Experimental Methods

274 Pages·1983·5.147 MB·English

by  HERBERT HERMAN (Eds.)

TREATISE EDITOR
HERBERT HERMAN
Department of Materials Science and Engineering
State University of New York at Stony Brook
Stony Brook, New York
ADVISORY BOARD
J. W. CHRISTIAN, F. R. S. P. B. HIRSCH, F. R. S.
Oxford University Oxford University
Oxford, England Oxford, England
M. E. FINE R. I. JAFFEE
Northwestern University Electric Power Research Institute
Evanston, Illinois Palo Alto, California
J. FRIEDEL E. I. SALKOVITZ
Université de Paris U.S. Office of Naval Research
Orsay, France Arlington, Virginia
A. GOLAND A.SEEGER
Department of Physics Max-Planck-Institut
Brookhaven National Laboratory Stuttgart, Germany
Upton, New York
J. J. HARWOOD J. B. WACHTMAN
Ford Motor Company National Bureau of Standards
Dearborn, Michigan Washington, D.C.
TREATISE ON MATERIALS SCIENCE
AND TECHNOLOGY
VOLUME 19
EXPERIMENTAL METHODS
PARTB
EDITED BY
HERBERT HERMAN
Department of Materials Science and Engineering
State University of New York at Stony Brook
Stony Brook, New York
MANUSCRIPT EDITOR
BARBARA R. HERMAN
1983
ACADEMIC PRESS
A Subsidiary of Harcourt Brace Jovanovich, Publishers
New York London
Paris San Diego San Francisco Sao Paulo Sydney Tokyo Toronto
COPYRIGHT© 1983 BY ACADEMIC PRESS, INC.
ALL RIGHTS RESERVED.
NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR
TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC
OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY
INFORMANTION STORAGE AND RETRIEVAL SYSTEM, WITHOUT
PERMISSION IN WRITING FROM THE PUBLISHER.
ACADEMIC PRESS, INC.
111 Fifth Avenue, New York, New York 10003
United Kingdom Edition published by
ACADEMIC PRESS, INC. (LONDON) LTD.
24/28 Oval Road, London NW1 7DX
Library of Congress Cataloging in Publication Data
Main entry under title:
Treatise on materials science and technology.
Vols. 3, 6-7, 9- have also special titles.
Includes bibliographies and index.
1. Materials. I. Herman, Herbert, ed.
TA403.T74 620.Π 77-182672
ISBN 0-12-341842-9 (v. 19 pt. B)
PRINTED IN THE UNITED STATES OF AMERICA
83 84 85 86 9 8 7 6 5 4 3 2 1
Contributors
Numbers in parentheses indicate the pages on which the authors' contributions begin.
WILLIAM J. BAXTER (1), Department of Physics, General Motors Research
Laboratories, Warren, Michigan 48090
VLADIMIR BOLDYREV1 (185), Institute of Physico-Chemical Foundations
of Processing of Raw Mineral Materials, Siberian Branch of the
Academy of Sciences of the USSR, Novosibirsk, USSR
RONALD GRONSKY (225), National Center for Electron Microscopy,
Lawrence Berkeley Laboratory, Berkeley, California 94720
ALLAN ROSENCWAIG2 (67), Lawrence Livermore Laboratory, University
of California, Livermore, California 94550
E. M. UYGUR (119), Metallurgical Engineering Department, Middle East
Technical University, Ankara, Turkey
'Present address: Derzhavina 18, Institute of Solid State Chemistry, Novosibirsk-91, USSR
630091.
2Present address: Therma-Wave, Inc., Fremont, California 94539.
v/z
TREATISE ON MATERIALS SCIENCE AND TECHNOLOGY, VOL. 19B
Exoelectron Emission
WILLIAM J. BAXTER
Department of Physics
General Motors Research Laboratories
Warren, Michigan
I. Introduction 1
II. Instrumentation 3
A. Measurement of Exoelectron Emission 4
B. Imaging of Exoelectron Emission 5
C. Specimen Preparation 14
III. Mechanisms of Exoelectron Emission 15
A. Photostimulated Exoelectron Emission 15
B. Dark Emission 24
C. Thermally Stimulated Exoelectron Emission 27
IV. Applications of Photostimulated Exoelectron Emission 30
A. Fracture of Oxide Films 30
B. Plastic Deformation 41
C. Fatigue 46
V. Summary 61
References 63
I. Introduction
The term exoelectron originates from the early investigations of Kramer
( 1949), who observed small emission currents from various materials under
a variety of conditions. These studies were stimulated, at least in part, by a
problem experienced with Geiger counters in the 1930s: Newly constructed
counters exhibited a high background count rate which eventually de-
creased to an acceptable level. This background emission was identified by
Lewis and Burcham (1936) as originating from the freshly machined metal
surfaces within the chamber of the counter. Kramer observed similar
currents from abraded metal surfaces, as well as during solidification of
1
Copyright © 1983 by Academic Press, Inc.
All rights of reproduction in any form reserved.
ISBN 0-12-341842-9
2 WILLIAM J. BAXTER
Wood's metal, and concluded that electrons were being emitted by exother-
mal processes. In the case of freshly machined or abraded metal surfaces the
electrons were presumably released during the process of reoxidation.
The advent of gas-filled counters and electron multipliers, which can
detect individual charged particles, resulted in a plethora of observations of
low-level emission currents, often of mysterious origin. Thus the term
"exoelectron" is now used in a wider sense to describe many different types
of emission phenomena, a considerable proportion of which do not involve
exothermal processes. Indeed there is now evidence that in a few cases the
currents are associated with the emission of negative ions rather than
electrons. Similarly, it has been proposed that the blackening of photo-
graphic emulsions during long-term exposure in air to freshly deformed
metal surfaces, an intriguing phenomenon known as the Russell effect,
should also be classed as an exoelectron effect.
Nevertheless, in most cases it is clear that the emitted particles are
electrons, and the common denominator is that they only appear if the
material has been previously subjected to certain treatments. Thus "exo-
electrons" have been observed from metals, semiconductors, and insulators
and have been associated with prior damage introduced by abrasion, plastic
deformation, fatigue cycling, optical radiation, or by the various forms of
ionizing radiation. In addition, "exoelectrons" are emitted during phase
changes, including gas-surface interactions.
To distinguish between these various types of exoelectrons, several no-
menclatures have been proposed. Since there are many distinctly different,
and some as yet unidentified, mechanisms of emission, a system of classifi-
cation in terms of the applied stimulus has obvious merits and has gradually
gained common acceptance. Thus the emission currents which may appear
if the sample is heated after the initial treatment (e.g., deformation) are
referred to as thermally stimulated exoelectron emission (TSEE). Similarly,
photostimulated exoelectron emission (PSEE) refers to the increase in
photoelectron current which may be observed after various sample treat-
ments. Some emission currents are only observed simultaneously with
deformation of the surface, so these are often called tribostimulated emis-
sion. However, in some of the earlier literature such emission currents were
considered to be spontaneous and referred to simply as "dark emission,"
"Kramer effect," or simply "exoelectrons." Another term sometimes en-
countered is chemiemission, which describes currents emitted during ad-
sorption or reaction of gases with surfaces. Some authors have proposed
more complicated systems of multiple prefixes (e.g., tribophotostimulated),
while others have refrained, perhaps more wisely, from the use of the term
exoelectron. Here the term is used in a generic sense, and the various types
are classified simply as TSEE, PSEE, and dark emission.
The subject can further be divided quite conveniently into two parts:
EXOELECTRON EMISSION 3
exoelectrons from metals and exoelectrons from nonmetallic materials. The
latter are very analogous to luminescence and can provide information on
the presence of electron traps in the surface layers of semiconductors and
insulators. They have been the subject of considerable investigation because
of their application to radiation dosimetry, and have been extensively
reviewed by Becker ( 1970, 1972) and Ramsey ( 1976). Thus it is appropriate
to restrict the present survey to exoelectron emission from metallic mate-
rials.
A complete account of the early work on exoelectron emission from
metals was described by Brotzen (1967), who surveyed the various circum-
stances resulting in the emission of exoelectrons and the various models
which had been proposed to account for the results. At that time the subject
was still in a state of confusion and was primarily of academic interest, but it
was clear that the oxide film on the surface of a metal played an important
role in the emission of exoelectrons. The early 1970s heralded important
advances in experimental techniques which clarified our understanding of
the emission mechanisms and demonstrated potential applications in mate-
rials research. This was particularly true in the case of PSEE; Arnott and
Ramsey (1971) defined the strong influence of the gaseous environment,
while Baxter and Rouze (1973b) formed direct images of PSEE which
unequivocally illustrated the key role of the fracture of the surface oxide
film. Concurrently, the development of scanning spot systems by Veerman
(1969) and Baxter (1973a) demonstrated the potential of PSEE as a tech-
nique for detecting the early stages of fatigue damage in metals. These
developments have been summarized by Baxter (1977c, 1979a,b).
Since then techniques have been further refined, a more detailed under-
standing of the behavior of surface oxide films has emerged, and the field has
benefited from the stimulus of additional investigators who have contrib-
uted to an improved understanding of the various forms of exoelectron
emission. These new findings are detailed in the appropriate sections, but
the main emphasis of this review is concerned with the demonstrated
applications of PSEE. This class of exoelectron emission can be imaged in a
photoelectron microscope to provide unique microscopic information on
the fracture of surface oxide films during deformation of the substrate metal.
Furthermore, the intensity of PSEE is a quantitative measure of the extent
and distribution of the surface deformation and provides a method of
assessing the onset and accumulation of the early stages of fatigue damage.
II. Instrumentation
It is important to appreciate that exoelectron emission is a surface phe-
nomenon. This is obviously true if the exoelectrons originate from the thin
4 WILLIAM J. BAXTER
layer of surface oxide, but it is also true in the case of electrons emitted from
the metal itself. Exoelectrons are emitted with energies of only a few electron
volts (Lohff, 1957; Kortov et al., 1970; Sujak and Gieroszynski, 1971;
Rosenblum et al, 1977b), and the escape depth for low-energy electrons is
less than 10 nm (Kanter, 1970). Thus all types of exoelectron emission will
be sensitive to the condition of the surface of the metal, and a primary
consideration in the design of apparatus is the need to maintain good
control of the gaseous environment of the sample for quantitative and
unambiguous results. In this regard, the results of some of the early investi-
gations conducted with gas-filled counters should be viewed with caution.
During the past decade most experiments have been conducted in vac-
uum chambers, the emitted particles being detected with electron multi-
pliers. Under these conditions the results are more reliable and easier to
interpret. The majority of the emitted particles are indeed electrons. This is
certainly true in the case of PSEE, which has been imaged (Baxter and
Rouze, 1973b, 1975a,b, 1976, 1978) and energy analyzed (Swami and
Chung, 1980) with electron optics. On the other hand, in the case of "dark
emission" Rosenblum et al. (1977b) have used magnetic deflection and
other discriminatory techniques to show that, at least in their experiments,
while the majority of the particles are electrons, they are accompanied by
negative ions and photons. Similarly, negative ions have been identified as
responsible for some TSEE from nonmetallic materials, and the same is
probably true for such emission from metals (Svitov and Krylova, 1976).
A. Measurement of Exoelectron Emission
With few exceptions the exoelectron currents are very small, that is, less
than 10~13 A, so the first stage of the detection system customarily provides
electron multiplication. The disadvantage of gas-filled detectors, such as
Geiger-Müller counters and proportional counters, has already been al-
luded to, namely, that the gas mixture may interact with the sample surface
and produce interfering and uncontrollable effects on the electron emission.
Thus detection with secondary electron multipliers in a vacuum environ-
ment is definitely preferable.
There are two types of commercially available electron multipliers, multi-
ple dynodes and continuous dynodes, which can provide electron multipli-
cations of up to a factor of 108. They do not respond well to low-energy
electrons, however, so it is important to preaccelerate the exoelectrons up to
a few hundred volts before they enter the electron multiplier. The output
from the multiplier can be used in the analog mode and measured by an
electrometer. However, since the gain of a multiplier is known to vary with
EXOELECTRON EMISSION 5
time, it is preferable to take advantage of the digital nature of the multiplier
output and count the pulses. Pulse-counting electronics have been devel-
oped to a high level for nuclear radiation measurements, and a wide
selection of suitable equipment is commercially available. In this digital
mode, individual electrons may be counted at rates of up to 106 sec-1, a
range of operation which is adequate for most purposes.
Emission currents have actually been measured directly in air without any
multiplication (Moore and Tsang, 1971;Hoenige/a/., 1971; Smith, 1975).
While the apparatus is simple, the physical processes involved are compli-
cated (e.g., the charge carriers are probably oxygen ions), so that interpreta-
tion of the results is very difficult.
The stimulation of the exoelectron emission is another very important
experimental consideration. One factor to guard against is the evolution of
gas, either from the sample or from other elements of the apparatus; this can
easily occur, particularly during heating of the specimen, as in TSEE, or
during any form of mechanical deformation of the specimen. Though the
gas may be released initially in the form of neutral molecules, these can be
immediately ionized by an ion gauge or ion pump, and the ions in turn can
traverse remarkably tortuous paths and be detected by the electron multi-
plier.
In studies of PSEE the choice of photon energy can have a strong effect on
the sensitivity. This is easily visualized from the relationship between
photon energy and the photoyield (the number of electrons emitted per
incident photon absorbed) from a metal. The example for nickel shown in
Fig. 1 is quite typical and illustrates the dramatic increase in photoyield as
the photon energy exceeds the threshold value for photoelectron emission
(the photoelectronic work function). Clearly, if the selected photon energy
corresponds to this steep portion of the curve, a small reduction of the
threshold (work function) can easily increase the photoemission (i.e., PSEE)
by an order of magnitude. Conversely, if the photon energy is much greater
than the work function and corresponds to the top portion of the curve, a
similar reduction of the work function will only result in a small fractional
increase of photoemission. This aspect is illustrated later in Sections IH,A,2
and IV,A,1, where the results of different investigators are compared.
B. Imaging of Exoelectron Emission
Techniques for displaying the spatial distribution of exoelectron emission
have revealed important detailed information regarding the mechanisms of
exoelectron emission and have aided in the interpretation of quantitative
measurements. This has been well illustrated by studies of metal fatigue,
6 WILLIAM J. BAXTER
4 6 8 10 12
PHOTON ENERGY (eV)
Fig. L The effect of photon energy on the photoelectric yield from nickel (Blodgett and
Spicer, 1966).
where the surface deformation is heterogeneous and there is a correspond-
ingly nonuniform distribution of PSEE. Thus the imaging of exoelectrons,
rather than the simple measurement of emission rate, constitutes a primary
technique. Here three methods will be described in some detail.
1. PHOTOGRAPHIC RECORDING
Krogstad and Moss (1965) have recorded dark emission directly on
photographic emulsion, and Forier et al (1971) have constructed a simple
camera to image TSEE from copper oxide. In both cases the electrons were
accelerated onto the photographic emulsion so that they could penetrate the
gelatin layer covering the silver halide crystals. Some interesting observa-
tions have also been reported in which an accelerating field is not required to
produce an image of surface deformation—the so-called Russell effect
(Russell, 1897). A photographic emulsion is applied to a freshly deformed
surface and after a few hours stripped off and developed. Baun (1975)
investigated the sensitivity of a number of emulsions and recommended
films which do not have the protective layer of gelatin. Such images have
been attributed to the emission of exoelectrons (Grunberg, 1953), but the
evidence seems to be somewhat circumstantial (see Section III,B,3).