- Email: [email protected]
Production and applications of nuclear track microfilters
0191-278X/89 $3.00 + .00 Pergamon Press plc
Nucl. Tracks Radiat. Meas., Vol. 15, Nos. 1-4, pp. 743-749, 1988 Int. J. Radiat. Appl. Instrurn., Part D Printed in Great Britain
PRODUCTION AND APPLICATIONS OF NUCLEAR TRACK MICROFILTERS P. VATER Kernchemie, FB 14, Philipps-Universit~t, D 3550 Marburg, FR of Germany
Abstract Nuclear T r a c k M i c r o f i l t e r s (NTM) are produced by irradiating a variety of Solid State Nuclear Track Detectors (SSNTD) such as mica, quartz, Polyvinylidenefluoride (PVDF), Makrofol and Kapton with heavy ions and subsequent etching with the appropriate etchant. The hole size and the porosity of the f i l t e r s can be preselected according to what is required. Properties of these f i l t e r s as well as already tested or possible applications w i l l be discussed. As an example, the use of Mica Track M i c r o f i l t e r s (MTM) in aerosol research w i l l be reported in more d e t a i l .
I. INTRODUCTION Based on the work of Fleischer, Price and Walker~ , about twenty years ago a new class of f i l t e r s called Nuclear Track M i c r o f i l t e r s appeared on the market. They were obtained by irradiating thin ( < 15 pm) plastic films with fission fragments at a reactor and subsequent etching with NaOH. The round-shaped discrete holes are of nearly uniform size. Meanwhile, these f i l t e r s are used on a large scale in d i f f e r e n t f i e l d s of application. Due to the material (polycarbonate) they are made from and the use of fission fragments to produce the holes, there are certain l i m i t s to t h e i r a p p l i c a b i l i t y . In particular, t h e i r heat resistance is limited to temperatures T ( 8 0 °C and t h e i r resistance against aggressive chemicals is also limited. Therefore, to extend the range of applications, in the seventies we started the production of Nuclear Track F i l t e r s using other materials. As bombarding particles, we used energetic heavy ions from accelerators. We have already published, the processes to manufacture Nuclear Track F i l t e r s from mica~ , o s c i l l a t i n g quartz 3 , PVDF~ , and Kapton~ and 3 6 8 and l i q u i d - l i q u i d separation 2,4¢} . the application of these f i l t e r s in aerosol research," This review is a recapitulation of the methods used to produce the f i l t e r s , and i t gives examples for t h e i r practical application.
2. PRODUCTIONAND BASIC PROPERTIESOF SOMENUCLEARTRACKFILTERS The Nuclear Track F i l t e r s we are dealing with were produced by the i r r a d i a t i o n of SSNTD f o i l s with heavy ions and subsequent etching with the appropriate etchant. All the irradiations took place at the accelerator UNILAC (GSl, Darmstadt), where ions up to uranium with maximum energies of 20 MeV/N can be obtained. There the required are~) density N (ions/cm 2) of the ions can be adjusted within the range of l ~ N (ions/cm 2) "~lO~ with an error of less than 10%. In special cases, the i r r a d i a t i o n of one sample with only one ion is possible. According to the maximum energy available, and depending on the SSNTD used, f o i l s of more than 150 Nm thickness can be penetrated by heavy ions. In Table l , the various etching conditions are given, whereby the exact etching times, of course, depend on the desired hole size and the thickness of the irradiated f o i l .
743
744 Table I.
P. VATER Etchingconditions to produce Nuclear Track F i l t e r s from various SSNTDf o i l s
f i l t e r material
supplier
etchant
muscovite mica
Jahre, Berlin
H2F2 (48%)
room
minutes
oscillating quartz (AT-cut)
Quartz-Technik, Daun
ION NaOH and then ION KOH *
120°C
hours
PVDF (solef l~))
Solvay, Bruxelles
6N KOH + O.l FW KMnO4
70oc **
days
KaptonO
Du Pont, Gen~ve
NaClO (I0% Cl)
70°C
hours
temperature
time
* One of the possible etchants. Also boiling NaOH, KOH, LiOH or stepwise etching with combinations of them can be used. The etching behaviour of this quartz is highly anisotropic, and therefore a broad variation of hole shapes depending on the etching procedure can be observed. ** In order to preserve the piezoelectrical properties of this f o i l , the etching temperature must be lower than 85 °C.
Some essential properties of the SSNTD materials and t h e i r respective etched f i l t e r s are compiled below: Muscovite mica. Up to now, mica f i l t e r s have been used most frequently by us. Mica distinguishes i t s e l f by i t s great resistance against aggressive chemicals and heat. The etching behaviour is well known. The cross-section of the e t c h e d holes is everywhere the same along the entire hole length (Fig. l ) . Mica can be etched also in boiling sodium hydroxide (NaOH) solution (also in addition to the H~F~ etching) to form hexagonal rounded-off entrances of the holes~ 4 Filters with hole sizes of a few nm as well as ones with hole sizes of 50 Nm have been produced. A disadvantage of mica f i l t e r s is their rather poor mechanical strength, in particular when the thickness is smaller than 60 Nm and the porosity larger than ~I0%. Oscillating quartz. Similar to mica, quartz exhibits the sam great r e s i s t i v i t y against aggressive chemicals. In order to minimize the temperature dependence of the eigenfrequency, AT-cutted quartzes were investigated. Compared to mica, the etching behavior is far more complex. Several etching conditions were explored. Several hole shapes were accordingly observed. A selection of them is shown in Fig. 2. The track etch rate is about one order of magnitude larger than the bulk e t c h rate giving rise to pores with an inner opening smaller than the opening at the surface.
Fig. 2.Holes in AT-cutted oscillating quartz. The etching conditiones are: A:I5N NaOH, 133°C, 15 h; B:ISN KOH, 130°C, lO h; C:ION NaOH, then ION KOH, 120°C (9 h each); D:I5N KOH (l h), then 15N NaOH (15 h), 130°C
Fig. l.Cross-section through a Mica Nuclear Track F i l t e r etched with H2F2.
NUCLEAR TRACK MICROFILTERS
Fig. 3.Holes in a PVDF Nuclear Track F i l t e r etched with 6N KOH + 0.I Fw KMnO4.
Fig.4.SEM-photo of etched holes in a Kapton Nuclear Track F i l t e r .
745
Polyvinylidene fluoride (PVDF). The t e f l o n - l i k e PVDF also exhibits high chemical r e s i s t i v i t y . The streched and rolled form (trade name "solef") has piezo/pyroelectrical properties. A solution of 6N KOH + O.l Fw KMnO~ was found to be the optimum etchant. The purple darkish precipitate on the PVDF surface observed after etching could be removed by reducing i t with K~S~Of solution. In order to keep the e l e c t r i c a l properties of the f i l t e r s unchanged, we kept the etching temperature at 70 °C. I f a possible user does not need these e l e c t r i c a l properties, the etching can be performed at higher temperatures, which reduces d r a s t i c a l l y the etching time. The cone angle of the holes seems to be rather small ( i t has not yet been measured accurately). As an example, after two days etching of a 40 pm thick f o i l holes could be observed having diameters of -v3.3 pm at the surface, and ~ 2 . 5 pm in the center of the f o i l . Fig. 3 is a microphoto of etched tracks in a PVDF-foil. Kapton ( polyimide f o i l , trading name of "Du Pont"). Kapton is a material with good c-~-~-i'~al r e s i s t i v i t y , e.g. against benzole, toluene, and methanol. I t has been used in a wide temperature interval rangig from 4K to 673K (-269 °C to 400 %). I t is rather resistant against high doses of radiation, the mechanical properties are known to be not very temperature dependent. The cone angle of a f o i l irradiated with 16.5 MeV/N ~SU ions and etched with NaClO solution at 70 °C was found to be less than 3.5 ° . Fig. 4 gives an example of etched holes in a Kapton f o i l .
3. SOME APPLICATIONS OF NUCLEAR TRACK FILTERS F i l t r a t i o n of bacteria and p a r t i c l e s . M a k r o f o l f i l t e r s have been used to f i l t r a t e water obtained from a natural water pool. The bacteria in the f i l t r a t e were c u l t i v a t e d to grow i n t o colonies, which were counted a f t e r a c e r t a i n time. I t has been demonstrated that Makrofol f i l t e r s with pore sizes less than 0.6 pm are able to remove 100% of a given group of bacteria. In food chemistry, complicated procedures are applied to measure the size of f l o a t i n g p a r t i c l e s in l i q u i d s . The use of nuclear track f i l t e r s o f f e r s a simpler method. In an experiment, i t has been shown t h a t mica f i l t e r s having hole sizes smaller than 34 pm are s u i t a b l e to separate suspended p a r t i c l e s from a l c o h o l i c suspensions of St. Johns bread essence and cola nut essence. I t can be concluded that the p a r t i c l e s in question must be larger than 34 pm. Separation of two s t r o n g l y mixed phases (emulsion). In some i n d u s t r i a l processes or in a n a l y t i c a l chemistry, there e x i s t s the problem of separating the two phases of an emulsion. As an example we have t r i e d to e x t r a c t the inorganic phase out of an emulsion consisting of 30% t r i b u t y l phosphate (TBP) in kerosene as organic phase and 2N HN03 as inorganic phase. Mica f i l t e r s having hole sizes around 5 pm are well suited to e x t r a c t the aqueous phase only. Fig. 5 shows the f i l t r a t i o n u n i t and the course of the experiment. Only the aqueous phase (dyed with blue ink f o r the purpose of demonstration) passes through the f i l t e r and is collected in the lower f l a s k . The emulsion in the upper cup becomes decoloured. At the end ( a f t e r ]0-20 minutes), the aqueous phase i s completely separated from the organic phase. This l a t t e r phase remains in the upper cup. Recently, Kapton f i l t e r s were applied in the same type of experiment. There, the organic phase passes through the f i l t e r s having pore sizes d of 3 pm i d ~ 13 pm. The inorganic phase remains in the upper cup. A complete separation has been proven using radiochemical methods.
746
P. V A T E R
I
50 mm
I
stirrer
" ~ ,
gloss cup
~:3~ -
nn~-~ ,~ I
"
spring filter holder with: o - rings mico filter supporting grid
collection flask
Fig. 5.Left:
Schematic representation of the extraction apparatus used. Right: A sequence of photos showing the extraction of the aqueous phase out of the emulsion with TBP in kerosene. At the beginning, the emulsion is poured into the cup (a)~ the dark dyed aqueous phase passes through the f i l t e r and is collected in the lower flask, the emulsion in the upper cup becomes decoloured (b and c); at the end, the aqueous phase is completely separated from the organic phase. The latter remains in the upper cup (d).
Mass spectrometric gas analysis. Mass spectrometry principally provides the necessary standards for multicomponent ~as analysis: high sensitivity for nearly all components of gas mixtures (relative up to t O - l ) . Its more widespread adoption in instrumental analysis is hindered by problems arising with the gas inlet of the mass spectrometer: high consumption of the analyte gas (capillaries) and fractionation of gas mixtures containing components with widely varying atomic or molecular weights (needle valves). A theoretical approach to the problem would be the use of a hyperfine hole with short distance of permeation. The experimental realization is Mica Track Microfilters with a few fine holes. They allow the required pressure reduction (from the analyte pressure to less than 10 -5 mbar in the inlet) without fractionating the composition of the analyte gas. Linear response of the mass spectrometer (Fig. 6) is obtained for gases with drastically different molecular weight
'=so2
80
• _~
e=H2
'
.
' f
60
.¢) o /-0
20
100
200 300 p/tabor
/.00
Fig.6.Correlation between the calibrated pressures of SO,and H%, respectively, and the mass spectrometric response.
N U C L E A R T R A C K MICROFILTERS
747
H~ and SO~ ) in a wide range of partial pressures. The consumption of the analyte gas is extremely low. Based on this new i n l e t system, an analytical technique for quantitatively determining gas mixtures was developed and is used for routine gas analysisfl¢ Aerosol mass determination. F i l t e r s made from o s c i l l a t i n g quartzes can be used to determine aerosol masses. For this purpose, the surfaces of the quartz f i l t e r have to be coated with electrodes which are then connected with an o s c i l l a t i o n c i r c u i t . The f i l t e r is installed in an aerosol sampling unit. I t has been proven that the quartz f i l t e r s can be employed as oscillators. Their moisture s e n s i t i v i t y and temperature coefficients are similar to those of untreated quartzes. Their typical mass response is approx, l Hz ng-4 cm-~ . A typical frequency response curve during mass loading is shown in Fig. 7. This method has the potential to detect t i n y aerosol particles of a few nano-grams weight almost instantaneously.
A~'Beginof
20 MHz
L
Pressure
~ Beginof
( t. p, 30rob) Aerosol Throughput
8
I/3
E
0
n
d
of Expetimlmt
ol
P
; Time (Hours)
Fig. 7.Response curve of the frequency change for a o s c i l l a t i n g Quartz Nuclear Track F i l t e r exposed to an aerosol particle containing gas flow. Measurement of absorbing aerosol. I t is a standard problem in a i r pollution control to measure the amount of aerosol suspended in gas. One conceivable method of measuring these parameters uses the piezo/pyroelectrical properties of PVDF f i l t e r s . Similar to the o s c i l l a t i n g quartz f i l t e r , both the surfaces of the PVDF nuclear track f i l t e r must be coated with electrodes. The measuring p r i n c i p l e is as follows: On the surface of a PVDF f i l t e r , aerosol particles are collected. The f i l t e r surface is i n t e r m i t t e n t l y irradiated with l i g h t pulses of a wavelength which is absorbed by the particles. The energy absorbed in a thin layer is proportional to the p a r t i c l e mass and gets converted to heat. PVDF absorbs this thermal radiation. The corresponding expansion causes surface charges on the electrodes. By using a current to voltage converter, they are converted into a signal which can be measured. Fig. 8 shows the result of a test experiment using copymachine toner dust as aerosol particles (mass concentration 4.37 mg/m~). The output signal clearly depends on the total f i l t e r load. Limited discrimination of various chemical species by changing the illumination wavelength seems to be possible. I
30
C
i
i
i
.
•
2O
o I 200
I I 600 1000 Total filter load (pg}
I 1~00
Fig. 8.0utput voltage as a f u n c t i o n of f i l t e r load. A p y r o e l e c t r i c a l PVDF f i l t e r was exposed to copymachine toner dust.
748
P. V A T E R
Measurement of aerosols carrying r a d i o a c t i v i t y . In recent years, we have used Mica Track Microfilters in a Cascade Fractionator (MMCF) to collect aerosols carrying radioactivity from the environmental a i r inside a nuclear reactor fuel elements production plant. After the aerosol sampling (few m3 of a i r ) the mass load of the f i l t e r s is determined by weight, and the total collected alpha a c t i v i t y is measured with a methane flow counter. The geometrical size distribution of uranium particles can be obtained by investigating the particles collected on the f i l t e r s with help of a scanning electron microscope (SEM) together with an attached energy dispersive X-ray analysis system (EDXA). The following results for several experiments carried out at d i f f e r e n t times in the same plant could be obtained reproducible: The specific mass load and specific a c t i v i t y of air are in the order of (O.l-O.4)mg/m and (O.15-O.3)Bq/m3, respectively, and the median geometrical diameter C for the frequency d i s t r i b u t i o n of U-particles collected on the f i r s t f i l t e r is C-~ 2 pm. Two new methods have been applied to improve the evaluation of the loaded f i l t e r s : I. In a SEM, the rate of backscattered electrons (BE) depends on the atomic number of the "target". I f one detects only the BE, uranium, having the largest atomic number, exhibits the highest backscattering rate. Therefore, the image of an U-particle is the brightest one of all (see Fig. 9 ) . In t h i s way, the number of particles that have to be investigated by EDXA can be reduced drastically,when one is predominantly interested in uranium-bearing aerosols.
Fig. 9. SEM-photos of a loaded mica f i l t e r . One particle consists of Right: Secondary and backscattered electrons are ~tected. Left: The same position. Only backscattered electrons are detected. The U-particle is the brightest one. 2. A well-defined f i l t e r section is investigated. From a) the number of U-particles found in this area, b) the median value of t h e i r geometrical size, c) the isotopic composition of U being processed during sampling, d) the total f i l t e r area and e) the assumption that the U-particles consist of UO~ and are of spherical shape, the total alpha-activity of the f i l t e r load can be calculated. By comparing the result with that obtained with the methane flow counter, i t can be shown i f the examined fraction is representative of the total f i l t e r area. As an example, a second f i l t e r of a cascade was investigated as described above. Based on 7 U-particles, the a c t i v i t y was calculated to be (O.12~O.05)Bq compared to (O.ll±O.Ol)Bq measured with the methane flow counter. The result shows that the methods are suited to make the investigation of loaded f i l t e r s less time-consuming. 4. CONCLUSION Nuclear Track M i c r o f i l t e r s can e f f e c t i v e l y be used in many f i e l d s of application. The f i l t e r material to be selected is determined by the special requirements of the particular problem. Most of the f i l t e r s can be reused after cleaning, making their application more economical. I t is less conceivable to i n s t a l l these f i l t e r s for cleaning of large amounts of gas or l i q u i d s , but for special purposes t h e i r usefulness has been shown.
NUCLEAR TRACK MICROFILTERS
749
5. ACKNOWLEDGEMENT The survey given here is based on experimental work mostly performed at Kernchemie, University of Marburg (R. Brandt, R. Dersch, H. Duschner, M. Ganz, G. Feige, A. Khodai, M. Plachky, U. Schneider, G. Tress, P. Vater), partly in collaboration with K. Jamil, H.A. Khan, M.A. Sial (PINSTECH, Islamabad, Pakistan), E.U. Khan (Goma] University, D.I. Khan, Pakistan), S.-L. Guo, Z. Fan, X. Hao, J. Lin, S. Wang, Y. Wang, T.-C. Zhu ( I n s t i t u t e of Atomic Energy, Academia Sinica, Beijing, China), W. Dunkhorst, W. Hollander, E. Pape (Fraunhofer Institute of Toxicology and Aerosol Research, Hannover, F.R. of Germany), B. GenswUrger, R. Spohr, J. Vetter (GSI, Darmstadt, F.R. of Germany), G. Luthardt, W. Rudolph (NUKEM, Hanau, F.R. of Germany), E.-J. Langrock, E. K~ber (Kernchemie, Technische Hochschule, Leipzig, DDR), M. Walter (VEB Chemisches Werk M i l t i t z , DDR), J. Bucher, N. Edelstein (LBL, Berkeley, USA). The irradiations were made at UNILAC (GSI, Darmstadt). The help of Drs. R. Spohr and J. Vetter is gratefully acknowledged. The work was supported by the Bundesministerium fur Umwelt, Naturschutz und Reaktorsicherheit (Bonn).
6. REFERENCES I. Fleischer, R.L., P.B. Price, R.M. Walker (1975). Nuclear Tracks in Solids. University of California Press (and references therein) 2. Vater, P., G. Tress, R. Brandt, B. GenswUrger, R. Spohr (1980). Nuclear Track Microfilters Made of Mica. NIM 173, 205-210 3. Hollander, W., E. Pape, M. ~ , P. Vater, R. Brandt (1986). Aerosol Mass Determination with Nuclear Track F i l t e r s from Quartz Crystals. J. Aerosol Sci. 17, 859-87] 4. Khodai-Joopary, A., M.A. Sial, P. Vater, R. Brandt (lg87). Chemical Etching of Polyvinylidene Fluoride Films. GSI 87-1 (Scientific Report 1986), 236 5. Zhu, T.-C., P. Vater, R. Brandt (1988). Microtllters with Tiny Holes in Kapton. GSI 88-I (Scientific Report 1987) 6. Tress, G., P. Vater, H.A. Khan, R. Brandt, M. Kadner (1986). Eine Fraktionierkaskade mit Kernporenfiltern aus Glimmer zur F i l t r a t i o n heiBer, radioaktiver Abgase. Staub, Reinhaltung der Luft 46, 147-151 7. HoII~nder, W., W. Dunkhorst, R. Brandt, P. Vater (1987). Measurement of absorbing aerosols with piezo-electric f i l t e r s from Polyvinylidenefluorid (PVDF): First results. J. Aerosol Sci. 18, 907-910 8. Vater, P., R. Dersch, R. Brandt, G. Luthardt, W. Rudolph (1987). Detailed Chemical Analysis of Aerosols Carrying Radioactivity and Collected with Mica Track Microfilters. J. Aerosol Sci. 18, 931-934 9. SiM, M.A., K. Jamil, H.A. Khan, P. Vater, R. Brandt (1987). Mica Track Microfilters Applied for the Separation of Two Strongly Mixed Liquid Phases (Emulsion). Isotopenpraxis 23, 305-308 lO. Ganz, M., G. Feige,-~-N. Edelstein, J. Bucher, T.-C. Zhu, P. Vater, R. Brandt, J. Vetter (1988). Application of Kapton Nuclear Track Microfilters in Liquid-Liquid Extraction of Actinoide Elements. GSI 88-I (Scientific Report 1987) I I . Khan, H.A., N.A. Khan, R. Spohr (198]), NIM 189, 511-581 12. Duschner, H., U. Schneider, P. Vater, R . ~ (1988). Mass Spectrometric Gas Analysis: Nuclear Track Filters as components for the Gas Inlet System. GSI 88-I (Scientific Report 1987)
NT 15:1/4-WW
Nucl. Tracks Radiat. Meas., Vol. 15, Nos. 1-4, pp. 743-749, 1988 Int. J. Radiat. Appl. Instrurn., Part D Printed in Great Britain
PRODUCTION AND APPLICATIONS OF NUCLEAR TRACK MICROFILTERS P. VATER Kernchemie, FB 14, Philipps-Universit~t, D 3550 Marburg, FR of Germany
Abstract Nuclear T r a c k M i c r o f i l t e r s (NTM) are produced by irradiating a variety of Solid State Nuclear Track Detectors (SSNTD) such as mica, quartz, Polyvinylidenefluoride (PVDF), Makrofol and Kapton with heavy ions and subsequent etching with the appropriate etchant. The hole size and the porosity of the f i l t e r s can be preselected according to what is required. Properties of these f i l t e r s as well as already tested or possible applications w i l l be discussed. As an example, the use of Mica Track M i c r o f i l t e r s (MTM) in aerosol research w i l l be reported in more d e t a i l .
I. INTRODUCTION Based on the work of Fleischer, Price and Walker~ , about twenty years ago a new class of f i l t e r s called Nuclear Track M i c r o f i l t e r s appeared on the market. They were obtained by irradiating thin ( < 15 pm) plastic films with fission fragments at a reactor and subsequent etching with NaOH. The round-shaped discrete holes are of nearly uniform size. Meanwhile, these f i l t e r s are used on a large scale in d i f f e r e n t f i e l d s of application. Due to the material (polycarbonate) they are made from and the use of fission fragments to produce the holes, there are certain l i m i t s to t h e i r a p p l i c a b i l i t y . In particular, t h e i r heat resistance is limited to temperatures T ( 8 0 °C and t h e i r resistance against aggressive chemicals is also limited. Therefore, to extend the range of applications, in the seventies we started the production of Nuclear Track F i l t e r s using other materials. As bombarding particles, we used energetic heavy ions from accelerators. We have already published, the processes to manufacture Nuclear Track F i l t e r s from mica~ , o s c i l l a t i n g quartz 3 , PVDF~ , and Kapton~ and 3 6 8 and l i q u i d - l i q u i d separation 2,4¢} . the application of these f i l t e r s in aerosol research," This review is a recapitulation of the methods used to produce the f i l t e r s , and i t gives examples for t h e i r practical application.
2. PRODUCTIONAND BASIC PROPERTIESOF SOMENUCLEARTRACKFILTERS The Nuclear Track F i l t e r s we are dealing with were produced by the i r r a d i a t i o n of SSNTD f o i l s with heavy ions and subsequent etching with the appropriate etchant. All the irradiations took place at the accelerator UNILAC (GSl, Darmstadt), where ions up to uranium with maximum energies of 20 MeV/N can be obtained. There the required are~) density N (ions/cm 2) of the ions can be adjusted within the range of l ~ N (ions/cm 2) "~lO~ with an error of less than 10%. In special cases, the i r r a d i a t i o n of one sample with only one ion is possible. According to the maximum energy available, and depending on the SSNTD used, f o i l s of more than 150 Nm thickness can be penetrated by heavy ions. In Table l , the various etching conditions are given, whereby the exact etching times, of course, depend on the desired hole size and the thickness of the irradiated f o i l .
743
744 Table I.
P. VATER Etchingconditions to produce Nuclear Track F i l t e r s from various SSNTDf o i l s
f i l t e r material
supplier
etchant
muscovite mica
Jahre, Berlin
H2F2 (48%)
room
minutes
oscillating quartz (AT-cut)
Quartz-Technik, Daun
ION NaOH and then ION KOH *
120°C
hours
PVDF (solef l~))
Solvay, Bruxelles
6N KOH + O.l FW KMnO4
70oc **
days
KaptonO
Du Pont, Gen~ve
NaClO (I0% Cl)
70°C
hours
temperature
time
* One of the possible etchants. Also boiling NaOH, KOH, LiOH or stepwise etching with combinations of them can be used. The etching behaviour of this quartz is highly anisotropic, and therefore a broad variation of hole shapes depending on the etching procedure can be observed. ** In order to preserve the piezoelectrical properties of this f o i l , the etching temperature must be lower than 85 °C.
Some essential properties of the SSNTD materials and t h e i r respective etched f i l t e r s are compiled below: Muscovite mica. Up to now, mica f i l t e r s have been used most frequently by us. Mica distinguishes i t s e l f by i t s great resistance against aggressive chemicals and heat. The etching behaviour is well known. The cross-section of the e t c h e d holes is everywhere the same along the entire hole length (Fig. l ) . Mica can be etched also in boiling sodium hydroxide (NaOH) solution (also in addition to the H~F~ etching) to form hexagonal rounded-off entrances of the holes~ 4 Filters with hole sizes of a few nm as well as ones with hole sizes of 50 Nm have been produced. A disadvantage of mica f i l t e r s is their rather poor mechanical strength, in particular when the thickness is smaller than 60 Nm and the porosity larger than ~I0%. Oscillating quartz. Similar to mica, quartz exhibits the sam great r e s i s t i v i t y against aggressive chemicals. In order to minimize the temperature dependence of the eigenfrequency, AT-cutted quartzes were investigated. Compared to mica, the etching behavior is far more complex. Several etching conditions were explored. Several hole shapes were accordingly observed. A selection of them is shown in Fig. 2. The track etch rate is about one order of magnitude larger than the bulk e t c h rate giving rise to pores with an inner opening smaller than the opening at the surface.
Fig. 2.Holes in AT-cutted oscillating quartz. The etching conditiones are: A:I5N NaOH, 133°C, 15 h; B:ISN KOH, 130°C, lO h; C:ION NaOH, then ION KOH, 120°C (9 h each); D:I5N KOH (l h), then 15N NaOH (15 h), 130°C
Fig. l.Cross-section through a Mica Nuclear Track F i l t e r etched with H2F2.
NUCLEAR TRACK MICROFILTERS
Fig. 3.Holes in a PVDF Nuclear Track F i l t e r etched with 6N KOH + 0.I Fw KMnO4.
Fig.4.SEM-photo of etched holes in a Kapton Nuclear Track F i l t e r .
745
Polyvinylidene fluoride (PVDF). The t e f l o n - l i k e PVDF also exhibits high chemical r e s i s t i v i t y . The streched and rolled form (trade name "solef") has piezo/pyroelectrical properties. A solution of 6N KOH + O.l Fw KMnO~ was found to be the optimum etchant. The purple darkish precipitate on the PVDF surface observed after etching could be removed by reducing i t with K~S~Of solution. In order to keep the e l e c t r i c a l properties of the f i l t e r s unchanged, we kept the etching temperature at 70 °C. I f a possible user does not need these e l e c t r i c a l properties, the etching can be performed at higher temperatures, which reduces d r a s t i c a l l y the etching time. The cone angle of the holes seems to be rather small ( i t has not yet been measured accurately). As an example, after two days etching of a 40 pm thick f o i l holes could be observed having diameters of -v3.3 pm at the surface, and ~ 2 . 5 pm in the center of the f o i l . Fig. 3 is a microphoto of etched tracks in a PVDF-foil. Kapton ( polyimide f o i l , trading name of "Du Pont"). Kapton is a material with good c-~-~-i'~al r e s i s t i v i t y , e.g. against benzole, toluene, and methanol. I t has been used in a wide temperature interval rangig from 4K to 673K (-269 °C to 400 %). I t is rather resistant against high doses of radiation, the mechanical properties are known to be not very temperature dependent. The cone angle of a f o i l irradiated with 16.5 MeV/N ~SU ions and etched with NaClO solution at 70 °C was found to be less than 3.5 ° . Fig. 4 gives an example of etched holes in a Kapton f o i l .
3. SOME APPLICATIONS OF NUCLEAR TRACK FILTERS F i l t r a t i o n of bacteria and p a r t i c l e s . M a k r o f o l f i l t e r s have been used to f i l t r a t e water obtained from a natural water pool. The bacteria in the f i l t r a t e were c u l t i v a t e d to grow i n t o colonies, which were counted a f t e r a c e r t a i n time. I t has been demonstrated that Makrofol f i l t e r s with pore sizes less than 0.6 pm are able to remove 100% of a given group of bacteria. In food chemistry, complicated procedures are applied to measure the size of f l o a t i n g p a r t i c l e s in l i q u i d s . The use of nuclear track f i l t e r s o f f e r s a simpler method. In an experiment, i t has been shown t h a t mica f i l t e r s having hole sizes smaller than 34 pm are s u i t a b l e to separate suspended p a r t i c l e s from a l c o h o l i c suspensions of St. Johns bread essence and cola nut essence. I t can be concluded that the p a r t i c l e s in question must be larger than 34 pm. Separation of two s t r o n g l y mixed phases (emulsion). In some i n d u s t r i a l processes or in a n a l y t i c a l chemistry, there e x i s t s the problem of separating the two phases of an emulsion. As an example we have t r i e d to e x t r a c t the inorganic phase out of an emulsion consisting of 30% t r i b u t y l phosphate (TBP) in kerosene as organic phase and 2N HN03 as inorganic phase. Mica f i l t e r s having hole sizes around 5 pm are well suited to e x t r a c t the aqueous phase only. Fig. 5 shows the f i l t r a t i o n u n i t and the course of the experiment. Only the aqueous phase (dyed with blue ink f o r the purpose of demonstration) passes through the f i l t e r and is collected in the lower f l a s k . The emulsion in the upper cup becomes decoloured. At the end ( a f t e r ]0-20 minutes), the aqueous phase i s completely separated from the organic phase. This l a t t e r phase remains in the upper cup. Recently, Kapton f i l t e r s were applied in the same type of experiment. There, the organic phase passes through the f i l t e r s having pore sizes d of 3 pm i d ~ 13 pm. The inorganic phase remains in the upper cup. A complete separation has been proven using radiochemical methods.
746
P. V A T E R
I
50 mm
I
stirrer
" ~ ,
gloss cup
~:3~ -
nn~-~ ,~ I
"
spring filter holder with: o - rings mico filter supporting grid
collection flask
Fig. 5.Left:
Schematic representation of the extraction apparatus used. Right: A sequence of photos showing the extraction of the aqueous phase out of the emulsion with TBP in kerosene. At the beginning, the emulsion is poured into the cup (a)~ the dark dyed aqueous phase passes through the f i l t e r and is collected in the lower flask, the emulsion in the upper cup becomes decoloured (b and c); at the end, the aqueous phase is completely separated from the organic phase. The latter remains in the upper cup (d).
Mass spectrometric gas analysis. Mass spectrometry principally provides the necessary standards for multicomponent ~as analysis: high sensitivity for nearly all components of gas mixtures (relative up to t O - l ) . Its more widespread adoption in instrumental analysis is hindered by problems arising with the gas inlet of the mass spectrometer: high consumption of the analyte gas (capillaries) and fractionation of gas mixtures containing components with widely varying atomic or molecular weights (needle valves). A theoretical approach to the problem would be the use of a hyperfine hole with short distance of permeation. The experimental realization is Mica Track Microfilters with a few fine holes. They allow the required pressure reduction (from the analyte pressure to less than 10 -5 mbar in the inlet) without fractionating the composition of the analyte gas. Linear response of the mass spectrometer (Fig. 6) is obtained for gases with drastically different molecular weight
'=so2
80
• _~
e=H2
'
.
' f
60
.¢) o /-0
20
100
200 300 p/tabor
/.00
Fig.6.Correlation between the calibrated pressures of SO,and H%, respectively, and the mass spectrometric response.
N U C L E A R T R A C K MICROFILTERS
747
H~ and SO~ ) in a wide range of partial pressures. The consumption of the analyte gas is extremely low. Based on this new i n l e t system, an analytical technique for quantitatively determining gas mixtures was developed and is used for routine gas analysisfl¢ Aerosol mass determination. F i l t e r s made from o s c i l l a t i n g quartzes can be used to determine aerosol masses. For this purpose, the surfaces of the quartz f i l t e r have to be coated with electrodes which are then connected with an o s c i l l a t i o n c i r c u i t . The f i l t e r is installed in an aerosol sampling unit. I t has been proven that the quartz f i l t e r s can be employed as oscillators. Their moisture s e n s i t i v i t y and temperature coefficients are similar to those of untreated quartzes. Their typical mass response is approx, l Hz ng-4 cm-~ . A typical frequency response curve during mass loading is shown in Fig. 7. This method has the potential to detect t i n y aerosol particles of a few nano-grams weight almost instantaneously.
A~'Beginof
20 MHz
L
Pressure
~ Beginof
( t. p, 30rob) Aerosol Throughput
8
I/3
E
0
n
d
of Expetimlmt
ol
P
; Time (Hours)
Fig. 7.Response curve of the frequency change for a o s c i l l a t i n g Quartz Nuclear Track F i l t e r exposed to an aerosol particle containing gas flow. Measurement of absorbing aerosol. I t is a standard problem in a i r pollution control to measure the amount of aerosol suspended in gas. One conceivable method of measuring these parameters uses the piezo/pyroelectrical properties of PVDF f i l t e r s . Similar to the o s c i l l a t i n g quartz f i l t e r , both the surfaces of the PVDF nuclear track f i l t e r must be coated with electrodes. The measuring p r i n c i p l e is as follows: On the surface of a PVDF f i l t e r , aerosol particles are collected. The f i l t e r surface is i n t e r m i t t e n t l y irradiated with l i g h t pulses of a wavelength which is absorbed by the particles. The energy absorbed in a thin layer is proportional to the p a r t i c l e mass and gets converted to heat. PVDF absorbs this thermal radiation. The corresponding expansion causes surface charges on the electrodes. By using a current to voltage converter, they are converted into a signal which can be measured. Fig. 8 shows the result of a test experiment using copymachine toner dust as aerosol particles (mass concentration 4.37 mg/m~). The output signal clearly depends on the total f i l t e r load. Limited discrimination of various chemical species by changing the illumination wavelength seems to be possible. I
30
C
i
i
i
.
•
2O
o I 200
I I 600 1000 Total filter load (pg}
I 1~00
Fig. 8.0utput voltage as a f u n c t i o n of f i l t e r load. A p y r o e l e c t r i c a l PVDF f i l t e r was exposed to copymachine toner dust.
748
P. V A T E R
Measurement of aerosols carrying r a d i o a c t i v i t y . In recent years, we have used Mica Track Microfilters in a Cascade Fractionator (MMCF) to collect aerosols carrying radioactivity from the environmental a i r inside a nuclear reactor fuel elements production plant. After the aerosol sampling (few m3 of a i r ) the mass load of the f i l t e r s is determined by weight, and the total collected alpha a c t i v i t y is measured with a methane flow counter. The geometrical size distribution of uranium particles can be obtained by investigating the particles collected on the f i l t e r s with help of a scanning electron microscope (SEM) together with an attached energy dispersive X-ray analysis system (EDXA). The following results for several experiments carried out at d i f f e r e n t times in the same plant could be obtained reproducible: The specific mass load and specific a c t i v i t y of air are in the order of (O.l-O.4)mg/m and (O.15-O.3)Bq/m3, respectively, and the median geometrical diameter C for the frequency d i s t r i b u t i o n of U-particles collected on the f i r s t f i l t e r is C-~ 2 pm. Two new methods have been applied to improve the evaluation of the loaded f i l t e r s : I. In a SEM, the rate of backscattered electrons (BE) depends on the atomic number of the "target". I f one detects only the BE, uranium, having the largest atomic number, exhibits the highest backscattering rate. Therefore, the image of an U-particle is the brightest one of all (see Fig. 9 ) . In t h i s way, the number of particles that have to be investigated by EDXA can be reduced drastically,when one is predominantly interested in uranium-bearing aerosols.
Fig. 9. SEM-photos of a loaded mica f i l t e r . One particle consists of Right: Secondary and backscattered electrons are ~tected. Left: The same position. Only backscattered electrons are detected. The U-particle is the brightest one. 2. A well-defined f i l t e r section is investigated. From a) the number of U-particles found in this area, b) the median value of t h e i r geometrical size, c) the isotopic composition of U being processed during sampling, d) the total f i l t e r area and e) the assumption that the U-particles consist of UO~ and are of spherical shape, the total alpha-activity of the f i l t e r load can be calculated. By comparing the result with that obtained with the methane flow counter, i t can be shown i f the examined fraction is representative of the total f i l t e r area. As an example, a second f i l t e r of a cascade was investigated as described above. Based on 7 U-particles, the a c t i v i t y was calculated to be (O.12~O.05)Bq compared to (O.ll±O.Ol)Bq measured with the methane flow counter. The result shows that the methods are suited to make the investigation of loaded f i l t e r s less time-consuming. 4. CONCLUSION Nuclear Track M i c r o f i l t e r s can e f f e c t i v e l y be used in many f i e l d s of application. The f i l t e r material to be selected is determined by the special requirements of the particular problem. Most of the f i l t e r s can be reused after cleaning, making their application more economical. I t is less conceivable to i n s t a l l these f i l t e r s for cleaning of large amounts of gas or l i q u i d s , but for special purposes t h e i r usefulness has been shown.
NUCLEAR TRACK MICROFILTERS
749
5. ACKNOWLEDGEMENT The survey given here is based on experimental work mostly performed at Kernchemie, University of Marburg (R. Brandt, R. Dersch, H. Duschner, M. Ganz, G. Feige, A. Khodai, M. Plachky, U. Schneider, G. Tress, P. Vater), partly in collaboration with K. Jamil, H.A. Khan, M.A. Sial (PINSTECH, Islamabad, Pakistan), E.U. Khan (Goma] University, D.I. Khan, Pakistan), S.-L. Guo, Z. Fan, X. Hao, J. Lin, S. Wang, Y. Wang, T.-C. Zhu ( I n s t i t u t e of Atomic Energy, Academia Sinica, Beijing, China), W. Dunkhorst, W. Hollander, E. Pape (Fraunhofer Institute of Toxicology and Aerosol Research, Hannover, F.R. of Germany), B. GenswUrger, R. Spohr, J. Vetter (GSI, Darmstadt, F.R. of Germany), G. Luthardt, W. Rudolph (NUKEM, Hanau, F.R. of Germany), E.-J. Langrock, E. K~ber (Kernchemie, Technische Hochschule, Leipzig, DDR), M. Walter (VEB Chemisches Werk M i l t i t z , DDR), J. Bucher, N. Edelstein (LBL, Berkeley, USA). The irradiations were made at UNILAC (GSI, Darmstadt). The help of Drs. R. Spohr and J. Vetter is gratefully acknowledged. The work was supported by the Bundesministerium fur Umwelt, Naturschutz und Reaktorsicherheit (Bonn).
6. REFERENCES I. Fleischer, R.L., P.B. Price, R.M. Walker (1975). Nuclear Tracks in Solids. University of California Press (and references therein) 2. Vater, P., G. Tress, R. Brandt, B. GenswUrger, R. Spohr (1980). Nuclear Track Microfilters Made of Mica. NIM 173, 205-210 3. Hollander, W., E. Pape, M. ~ , P. Vater, R. Brandt (1986). Aerosol Mass Determination with Nuclear Track F i l t e r s from Quartz Crystals. J. Aerosol Sci. 17, 859-87] 4. Khodai-Joopary, A., M.A. Sial, P. Vater, R. Brandt (lg87). Chemical Etching of Polyvinylidene Fluoride Films. GSI 87-1 (Scientific Report 1986), 236 5. Zhu, T.-C., P. Vater, R. Brandt (1988). Microtllters with Tiny Holes in Kapton. GSI 88-I (Scientific Report 1987) 6. Tress, G., P. Vater, H.A. Khan, R. Brandt, M. Kadner (1986). Eine Fraktionierkaskade mit Kernporenfiltern aus Glimmer zur F i l t r a t i o n heiBer, radioaktiver Abgase. Staub, Reinhaltung der Luft 46, 147-151 7. HoII~nder, W., W. Dunkhorst, R. Brandt, P. Vater (1987). Measurement of absorbing aerosols with piezo-electric f i l t e r s from Polyvinylidenefluorid (PVDF): First results. J. Aerosol Sci. 18, 907-910 8. Vater, P., R. Dersch, R. Brandt, G. Luthardt, W. Rudolph (1987). Detailed Chemical Analysis of Aerosols Carrying Radioactivity and Collected with Mica Track Microfilters. J. Aerosol Sci. 18, 931-934 9. SiM, M.A., K. Jamil, H.A. Khan, P. Vater, R. Brandt (1987). Mica Track Microfilters Applied for the Separation of Two Strongly Mixed Liquid Phases (Emulsion). Isotopenpraxis 23, 305-308 lO. Ganz, M., G. Feige,-~-N. Edelstein, J. Bucher, T.-C. Zhu, P. Vater, R. Brandt, J. Vetter (1988). Application of Kapton Nuclear Track Microfilters in Liquid-Liquid Extraction of Actinoide Elements. GSI 88-I (Scientific Report 1987) I I . Khan, H.A., N.A. Khan, R. Spohr (198]), NIM 189, 511-581 12. Duschner, H., U. Schneider, P. Vater, R . ~ (1988). Mass Spectrometric Gas Analysis: Nuclear Track Filters as components for the Gas Inlet System. GSI 88-I (Scientific Report 1987)
NT 15:1/4-WW