EPA-650/3-75-006
February 1975
Ecological Research Series
STUDIES OF
FORMED BY HOMOGENEOUS NUCLEATION:
LIGHT SCATTERING
AND ELECTRON MICROSCOPY
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EPA-650/3-75-006
STUDIES OF SMALL METALLIC PARTICLES
FORMED BY HOMOGENEOUS NUCLEATION:
LIGHT SCATTERING
AND ELECTRON MICROSCOPY
by
H. P. Broida, J. D. Eversole,
and P. K. Hansma
University of California
Santa Barbara, California 93106
Grant No. 800845
Project No. 21AJX-004
Program Element No. 1AA008
EPA Project Officer: Dr. Jack L. Durham
Chemistry and Physics Laboratory
National Environmental Research Center
Research Triangle Park, North Carolina 27711
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
WASHINGTON, D. C. 20460
February 1975
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EPA REVIEW NOTICE
This report lias been reviewed by the N.ilicmal Environmental Research
Center - Research Triangle Park, Office of Research and Development.
KPA, and approved lor publication. Approval does not igmfy-that the
coiid nts neu'Ms.mly reflect Ihe views and policies ol the Environmental
Piolej lion Ageiuy, nor does mention ol trade n.ime.s or commerc-ial
products constitute endorsement or recommendation lor use.
RESEARCH REPORTING SERIES
Research reports of the Office ol Research and Development, U.S. Environ-
mental Protection Agency , have been grouped into series. These broad
categories were established to facilitate further development and applica-
tion ol 1'iivjronmental technology. Elimination of traditional grouping was
consciously planned to foster technology transfer and maximum interface
in related fields. These scries are:
1. ENVIRONMENTAL HEALTH EFFECTS RESEARCH
2. ENVIRONMENTAL PROTECTION TECHNOLOGY
3. ECOLOGICAL RESEARCH
4. ENVIRONMENTAL MONITORING
5. SOCIOECONOMIC ENVIRONMENTAL STUDIES
6. SCIENTIFIC AND TECHNICAL ASSESSMENT REPORTS
9. MISCELLANEOUS
This report has been assigned to the ECOLOGICAL RESEARCH series.
This series describes research on the effects of pollution on humans,
plant and animal species, and materials. Problems are assessed for
their long- and short-term influences. Investigations include formation,
transport, and pathway studies to determine the fate of pollutants and
their effects. This work provides the technical basis for setting standards
to minimize undesirable changes in living organisms in the aquatic,
terrestrial, and atmospheric environments.
This document is available to the public for sale through the National
Technical Information Service, Springfield, Virginia 22161.
Publication No. EPA-650/3-75-006
a
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ABSTRACT
This report describes basic research on the subject of formation and
detection of metallic particles ranging from 5 to 5000 ran in diameter.
Particulate matter is formed in a flowing inert gas by homogeneous
nucleation from the vapor phase. Metals used in the work done under
this grant include Na, K, Li, Rb, Cs, Cd, Mg, Zn,and Pb. Specific
objectives of this project are necessarily broad and open-ended as
present understanding of particle formation and growth mechanism is
still in an early state of development. Particulate matter with
diameters less than 100 nm plays an important role in atmospheric
pollution, and it was hoped that research would contribute to the
detection, measurement, and/or control of this problem.
Observations of plasma-resonance scattering of white light from alkali-
metal particles, the growth of Zn particles observed by electron
microscopy, and the development of a superconducting device: a small-
particle Josephson magnetometer, are significant results of this
project which have been published. Attempts to observe the Raman
scattering of laser light from Zn, Mg, and Cd particles, to isolate
macroscopic quantities of particles from the flow system, and to obtain
information from the 0 to 180 degrees angular light scattering were
not totally successful.
iii
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CONTENTS
Abstract
List of Figures
Sections
I Conclusions 1
II Introduction ' 2
III Experimental Apparatus 5
IV Plasma Resonance Light Scattering from Alkali Metal
Particles n
V- Electron Microscopy of Zn Particles 19
VI Small Particle Josephson Magnetometer 31
VII Observations of Laminar Flow in a Glass Tube 35
VIII "Matrix" Isolation of Na Particles 36
IX Raman Scattering 37
X Summary 3B
XI References 39
XII List of Publications ' 41
iv
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FIGURES
No. Page
1. Vacuum System Schematic 6
2 Standard Furnace Schematic 9
3 High Temperature Furnace Schematic 10
4 Scattered Light Spectra of Na IS
5 Scattered Light Spectra of K and Rb 15
6 Observation Chamber and Particle Collecting Apparatus
Schematic 20
7 Zn Particles 1200 run across 22
8 Indented Side views of Zn Particles 24
9 Summary of Three Size Categories of Zn Particles 25
10 Example of Agglometated Particle 27
11 Results of O2 Contamination 28
12 The Smallest Size Range of Particles Observed 30
13a Magnetic Field Dependence of the Critical Current of a
Small Particle Array 32
b Magnetic Field Dependence of the Critical Current of
Two Coupled Small Particle Arrays • 32
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SECTION I
CONCLUSIONS
Identification of particle-composition by plasma-resonance light
scattering, or by Raman scattering from the particles is not con-
sidered feasible with present state-of-the-art techniques for
industrial or commercial applications. Correlation of plasma-
resonance light scattering data to electron microscopy may provide
important information on the fundamental processes of particle nucleation
and growth. Optical structure should also exist for certain semi-
conductor materials arising from interband transitions rather than
plasma-resonance which again would provide basic information rather
than direct application. ~
Relative concentrations of carbon-based particles could conceivably
be measured utilizing plasma resonance light scattering. However,
more research is required to make a definate statement on this
possibility.
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SECTION II
INTRODUCTION
GENERAL
One result of current industrial activity in the United States has
been the introduction of approximately 107 tons per year of particulate
matter into the atmosphere.1 Current particulate air pollution is
very much in evidence as can readily be seen in industrial smoke
emissions and in the turbidity of the atmosphere in the vicinity of
cities. Of the emitted particles, the smaller, less visible particles
in the size range below 0.1 y are the most difficult to control and
detect. These very small particles have received little attention
as to creation, detection and control.*~5
The increase in the number of very small particles over the natural
background level, characteristic of cities, seems to be entirely the
result of combustion. Exanining some cases wheie particle size has
been measured at the particles source, Silverman, et al. have found
that, by number, the majority of particles emitted by an open hearth
furnace are below 100 nm diameter. Lee, et al. are cited3 as finding
that 90% by weight of automobile exhaust lead is emitted in the form of
particles with diameters less than 500 nm. It is possible that any
process of incineration or combustion that is able to vaporize metals
or metallic impurities may produce very small metallic or metallic
oxide particles.
Atmospheric particles are routinely monitored in many cities using
methods such as high-volume samplers, tape samplers, or turbidity
measurements.*~5 In general, these present monitoring methods
emphasize the larger, more massive fraction of particles of sizes
greater than 1000 nm. Subsequent measurements of the chemical com-
position of collected material, of course, tends again to emphasize
the more massive materials, which may not be indiciative of the
majority of particles.
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At present, no reliable optical method exists for characterizing
particles less than 100 nm diameter and the only direct method in
use is the electron microscope after collection.3'7 Other less direct
methods such as observing condensation of water vapor on small
particles after rapid expansion8'9 and measurements of mobility of
charged particles in an electric field10 have been used for observing
and characterizing very small particles.
These very small particles which have been somewhat ignored by present
observational techniques are nevertheless important for the following
reasons: 1) Because of their small size, very small particles
have settling velocities on the order of 10~5 cm/sec and consequently
have large residence times in the atmosphere. 2) Very small primary
particles can escape control efforts at the source and subsequently
grow to become larger particles either through direct combination
or by participation in photochemical reactions. 3) Very small
particles may be more injurious to health because they can be trans-
ported to the pulmonary and tracheobronchial regions of the respira-
tory tract, whereas larger particles can be deposited in the naso-
pharyngeal region where they can be removed by cilary action.3'**
One of the basic factors that has hampered studies of particles,
especially growth mechanisms, has been the inability to generate
particles in a controlled manner. At Santa Barbara, we have developed
a method for producing metallic particles from a metal vapor flowing
in a controlled atmosphere at temperatures from 300 to 2000 K. In
initial studies, particles have been produced with sizes ranging
from 5 to 1000 nm of the following materials: Pb, Zn, Na, K, Rb, Cs,
Mg, Cd and Li.
Measurements and detection of particles less than 100 nm diameter in
our system have been made using white light or laser light scattering
and electron microscopy. In the case of very small metallic particles,
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a maximum in the white light scattered intensity as a function of
wavelength is seen, corresponding to plasma resonance of the conduc-
tion electrons of the particles. This scattered light is highly
polarized. The wavelength and shape of the plasma resonance scattering
should depend on the material, size, and shape of the particles.
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SECTION III
EXPERIMENTAL APPARATUS
GENERAL
Particles are produced by evaporating bulk metal in a heated flowing
inert gas. When the gas carries metal vapor to a cooler region of the
flow system, a supersaturated condition of the metal vapor can be
produced. At sufficiently low temperature, spontaneous condensation
of small "solid" particles can result. Electron microscopy of such
particles shows size ranges foom 5 to 5000 nm, depending on the
production and collecting conditions of the system.
The experimental apparatus is illustrated in Fig. 1. The basic system
consists of a furnace where the metal of interest is heated, and an ~
observation chamber above the furnace where the particles are formed.
Both of these are enclosed in, and partly made up of a vacuum flow
system. It is convenient to describe the apparatus by separating
these elements and discussing first, the vacuum flow system; and
second, the furnace.
VACUUM FLOW SYSTEM
Vacuum flow systems are constructed from commercially manufactured
stainless-steel tubing available in 5 and 10 cm diameters. "0-ring"
grooves are cut into flanges on the ends of these pieces so that
different pieces may be joined together with a vacuum seal by simply
placing an "0-ring" between them. Special thumb-screw clamps hold
the pieces in place. The stainless steel tubing may be obtained as
crosses, tees, ells, weIdable ferrules, and blank-offs; so that a
great variety of configurations may be quickly and easily constructed.
Modifications necessary for valves, windows, pressure gauges, copper
tubing adapters, flexible vacuum lines, and pump flanges are straignt-
forward and simple shop work with the purchased blank-offs and weldable
ferrules. The entire vacuum system therefore, becomes standardized
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A
t
B
Fig. 1 Vacuum system schematic: A) to vacuum pump, B) filter,
C) furnace, D) heater electrical contacts, E) windows, F) cooling water
ports, and G) carrier gas inlet.
6
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to 5 and 10 cm components with the flange, "0-ring", and clamp seal.
The in-line filter assembly is the only part which is necessary to
completely fabricate.
The central part of the apparatus is a 10 cm 4-way cross which forms
the observation chamber. This cross has been modified by adding two
5 cm ferrules to form a 6-way cross. (See Fig. 1). The furnace
protrudes into the bottom of this observation chamber and different
housing is required below this cross depending on which furnace is
used. In the horizontal plane the observation chamber will typically
have three windows and a butterfly valve connected. The top port of
the cross connects to the vacuum pump via a filter/ valve/ pressure
gauge, and flexible vacuum hose. Apparatus for taking samples for
electron microscopy may be attached to the other side of the butterfly
valve, and this will be discussed in Section V. (See Fig. 6).
FURNACE
The standard furnace design could achieve a maximum temperature of
900°-1000eC. In later designs temperatures in the 1600°-1800°C range
were reached, hence a distinction between the "standard" and "high
temperature" furnaces must be made. However, in both types of furnaces,
there is the basic design of an indirectly heated crucible. The major
difference is that higher tempteratures were achieved in part by a
smaller size to minimize conduction losses/ so that the component size
of the high-temperature furnace is less than half the scale of the
standard furnace. In both cases an alumina crucible containing the
metal of interest is supported inside an alumina tube, and this outer
tube is heated electrically by a tungsten wire. Heat is transferred
from the wall of the tube to the crucible by the inert carrier gas
introduced at the bottom of the tube. The mixture of carrier .gas and
metal vapor exits through a nozzle at the top of the tube to the
observation region. Cooling occurs because of conduction to the walls,
which are at room temperature. .
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In the standard furnace design, this heater assembly is housed inside
two concentric stainless steel heat shields (7 and 8 cm dia. see Fig. 2)
The entire furnace is supported on stainless steel rods above a 10 cm
blank off. The blank-cff is modified to accept the carrier gas and
electrically insulated stainless steel tubes which also carry
cooling water to the filament contacts. This furnace is enclosed
in a 10 cm tee which has been wrapped with copper tubing to
carry cooling water, and is attached to the bottom of the
observation chamber cross. When assembled, the top of the furnace
protrudes into the bottom of the cross.
In the high temperature furance design, the heater assembly rests
directly on a copper base-plate. The copper plate accomodates the
carrier gas and electrical connections, and is housed in a 10 cm
ferrule (see Fig. 3) so that the entire plate comes into contact
with cooling water. The metal heat shields are eliminated and
replaced by another alumina tube (5 cm diameter), and zirconia
thermal insulation is placed inside and outside this alumina tube.
This furnace assembly is compact enough to fit directly into the
bottom of the observation chamber cross. Greater details of furnace
design and construction as well as other uses are contained in Ref. 12.
8
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ALUMINA
TUBE
CRUCIBLE
WITH
METAL
SAMPLE
Fig. 2 Standard furnace schematic.
9
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ZIRCONIA
FELT
hUO
ALUMINA
TUBE
CRUCIBLE WITH
METAL SAMPLE
ZIRCONIA
TUBE
COPPER PLATE
10mm
CARRIER
GAS
Fig. 3 High temperature furnace schematic.
10
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SECTION IV
PLASMA-RESONANCE LIGHT SCATTERING FROM ALKALI METAL PARTICLES
INTRODUCTION
The first experiments performed under this grant investigated the
light-scattering characteristics of particles made from alkali metals.
The conduction electrons of the alkali metals are known to approximate
a free-electron-gas model. One may calculate the scattering cross
section for a small spherical particle using this model to supply the
index of refraction as a function of wavelength.*3~*7 The result is
an anomalously large cross section at a particular light frequency
which is resonant with the collective motion of the conduction electrons
in the small particle. For the alkali metals this resonance occurs in
the visible region of the spectrum. This phenomena of collective
motion is called plasma-resonance and has been observed in various bulk
metals by inelastic electron scattering, and by light scattering in
thin films. However, the position (frequency) and width of this
resonance should be a measurable function of particle size and shape,
as well as composition. Plasma-resonance light scattering in small
particles has been observed previously by various groups, but these
experiments were the first observations made on particles that were
forming in situ in free space and continuing to grow in size.
EXPERIMENTAL
Particles were formed using the standard furnace and vacuum apparatus
described previously. Temperature of the furnace was adjusted empiri-
cally to obtain particle formation and measurements indicate metal
equilibrium vapor pressures between 0.1 and 10 torr. Under typical
operating conditions, metal was evaporated from the crucible at about
0.2 g/h. Lithium, sodium, and potassium were obtained commercially
•
as sticks of 99.95% purity, packed under oil. Rubidium and cesium were
obtained in glass vials with break-seal necks. The seals were broken
from outside the furnace just before experiments were begun. Light
scattering was observed by focusing light from a 450-W xenon arc lamp
11
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directly into the observation chamber/ and collecting the scattered
light at 90° to the incident beam by a monochromator. Light from
the monochromator was detected with a photomultiplier, and its
intensity vs. wavelength displayed on a chart recorder. Greater
detail of procedures and results is available in Ref, 18.
RESULTS
A weak resonance peak was observed for Lithium at 270 run. Due to the
weak signal no measurements of relative peak shift or width could be
made. The high background of scattered light was attributed to
reaction products of the liquid lithium with the alumina in the furnace
as the alumina crucible was destroyed in less than 1 h of operation.
Two different resonant peaks have been observed with sodium particles:
at 310 nm and at 400 nm. Subsequently to the published paper on these
experiments it was found that the 310 nm peak is caused by the presence
of a small amount of oxygen in the system. Therefore, contrary to the
original published paper,18 only the 400 nm peak can be considered as
due to pure sodium particle plasma-resonance.
The 400 nm peak is easily observed and relative shifts in the peak were
produced by changes in experimental parameters. A reduction in flow
rate of the carrier gas (N2) by a factor of 5 (from 2.5 to 0.5 cm3/sec)
was found to produce a shift of about 2 nm to a shorter wavelength.
Pressure induced shifts were somewhat: more pronounced. The "400 nm"
peak shifts from 403.0 nm at 1.3 kPa to 384.0 nm at 4.0 kPa, both at a
flow of 2.0 cm3/sec (see Fig. 4). There was also a furnace-temperature
dependence shifting from 385 to 419 nm as the temperature increased
from 700 to 800 K at a constant flow of 2 cm3/sec.
Attempts were made to obtain electron micrographs of sodium particles
collected from directly above the furnace under conditions which gave
sharp resonance scattering. The resulting photographs were not very
12
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oo
UJ
or
400 500
WAVELENGTH (NM)
600
Fig. 4 Scattered light spectra of Na (uncorrected) at constant flow
rate and furnace temperature showing the effects of increasing pressure,
Curves have been displaced vertically for clarity.
13
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definitive, apparently because the Na particles react rapidly with
atmospheric moisture and are easily melted by the electron beam in
the microscope. Definite correlation of changes in particle size
and shape with shifts in the resonant frequency would have been
extremely important.
Potassium behaved qualitatively similar to sodium. The plasma
resonance for potassium particles is around 570 nm and appeared as
an orange glow (see Fig. 5) . As the pressure changed from 53 pa
to 267 Pa the peak shifted from 568.5 nm with a 42 nm width, to
577.0 nm with a 60.5 nm width. At constant pressure and flow rate
the peak shifted from 571.5 to 558.0 nm and narrowed from 50.5
to 37 nm as the furnace temperature was lowered from normal operat-
ing temperatures P* 600 K) to a point where no particles appeared.
A weak peak was also recorded at 523.0 nm but this has not yet been
satisfactorily explained.
Resonant scattering behavior was more difficult to obtain for rubidium
particles. A resonance peak was observed at 625.0 nm (see Fig. 5)
at a constant flow rate and furnace temperature and a pressure of 67 Pa.
An increase in the pressure to 530 Pa shifted the peak to 631.5 nm and
broadened it. Increasing the furnace temperature had a similar effect.
No resonance scattering was found with cesium vapor. Under some
conditions, a faint red glow possibly 'due to Cs2 was seen by eye.
Very strong nonpolarized white scattering was seen under all normal
conditions .
DISCUSSION
As mentioned in the introduction to this section the position of
•the plasma resonance may be calculated on the basis of a free electron
gas model for a spherical particle of a given size:
[l-3/2(wpi)2] (1)
14
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Rb
540
660
580 620
WAVELENGTH (NM)
Fig. 5 Scattered light spectra of K and Rb, unreduced data.
displaced vertically for clarity.
15
Curves
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where WR is the resonant frequency, and w is the plasma resonance
frequency for the bulk material. The relaxation time T is related to
the half-width of the observed resonance, AA, by I/WRT » AA/AR. Since
it may be argued that in a very small particle the mean free path of
a free electron will be limited primarily by the walls of the particle
and therefore:
T » R/VF (2)
where R is the radius of the particle and VF is the Fermi velocity of
the electrons. Eliminating T and WR from these equations gives:
R = 6lICVF/AAw2 (3)
Table 1 summarizes the observed resonant peak positions of the alkali
metals, and the expected wavelengths using Eq. 1 (AR-Calc). The second
column gives the experimentally measured plasma wavelengths for the ..
bulk metal, and column three is the calculated extinction peak resonance
of a spherical particle with a 2.5 run radius based on empirical
measurement of the dielectric constant as a function of wavelength
instead of the free-electron-gas model. Column five is the observed
resonance wavelengths and illustrates the breakdown of the free-
electron-gas model as the atomic number increases. Column six contains
observed widths of the resonances, and the last column indicates the
size of the particles as calculated from Eq. 2.
In the absence of an exact calcuation taking into account the effects
of damping, it is difficult to accurately determine the size of the
alkali metal spheres from scattering data. Radii given in Table I
may be considered to be upper limits since the effect of other
relaxation mechanisms would be to narrow AA and give an artifically
large R. The work of Doyle and Agarwal19 calculated the extinction
coefficient for particles from 2.5 to 40 nm radius. Their results
show that for less than 10 nm radius there is only a slight shift of
resonance peak position to longer wavelength but the peak narrows,
16
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RESONANCE PEAK WAVELENGTHS AND PARTICLE DIAMETERS
(nanometers)
Metal
Li
Na
K
Rb
Cs
XP
155
218
292
322
363
*R
(calc)
269
375
495
557
627
*ext
(calc)
• • •
370
545
625
665
Aobs
270
384
309
568
523
625
...
*Aobs
• • •
47
40
55
.. '•
R
• • •
6.5
11.5
10
Table 1. Summary of observed and calculated resonance peak wavelengths
and minimum particle diameters. Ap is the experimental bulk plasma
resonance wavelength, AR is the resonance wavelength for a 5-nm spherical
particle caclulated using Eq. 2, A^j. is the extinction maximum for
a 2.5 nm spherical particle (Ref. 14), Aobs is the observed resonance
maximum, AAobs is the observed resonance half-width, a:.d R is the
particle radius calculated from AAQbs using Eq. 3.
17
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as is expected from Eq. 3. Above 10 nm, the peak shifts more
rapidly to longer wavelength accompanied by increasing half-width.
No resonances which narrowed while remaining near the same wavelength
were observed, implying that the smallest observed particles were greater
than 5 nm. The prominent shifts of the resonances to longer wave-
lengths are interpretable as due to increasing particle size, while
the relatively narrow resonance width indicates the formation of particles
particles of uniform size.
18
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SECTION V
ELECTRON MICROSCOPY OF Zn PARTICLES
GENERAL
Attempts at electron microscopy of the alkali metal particles led to
consideration of other metals, as previous results indicated clearly
defined crystalline forms with many other metals. Pb, Zn, Cd, and
Mg have high enough vapor pressures to be used in the standard
furnace and of these Zn, Cd and Mg yielded electron micrographs
with clear crystalline shapes. Pb appeared to form small spheres
with low contrast. None of these metals exhibits a plasma resonance
frequency in the visibie region of the spectrum. Zn was chosen for
some extensive study and its plasma resonance was calculated to lie
in the near ultra-violet/ but has not yet been observed. Recent
data20 indicates that the resonance should occur in the vacuum UV at
about 130 nm.
EXPERIMENTAL
The furnace and vacuum apparatus used in these experiments was the same
as in the alkali metal experiments with the addition of a butterfly
valve to the main chamber to facilitate electron microscope samples
(see Fig. 6).
Sample collections were taken on a thin Parlodian plastic film supported
by standard electron microscopy 200-mesh copper grids of 3-mm diameter
(corresponding to ~ 0.1-mm squares). Grids were mounted on the tip
of a movable 3-mm stainless steel rod. The rod served as an adjustable
feedthrough to the outside of the system and was sealed with rubber
0-rings so that the rod and grid assembly could be slipped into and out
of the observation region without introducing an air leak into the
vacuum (Fig. 6). During a typical experiment of 2 or 3 h, up to 20
samples would be taken by using the side chamber as an air lock,
so that the main system was unaffected by changing grids. Collection
times were varied according to the position in the chamber and flow
19
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PUMP
SAMPLE
COLLECTOR
PUMP
LAMP
OBSERVATION
WINDOW
Fig. 6 Observation chamber and particle collecting apparatus
schematic.
20
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conditions. Most collection times were from 1-4 min with shorter times
at close distances to the nozzle. The mounted grid was maintained in
the side chamber with the valve closed until it was ready to be moved
into position for collection.
When the collection was completed, grids were taken to the electron
microscope and scanned at low magnification for evidence of deposit.
Higher magnifications (up to 50,000 X) were used for observation and
transmission photography. Some 150 samples were collected in this
study, and typically 3 or 4 photographs were taken to characterize
each sample. Reference 21 is a paper published on many of these
experiments.
RESULTS
Samples were obtained with variations in carrier gas flow rate, furnace
temperature, pressure, and position in the observation chamber. It was
hoped that a correlation of particle characteristics to one of these
variables would be evident. However, changing something like the
flow rate was found to modify all of the physical parameters of the
system (such as, supersaturation ratio, initial atom concentration,
etc.). Therefore, an exact prescription of the physical parameters
needed for control of nucleation was not developed. The critical
factor in determining the size of the particles was time, or the age
of the particles.
On the basis of frequency of occurrence, the most probable shape of
the observed Zn particles is described as a section of a regular
hexagonal prism. The particles had two stable orientations on the
parlodian plastic. These two orientations are designated the
"end-on" and "side" views corresponding to hexagonal and rough
rectangular silhouette, respectively. Figure 7 is an example of
some large particles P* 1200 nm diameter), two of which show a
layered rectangular side shape.
21
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Fig. 7 These particles are single Zn crystals about 1200 nm across.
The two hexagons represent "end on" views, while the other two have the
same shape but are rotated out the page by 90° to present a "side view
(6000 K).
22
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Electron diffraction on one single hexagon indicates that the central
axis of the hexagonal prism is perpendicular to a simple crystallo-
graphic plane. The hexagonal cross section occurs in all the samples
taken (excepting cases of contamination).
Other shapes also occur, such as triangles, irregular hexagons and
pentagons. The triangular shape is consistent with the hep structure
of bulk Zn; it commonly appears, but is always less abundant than
the hexagon. If one expects the particles to approach a spherical
shape, the triangular shape of viewed as a possible but less probable
configuration.
As the size of the particle increases, the sides of the hexagonal
prism usually becomes indented, which results in an hourglass- or
butterfly-shaped side view as in Figs. 7 and 8. Particles ranging
in size from 20 to 500 run appear to be layered.
In Fig. 9 are three-dimensional sketches with corresponding side
views, summarizing the various shapes and size groups of the observed
particles. The smaller particles were found at short times
(distance/flow rate) form the tip of the nozzle, typically within
1-3 cm, while the larger particles were found at greater distances.
The collected particles were generally very uniform, with sizes
varying by a factor of 3. Some pictures show two distinct size
populations separated by an order of magnitude. Eddy currents and
lack of laminar flow in the collection region could account for
this variation.
The largest sized particles (500 to 5000 nm diameters) were usually
identifiable as an agglomeration of smaller particles. However, it
was not always obvious whether a particle was a composite or a
single crystal. The striking effect with the larger particles
is that they usually retain the basic hexagonal prism shape.
23
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II *
i
a
Fig. 8 Indented or butterfly side views typical of medium sized
(100 nm diam.) hexagons (40,000 x).
24
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SIZE
RANGE
PROJECTION v
5-50 NM
DIAMETER
50 - 500 NM
DIAMETER
500-5000 NM
DIAMETER
(AGGLOMERATES)
UJ
o
i
ro
XT
UJ
UJ
UJ
o
i
Q
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Fig. 9 Summary of the three observed size categories of hexagonal-
plane particles and corresponding general shapes in three-dimensional,
end-on, and side projections.
25
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Figure 10 is a clear example of an agglomerated particle with hexagonal
shape made from smaller hexagonal prrtides. This phenomenon is an
example of agglomeration that occurs in the gas phase, since under
normal operating conditions there are some discrete particles
directly visible in the observation region.
The deleterious effect of 02 on the crystalline shape of the particles,
described previously by several investigators,6"® was confirmed in
this system by means of an air leak. Unless care was exercised in
the sample collection process, a small amount of air would yield
results similar to Fig. 11. A careful study of the effect of controlled
amounts of 02 has not been made, but the oxygen may coat the nucleated
particles epitaxially and thereby prevent further growth except by
agglomeration. This suggestion follows from the observation that light
scattered from the particles becomes highly polarized when 02 is
introduced into the collection region. Complete polarization at a 90°
scattering angle is associated with the Rayleigh scattering limit
of very small particles (small compared to wavelength). Also, isolated
particles were rarely observed in contaminated samples; however, when
they occurred they were always 5 nm or less in diameter (Fig. 11).
Diffraction patterns of such contaminated samples are similar to those
obtained with pure crystals, but detailed comparisons have not been
made.
DISCUSSION
The most probable shape of the observed Zn particles has been explored
in some detail. It basically consists of a section of a hexagonal prism,
but as illustrated in Fig. 9 the shape is a function of the absolute
size of the particle. In the growth or aging process, agglomeration
is a significant mechanism and it becomes more pronounced at higher
furnace temperatures.
Since a considerable amount of theoretical work is available on
26
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Fig. 10 Good example of a single large (400 nm) particle agglomerated
from (20 nm) hexagonal particles in the gas phase (40,000 x) .
27
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i V . »
t*
1.
"m «*,-:•-
. ' -.;: -' fc „ » , v. *,
»%»:•" **
- * .. " .
"'
Fig. 11 Q£ contaminated sample showing the pronounced tendency for
particles to clump together. The single particles are 5 nm or less
in diameter, while the large clumps are on the order of 500 nm across
C40,000 x).
28
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homogeneous nucleation, information concerning the nucleation of
these small crystals is of interest. From this point of view, further
studies with the electron microscope seem inadequate. The smallest
particles observed with our instrument were on the order of 5 run in
diameter Fig. 12 which corresponds to about 10,000 atoms by using the
density of bulk Zn. However, at this magnification, measuring the
diameter on the photograph only puts an upper limit to the actual
size. This is consistent with the specifications of our electron
microscope, which in principle can detect a particle of 1 nm diameter.
In order to adequately study the structure of a particle, its diameter
must exceed this minimum resolution by an order of magnitude. Figure
12 illustrates this conclusion; the smallest clearly shaped hexagons
are about 10 nm across. Electron diffraction analysis and other _
electron microscopy techniques can be applied to yield much more
information, but it would be very desirable to discover in situ
methods of analysis as well. Theoretical considerations give a
relation for the smallest size of stable particle in terms of the ratio
of the actual metal vapor pressure to the equilibrium vapor pressure
at the same temperature, Jcnown as the supersaturation satio. As the
supersaturation ratio increases the smallest size for a stable particle
decreases. The temperatures in the collection, chamber yield super-
saturation ratios of roughly 100-1000. These numbers predict a smallest
size of particle containing on the order of 10 atoms, which points
out the inadequacy of using the electron microscopy exclusively to
probe nucleation. Thus, results remain at a qualitative level. The
uniformity of sizes in a given sample encourages the idea that quanti-
tative measurements can be compared to theoretical calculations. Time
may be parametrized by distance in the flow system to allow a measurement
of rates of nucleation and growth.
29
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' *!• ~/Sl&S8&
• • •*.•
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SECTION VI
SMALL PARTICLE JOSEPHSON MAGNETOMETER
Use of electron tunneling effects in small particles at low temperature
was viewed as a possible new way to determine size and size distri-
bution. Measurements were made on thin film, stable arrays with large
numbers of small selenium particles produced by evaporating the metal
in an inert gas at low pressure. Particle sizes as revealed by electron
microscopy ranged from 10 to 1000 nm, but were usually uniform and
typically centered around 200 nm. These arrays showed potential for
investigation of phase coherence properties of large numbers of coupled
junctions and also for application as reliable, sensitive magnetometers.
Reference 22 contains many of the experimental details.
Samples with resistances less than 100,0000/square all had nonzero
critical currents below 3.77 K. The critical currents varied as
R~n where R is the resistance and n = 1.5 ± 0.2 with a 1 K /square
sample having a critical current of approximately 50 yA. The nonzero
voltage part of the I(V) curve could be hysteretic or not depending
on sample resistance, temperature, and particle size. For particles
formed in 100 v air, samples with resistances above 500 ft/square were
not hysteretic at any temperature (down to 1.07 K) for bias voltages
less than a few mV. Lower resistance samples would be nonhysteretic
near their transition temperature but become hysteretic as the tempera-
ture was lowered.
A simple model22 can be used to explain perhaps the most interesting
characteristic of these samples, their response to magnetic fields.
Figure 13(a) shows that the critical current is a multiply periodic
function of a magetic field applied perpendicular to the plane of the
array.
The critical current, IG/ was monitored by applying a d.c. bias current
just less than Ic and an a.c. modulation current large enough so
31
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-8
(b)
-4-20246
APPLIED MAGNETIC FIELD (MILLIGAUSS)
6
-2-10123'
APPLIED MAGNETIC FIELD (MILLIGAUSS)
Fig. 13a The magnetic field dependence of the critical current of a
small particle array 0.32 mm long x approximately 0.03 mm wide that was
cut with a scribe from a wider film of small particles. The amplitude
of the most rapid periodicity is approximately 0.4 yA.
Fig. 13b The magnetic field dependence of the critical current of two
coupled small particle arrays, each 0.32 mm long x approximately 0.05 mm
wide. The most rapid periodicity has an amplitude of 0.24 yA and a
magnetic flux period of 2.1 ± 0.2 x 10~7 g cm2.
32
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that Ic was exceeded during part of each a.c. cycle. The a.c.
voltage across the sample, which was measured with a lock-in
amplifier, is a non-linear measure of Ic. The non-linearity was
due to the fact that the dynamic resistance for I > Ic is a non-
linear function of I. The most rapid magentic field periodicity is
on the order of *Q/A where A is the area of the array and *Q = V2e
is one flux quantum, but many longer periodicities, up to over 100
$g/A for a typical sample, are present.
The probable explanation is that the many periodicities result from
quantum interference effects2 3 around the many loops formed by pairs
of current paths. The largest area loop, defined by the outermost
current paths, would be expected to be of the order of the sample area.
As an example, unambiguous measurements of the magnetic flux periodicity
could be made with a double junction geometry shown schematically in
the insert of Fig. 13(b). For these samples, a magnetic field period
$0/A. is the loop area, was observed superimposed on an envelope with
periodicities 24>0/A, where A is the area of the individual arrays.
(Analogous to a double slit diffraction pattern with finite slit width.)
Figure 13(b) shows experimental results for the best sample. Other
samples had the same general features, but the amplitude of the rapid
periodicity was smaller. Probably this was due to inequality in the
critical currents through the two arms. The mean flux period for
samples of this geometry was 2.0 X 10~7 G cm2 with a standard deviation
of 0.2 X 10~7 G cm2 («0 = 2.07 X 10"7 G cm2).
For use as magnetometers, the typical sensitivity of good samples was
a few parts in 10"5 G with a 1 sec time constant. Some samples had
sensitivities as great as a few parts in 10~6 G. One very small area
sample had a magnetic flux sensitivity of 1.5 X 10~4 4g. The sensi-
tivities obtained now, though not comparable to those of the best
superconducting devices,24* are adequate for a wide range of applications.
33
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The arrays have not degraded with room temperature storage and survive
frequent thermal cycling with no protection from condensed atmospheric
moisture.
In summary, we have produced and studied arrays containing "lO6 small,
weakly coupled particles each ^200 nm in diameter. The I(V) and IC(H)
characteristics can be qualitatively understood with a simple model
which simulate the array by a set of n current paths in parallel,
each with in Josephson weak links in series. The arrays show potential
as practical magnetometers in that they are easy to fabricate, operate
with d.c. electronics, and presently have sensitivities of a few parts
in 10~5 G.
34
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SECTION VII
OBSERVATIONS OF LAMINAR FLOW IN A GLASS TUBE
One of the unique characteristics of this system of nucleating
particles is that it occurs in a steady flow. Eddy currents in the
particle flow are readily observed in the apparatus described previously.
However, if these eddy currents could be eliminated then an obvious
step would be to look at light scattering from different points down-
stream. A glass tube (10 cm diameter) was devised for this purpose
to replace the observation chamber cross. Since there were no
irregularities in the wall surface, no eddy currents were formed and
the particle flow was unidirectional up the tube. However; deposition
by diffusion gradually clouded the walls. Further attempts added more
gas flows to keep the walls clean, but these resulted in unstable ~
oscillatory particle flows. Stable flows with unclouded walls never
exceeded more than about 30 minutes of experiment time. Data that
was obtained did not indicate a detectable consistent change as a function
of distance along the glass tube. Nucleation and growth seem to occur
close to the furnace exit, and in terms of the plasma-resonance are not
large effects further downstream. The glass tube apparatus also
allowed the possibility of 0-180° angular light scattering from the
particles. However, this experiment was never initiated due to the
large amount of computer and programming time required for data reduction
coupled with the small amount of expected information.
35
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SECTION VIII
"MATRIX" ISOLATION OF Na PARTICLES
The isolation and entrapment of small particles was pursued as a technique
to facilitate other experiments such as Raman laser scattering, NMR,
and ESR. This was accomplished by a cryogenic method of freezing the
particles and carrier gas to a solid. The particles and carrier
gas are exposed to a cold F* 20 K) copper plate. Gas coming in contact
with the plate will undergo a phase change to the solid state. The
metal particles originally in the gas will be entrapped in their
"matrix" of frozen gas. Since particle apparatus and the cryogenic
apparatus required greatly different gas pressures, a "pulsed" technique
was employed for matrix deposition. A large valve connecting the two
systems was opened for short periods of time f* 1 sec) with long intervals
in between to allow the cryogenic system to restabilize. Although
matrix deposition was observed, and particle entrapment was indicated
by weak plasma resonance light scattering, a good thick matrix was never
obtained. After just a few deposition pulses, no further deposition
occurred. Since the temperature of the cryotip copper plate continuously
rose during deposition it was concluded that the cryogenic apparatus
did not have sufficient power dissipation to adequately carry out such
experiments.
36
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SECTION IX
RAMAN SCATTERING
Initially, observation of Raman Scattering from particles illuminated
by an Argon ion laser beam (1 watt) was attempted in situ. No
Raman signals were observed and estimates of the particle density above
the furnace (1010 - 1013/cc) indicated that this would be a difficult
approach assuming that the Raman cross section of the particles
was comparable to that of N2 or 02 gas. Therefore further attempts
employed the matrix isolation technique of collecting the particles
in a larger concentration than the gas phase. Various lines of a 1
watt Argon ion laser were focused onto the matrix of particles, and
a scanning double monochromator with a low-noise phototube was employed
for detection but no lines attributable to the particles were observed.
37
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SECTION X
SUMMARY
Studies performed under this grant strongly indicate that plasma
resonance light scattering may be a significant tool for obtaining
data on the subject of homogeneous nucleation. Plasma resonance peaks
and widths for alkali metal particles have been observed to change
systematically with system parameters; and electron microscopy of Zn
and other metal particles has revealed sizes and characteristic
geometric shapes. Correlation of these observations may result in
determination of size and/or shape of small particles by careful
measurement of the position and width of the plasma resonance. This,
in turn, would directly provide information to compute nucleation and -
growth rates in a steady-state system. Information on particle
formation obtained for metals with a plasma resonance could be
generalized to other substances.
38
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SECTION XI
REFERENCES
1. Report of the National Bureau of Standards Study Group on Air
Pollution, Measures of Air Quality Program (National1Bureau of
Standards, 1970).
2. Project Clean Air Task Force Assessments (University of California
1970}.
3. Air Quality Criteria for Particulate Matter, National Air Pollution
Control Administration Publication AP-49 (1969). Over 300
references listed here.
4. Air Pollution Engineering Manual, Public Health Service Publication
No. 999-AP-40 (1967).
5. Control Techniques for Particulate Air Pollutants, National Air
Pollution Control Administration Publication No. AP-51.
6. L. Silverman, Air Repair £, 189 (1955).
7. E.R. Frank and J.P. Lodge, J. Microscopic 4_, 449 (1967).
8. T.A. Rich, L.W. Pollak and A.L. Metnieks, Geofis. Pura. Appl. 44,
233 (1959).
9. W.J. Megaw, J. Rech. Atmopheriques £, 53 (1966).
10. C. Orr, F.K. Kurd, W.P. Hendrix and C.E. Junge, J. Meteorol. 15,
240 (1958).
11. Task Group on Lung Dynamics, Health Physics 12, 173 (1966).
12. West, J.B., Bradford, R.S. Jr., Eversole, J.D., and Jones, C.R.
Rev. Sci. Instr. 46, 164 C1975).
13. S. Yamaguchi, j. Phys. Soc. Japan 15_, 1577 (1960).
14. D.P. Gilra, Collective Excitations in Small Particles and Astrono-
mical Applications, Ph.D. Thesis, University of Wisconsin 1972.
15. W.T. Doyle, Phys. Rev. Ill, 1067 (1958).
16. M. Kerker, The Scattering of Light and Other Electromagnetic Radiation
(Academic, New York, 1969).
17. H.C. Van de Hulst, Light Scattering by Small Particles (Wiley, New
York, 1957).
18. D.M. Mann and H.P. Broida, J. Appl. Phys. 44, 4950 ''973).
39
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19. Doyle, W.T. and Agarwal, A., J. Opt. Soc. Am._55_, 305 (1965).
20. Hosteller, L.P. Jr., and Wooten, F.f Phys. Rev. 171, 743 (1965).
21. Eversole, J.D. and Broida, H.P. J. Appl. Phys. 45, 596 (1974).
22. Hansma, P.K., Sol. State Comm. JL3_, 397 (1973).
23. References and a comprehessible summary are given by
Mercereau, J.E., Superconductivity (ed. by Parks, R.D.) Marcel
Dekker, New York (1969) .
24. An excellent summary of work on these devices is contained in
several articles in the special issue of Proc. IEEE on Cryogenics,
published January 1973.
40
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SECTION XII
LIST OF PUBLICATIONS
1. D.M. Mann and H.P. Broida, J, Appl. Phys 44^ 4950 (1973).
2. P.K. Hansma, Sol. State Comm. 13, 397 (1973).
3. J.D. Eversole and H.P. Broida, J. Appl. Phys. 45, 596 (1974)
41
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
REPORT NO
EPA-650/3-75-006
3 RECIPIENT'S ACCESSIOr»NO
TITLE AND SUBTITLE Studies of Small Metallic Particles
Formed by Homogeneous Nucleation — Light Scattering
and Electron Microscopy
5 REPORT DATE
FEBRUARY. 1975
6 PERFORMING ORGANIZATION CODE
AUTHOR(S)
8 PERFORMING ORGANIZATION REPORT NO
Jay D. Eversole, Paul K. Hansma and H. P. Broida
PERFORMING ORGANIZATION NAME AND ADDRESS
Department of Physics
University of California
Santa Barbara, California 93106
10 PROGRAM ELEMENT NO
1AA008
TT CONTRACT/GRANT NO
Grant No. R-800845
2 SPONSORING AGENCY NAME AND ADDRESS
Office of Research and Monitoring
U. S. Environmental Protection Agency
Washington, D. C. 20460
13 TYPE OF REPORT AND PERIOD COVERED
Final Report, 3/72 - 2/75
14 SPONSORING AGENCY CODE
5 SUPPLEMENTARY NOTES
6 ABSTRACT
This report describes basic research on the subject of formation and detection of
metallic particles ranging from 5 to 5000 nm in diameter. Particulate matter is
formed in a flowing inert gas by homogeneous nucleation from the vapor phase.
Metals used in the work done under this grant include Na, K, Li, Rb, Cs, Cd, Mg,
Zn, and Pb. Specific objectives of this project are necessarily broad and open-
ended as present understanding of particle formation and growth mechanism is still
in an early state of development. Particulate matter with diameters less than
100 nm plays an important role in atmospheric pollution, and it was hoped that
research would contribute to the detection, measurement, and/or control of this
problem.
Observations of plasma-resonance scattering of white light from alkali metal
particles, the growth of Zn particles observed by electron microscopy, and the
development of a superconducting device: a small-particle Josephson magnetometer,
are significant results of this project which have been published. Attempts to
observe the Raman scattering of laser light from Zn, Mg, and Cd particles, to
isolate macroscopic quantities of particles from the flow system, and to obtain
information from the 0 to 180 degrees angular light scattering were not totally
successful. —
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b IDENTIFIERS/OPEN ENDED TERMS C COSATI I Ickl/Group
particulate
particle
nulceation
plasma-resonance
sodium (Na); potassium (K); zinc (Zn)
13 DISTRIBUTION STATEMENT
Release Unlimited
19 SECURITY CLASS (This Report)
!1 NO OF PAGES
20 SECURITY CLASS (Thispage)
Unclassified
22 PRICE
EPA Form 2220-1 (9-73)
42
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