Plasma-assisted electron emission from (Pb,La)(Zr,Ti)O3 ceramic cathodes

Plasma-assisted electron emission from (Pb,La)(Zr,Ti)O3 ceramic cathodes D. Shur and G. Rosenman Department of Electrical Engineering-Physical Electronics, Tel-Aviv University, Faculty of Engineering Tel-Aviv 69978, Israel

Ya. E. Krasik Physics Department, Weizmann Institute of Science, Rehovot 76100, Israel

V. D. Kugel 187 Materials Research Laboratory, Pennsylvania State University, University Park, Pennsylvania 16802-4801

~Received 21 August 1995; accepted for publication 8 November 1995! Strong pulsed electron emission has been observed from 12/65/35 lead lanthanum zirconate titanate ceramic composition in two different nonswitched phases at room temperature and at the temperature 100 °C. The electron emission parameters of this composition appear to be independent of phase for the two phases investigated. Fast photography and direct observation show that the strong electron emission occurs from the surface discharge plasma. The new experimental data make it possible to demonstrate the validity of the Child–Langmuir law for this electron emitter. A pulsed plasma lead lanthanum zirconate titanate ceramic cathode with burst frequency up to 100 kHz and collector current density up to 10 A/cm2 is developed. © 1996 American Institute of Physics. @S0021-8979~96!02304-3#

I. INTRODUCTION

Ferroelectric electron cathodes have come to the fore recently after the observation of a copious electron emission current density as high as 105 A/cm2 in a nanosecond time scale.1 This result and all other numerous data2–7 were obtained by using lead zirconate titanate ~PZT! and lead lanthanum zirconate titanate ~PLZT! ferroelectric ceramics. It was shown that the electron current considerably exceeded the Child–Langmuir current.4 –7 The effect was ascribed to the fast spontaneous polarization switching. Nevertheless the emission also occurred above the Curie point.3,6 It should be noted that no experimental evidence was presented for electron extraction from a free surface of a ferroelectric ceramic during polarization reversal. It was mentioned in Ref. 7 that plasma assisted emission is possible from PLZT cathodes. In Ref. 8 the strong emission observed from PLZT ceramic was referred to the plasma emission initiated by fusion of tips located on the electrode or bare ceramic regions. Giant electron emission charge considerably exceeding the value of spontaneous polarization for the ferroelectric TGS crystal was ascribed to the plasma formation.9 A ‘‘weak’’ ferroelectric electron emission caused by spontaneous polarization switching was observed earlier for several ferroelectrics crystals.10–12 The effect took place in the ferroelectric phase only and no emission was recorded above the Curie temperature.12 Visualization of the electron flux by using a position sensitive electron detector made it possible to demonstrate that emission of electrons occurs from the places of domain nucleations and under sideways motion of the domain walls.11 Measured electron emission current density was about 1027 A/cm2.12 It was shown that this ‘‘weak’’ emission current represents the electron screening current into a vacuum.13 J. Appl. Phys. 79 (7), 1 April 1996

The large difference of twelve orders of magnitude between ‘‘strong’’1–7 and ‘‘weak’’10–13 electron emission allows us to assume a quite different physical mechanism for the observed effects. It should be noted that dielectric cathodes were developed about 25 years ago.16 –21 They were based on the severe lowering of the breakdown voltage for vacuum gaps with dielectrics.14,15 Investigations of a surface discharge mechanism at a dielectric vacuum interface were conducted in Refs. 16 and 17. The emission of electrons from the plasma of a discharge along a barium titanate surface in vacuum was reported in Ref. 18 without any mention of polarization switching of this ferroelectric material. A high-density pulsed electron emission current up to 103 –104 A/cm2 was obtained from barium titanate cathodes.19,20 These cathodes were later used as powerful electron sources.20 A better understanding of the phenomena may open an avenue for successful application of this electron emission both for flat emission panel displays22 and powerful ferroelectric cathodes.1–7 In this work we present a new experimental study of electron emission from the ceramic composition PLZT 12/65/35 in two different nonferroelectric phases ~antiferroelectric-slim-loop phase boundary and paraelectric phase!.23 The effects of polarization switching are eliminated in nonferroelectric phases. II. EXPERIMENTAL TECHNIQUES

The experimental setup is shown schematically in Fig. 1. It consisted of trigger and collector circuits. A grid collector was made of stainless-steel 50-mm-thick grid. It was located at a distance of 3.5 mm from the sample surface. For some measurements a Pilot B scintillator with fluorescent time 10 ns was pressed behind the grid for electron beam observation. The vacuum viewport made it possible to observe visually and to photograph by fast frame camera ~50 ns frame!

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© 1996 American Institute of Physics

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FIG. 2. Experimental data show the trigger voltage ~trace TR1A; 500 V/div!; collector ~emission! current pulse ~trace TR4A; 1 A/div!; sample current waveform ~trace TR3A; 5 A/div!. The time scale is 250 ns/div. The positive trigger voltage V tr511700 V with pulse duration 400 ns is applied to the rear contact of the sample. The collector voltage is V col51000 V. The maximal collector current is I col'2.2 A. The emitted electron charge is Q em'320 nC. The vacuum gap is 3.5 mm.

FIG. 1. Schematic circuit of the experimental setup. The digital oscilloscope Gould 4074 ~Gould Instrument Systems Ltd., Roebuck Road, Hainault, Ilford, Essex IG6 3UE, England!, fast high-voltage transistor switch HTS 31-GSM from Eurotek Inc. ~10 Beth Lane Morganville, NJ 07751!, and function pulse generator Tabor 8551 ~Tabor Electronics Ltd., P.O. Box 901 Haifa, Israel! were used.

the process within the vacuum gap or on the scintillator surface. The dc voltage V col applied to the collector was varied from 1200 V up to 13000 V. The charging time constant of the collector capacitor tcol5R col•C col was 2.5 ms. We tested the collector circuit by pulses of nanosecond time scale with rise time