Photomicrographic and Microradiographic Method for Qualitative Analysis of Solid Propellants Tin Boo Yee.. ProDulsion Laboratory, Research and Development Directorate, U. 5. Army Missile Command, . Redstone Arsenal, Ala.
METHOD
is described here for the
A qualitative analysis of a solid propellant without destroying the propellant sample. The method depends on the use of polarized light to distinguish the ammonium perchlorate crystals and soft x-rays to distinguish the nitrocellulose hall powders and aluminum particles from other ingredients in the propellant. EXPERIMENTAL
The propellant used in this experiment ha? a composition of nitrocellulose hall powders, ammonium perchlorate rrystals, aluminum particles,
Figure 1A. Photomicrogroph of a thin section of propellant, showing the ingredients of the propellant (X45). B. Photomicrograph token with polarized light of the same thin section as A, and showing only the ammonium perchlorate crystals, represented b y the white areas 1x45). C. Same as B except the sample has been rotated 45' 1 x 4 5 )
772
.
ANALYTICAL CHEMISTRY
plasticizer, and a small amount of stabilizer. Thin sections of the propellant of about 20 microns thick were sliced with a microtome. (If a microtome is not available, a Tabor blade can he used.) Apparatus. A Bausch and Lomb photomicrographic camerst unit, Model L, rvith a Bausch and Lomh microscope, equipped with an analyzer and polarizer, was used for taking the photomicrographs. hlicroradiographic pictures were taken with a Philips Electronics contact microradiographic unit. Procedure. A thin section of, the propellant was placed on a microscope slide. Transmitted light was used for taking the pictures. An exposure of 2 seconds on Kodak Royal Pan film wa8 used for the photomicrograph, Figure In. To take a picture of the thin section of propellant to show only the ammonium perchlorate crystals, the thin section was placed between the analyzer and polarizer of the microscope. Transmitted light and an exposure of 12 seconds on Kodak Commercial film were used for the photomicrographs shown in Figure 1, B and C. Soft x-rays of about 5 A. can be used to differentiate the nitrocellulose hall powders from other ingredients ( 1 ) in a thin section of propellant. The Philips Electronics contact microradiographic unit can produce very soft radiation up to about 8A. wavelength. The Kodak film, 6494, was cut into a circular shape to fit the film holder. A thin section of the propellant was placed on the emulsion side of the film. The film with the thin section on it was placed in the film holder, which was placed in the specimen chamber next to the window of the x-ray tube. The air in the specimen chamber must he pumped out to prevent the absorption of the soft radiation by the air. Exposure data used for the microradiogram shown in Figure 2 were 4 kv., 2 ma., and 10 minutes. The contact microradiogram was enlarged by photomicrographic techniques. The finished enlargement has the same appearance as the contact microradiogram. Both ammonium perchlorate crystals and aluminum particles absorbed x-rays. Little radiation passed through them to espose the film. Because the areas under the ammonium perchlorate crystals and aluminum particles received little or no exposure from the radiation, the developed microradiogram will show white are= under these ammonium perchlorate crystals and aluminum particles. To have the white arPas represent only the aluminum particles on the microradiogram, the animnniuin perchlorate crystals must he removed from the thin section before the microradiogram is made.
Figure 2. Microradiograph showing the undissolved nitrocellulose ball powders (dark round and irregular shaped areas) in a thin section of the propellant 1 x 4 5 )
Ammonium perchlorate is soluble in water, while aluminum is not. The thin section was soaked and washed in a beaker of water several times to remove the ammonium perrhlorate crystals. The thin section was airdried and a microradiogram w a s made of the dried thin section as described in this paper. The exposure data used for the microradiogram shown in Figure 3 were 4 kv., 2 ma., and 14 minutes. DISCUSSION
Figure 1A is a photomicrographic picture of a thin section of the propellant. I t is difficult to identify any ingredient in this picture. Figure 1, B and C, shows photomicrographic pictures of the same thin section, made with the thin section placed between the analyzer and polarizer of the microscope. Ammonium perchlorate crystals have the ahility to polarize the light to give whit,e areas in the photomicrographs, while other ingredients in the propellant do not have this ability so the areas represented by the latter will be dark.
Figure 3. Microradiogreph showing aluminum particles (white oreas) in a thin section of the propellont 1 x 4 5 )
Because ammonium perchlorate is anisotropic (4,it is necessary to take one picture, Figure IC, 45" from Figure 1B. Grains a t cxtinction positions in Figure 1B will show up in Figure 1C. The white areas in the photomicrographs, Figure 1, Band C, represent the ammonium perchlorate crystals in the thin section of the propellant. Careful examination of the photomicrographs in Figure 1 will reveal corresponding grains of ammonium uerchlorate in the three pictures. The round and irrecularlv shaoed dark areas in Figure 2 represent t h e undissolved nitrocellulose ball powders in the thin section of the propellant.
Nitrocellulose ball powders absorbed less x-r ays than the surrounding materials Alumirmm particles absorb almost llLL1. . . ~~icrerure, U L ~ K radiation all soft x-rays; LL..~-c~-. can pass through them to expose the film. The areas under the aluminum particles will be white on the developed microradiogram. The smaller white areas in Figure 3 represent the aluminum particles, while the larger areas probably represent aggregations of aluminum particles. The method described in this paper can be used to take photomicrographs and microradiographs of 8 thin section of a propellant to show the size, shape,
and position of such ingredients as ammonium perchlorate crystals, aluminum particles, and nitrocellulose ball powders in the thin section. LITERATURE CITED
(1) Hess, W. M., Nmelco Reporter, 9, No. 1, 3 (1962).
(2) Rigby, G. R., "The Thin-Section Mineralogy of Ceramic Materials," p. 39, The British Refractories Research Aasociation, 1948.
This paper has been clewed for open publication by the Directorate for Security Review, Department of Defense. Division of Analytical Chemistry, 149th Meeting, ACS, Detroit, Mich., April 1965
Removable Mylar Window for Electron-Probe Microanalyzer
,
.L
T. E. Reichard and W. 5. Coakley, Central kesearcn veporrrnenr,
of most microanalyzers are housed in separate vacuum chambers which are connected to the electronbeam column via a relatively small channel through which x-rays pass from specimen to spectrometer. It is advantageous to have this channel, at different times, either completely open, solidly closed, or separated by a thin Mylar vacuum-tight window. An unobstructed opening is essential for passage of the longer x-ray wavelengths used for analysis of light elements; a Mylar window is needed when the spectrometer is opened for alignment of diffracting crystals, counter tubes, etc.; and the closed position is convenient for overnight shutdown. A three-position valve which provides for these three conditions, "instantly" interchangeable under vacuum, has been installed in our electron probe. The modification was made at relatively low cost, nith very little alteration of the existing instrument, and with only a one-day interruption of normal operation. Although this valve was designed specifically for the Cambridge unit ( I , 2 ) , variations of it may be applicable to other electron probes. H E X-RAY
I
I .
._._. I .r _ c.
monsonro
LO.,
JL
I uis, Mo. lo!
AIR VENT
SPECTROMETERS
Telectron-probe
i&79 .
I
.
I
/PLUG
,
-
POSITIONING
SPECIMEN 6-MICRON MYLAR I
t
-I%
Figure 1.
I
IINCIi
~..
I Cross sedtinn thrnunh :entr !r of valve, shown in open position
The principal components are shown in Figure 2-i.e., the valve body with a rectangular O-ring groove cut into its inside cylindrical wall, and the plug with its end caps. The longitudinal edges of the two rectangular plug ports are rounded slightly and polished, bo ride smoothly over the gremed O-ring. A sheet of 6micron-thick Mylar is cemented over one of the plug ports with epoxy resin. The end caps are attached with epoxy also, so they can be removed via epoxy solvent if the Mylar needs to he replaced. The valve
is operated by a small lever which keys into a square hole in one end cap. I n the Cambridge electron probe, the valve is mounted inside the brass tuhe which couples the spectrometer to the base of the lens column just above the specimen chamber. This coupling tube, which previously held the permanent Mylar window, is modified to serve as the valve housing, The opera& ing lever is brought out through a small O-ring seal between the objective-lens housing and the x-ray aperture assembly. The coupling tube and the
,I
in the configuration of a stopcock with an O-ring s r d aronnd its bore opening. Figure 1 shows a cross section through the center of the valve, in open position, h mechanical stop aligns t,he hlylar window and "open" posit,ions a t the two limits of a 90-degree rotation of the ryl'ndrical plug. I n the center (454egrre) position, the port is solidly hlnnkrrl off. h small vrnt is provided to allow air to rsrape from the sliace undm the Mylar r h r n bhe spectrometer is pre-cracuated with t,he valve in closed posit,ion.
Valve Figure 2. body and Plug
VOL 37. NO. 6, MAY 1965
773
