This paper describes the characteristics of microbatteries suitable for use in a hybrid micropower supply for powering autonomous MEMS and other microsystems. The hybrid power supply includes an energy conversion device, microscopic batteries for energy storage, and control/interface circuitry. Comparison of the hybrid approach with single power sources (either a battery or energy conversion device alone) shows that it offers several potential advantages including reduced size, increased flexibility, long lifetime and increased reliability. Such an approach is well suited to the expected duty cycles of remote microsensors. Realization of the advantages of a hybrid system depends on the availability of a battery with the required characteristics. Initial experimental results demonstrate the feasibility of fabricating microbatteries with the proper characteristics and the use of these batteries as part of a hybrid micropower supply. It is anticipated that hybrid micropower supplies with suitable microbatteries will play a critical role in the successful implementation of a wide variety of autonomous microsystems.

Lacking from much of MEMS technology is a viable strategy for supplying power. In particular, MEMS intended to be autonomous require microscopic energy storage. The purpose of this paper is to discuss the design of microscopic batteries to fill this need. Energy storage strategies for MEMS, and design considerations for batteries, are reviewed. The experimental results of initial microscopic battery demonstration is also presented. A range of secondary battery chemistries were demonstrated in miniature or microscopic prototype cells, with both solid and liquid electrolytes. Their various attributes and performances were assessed, and two different chemistries were selected for initial development, Ni/Zn and Li/Ion. Successful versions of the sealed Ni/Zn and unsealed Li-ion cells have been demonstrated.The batteries are fabricated using existing MEMS fabrication procedures, so that complete integration is possible. These batteries offer far more energy storage capability than a capacitor, sufficient to operate many MEMS devices for extended time periods without a significant recharge. Autonomous and remote MEMS devices thus become feasible. The use of integrable microscopic batteries in MEMS represents a significant enablement of MEMS technology.

A mathematical model has been developed in order to simulate the three-dimensional, transient behavior during discharge of a spirally wound lead-acid battery cell designed for use in hybrid-electric vehicles. Several aspects of battery behavior are predicted, including potential and current distributions, heat generation rates, concentration changes, and temperature. The influence of the lead grids which support both electrodes has been described and found to have a significant impact on both the power available from the cell and heat generation rates in the cell. The model predictions compared well with available experimental data (current-voltage-time). Models such as the one presented in this paper represent a valuable tool for understanding battery behavior, solving problems with existing battery designs, and generating and optimizing new designs. (C) 1999 The Electrochemical Society. S0013-4651(98)06-005-4. All rights reserved.

This paper outlines fabrication and evaluation of a thin-film, rechargeable microbattery that provides sufficient power for many autonomous MEMS applications.Initial prototype microbattery 0-9640024-2-6/hh1998/$20©1998TRF

The Plante method for forming the lead dioxide electrode represents one of the oldest known electrochemical processes, yet there remain gaps in our understanding of the mechanisms involved. Several mechanistic explanations have been presented which provide a framework for understanding these processes. Notable among these are the efforts of Reutschi, Pavlov, Lazarides, and Hampson, and Valeriote and their co-workers. One key element of their models is the existence of regions of high pH within the anodized layer on the lead surface. These varying pH conditions are explained by inhibited mass transfer of liquid-phase species. In this paper, the various models are assimilated, and a mathematical model of liquid-phase mass transfer in the anodized layer is presented. This model is solved for conditions both with and without the lead-solubilizing species (perchlorate) present. Solutions to the model without perchlorate present show that, within an idealized pore in the anodized region, sharp increases in pH occur at the solid/liquid interface, and that at that point sulfate ions are depleted and alkaline conditions can exist. At this interface, the ionic strength of the solution drops to a minimum. With perchlorate present, the minimum in ionic strength occurs away from the interface and moves farther out with time. These model predictions lend support for the theories that inhibited mass transfer leads to regions of high pH as well as conditions of low ionic strength, where electrolysis of water and hence OH- production can occur.

The design of a battery with maximum specific power to be discharged for 0.01s or less was explored. Key elements of the design are bipolar construction, using thin components with high electronic conductivity in the bipolar separator and high ionic conductivity in the electrolyte, and the use of an electrochemical couple with high open‐circuit potential and fast electrode kinetics. Bipolar lead‐acid stacks were assembled which showed specific powers of 100–800 kW/kg with current densities of up to 10–40 A/cm2 for up to 100 μs. Single lead‐acid cell tests showed that acid concentration, separator thickness and conductivity, discharge potential, and formation time all had a major impact on the cell power output. Tests showed that these cells are capable of well over a million shallow discharge/charge cycles. Evidence indicates that formation severely reduces cell current densities after 200–400 μs of discharge. In the first 200 μs, concentration depletion at the reaction interface appears to be a factor in current decline.

Two-color optical pyrometers have been used to measure the temperature of reacting pulverized coal particles. An analysis of such measurements was performed to determine the effect of several possible conditions on the measured temperature. The conditions investigated were the use of a single photomultiplier to alternately measure the radiant emission at the two selected wavelengths, the presence of soot, light extinction, the choice of wavelengths used to compute the two-color temperature, and non-uniform particle clouds. A computer model of a one-dimensional coal particle cloud was written for this analysis. Results of calculations showed that artificially high temperatures can result if a pyrometer with a single photodetector is used to measure temperatures in a rapidly fluctuating flame. Emission by soot in the coal particle cloud caused unrealistically high temperature measurements. Light absorption by soot lowered the two-color temperature, but not enough to compensate for the rise in observed temperature caused by soot emission. When the wavelengths used are in the visible spectrum. the hotter particles are weighted much more heavily than when the wavelengths are in the infrared region. The use of Weins Law, a valid approximation to Planck's Law in the visible spectrum, causes substantial error for longer wavelengths. Finally, the two-color temperature of a non-isothermal cloud was weighted most heavily by the hotter particles, depending upon the wavelengths used for the measurement. From this analysis important questions have been raised as to the validity of such measurements under transient conditions and during devolatilization.