We are developing a hard x-ray microscope for direct observation of solidification dynamics in metal alloys and metal matrix composites. The Fein-Focus Inc. x-ray source was delivered in September and found to perform better than expected. Confirmed resolution of better than 2 micrometers was obtained and magnifications up to 800X were measured. Nickel beads of 30 micrometer diameter were easily detected through 6mm of aluminum. X-ray metallography was performed on several specimens showing high resolution and clear definition of 3-dimensional structures. Prototype furnace installed and tested

Kaukler, William F.

Rosenberger, Franz E.

English

NASA Technical Reports Server

NASA-CR-19T884
2 ND SEMI-ANNUAL PROGRESS REPORT
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NASA CONTRACT NAS8-40185
X-RAY TRANSMISSION MICROSCOPE DEVELOPMENT
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Period of Perfromance
03/01/94 - 02/28/95
Principle Investigator
WILLIAM F. KAUKLER
Co-Investigator
FRANZ E. ROSENBERGER
Center for Microgravity and Materials Research
The University of Alabama in Huntsville
Huntsville, AL 35899
https://ntrs.nasa.gov/search.jsp?R=19950016599 2020-06-16T08:16:04+00:00Z
X-Ray Microscope for Solidification Studies
Introduction:
This report covers the second 6 month period for the year March 1, 1994 to
February 28, 1995. The material outlined in this semi-annual report continues from the
previous semi-annual report.
The Fein Focus Inc. x-ray source was delivered in September and coincides
with the beginning of the second 6 month effort. As a result, and as outlined in the
statement of work, this period was dedicated to the evaluation, testing and calibration
of the x-ray source.
In addition, in this period the modeling effort was continued and extended by
the Tiger series of Monte-Carlo simulation programs for photon and electron
interactions with materials obtained from the Oak Ridge RISC Library. Some further
calculations were also made with the absorption model.
Preliminary X-Ray Source Evaluation:
In a word, the performance of the x-ray source was superb. After installation
and some fine tuning, the device was shown to be an excellent performer that in some
ways exceeded advertised specifications.
Using the supplied tri-focus x-ray image intensifier, and the borrowed gold line
resolution grid, it was ultimately possible to resolve the 2 micron lines on the grid in
real time. It became clear that to achieve the highest resolutions possible from this
instrument, limited by the intensifier, one needed to maintain the specimen placement
within the first 4 to 5 mm nearest the source. The magnifications exceeded 500 X
when specimens were placed with the nearest mm to the target. The grid was
radiographed on Dupont 55 film and showed clearly the 2 micrometer lines. The
clarity was so good that it can be assumed the resolution approaches 1 micrometer or
better (with film). What was unexpected was that this resolution was available at the
maximum power the machine would operate with. The input acceleration of 50 kV and
400 microamps offered 20 watts of input power. Due to internal computer imposed
limitations, the power level drops off on either side of this point. The maximum power
input was determined and is plotted in Figure 1. At this time, the reason for power
drop off below 50 kV is not known. Above 50 kV, the 5 micrometer thick W target is
protected by cutting the current as acceleration increases.
From prior communications with Fein Focus Inc. the power density of electrons
onto the target was ostensibly determined by the limitations of damaging the W target
given a limit to the removal of excess heat. The curves for the other machines they
build approximately showed a linear relation with power level in watts and the same
figure for the minimum focal spot size in micrometers (i.e. a 10 micron spot could
tolerate 10 watts of power, and a 1 micron spot only 1 watt of power maximum). For
the performance levels of resolution desired (sub-micron spot size), a low x-ray power
would be expected. It seems that the electron optics of this High Definition tube
significantly reduce the electron flux to the focus spot during the beam conditioning
process. As a consequence,it hasbeenobservedthat after prolonged operation (1
hour) at maximum power, the outer target surface was merely warm to the touch.
Compare this to the uncomfortably high heat emanating from the other transmission
tube target used for the feasibility studies not even operating at full power. This
evidence supports the deduction that the power of the electron beam reaching the target
is far less than that being generated at the filament.
POWER
W
Figure 1.
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ACCELERATION kV
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Fein Focus Inc. High Definition tube maximum power vs. electron
acceleration over the full acceleration range.
Further experimenting showed that for real-time observation of specimens,
acceleration voltages of 45 or at least 40 kV were needed. If the cooled CCD camera
were used, the images could be detected down to 20 kV with significant increases in
exposure time (several minutes). The low power output of this tube also limits
observation in real time to thin metal specimens. The denser and thicker specimens
require the higher voltages such as 80 or more kV for ease of observation.
A variety of specimen types were examined with interesting results. For
example, real-time observation through 1 mm of tin or HgCdTe was not possible. Thin
chips of BiSrCuO high Tc superconducting material were equally difficult to image.
Thin sheet metal boxes made from steel, copper, brass or nickel did not prevent
detection of the contents. Nickel beads of 30-100 micrometer diameter were easily
imaged in real time and could be detected through 6 mm of AI. This showed the high
contrast was available when such beads will be used for particle pushing experiments.
Five mm diameter specimens grown in the ADSF furnace were examined. Iron and
steel specimens did not reveal any interesting structures. However, old specimens of
Al-ln-Sn alloys were proving very interesting and the x-ray microscope views provided
tar more information than any metallography could ever have permitted. It seems the
monotectic metal alloys, particularly Al-ln and AI-Pb offer the most dramatic and
informative microstructures.
Real-timeobservation of materials is truly informative. However, the best
images were obtained when the cooled CCD camera was used where longer exposures,
more gray level resolution and higher camera resolution produced such results that the
x-ray intensifier was the limiting link in the imaging chain. The higher resolution and
contrast offered by film radiography was explored with exemplary results. The near
infinite depth of field the x-ray source offers permits views of materials few people
have seen. The one micrometer (at the film) resolution of the radiographic film does
not limit the image quality of any specimen in this instrument. Images as clear as those
obtained from contact radiographic methods were obtained. Short exposures of a
minute or so yielded the best results. However, good radiographs were taken using 15
or more minutes of exposure. However, the mechanical stability of the film, the
specimen and the source sometimes was inferior to the source resolution and blurring
Occurs.
Measured Characteristics of Fein Focus Inc. High Definition Tube
Part of the characterization of the instrument carried out was the measurement
of certain important variables that were not supplied by the Fein Focus Inc. people.
Examples are output fluxes, magnification range, resolutions, power level and more as
a function of acceleration voltage, current, and specimen to detector geometry.
A set of measurements were made to determine the magnification available.
The measurements are compiled in a graph where the source and intensifier positions
are fixed (as they always are). The calibrated wires and bars used to make the
measurements were extremely precise and errors of a thousandth of an inch are the
most one will see. The magnification as a function of distance from the target exterior
onto the intensifier is plotted in Fig. 2.
Magnification
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Figure 2.
Eq
Gap inmm
Magnification vs. distance from target surface.
As one can see, the significant magnificationcan only be achievedwithin the
first 5 mm (1/4 of an inch) from thetarget. This is for thefixed distancefrom target to
intensifier tube of 21 inches. The measurementscould not be madehigher than the
graph showsalthough 800 X was achievedby another means. Shimmingcalibrated
wires to thetargetsurfaceprovidedtheplot in Figure 3. Thehigher magnificationscan
be seen. The diameterof the wires themselveswere in all casesaccountedfor in the
distancecalculations.
An importantconsequenceis the high significanceof the relative depth of the
featureswithin the specimenwhen the specimenis in this 5 mm zone. A 1 mm
specimen,suchas the kind anticipatedfor solidification studiescan have a features
displayedseveralhundredtimesdifferent in magnification. This will not be as greata
problem in the furnace since the furnace walls will imposea gap of at least5 ram,
some magnification difference will occur but not as unmanageablyas when the
specimenis magnified400 X.
80O
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Magnification - ]
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200
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Gap in inches
Figure 3. Magnification measured near the target surface.
Figure 4 is shown to compare the linearity of the measured magnification compared to
a calculated one. Multiple measurements do show some scatter, but the line is near
straight.
measured
mag.
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600
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Figure 4. Measured magnification plotted against calculated magnification using
similar triangles geometry.
The specimen table between the tube and intensifier can be moved down to
provide 10 inches of workspace. The motors for this table are expensive and precise
instruments. Each motor, although not set up for this, has installed an encoder which,
if properly connected can be read and accurate table position obtained. The CNC
option which would do this was not purchased but the encoders only need electronics to
read them out. The joystick positioning control works effortlessly. However, at the
high magnifications obtainable with the High Definition tube, the rate of minimum
motion is still to high and positioning of a feature in the viewing screen is difficult.
Some means of slowing this motion (in x and y only) is needed even at the expense of
not being able to go as fast at the highest velocities.
A measure of the flux vs. input current was made for 30 and 50 kV using the
ionization survey meter. The plots shown in Fig. 5 and 6 show a near linear
dependence of output with current in the low ranges.
Flux in
R/hr
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Figure 5. Measured flux vs. input current at 30 kV.
Flux in
R/hr
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Input Current uA
Figure 6. Measured flux vs. input current at 50 kV. The curve is caused by
the meter becoming saturated at 6.5 R/hr.
Prototype Furnace Installation
The effort for the last two months has been to install the prototype x-ray
furnace. The second year work will involve developing a better furnace, but at this
date, it is already known that certain features are needed for any furnace. To this end,
the work has been to install into the x-ray cabinet, the appropriate feed throughs for
power, cooling water and sensors. The furnace will be modular and all the flexible
fittings will have quick disconnects. To prevent serious damage from free water within
the cabinet, the x-ray intensifier tube has to be covered with a watertight seal that will
not attenuate the x-ray signal. This was installed as a double hoop arrangement around
the intensifier opening to the shielded cabinet. The hoops hold thin plastic film across
the intensifier tube face and thus prevent water from leaking out. High current (100A)
feedthoughs were installed in a film access panel in the side of the shielded cabinet. A
machined plate was prepared that holds the feedthroughs as well as the quick water
disconnects. The original plan to use tap water for furnace cooling was abandoned.
Circulated, temperature controlled water will be used instead. Care was required to
ensure that the feedthroughs did not interfere with the outer cabinet walls. In addition,
the feedthrough area was shielded with an additional 1/8 inch lead sheet with some
smaller pieces fitted around the feedthroughs themselves.
The plan then is to permit fitting of the furnace within the cabinet easily and
safely. The furnace will be mounted to the
that scanning of the specimen and changing
and precisely. Limit switches will be needed
motorized platform inside the cabinet so
magnification can be performed remotely
to prevent pushing the furnace into the x-
ray tube although this feature may already be installed and simply need readjusting. At
a later time, gas fittings will be needed for the higher temperatures and alloys that
oxidize readily. The thinner wires for thermocouples and translator motor will be
passed into the cabinet from the rear access panel. The thin wall structure of the x-ray
windows of the furnace will not permit vacuum operation unless the source is sealed
into the furnace housing. Testing of furnace designs may be needed in the future and
the use of the quick disconnects will make the experiments simpler to perform.
Other Concerns
Detailed information about the High Definition tube is slowly coming from Fein
Focus Inc. They supplied a 3 micrometer thick target to see if definition and flux
levels are improved. Preliminary results are that definition is higher but flux is not
significantly so.
A number of questions about the operation of the tube are not satisfactorily
answered. Why are the filaments are burning up so quickly? Why is filament life
indicated at levels of 35% for a new unit? Why is a good vacuum 140% and what does
it mean in real numbers? Where do all the electrons emitted by the filament that dont
hit the target go? How do you focus the beam at accelerations below which the
intensifier can operate? Why does centering the beam alter the optimum focus? Why
does the beam need to be centered so often?
REPORT DOCUMENTA I IUN I-'ABI- IOUSNo.07040,.
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Oavrs H)qtw_a¥, 5ude 1204 Arhnqtc-n. VA 2220-4302. and to the Office of Mana_lement and Oudqet. Pa_ Reduc_on Prolect {0704-0138). Wasn_on OC 20503
1. AGENCY USE ONLY (lea_ 0tanK) 12. REPORT DATE REPORT TYPE AND DATES COVERED
I Ol March 1995 3/i/95
4. TITLE AND SUBTITLE
X-ray Transmission Microscope Development
6. AUTHOR(S)
William F. Kaukler, Franz E. Rosenberger
7. PERFORMING ORGANIZATIONNAMEiS) AND ADDRESS(ES)
Center for Microgravity and Materials Research
The University of Alabama in Huntsville
Huntsville, Alabama 35899
§.SPONSORING/MONITORING AGENCYNAME(S) ANDADDRESS(ES)
NASA/MSFC Procurement Office
AP 37-4
3. Semi-Annual Interim 9/I/94 -
5. FUNDING NUMBERS
C NAS8-40185
8. PERFORMING ORGANIZATION
REPORT NUMBER
10. SPONSORING/MONITORING
AGENCY REPORTNUMBER
11. SUPPLEMENTARYNOTES
Second Interim Report
12a. DISTRIBUTIONIAVAILABIUTY STATEMENT
12b.
DISTRIBUTION CODE
13. ABSTRACT (Maximum 200 wordsl
We are developing a hard x-ray microscope for direct observation of solidification
dynamics in metal alloys and metal matrix composites. The Fein-Focus Inc. x-ray source
was delivered in September and found to perform better than expected. Confirmed resolution
of better than 2 micrometers was obtained and magnifications up to 800X were measured.
Nickel beads of 30 micrometer diameter were easily detected through 6mm of aluminum.
X-ray metallography was performed on several specimens showing high resolution and clear
definition of 3-dimensional structures. Prototype furnace installed and tested.
14. SUBJECT TERMS
X-ray Detectors
15. NUMBER OFPAGES
__6.'PRICECODE
X-ray Microscopy
Solidification
17. SECURITY CLASSIFICATION
OF REPORT
Unclassified
X-ray Absorption
18. SECURITY CLASSIFICATION
OF THISPAGE
Unclassified
NSN 7540 01 280 5500
18. SECURITY CLASSIFICATION I20.OF ABSTRACT
Unclassified
LIMITATION OF ABSTRACT
Stanaato Form 289 trey 2-89)
Prescn0ed 13yANSI Std 239-18
298-102


X-ray transmission microscope development

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