Positron annihilation spectroscopy (PAS) is a non-destructive technique that has been used to characterize negatively charged and neutral monovacancies, vacancy clusters, and nanovoids in a broad array of materials. This is done by measuring the lag time between the generation of free positrons via pair production during irradiation of a tungsten moderator with the high-energy gamma emission from a PULSTAR fission source and the emission of gamma rays during the annihilation process between these positrons and electrons in the material.  Because the lifetime of a free positron is dependent on the electron density at its site of annihilation, the presence of defects may be detected based on the difference in the lifetimes of positrons that have become trapped at defect sites and those that have annihilated in the bulk of the material.  The defect concentration is proportional to the rate of positron trapping by defects and can therefore be estimated from PAS measurements. In the proposed work, we plan to study thin film material capacitors manufactured at Army Research Labs consisting of lead zirconate titanate, PbZr_xTi_1-xO_3, deposited on platinized silicon wafers. The PZT layer will be approximately 1 micron thick, with a density of ~7.5 g/cm^3. These ferroelectric materials will serve as the actuator material in micro- and nano-electromechanical logic devices because of their electromechanical response behavior. The overarching objective of this experiment is to assess the effects of radiation exposure on the ferroelectric material, PZT, to determine how operability will be impacted by extreme environments. The samples to be analyzed will be varied in composition, top electrode, exposure source, and total dose resulting in a total of 20 samples. The sample details are summarized in the following list: 1. Compositions: PbZr_0.52Ti_0.48O_3 and PbZr_0.30Ti_0.70O_3 2. Radiation Sources: Gamma and Proton 3. Top Electrodes: None, Pt, and IrO2 4. Exposure Doses: 0, 2.5, 5, and 10 Mrad (Si) The Gamma-irradiated samples will be irradiated at the Naval Research Laboratory (NRL) using their Co-60 irradiation facilities with a dose rate of 500 rad(Si)/s. The proton irradiated samples will be irradiated at Auburn University, in conjunction with NRL, using their 3 MeV proton beam at a flux range of 10^9 - 10^11 p/s cm^2. Multiple PZT compositions will be studied to analyze the effects of irradiation near and away from the polymorphic phase boundary (PZT 52/48). Ferroelectric domain switching is more easily observed at compositions located away from this boundary, therefore the effects of irradiation should be more easily observed, but the extent to which the energetics of vacancy formation under irradiation change between these compositions is not yet known. The main goal of the work is to observe the effects of irradiation on the ferroelectric PZT actuator material (no top electrode) used in the mechanical logic devices. However, it is also desired to perform depth profiling on the samples with top electrodes. This technique will be conducted in order to observe the defect content in the electrode materials and at the PZT-electrode interface. In conclusion, the end goal of the proposed work is to define a relationship between irradiation and defect content in ferroelectric PZT thin film capacitors, both with and without top electrodes.

Ferroelectrics constitute a class of electromechanical materials with applications in a diverse range of devices that have the potential to improve the operation of next generation nuclear energy facilities, including low power consumption integrated circuits, ultrasonic sensors for non-destruction inspection, and mechanical actuators for remote control. The radiation survivability requirements for electronics operating in environments with elevated radiation doses, in both standard operating and accident conditions in nuclear facilities may place strictures on ferroelectric-based technology implementation. In the case of logic relays, radiation damage has been extensively studied in traditional complementary metal oxide semiconductor (CMOS) devices and hardening techniques have been implemented to improve device survivability for use in environments with elevated radiation doses, but as the demand for reduced power consumption and higher processor density grow, alternative transistor designs with fundamentally different implementations, such as the incorporation of ferroelectric-based mechanical relays, are required. While radiation survivability studies on mechanical relays have shown promising results, little is understood about the defect generation mechanisms by which radiation damage occurs in these devices. The susceptibility of these devices to radiation damage may be of even greater importance in the case of small modular reactors and other next generation plant designs, for which the fuel cycles may be longer, exposing electronics to larger accumulated doses before routine inspection [1].



In order to elucidate point defect generation mechanisms in irradiated ferroelectrics, the proposed work will focus on characterizing and quantifying vacancy formation in lead zirconate-titanate (PZT) thin film capacitors by using positron annihilation spectroscopy (PAS). Measured vacancy contents will be compared to ferroelectric properties of these capacitors in order to relate any degradation in these properties to observed changes in vacancy contents under irradiation. Understanding the mechanisms by which these degradation processes occur could point to potential methods for radiation hardening that may be successfully implemented in the future devices.



[1] Clayton D., Wood R.. The Role of Instrumentation and Control Technology in Enabling Deployment of Small Modular Reactors. Proceedings of the Seventh ANS Intl. Topical Meeting on Nuclear Plant Instrumentation, Control and Human-Machine Interface Technologies (NPIC&HMIT 2010), American Nuclear Society, Las Vegas, Nevada, 7–11 November 2010.