A small object is coated by holding it in the pressure well of an acoustic standing wave pattern, and then applying a mist of liquid coating material at low velocity into the pressure well. The pressure gradient within the well forces the mist particles to be pushed against the object. A lower frequency acoustic wave also can be applied to the coated object, to vibrate it so as to evenly distribute the coated material. The same lower frequency vibrations can be applied to an object in the shape of a hollow sphere, to center the inner and outer surfaces of the sphere while it remains suspended

Lee, M. C.

English

NASA Technical Reports Server

United States Patent [19] 
Lee 
OTHER PUBLICATIONS 
J. E. Shelbv, “Development of Glasses for application 
[54] CONTACTLESS PELLET FABRICATION 
[75] Inventor: Mark C. Lee, La Canada, Calif. 
[73] Assignee: The United States of America as as Laser F k o n  Targeis”, Sandia Laboratories, SAND 
76-8325, Jan. 1977. 
J. P. Reedy, “Selection and Measurement of Micros- 
peres Laser Targets Based on Refraction”, Journal of 
Applied Physics, v. 47, No. 61, Jun. 1976, p. 2503. 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 314,702 
[22] Filed: Oct. 26, 1981 
[51] 
1521 
Int. Cl.3 .......................... B05D 3/00; B05D 1/00 
U.S. C1. ........................................ 427/57; 65/213; 
427/6;427/346;427/421;427/426; 118/50; 
1 18/50.1; 1 18/57; 1 18/62; 1 18/30Q 1 18/500 
Field of Search ..................... 427/6, 57, 346, 421, 
427/426; 118/50, 50.1, 57, 62, 300, 500 
[58] 
~561 References Cited 
U.S. PATENT DOCUMENTS 
2,833,241 5/1958 Crowley et al. ...................... 118/62 
3,365,315 1/1968 Beck et al. .......................... 65/22 X 
3,640,767 2/1972 Fulton et al. 
3,882,732 5/1975 Fletcher et a1 
4,017,290 4/1977 Budrick et al. ...................... 65/21.4 
4,052,181 10/1977 Fletcher .................................... 65/2 
4,218.921 8/1980 Oran et al. 308/10 
4,257,799 3/1981 Rosencwaig et al. .............. 65/22 X 
4,290,384 9/1981 Aussehnitt ............................ 427/57 
Primary Examiner-Shrive P. Beck 
Attorney, Agent, or Firm-John R. Manning; Paul F. 
McCaul; Thomas H. Jones 
ABSTRACT 
A small object (12, FIG. 2) is coated by holding it in the 
pressure well (20) of an acoustic standing wave pattern, 
and then applying a mist of liquid coating material (42) 
at low velocity into the pressure well. The pressure 
gradient within the well forces the mist particles to be 
pushed against the object. A lower frequency acoustic 
wave (for oscillator 28, FIG. 4) also can be applied to 
the coated object, to vibrate it so as to evenly distribute 
the coated material. The same lower frequency vibra- 
tions can be applied to an object in the shape of a hollow 
sphere, to center the inner and outer surfaces of the 
sphere while it remains suspended. 
6 Claims, 7 Drawing Figures 
34 32 36 30 38 
https://ntrs.nasa.gov/search.jsp?R=19840008872 2020-03-22T08:02:28+00:00Z
U,S. Patent Jan. 18, 1984 9 
HEAT 
VIBRATE MOLTEN 
HOLLOW S P H E R E  
2 
4,425,376 
1 
CONTACTLESS PELLET FABRICATION 
ORIGIN O F  THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(42 Stat. 435; 42 USC 2457). 
BACKGROUND O F  THE INVENTION 
Certain applications require the even coating of small 
objects or pellets. For example, inertial confinement 
fusion reactors require hollow spheres, containing hy- 
drogen isotopes, and evenly coated with many layers of 
ablative materials. The object cannot be held by another 
solid object during coating or else there will be a defec- 
tive spot in the coating. Contactless fabrication can be 
accomplished by applying a coating while the object is 
in free fall, but it is then difficult to apply a coating to 
the moving object, and there is some unevenness in the 
coating due to the wind passing the dropping object 
when there is air in the drop tower. It is also difficult to 
evenly coat a small object and with a controlled amount 
of coating material, since spraying of many droplets 
against the object may result in loss of an unknown 
amount of material, and the force of a spray may knock 
the object out of position. A technique which facilitated 
the precise coating of a small object, as well as other 
fabricating techniques, would be of considerable value. 
SUMMARY O F  THE INVENTION 
In accordance with one embodiment of the present 
invention, a process is provided for fabricating a small 
object without contacting it. Coating of the object can 
be accomplished by first acoustically suspending the 
object by applying resonant acoustic energy to a gas- 
filled area to hold the object within a pressure well of a 
standing wave pattern. A multiplicity of fine quantities 
of coating material, or mist thereof, can be applied at 
low velocity to the pressure well. The pressure gradient 
within the well then forces the coating material against 
the suspended object. 
The coating material can be more evenly distributed 
about the object, by applying a low frequency acoustic 
wave that oscillates the coating material to more evenly 
distribute the coating thereon. Where the object is a 
hollow sphere, such resonant oscillations also can be 
utilized in the formation of the hollow sphere while it is 
molten, to position its inner and outer spherical surfaces 
concentric to one another. 
The novel features of the invention are set forth with 
particularity in the appended claims. The invention will 
be best understood from the following description when 
read in conjunction with the accompanying drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a sectional side view showing a coating 
FIG. 2 is an enlarged view of a portion of FIG. 1. 
FIG. 3 is shows one step in the production of a hol- 
FIG. 4 shows another step in the production of the 
FIG. 5 shows another step in the production of the 
process of the present invention. 
low sphere utilizing the method of the invention. 
hollow sphere, following the step of FIG. 3. 
hollow sphere, following the step of FIG. 4. 
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2 
FIG. 6 is an enlarged sectional view of another 
FIG. 7 is a perspective view of another fabricating 
method of the invention. 
apparatus and method of the invention. 
DESCRIPTION O F  THE PREFERRED 
EMBODIMENTS 
FIGS. 1 and 2 illustrate a system 10 which can be 
used to acoustically levitate a small object 12. The sys- 
tem includes a large dish 14 with transducers 16 
thereon, and a reflector 18 positioned near the focus of 
the dish, with air or other gaseous medium between 
them. The system produces a converging acoustic field 
with a pressure well at 20 which is located slightly 
beyond the focal point 22 of the dish 14. A large static 
pressure gradient lies around the pressure well 20 with 
the pressure increasing in every direction away from 
the center 23 of the well, and the object 12 is held close 
to the center of the pressure well. Where an acoustic 
wave of high intensity is propagated, the effect of grav- 
ity is to lower the object 12 only slightly. Where the 
object 12 is a hollow glass sphere, it lies substantially at 
the center of the pressure well. Such hollow spheres, 
with pressured hydrogen gas isotopes therein and a 
coating of an ablating material, are useful as targets in 
inertial confinement fusion reactors. 
FIGS. 3-5 illustrate some of the steps in the fabrica- 
tion of a fusion reactor target which includes a coated 
hollow sphere. A glass frit, such as of Pyrex containing 
a blowing agent, is placed in the pressure well of a 
levitating system of the type shown in FIG. 1. Heat is 
applied, as by a laser beam indicated by the arrow 24, 
(FIG. 3) to blow the frit into a small hollow body. FIG. 
4 shows a next step in the process, wherein the acousti- 
cally levitated hollow body at 12b is vibrated to form it 
into an accurately shaped hollow sphere with concen- 
tric inner and outer surfaces. In a system of the type 
shown in FIG. 1, with a dish 14 of about 12 inches 
diameter, a levitating frequency of about 100 kHz may 
be utilized (although there are several resonant frequen- 
cies of that order of magnitude that can be used). A 
small hollow glass sphere of a diameter on the order of 
a millimeter, may have a natural frequency of oscilla- 
tion on the order of 100 Hz. By applying both a relative 
low frequency such as 100 Hz to vibrate the levitated 
object at 12b as well as the much higher levitating fre- 
quency, the object can be vibrated without contacting 
it. 
In FIG. 4, a levitation oscillator 26 is used to apply a 
high levitating frequency such as 100 kHz to the trans- 
ducers on the dish 14. In addition, a vibration oscillator 
28 is used, which provides a much lower frequency 
such as 100 Hz to the same transducers. The low fre- 
quency from the oscillator 28 is applied while the object 
at 12b is still molten, but is decreasing in temperature 
towards a hardened state. The resulting hollow sphere 
at 126 tends to have spherical surfaces regardless of the 
vibrations, but the vibrations serve to cause the inner 
and outer spherical surfaces to lie concentric to one 
another, to produce a hollow sphere with substantially 
uniform wall thickness. The fact that the vibrations are 
performed on the molten sphere without a wind blow- 
ing thereby (as can occur when the object is vibrated 
during its fall in an air-filled drop tower) helps to avoid 
deformities from the desired spherical configuration. 
The hollow sphere can be filled with pressured hy- 
drogen gas by a variety of techniques, such as by plac- 
ing the hardened sphere in a pressure chamber contain- 
4,425,376 
3 4 
ing the hydrogen gas isotopes, for a few days until the form a chamber of square crw, c-ection, and three trans- 
hydrogen permeates the sphere. ducers 52, 54, and 56 are used to generate acoustic 
After the sphere has been formed, it must be coated waves resonant to the length L, width W and height H 
with a thin layer of ablative material such as a metal of of the chamber. Each of the transducers such as 52 is 
moderate melting temperature. This can be accom- 5 shown connected to the chamber through a tube 58 o f a  
plished, as shown in FIG. 5, by holding the sphere at length equal to one-half of the wavelength of sound 
in acoustic suspension, while a mist of the molten transmitted therethrough, to facilitate coupling. In FIG. 
coating material is applied to the sphere. This technique 7, a coating is shown being applied to a hollow sphere 
is shown in greater detail in FIGS. 1 and 2, which show 12, by a stepped horn atomizer 30 of the type shown in 
that the coating material is applied by the use of a 10 FIG. 1. 
stepped horn atomizer 30. The atomizer 30 includes a Thus, the invention provides a system for processing 
pair of piezoelectric transducers 32 driven at 20 kHz, a small objects such as hollow spheres, while avoiding 
backup rod 34 behind the transducers, a forward rod 36 contact therewith during much of the processing. The 
in front of the transducers, and a stepped horn 38 in coating of the sphere can be accomplished by acousti- 
front of the forward rod. A small quantity 42 (FIG. 2) 15 cally levitating the sphere in a pressure well, and apply- 
Of a coating ing a mist of coating material to the pressure well, so 
40 of the rod, with the horn and coating materia1 at a that the pressure gradient presses the mist of coating 
temperature above the point Of the coating material against the object. The object can be vibrated, 
can be applied to the extreme 
material. The transducers 32 are driven through an by applying soundwaves of a much lower frequency 
oscillator, which causes breakup of the drop 42 into a 20 than used for levitation and which is resonant to the 
The mist includes very fine droplets of diameters on the coating. Such vibrations are also useful while a hollow order of one micron, with a volume that is a plurality of sphere is still in a molten state, to help center the inner 
ble distance. The transducers of the atomizer are driven 
after the tip 40 has been placed substantially within the 
pressure well around the sphere 12. The pressure gradi- 
ent in the well forces the mist of coating material 
towards the center of the pressure well, where the 30 
sphere 12 is located, to thereby coat the sphere. 
After the sphere has been coated, and at a time when 
the coating material is flowable, as at a time soon after 
the laser heat beam 24 was applied, low frequency (e.g. 
mist which emerges in front Of the tip 40. object, to object vibrations that help distribute the 
Orders Of magnitude less than that Of the sphere, so the 
drop1ets move at high and outer surfaces of the sphere while the object is out no for any apprecia- 25 of contact with any solid and there is 
wind on the object. 
Although particular embodiments of the invention 
have been described and illustrated herein, it is recog- 
nized that modifications and variations may readily 
occur to those skilled in the art and consequently, it is 
intended that the claims be interpreted to cover such 
modifications and 
What is 'Iaimed 
100 Hz) acoustic energy is applied to them. This pro- 35 
duces vibrations of the coating material to more evenly 
1. A process for coating an object comprising: 
applying F3onant acoustic energy to a gas-fi11ed area, 
that establishes a standing wave pattern with at 
least one pressure well at which there is a pressure 
gradient with pressure increasing in every direc- 
tion away from the center Of the pressure 
distribute it. A low frequency is chosen which produces 
such vibrations of the coating material. The frequency, 
which depends on the density, surface tension, and total 
mass of the coating material can be determined by ob- 40 
serving it through a microscope while varying the low 
frequency until coating material oscillations are ob- 
served. When the coating material has cooled, the hol- 
low sphere can be tested to determine whether it is 
ready for use as a target in an inertial confinement fu- 45 
sion reactor. 
A variety of acoustic suspension systems can be uti- 
lized to acoustically levitate the sphere or other object 
during processing. The system of FIG. 1 can be modi- 
fied by using a long trough instead of the dish 14, and 50 
moving the object along the trough by moving the 
reflector 18, to bring the sphere to various processing 
stations above the trough. 
While many coating materials can be easily formed 
into a mist, it is often difficult to do so with some metals. 5 5  
A quantity of such metal or other coating material such 
as is shown at 60 in FIG. 6, of moderately high melting 
temperature, can be applied by holding it in a vacuum 
chuck 62. The chuck is moved to bring the material at 
60a against the acoustically levitated sphere 64, and the 60 
coating material is melted while it is in contact with the 
sphere as by applying a laser beam 66. While the coating 
material is still molten, a low oscillation frequency reso- 
nant to the coating, scch as of an order of magnitude of 
100 Hz, is applied to vibrate the coating material to 65 
evenly distribute it on the surface of the sphere. 
It is also possible to use a substantially closed cham- 
ber of the type shown at 50 in FIG. 7, wherein the walls 
suspending an object in said Pressure well; and 
applying a coating to said object, including introduc- 
ing a multiplicity of fine quantities of coating mate- 
rial to a region containing said object, whereby to 
allow the Pressure around the we*1 to press the fine 
coating material quantities against the object. 
2. The process described in claim 1 wherein: 
said Step of applying a coating includes applying a 
controlled amount of a liquid coating material to 
the tip of an atomizer device, placing said tip 
within said pressure well, and atomizing said coat- 
ing material. 
3. The process described in claim 1 including: 
applying additional acoustic energy to said suspended 
object, after said coating has been applied and 
while said coating is in a liquid state, of a frequency 
which is lower than said resonant frequency and 
which creates oscillations of said coating material, 
whereby to evenly distribute said coating. 
4. A process for coating an object comprising: 
applying resonant acoustic energy to a gas-filled area, 
that establishes a standing wave pattern with at 
least one pressure well at which there is a pressure 
gradient with pressure increasing in every direc- 
tion away from the center of the pressure well; 
suspending an object in said pressure well; 
applying a quantity of solid coating material to said 
object; and 
4,425,376 
5 6 
applying heat to said solid coating material while it 
lies on said suspended object, to melt the coating 
6.  Apparatus for coating an object, comprising: 
an acoustic levitator which includes at least two walls 
lying opposite one another, and means for estab- 
lishing an acoustic standing wave pattern with a 
pressure well, between the walls; and 
means having a tip, for forming a drop of liquid at the 
tip into a mist at a location directly in front of said 
tip, said means being positionable with said tip 
between said walls and in substantially said pres- 
sure well to apply the mist to said pressure well. 
material. 
5. The process described in claim 4 including: 
applying acoustic energy of a second frequency much 
lower than the frequency of said energy that estab- 
lishes a standing wave pattern, and which produces 
oscillations of said coating material, to said object 
with said coating material thereon while said mate- io 
rial is molten. * * * * *  
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Contactless pellet fabrication

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