THE UNIVERSITY OF CHICAGO ENGINEERING CENTER

5640 S. Ellis Avenue, Chicago, IL 60637

Phone: (773) 834-3773

Fax: (773) 702-8212

<wang@oddjob.uchicago.edu>

# MEMO

**To:** Al Harper

**From:** Shu-i Wang

**Cc:** Tim Rennick, Steve Heimsath

**Date:** 4/6/01

**Re:** Vacuum Window Tests

We tested three materials as vacuum window candidates for HAWC. The
materials were Ultra High Molecular Weight Polyethylene (UHMWPE), High
Density Polyethylene (HDPE) and Polypropylene (PP) in copolymer form.
The candidates were chosen by their transmission from 50-250 microns,
mechanical properties, price and availability. Materials with high
transmission at room temperature in the 50-250 microns wavelength range
are almost non-existent. Even the three materials we tested are quite
lossy. To maximize the window transmission, we tested the feasibility of
using the candidate materials in thinner forms than we have used in the
past.

Table 1 lists the published mechanical strength and stiffness of the
three materials. As a comparison, chemical vapor deposition (CVD)-grown
diamond is also included in the Table. We mechanically tested 0.5mm
thick windows made of UHMWPE and PP, and a 0.83mm (thinnest available)
thick HDPE window. The windows had a 3-inch diameter clear aperture.
Figure 1 shows the window mount and the vacuum test fixture. The windows
were vacuum cycled to test their deflections under vacuum and their
effectiveness as a vacuum seal. The window sag (deflection) under vacuum
was measured with a dial indicator as a function of radius and time.
Figure 2 shows a photo of the window measurement setup. The window
curvatures tend to approached an asymptotic value after several days of
vacuum pumping. After a successful vacuum test, the windows were
pressurized by water to test the burst strength.

Table 1. Mechanical Properties of Window Materials.

+-----------------+---------------+----------------+------------------+
| > Material      | Tensile       | Tensile        | Poisson\'s       |
|                 | Strength, S   | Modulus, E     | Ratio, NU        |
+-----------------+---------------+----------------+------------------+
| > UH            | 38 -- 48 MPa  | > 0.2-1.2 GPa  | 0.4              |
| MWPE[^1]^,^[^2] |               |                |                  |
+-----------------+---------------+----------------+------------------+
| > HDPEError:    | 22 -- 31 MPa  | 0.5-1.2 GPa    | 0.4              |
| > Reference     |               |                |                  |
| > source not    |               |                |                  |
| > found^,2^     |               |                |                  |
+-----------------+---------------+----------------+------------------+
| > PP-           | 27 -- 38 MPa  | 0.9-1.5 GPa    | 0.4              |
| CopolymerError: |               |                |                  |
| > Reference     |               |                |                  |
| > source not    |               |                |                  |
| > found^,2^     |               |                |                  |
+-----------------+---------------+----------------+------------------+
| > (             | 200 MPa       | 1000-1140 GPa  | 0.07             |
| CVD-Diamond)[^3 |               |                |                  |
| ]^,^[^4]^,^[^5] |               |                |                  |
+-----------------+---------------+----------------+------------------+

![](media/image1.wmf){width="5.99375in" height="4.634027777777778in"}

Figure 1. Window Test Mount Assembly.

![](media/image2.wmf){width="4.160416666666666in" height="3.125in"}

Figure 2. Setup for Measuring Window Sag Under Vacuum.

Ultra-High Molecular Weight (UHMWPE) is similar to HDPE and PP in
deflection and strength. It has outstanding toughness, and has a better
abrasions resistance than HDPE. A 0.02\" (0.5mm) thick sheet of UHMWPE
was procured from the McMaster-Carr Supply Company[^6] for our test. We
were not able to find transmission data for UHMWPE in the literature,
and the transmission data we found for HDPE and PP had significant
variations[^7]^,^[^8]^,^[^9]^,^[^10]^,^[^11]^,^[^12]. We therefore sent
samples of our window materials to the GSFC IR Spectroscopy Lab[^13] to
have the transmissions measured. Figure 3 shows the measured
transmission of a piece of the 0.5mm UHMWPE material.

![](media/image3.png){width="5.4in" height="4.0875in"}Figure 3.
TransmissionError: Reference source not found of 0.5mm Thick
UHMWPEError: Reference source not found.

To test the mechanical properties, we vacuum cycled a 3-inch clear
aperture diameter UHMWPE window 6 times. The center and half radius
window sag were measured for cycles 1, 3, and 6. The results are shown
in Figure 4. The data were fitted with a fourth order polynomial (the
theoretical form for the deflection). The fitted window surface was then
added to the HAWC optics model in the Zemax Optical Design Program[^14]
to calculate a best-fit spherical radius for the window and to evaluate
the system image quality. The spherical radius varied from 100.9mm to
85.6mm from cycles 1-6. Figure 5 shows the change in best-fit spherical
radius with vacuum cycles. The maximum center-edge deflection was
0.375\" after 6 cycles. This amount of deflection was worrisome since
the window had only been steadily pumped for 1.5 hours and we expected
the deflection to increase at the same rate before settling toward an
asymptotic value after about 20 hours on the pump. (This trend was
observed in the other two materials tested.) Otherwise, UHMWPE looks
promising mechanically. The window survived 3 atmospheres in our water
test. (Our test fixture leaked at 3 atm., causing the pressure test to
be halted, while the window showed no sign of failure.)

![](media/image4.wmf){width="5.75in" height="4.197222222222222in"}Figure
4. Vacuum Testing of UHMWPEError: Reference source not found Window Sag.

![](media/image5.wmf){width="5.8597222222222225in"
height="4.200694444444444in"}Figure 5. Change in Best-Fit Radius With
Vacuum Cycles for 0.02\" UHMWPEError: Reference source not found.

High Density Polyethylene (HDPE) is the most proven of all our tested
window materials. Thicker HDPE windows had been used in several UC
instruments. At the time of mechanical testing, we could not obtain HDPE
thinner than 0.03\" off-the-shelf. The window we tested was cut from a
sheet of 0.033\" HDPE from Abbott Plastics & Supply Co[^15]. This sheet
of HDPE seems to be from Poly Hi Solidur[^16], but I could not get the
particular trade name. The measured transmission of this material is
shown in Figure 6. We have since procured sheets of both 0.3\" and
0.02\" thickness HDPE from Cadillac Plastic[^17]. The Cadillac Plastic
HDPE sheets were manufactured by Spartech Plastics[^18]. The
transmissions of the Spartech Plastic materials are shown in Figures 7 &
8 for the 0.3\" and 0.02\" HDPE sheets, respectively. The 0.3\" HDPE
manufactured by Spartech Plastic has about a 0-1% higher transmission
than the Abbott Plastics sample from 50-250 μm, and about a 5% higher
transmission at 500cm^-1^ (20 μm). The higher transmission is most
likely due to differences in material purity. It would be interesting to
see if the transmissions converge as the materials age. The Spartech
Plastic 0.02\" HDPE has about a 1-5% higher transmission than the 0.3\"
HDPE from 50-250 μm. This agrees with our theoretical calculations of
HDPE absorption. Compared with the 0.02\" UHMWPE sample, the Spartech
Plastic 0.02\" HDPE has about a 0-1% lower transmission from 50-250 μm,
and about a 5% lower transmission at 20 μm. This difference in
transmission is about the same as the variation observed between the two
0.3\" HDPE sheets from the two manufacturers. We therefore expect very
similar transmission between UHMWPE and HDPE.

![](media/image6.png){width="5.409722222222222in"
height="4.101388888888889in"}Figure 6. TransmissionError: Reference
source not found of 0.033\" Thick HDPE from Abbott Plastics & Supply Co.

![](media/image7.png){width="5.409722222222222in"
height="4.104166666666667in"}Figure 7. TransmissionError: Reference
source not found of 0.033\" Thick HDPE manufactured by Spartech Plastic.

![](media/image8.png){width="5.409722222222222in"
height="4.104166666666667in"}Figure 8. TransmissionError: Reference
source not found of 0.5mm Thick HDPE manufactured by Spartech Plastic.

The 0.033\" HDPE showed the best mechanical performance of the three
materials we tested. After vacuum pumping for 2-3 days, the window
settled into a fairly constant deflected shape. Figure 9 shows the
change in window curvature with time. The change in best-fit spherical
radius with time is plotted in Figure 10. Figure 10 shows that the RMS
Residual is increasing with time. This increase in asphericity is
predicted by the window\'s 4^th^ order polynomial theoretical form.
However, the measured window curvature is best modeled by a 5^th^ order
polynomial. The window shows insensitivity to mounting configurations
and edge conditions. Short-term burst strength is very good at better
than 7 atm.

The image degradation due to the aspheric windows were calculated for
the last three sets of measurements, at T=49hr. 45min., T=121hr. 45min.
and the 2^nd^ pump down. The optics was optimized with the criteria of
best performance with all 3 window profiles without refocusing the
telescope. The image degradation was the worst in Band 1, with a
worst-case increase in rms spot diameter of 65 microns in-field, and 91
microns in the 5-Airy fields. The largest variation in rms spot diameter
between the three sets of measurements was about 15 microns. Even with
the expected worst case degradation of the optics due to the aspheric
window profile, the HAWC optics are still diffraction limited.

![](media/image9.wmf){width="5.9944444444444445in"
height="3.7152777777777777in"}

Figure 9. Vacuum Testing of 0.033\" HDPEError: Reference source not
found Window Sag.

![](media/image10.wmf){width="5.971527777777778in"
height="3.6770833333333335in"}Figure 10. Change in Best-Fit Spherical
Radius With Vacuum Cycles for 0.033\" HDPEError: Reference source not
found.

Polypropylene (PP) comes in homopolymer (brittle below 0 degree C) and
copolymer formulations. We used a 0.02\" piece of copolymer PP from
GoodfellowsError: Reference source not found. The measured transmission
for the 0.02\" thick PP is shown in Figure 11.

![](media/image11.png){width="5.409722222222222in"
height="4.104166666666667in"}Figure 11. TransmissionError: Reference
source not found of 0.5mm thick PP.

Figure 12 shows the change in window curvature with time. The best-fit
spherical radius is plotted in Figure 13. Of the three windows tested,
the 0.02\" PP window was the most spherical in shape. However, the
0.02\" PP showed poor mechanical performance due to edge sensitivity.
After 2 days of vacuum pumping, micro-cracking developed around the edge
and lead to a failed helium leak check. Since the average transmission
of PP is lower than PE of comparable thickness in our wavelength region,
there is no advantage in using PP as our window material.

![](media/image12.wmf){width="5.99375in"
height="3.6770833333333335in"}Figure 12. Vacuum Testing of 0.02\" PP
Window Sag.

![](media/image13.wmf){width="5.964583333333334in"
height="3.6041666666666665in"}Figure 13. Change in Best-Fit Spherical
Radius With Vacuum Cycles for 0.02\" PP.

As an interesting investigation of alternative window materials, we
looked into the possibility of using chemical vapor deposition
(CVD)-grown diamond as the window material. CVD diamond can be made to
3-inch diameters of 1mm thickness. Diamond has no absorption lines in
our wavelength region. High quality CVD diamond has optical properties
similar to those of Type IIa single-crystal diamond. Its 1-phonon
vibrations centered around 1000 cm^-1^ are forbidden by the perfect
tetrahedral symmetry of the carbon atoms. Therefore, the transmission of
diamond is flat beyond the 2-phonon absorption region centered around 5
micronsError: Reference source not found. With a proper AR coating, the
transmission of CVD diamond could be quite high and uniform in our
wavelength region. A nonlinear finite element analysis was performed to
determine the deflection under 1 atmosphere pressure of a 3.0-inch
diameter 1-mm thick diamond window. The analysis used shell elements
clamped around the edges. The maximum deflection/sag calculated for the
diamond window was 0.0015\", as shown in Figure 14. The maximum stress
(at 14572 psi) was around the edges. This level of stress gives a safety
factor better than 4, based upon the maximum quoted tensile strength of
420-630 MPaError: Reference source not found. With its very high
stiffness and expected brittle behavior, the diamond window sag should
not creep like the polymer windows, and its curvature would be very
repeatable. However, the high cost of diamond windows, around \$60,000
for a 3\" window, discouraged us from pursuing this option.

![](media/image14.wmf){width="6.029861111111111in"
height="3.714583333333333in"}

Figure 14. Calculated Sag Profile of a 3 inch Diameter Diamond Vacuum
Window.

Of the materials tested, the 0.033\" HDPE window showed the best
mechanical performance. We plan on using the Spartech Plastic 0.03\"
HDPE as the window material for the flight dewar, since we have the
material certification for this materialError: Reference source not
found. Its measured transmission was actually higher than we had
predicted using absorption values from Chantry et al.Error: Reference
source not found. We will rerun the mechanical tests for this particular
sheet of material as part of the FAA certification procedure. Although
additional mechanical testing may be performed to test the window
curvature variations between samples to obtain the statistical tolerance
between windows, we expect the variations to be small. The long-term
aging effects may also be tested by accelerating the window life tests
at a slightly elevated temperature. This should be followed by a burst
test to look for residual strength. The results of these tests will be
checked against the tolerance analysis to make sure we don\'t exceed the
window tolerance specifications.

[^1]: Goodfellow Web Site: <http://www.goodfellow.com/> (7/00).

[^2]: Avallone, E. and Baumeister, T. Mark's Standard Handbook for
    Mechanical Engineers, 10^th^ Ed., McGraw-Hill, pp. 6-196 -- 6-197
    (\'76).

[^3]: Private Communication with Joseph Yehoda of Diamonex, Inc.
    (11/99).

[^4]: Coherent Auburn Group Catalog, p. 206 (11/99).

[^5]: \"Materials for infrared windows and domes: properties and
    performance\", D. C. Harris, SPIE Optical Engineering Press,
    Washington (\'99).

[^6]: McMaster-Carr Supply Company Web Site: <http://www.mcmaster.com/>
    (7/00).

[^7]: \"Far infrared & millimeter-wave absorption spectra of some
    low-loss polymers,\" G. Chantry, J. Fleming & P. Smith, Chem Phys
    Letters, Vol. 10, No. 4, pp. 473-477 (8/71).

[^8]: \"Precise dielectric measurements of low-loss materials at
    millimeter and submillimeter wavelengths,\" M. Afsar and G. Chantry,
    IEEE Transactions on Microwave Theory and Techniques, MTT-25. pp.
    509-511 (6/77).

[^9]: \"Optical constants of far infrared materials. 3: plastics\", D.
    Smith & E. Loewenstein, Applied Optics, Vol. 14, No. 6, pp.
    1335-1341 (6/75).

[^10]: \"Absorption spectra of polypropylene in the millimetre and
    submillimetre regions\", G. Chantry, J. Fleming & G. Pardoe,
    Infrared Physics, Vol. 11, pp. 109-118 (\'71).

[^11]: \"The far infrared optical constants of some common polymers\",
    J. Birch, NPL Report DES 111 (6/91).

[^12]: \"The far infrared transmission of polyethylene at reduced
    temperatures\", N. Stone, D. Williams, Applied Optics, Vol. 5, No.
    2, pp. 353-354 (2/66).

[^13]: Private Communications with Ken Stewart of the GSFC IR
    Spectroscopy Lab (3/01).

[^14]: Focus Software, Inc. Web Site:
    [http://www.focus-software.com](http://www.focus-software.com/)
    (7/00).

[^15]: Abbott Plastics, Rockford, IL. Phone: (800)850-8551. (6/00).

[^16]: Poli Hi Solidur Web Site:
    <http://www.polyhisolidur.com/Default.asp> (7/00).

[^17]: Cadillac Plastic Web Site:
    [http://www.cadillacplastic.com](http://www.cadillacplastic.com/)
    (10/00).

[^18]: Spartech Plastics, Conneaut, OH. Phone: (440)599-8175. Virgin
    grade high density polyethylene resin, reference batch number
    05/01/00-35-60-10, FPI #3500035296 (10/00).