy using radioisotope actuation of piezoelectric unimorph cantilevers. The stored electromechanical energy is efficiently integrated over a reciprocation period, and discharged to generate pulsed electrical power through the piezoelectric with energy conversion efficiencies as high as 3.97%. The corresponding radioisotope kinetic to electromechanical energy conversion and electromechanical to electrical energy conversion efficiencies are ≈ 4.6% and ≈ 85% respectively.

Radioisotope Direct Charging: Autonomous Wireless Sensors

The lifetime of autonomous wireless sensor microsystems can be increased either by increasing the energy capacity of microbatteries, or by decreasing the power consumption of on-board sensors and wireless transmitters. However, while radioisotope microbatteries do provide higher energy capacity energy sources compared to conventional electrochemical microbatteries, increasing the energy capacity of radioisotope microbatteries requires higher activities of radioisotope fuels. This undesirably raises safety concerns, and often limits the use of such microbatteries to specialized applications that can tolerate the lower radiation safety. Therefore, the application of radioisotopes to realizing zero-power wireless sensors was explored. Previously demonstrated applications utilizing radiation from radioisotopes for low-power sensing include the smoke detectors and electron capture detectors that employ the radiation for ionization of gas molecules. Such applications can function even with <1 millicurie of radioisotope, and consequently are safe enough for widespread deployment.

Radioisotope Thin Films for Microsystems

Radioisotopes can be employed in microsystems in a variety of ways to exploit the many unique properties of radioactivity:1. The kinetic energy of the emitted radiation can be converted into electrical energy for micropower generation 2. The energetic charged particles emitted can be directly collected and charge-separation based electrostatic actuation of cantilevers can be enabled for autonomous sensors 3. The near constant probability of decay of nuclei could be utilized for realizing frequency standards.

Radioactivity and Radioisotopes

Radioactivity was discovered in 1896 by Henri Becquerel, when he found that a mixture of uranium salts emitted a mysterious penetrating radiation that passed through thin sheets of metal. Since then, radioactivity has been applied in many diverse areas within the fields of industry, agriculture, medicine, and the military. In industry, sealed radioactive sources are used in industrial radiography, gauging applications, and mineral analysis. Short-lived radioactive material is used in flow tracing and mixing measurements. In agriculture, gamma sterilization is used for food preservation and sterilization of bulk commodities. In medicine, gamma sterilization is also used for sterilizing medical supplies, but more importantly, radioisotopes are indispensable in both diagnosing and treating some diseases including cancer. However, the most prevalent use of radioactivity is in the generation of electrical power. Applications benefiting from nuclear power generation range from industries and residences to submarines and deep space probes.

Radioisotope Decay Rate Based Counting Clock

Precise timing and frequency sources are vital in a wide range electronic-based systems such as communication networks and global positioning systems. These applications constantly demand reductions in size, weight and power (SWaP) while improving the precision of time or frequency references. Historically, clocks based on electromagnetic oscillations of atoms have provided the most precise method of timing events lasting longer than a few minutes.These oscillations are so precise that in 1967 the unit of time the second – was redefined to be the time taken for a Cs atom in a particular quantum state to undergo exactly 9,192,631,770 oscillations. While the long-term precision of atomic clocks is unsurpassed, the size and power required to run these devices has prevented their use in a variety of areas, particularly in those applications requiring portability or battery operation. The NIST 17 F-1 primary standard, for example, occupies a large optical table and requires many hundreds of watts to operate. The state-of-the-art in compact commercial atomic frequency references are Rb vapor-cell devices with volumes near 100 cm3 that operate on a few tens of watts of power and cost about 1–3 thousand dollars.The long-term stability of atomic clocks including is based onthe ability to interrogate a fundamental time constant – the hyperfine resonance frequency of ground level transitions1. It is thus natural to extend this idea of interrogating other time constants to realize clocks with good long-term stabilities.

Radioisotope Micropower Generation: 3D Silicon Electronvoltaics

In this chapter, the application of high power density (1–2 W/cc) and high energy density (100–200MJ/cc) 147Pm fuel in 1–20mW/cc, 5 year lifetime microbatteries is explored. The resulting >220 W-h/cc microbatteries could potentially replace the currently employed 1–10 W-h/cc electrochemical batteries [18] and enable longer lifetime autonomous microsystems

Radioisotope Micropower Generation: Microfabricated Reciprocating Electro-Mechanical Power Generators

In this chapter, microfabricated radioisotope power generators that employ 100.3 year half-lifetime 63Ni radioisotope fuel for realizing long operational lifetime are presented. The generators utilize energetic electrons emitted from 63Ni thin films to electrostatically actuate reciprocating piezoelectric unimorph cantilevers, converting the emitted radioisotope energy to electromechanical energy stored in the deformed unimorph cantilever, and piezoelectricity to convert the resulting mechanical strain into electrical charges, converting the stored electromechanical energy to extractable electrical energy. The reciprocating piezoelectric unimorph cantilevers go through charge-discharge-vibrate cycles, and efficiently integrate the low ≈300nW output from low activities (≈2.9millicurie) of safe 63Ni thin films to generate 0.25% duty cycle 12.95 μW power pulses (across an optimal load impedance of 521 kΩ) potentially useful for pulsed sensor microsystems.