EPA-650/3-75-006

February 1975
Ecological Research Series
 STUDIES OF
FORMED BY HOMOGENEOUS NUCLEATION:
                    LIGHT SCATTERING
           AND ELECTRON MICROSCOPY
                               s?
                          SB
          UJ
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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-
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          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
       UJ
  O
       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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