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The production of porous glass microspheres by the nuclear track technique
0191-278X/87 $3.00 + .00 Pergamon Journals Ltd.
Nucl. Tracks Radiat. Meas., Vol. 11, No. 6, pp. 289-293, 1986 Int. J. Radiat. Appl. lnstrum., Part D Printed in Great Britain
THE PRODUCTION OF POROUS GLASS MICROSPHERES BY THE NUCLEAR TRACK TECHNIQUE* B. STEPHEN CARPENTER
Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, MD 20899, U.S.A. CSABA HORVATH
Department of Chemical Engineering, Yale University, New Haven, CT 06520, U.S.A. and CORAZONR. VOGT~" Environmental Trace Substances Research Center, University of Missouri-Columbia, Columbia, MO 65201, U.S.A.
(Received 25 July 1985; in revised form 24 February 1986)
Abstract--The nuclear track technique (NTT) is used to produce porous glass microspheres. The nuclear tracks randomly penetrate the material so that the resultant pores are interconnected. The result of this process is the creation of latent, radiation-damaged regions by the charged particles emitted from the neutron-induced fissioning of z35U,an isotope of uranium which in trace quantities either naturally occurs in or surrounds the microspheres. The damaged regions, or "tracks" are then enlarged to optically visible tracks with the aid of a light microscope by chemicallyetching the material. The number of tracks or pores created both at the surface and within the microsphere is dependent upon the neutron fluence used to induce the fission of 235U,provided that the bulk uranium is constant in the microspheres. Pore diameter is determined by the concentration of the etching solution and the etching time.
INTRODUCTION
IN THE past the production of porous glass microspheres or porous surfaces has been commonly based on the principle of phase separation of boron-oxide rich regions from a silica matrix. The boron, caused to migrate to the surface of the silica, is then dissolved with water or aqueous solutions leaving cavities that had been previously occupied by the born rich phase. Another technique for making porous glass surfaces has been based on the treatment of the glass surface with the vapor of a silica-dissolving agent. This method produces materials that are superficially porous, i.e. the pores do not penetrate the material but are limited to the surface (Hammel and Allersma, 1974; Elmer and Tischer, 1974; Eaton, 1974a and 1974b; Pretorius and Schieke, 1979; Haller, 1970; Deckman et al., 1983). This work will describe the use of the nuclear track technique (NTT) to produce porous glass microspheres or porous glass surfaces on substrates greater than 4 0 # m thick. The basis of this process is the creation of latent radiation damage regions within the matrix of the material by neutron-induced charged-particle emission. The charged particles can be protons, tritons, alphas or fission fragments. In this work, fissioning fragments are the charged particles emitted from the neutron induced fissioning of
235U, an isotope of uranium which in trace quantities is naturally occurring in the microspheres. The damaged regions, or "Tracks" are then enlarged by chemically etching the material. The number of tracks or pores created both at the surface and within the microsphere is dependent upon the neutron fluence used to induce the fission of 235U, provided that the bulk uranium concentration remains constant in the microspheres. Pore diameter is determined by the concentration of the etching solution and the etching time.
EXPERIMENTAL
In this study the substrate was commercially available soda-lime glass microspheres of nominal composition 70% SiO2, 20% Na~O, 10% CaO (weight percent), and other trace elements, and ranging in size from 20 to 40 p m dia. For the neutron irradiations, 0.5-1.0 g portions of microspheres were placed in 2/5 dram poly vials, heat sealed, and placed in irradiation containers.
Irradiation facilities Two research reactors were used: National Bureau o f Standards Research Reactor (NBSR). The RT-4 pneumatic transfer system of this
*Patent Application Pending. tCurrent Address: Advanced Technology Research, Kellogg Company, Battle Creek, MI 49017, U.S.A. 289
290
B. S T E P H E N C A R P E N T E R
et al.
Table 1. Pore production conditions and results
Sample I 2 3 1 3 3 3-4 4 54 5 5 5 7 5 10 6 7 8 9
Irradiation time (s)
Etchant/time (min)
Bead dia. range (/zm)
Pore dia. range (~m)
Mean pore dia. (~m)
300* 150' 75* 75* 75* 720* 180t 180? 180t 180t 180t 180t 180t 180?
1/4 I/4 I1/5 III/5 IV/7.5 I/4 IV/10 III/0.5 IIl/3 V/7.5 IV/5 II1/5 II/5 I/5
23.8 41.1 28.2 32.8 30.5M2.6 27.5 39.6 27.5 39.6 22.5M1.6 23.8 37.5 37.5M1.7 30.8~43.7 28.4~0. I 28.1 39.1 33.9 43.6 28.5 37.5 30.8 33.3
0.71M.96 1.69 1.01 16.45 5.28 0.79 6.84 2.20 0.55 2.20 1.36 0.72 3.60 1.48 0.45-19.3 2.26 0.98 3.41 1.51 < 1.0 < 1.0 0.5(~9.68 1.58 0.48-3.98 1.44 0.41 5.04 1.56 1.08 10.8 2.75 1.12 9.44 2.97 3.60-9.20 7.88
*National Bureau of Standards Research Reactor (NBSR). ?Missouri University Research Reactor (MURR). Etchants: I--5% HT; I1 2% HF: 11I--1% HF: I V ~ . 5 % V--HF:HCI: HNO3: H20, 1:2:3:118.
10 M W reactor was used. This position h a d a n e u t r o n fluence rate o f 1.33 x 10 L3n cm 2 s 1 with a c a d m i u m ratio for gold of 87. Irradiations were performed at this rate a n d at a reduced rate (approximately 80% of this level) as the reactor was occasionally operated at a reduced 8 M W power level (currently this reactor is licenced to operate at 20 M W a n d the n e u t r o n fluence rate in RT-4 is 2.76 x 1013ncm 2 s l . Irradiation times at the N B S R were 75, 150 a n d 300 s at 10 M W a n d 12 min at the reduced power level.
Missouri University Research Reactor (MURR). The G-2 p n e u m a t i c transfer system of the 10 M W reactor was used. This position is within the reactor core a n d has a n e u t r o n flux of 5.0 x 10 j3 n cm 2 s with a c a d m i u m ratio for gold of 10. In this facility the irradiation time was 180 s.
Etching procedure After irradiation, the microspheres were stored for two weeks to allow residual radioactivity to decay. The microspheres were then removed from the poly vials a n d placed in a 2 9 m m dia. x 1 0 4 m m long r o u n d b o t t o m plastic centrifuge tube. T h e n 3 ml of e t c h a n t solution was added to the tube a n d the mixture was agitated at 22"C. E t c h a n t solutions were p r e p a r e d from N B S purified reagents of H F , HCI or H N O 3 a n d diluted with triple-distilled water. E t c h a n t compositions, concentrations, and times are given in Table 1. Next, the microspheres were separated by sedimentary centrifugation a n d the s u p e r n a t a n t discarded. The microspheres were then rinsed with 30 ml of distilled water, agitated, centrifuged a n d the supern a t a n t discarded. The washing procedure was repeated four times. The final rinse used either e t h a n o l or acetone to hasten the air drying. After the first set o f microspheres was etched, the procedure was modified to include the addition of the 3 0 m l of distilled water to the tube in order to dilute the
HF;
e t c h a n t solution and thereby terminate the etching process. The effect ,~f varying the type of e t c h a n t composition, e t : h a n t concentrations, a n d etching time was also investigated. The aqueous hydroflouoric acid c o n c e n t r a t i o n s o f the etchants was 0.5, 1, 2 and 5 % a n d the etching times were 0.5, 1.0, 3.0, 4.0, 5.0, 7.5 and 10.0 min. Two e t c h a n t mixtures, containing N B S purified reagents H F , HC1, H N O 3, a n d tripledistilled water at volume ratios of 1 : 2 : 3 : 6 a n d 1 : 2 : 3 : 1 1 8 were also used at etching times of 0.5, 1.0 and 7.5 rain. After etching, an aliquot from each b a t c h of etched microspheres was placed o n a scanning electron microscope (SEM) stub a n d s h a d o w e d with gold. The S E M microscopy, using a n A m r a y 1600 T, was performed at the Geology D e p a r t m e n t of the University of Missouri a n d examples o f the p o r o u s microspheres are penetrated in the p h o t o m i c r o g r a p h s in Fig. 1.
Uranium determination Epoxy resin mounts, 25 m m in dia., were prepared by coating a sheet of lead, 10 cm x 10 cm a n d 2 m m thick, with a thin film of epoxy resin o n t o which Bakelite ring forms, 2 5 . 4 m m O D x 2 2 . 2 m m ID, were placed. T h e n 25 mg of microspheres were placed within the ring forms followed by a layer of epoxy resin to cover the microspheres. After the resin h a d set, the m o u n t s were removed from the forms a n d polished to expose p o r t i o n s o f the e m b e d d e d microspheres. Next, muscovite mica track detectors were placed over the m o u n t e d microspheres. This combin a t i o n was placed in a polyethylene bag which was hermetically v a c u u m sealed to ensure intimate contact between detector and m o u n t . Glass s t a n d a r d wafers, N B S S R M 612 a n d 614, with k n o w n u r a n i u m c o n c e n t r a t i o n (Carpenter, 1972 a n d N B S Certificate S t a n d a r d Reference Material 610-616, 1973) of
P R O D U C T I O N OF M I C R O S P H E R E S BY N U C L E A R T R A C K T E C H N I Q U E
291
Table 2. Uranium concentration determined in the microspheres
Sample number SB-lt SB-5t SB-7t SRM-614-3:~ SRM-614-1:~ SRM-612-2§ SRM-612-3:~
Irradiation time (s) 300 75 300 75 300 150 75
Average track density (track cm-2) * 5.507 x 1.202 × 2.104 x 2.731 x 1.079 x 2.096 × 1.005 x
103 10 3
103 103 104 105 105
Uranium concentration (ppb)* 161.9 141.4 61.9 834.5 824.1 37.35 × 1 0 3 35.8 X 1 0 3
*Measurement precision _+3%. tMicrospheres with assumed 235U isotope abundance of 0.7205. :~SRM-614 glasses and SRM-612-3 glass used as an unknown with 235U isotope abundance of 0.2792 and 0.2392, respectively. §SRM-612 used as standard with 235Uisotope abundance of 0.2392.
37.38ppm and 0.823 ppm and 235U atom percent abundances of 0.2392 and 0.2792, respectively, were also covered with mica detectors and sealed in the same manner as the epoxy mounts. Then both the mounts and standard glasses were placed in an irradiation container and irradiated in the NBSR RT-4 pneumatic tube for 300s at a neutron fluence rate of 1.33 x 1013ncm-2s -l. Following the irradiation, residual radioactivity was allowed to decay for 3 weeks before the mica detectors were separated. Then the detectors were etched in 48% H F for 45min at 22°C, rinsed in distilled water, and air dried. The etched detectors were then mounted on microscope slides for track counting with the aid of an optical transmission light microscope at a magnification of 1200 × . The tracks in both detectors, standard glass and microspheres, were counted under the pre-established conditions of either a minimum of at least 1000 tracks or 50 random fields of view. The random fields of view were of an area of 2.54 x 10 -4 cm 2. An example of the track densities obtained from the counting of the detectors is given in Table 2. R ES UL T S AND D I S C U S S I O N The uranium concentration of the microspheres was determined from the mica detectors which covered the epoxy mounts. The results are given in Table 2 and indicate that the uranium distribution in the microspheres was inhomogeneous. As a result of this inhomogeneity in the trace uranium concentration, which varied from 62 ppb to 162 ppb, there was a variation in the surface track density of the porosity of the microspheres. This variation is evident in the SEM photomicrographs of Fig. 1, which shows a random sampling of one of the irradiated batches. Track density, or porosity, is controlled by both U concentration and neutron fluence (the total number of neutrons per cm2). As the neutron fluence increases, the induced fissioning of 235U will increase, The range of the fission fragment will also affect the surface track density or the porosity because the
track is created by the finite distance the fragment traverses as it comes to rest. This finite distance limits the depth from which pore-producing fragments can be generated. If the distance travelled is too short for the fragment to cross the surface of the microsphere or if the latent track is so situated that it is not reached when the bulk material is removed during etching, then an etchable track will not be created. Etching conditions were varied to observe how they affect pore size. Pore diameters were found to be controlled by both the concentration of the etchant and the etching time. The conditions used and the resultant pore size ranges are given in Table 1. The pores generated in this study covered a wide range from 0.41 to 16/~m and there is evidence that the range can be even greater. Bean and co-workers made pores as small as 30 A in mica detectors (Fleischer et al., 1975; Bean et al., 1970). Pores smaller than those found in this study could be made by using milder etchant concentrations and/or shorter etching times. Larger pores could be made by using stronger etchants and/or longer etching times. Figure 2 illustrates that at constant etching times pore size will vary as etchant concentration is changed. CONCLUSION The nuclear track technique can be successfully used to form pores in soda lime glass microspheres. Pore density depends on the uranium concentration and on the neutron fluence exposure. If the 235U concentration is insufficient to produce the desired pore density an alternate means of overcoming this limitation would be to suspend the microspheres in, or adsorb onto their surfaces, a medium with an elevated uranium concentration. In addition, the pore sizes can be regulated by choosing the appropriate etchant concentration and etching time. These porous microspheres can be stabilized for chromatographic use or can be used as an inert carrier for the immobilization of enzymes, catalysts, or drugs that are useful as chemical reactors or for
292
B. S T E P H E N C A R P E N T E R
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F~G. 1. Photomicrographs of porous microspheres. (a) sample CV-2 was irradiated for 150 s at NBSR and etched with 5% HF for 4rain; (b) sample CV-7 was irradiated for 180s at MURR and etched with 1% HF for 5 min; and (c) sample CV-6 was irradiated for 180 s at MURR and etched with 0.5% HF for 5 min.
localized medical treatment. Their a d v a n t a g e over p o r o u s microspheres p r o d u c e d by n o n - i r r a d i a t e d techniques is that the pores are interconnected rather t h a n superficially located at the surface. This total porosity is preferred over surface porosity because it m a k e s the a n c h o r i n g of the organic c o m p o u n d s more
stable and more accessible. In addition, when used in a pressurized flow-through system, these totally porous microspheres should provide a m i n i m u m build-up of back-pressure. Finally, devoid of i m m o bilized organic c o m p o u n d s , these materials also can be used as molecular sieves.
PRODUCTION
OF MICROSPHERES
BY N U C L E A R
TRACK
TECHNIQUE
293
REFERENCES 0
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FIG. 2. Mean pore diameter vs HF etchant concentration.
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Bean C. P., Doyle M. V. and Entine G. (1970) Etching of submicron pores in irradiated mica. J. Appl. Phys. 41, 1454. Carpenter B. S. (1972) Determination of trace concentration of boron and uranium in glass by the nuclear track technique. Analyt. Chem. 44, 600. Deckman H. W., Halpern G. M. and Dunsmuir J. G. (1983) Method for filling hollow shells with gas for use as laser fusion targets. U.S. Patent 4,380,855. Eaton D. L. (1974a) Porous glass chromatographic support. U.S. Patent 3,792,987. Eaton D. L. (1974b) Porous glass chromatographic support. U.S. Patent 3,790,475. Elmer T. H. and Tischer R. E. (1974) Porous glass support for automotive emission control. U.S. Patent 3,804,647. Fleischer R. L., Price P. B. and Walker R. M. (1975) Nuclear Tracks in Solids: Principles and Applications, University of California Press. Haller W. (1970) Material and method for performing steric separations. U.S. Patent 3,549,524. Hammel J. J. and Allersma T. (1974) Stable crush resistant microporous catalyst support. U.S. Patent 3,843,341. National Bureau of Standards, Certificate Standard Reference Material 610 through 616, Trace Elements in Glass (1973). Pretorius V. and Schieke J. D. (1979) Stationary phase surface for chromatography. U.S. Patent 4,169,790.
Nucl. Tracks Radiat. Meas., Vol. 11, No. 6, pp. 289-293, 1986 Int. J. Radiat. Appl. lnstrum., Part D Printed in Great Britain
THE PRODUCTION OF POROUS GLASS MICROSPHERES BY THE NUCLEAR TRACK TECHNIQUE* B. STEPHEN CARPENTER
Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, MD 20899, U.S.A. CSABA HORVATH
Department of Chemical Engineering, Yale University, New Haven, CT 06520, U.S.A. and CORAZONR. VOGT~" Environmental Trace Substances Research Center, University of Missouri-Columbia, Columbia, MO 65201, U.S.A.
(Received 25 July 1985; in revised form 24 February 1986)
Abstract--The nuclear track technique (NTT) is used to produce porous glass microspheres. The nuclear tracks randomly penetrate the material so that the resultant pores are interconnected. The result of this process is the creation of latent, radiation-damaged regions by the charged particles emitted from the neutron-induced fissioning of z35U,an isotope of uranium which in trace quantities either naturally occurs in or surrounds the microspheres. The damaged regions, or "tracks" are then enlarged to optically visible tracks with the aid of a light microscope by chemicallyetching the material. The number of tracks or pores created both at the surface and within the microsphere is dependent upon the neutron fluence used to induce the fission of 235U,provided that the bulk uranium is constant in the microspheres. Pore diameter is determined by the concentration of the etching solution and the etching time.
INTRODUCTION
IN THE past the production of porous glass microspheres or porous surfaces has been commonly based on the principle of phase separation of boron-oxide rich regions from a silica matrix. The boron, caused to migrate to the surface of the silica, is then dissolved with water or aqueous solutions leaving cavities that had been previously occupied by the born rich phase. Another technique for making porous glass surfaces has been based on the treatment of the glass surface with the vapor of a silica-dissolving agent. This method produces materials that are superficially porous, i.e. the pores do not penetrate the material but are limited to the surface (Hammel and Allersma, 1974; Elmer and Tischer, 1974; Eaton, 1974a and 1974b; Pretorius and Schieke, 1979; Haller, 1970; Deckman et al., 1983). This work will describe the use of the nuclear track technique (NTT) to produce porous glass microspheres or porous glass surfaces on substrates greater than 4 0 # m thick. The basis of this process is the creation of latent radiation damage regions within the matrix of the material by neutron-induced charged-particle emission. The charged particles can be protons, tritons, alphas or fission fragments. In this work, fissioning fragments are the charged particles emitted from the neutron induced fissioning of
235U, an isotope of uranium which in trace quantities is naturally occurring in the microspheres. The damaged regions, or "Tracks" are then enlarged by chemically etching the material. The number of tracks or pores created both at the surface and within the microsphere is dependent upon the neutron fluence used to induce the fission of 235U, provided that the bulk uranium concentration remains constant in the microspheres. Pore diameter is determined by the concentration of the etching solution and the etching time.
EXPERIMENTAL
In this study the substrate was commercially available soda-lime glass microspheres of nominal composition 70% SiO2, 20% Na~O, 10% CaO (weight percent), and other trace elements, and ranging in size from 20 to 40 p m dia. For the neutron irradiations, 0.5-1.0 g portions of microspheres were placed in 2/5 dram poly vials, heat sealed, and placed in irradiation containers.
Irradiation facilities Two research reactors were used: National Bureau o f Standards Research Reactor (NBSR). The RT-4 pneumatic transfer system of this
*Patent Application Pending. tCurrent Address: Advanced Technology Research, Kellogg Company, Battle Creek, MI 49017, U.S.A. 289
290
B. S T E P H E N C A R P E N T E R
et al.
Table 1. Pore production conditions and results
Sample I 2 3 1 3 3 3-4 4 54 5 5 5 7 5 10 6 7 8 9
Irradiation time (s)
Etchant/time (min)
Bead dia. range (/zm)
Pore dia. range (~m)
Mean pore dia. (~m)
300* 150' 75* 75* 75* 720* 180t 180? 180t 180t 180t 180t 180t 180?
1/4 I/4 I1/5 III/5 IV/7.5 I/4 IV/10 III/0.5 IIl/3 V/7.5 IV/5 II1/5 II/5 I/5
23.8 41.1 28.2 32.8 30.5M2.6 27.5 39.6 27.5 39.6 22.5M1.6 23.8 37.5 37.5M1.7 30.8~43.7 28.4~0. I 28.1 39.1 33.9 43.6 28.5 37.5 30.8 33.3
0.71M.96 1.69 1.01 16.45 5.28 0.79 6.84 2.20 0.55 2.20 1.36 0.72 3.60 1.48 0.45-19.3 2.26 0.98 3.41 1.51 < 1.0 < 1.0 0.5(~9.68 1.58 0.48-3.98 1.44 0.41 5.04 1.56 1.08 10.8 2.75 1.12 9.44 2.97 3.60-9.20 7.88
*National Bureau of Standards Research Reactor (NBSR). ?Missouri University Research Reactor (MURR). Etchants: I--5% HT; I1 2% HF: 11I--1% HF: I V ~ . 5 % V--HF:HCI: HNO3: H20, 1:2:3:118.
10 M W reactor was used. This position h a d a n e u t r o n fluence rate o f 1.33 x 10 L3n cm 2 s 1 with a c a d m i u m ratio for gold of 87. Irradiations were performed at this rate a n d at a reduced rate (approximately 80% of this level) as the reactor was occasionally operated at a reduced 8 M W power level (currently this reactor is licenced to operate at 20 M W a n d the n e u t r o n fluence rate in RT-4 is 2.76 x 1013ncm 2 s l . Irradiation times at the N B S R were 75, 150 a n d 300 s at 10 M W a n d 12 min at the reduced power level.
Missouri University Research Reactor (MURR). The G-2 p n e u m a t i c transfer system of the 10 M W reactor was used. This position is within the reactor core a n d has a n e u t r o n flux of 5.0 x 10 j3 n cm 2 s with a c a d m i u m ratio for gold of 10. In this facility the irradiation time was 180 s.
Etching procedure After irradiation, the microspheres were stored for two weeks to allow residual radioactivity to decay. The microspheres were then removed from the poly vials a n d placed in a 2 9 m m dia. x 1 0 4 m m long r o u n d b o t t o m plastic centrifuge tube. T h e n 3 ml of e t c h a n t solution was added to the tube a n d the mixture was agitated at 22"C. E t c h a n t solutions were p r e p a r e d from N B S purified reagents of H F , HCI or H N O 3 a n d diluted with triple-distilled water. E t c h a n t compositions, concentrations, and times are given in Table 1. Next, the microspheres were separated by sedimentary centrifugation a n d the s u p e r n a t a n t discarded. The microspheres were then rinsed with 30 ml of distilled water, agitated, centrifuged a n d the supern a t a n t discarded. The washing procedure was repeated four times. The final rinse used either e t h a n o l or acetone to hasten the air drying. After the first set o f microspheres was etched, the procedure was modified to include the addition of the 3 0 m l of distilled water to the tube in order to dilute the
HF;
e t c h a n t solution and thereby terminate the etching process. The effect ,~f varying the type of e t c h a n t composition, e t : h a n t concentrations, a n d etching time was also investigated. The aqueous hydroflouoric acid c o n c e n t r a t i o n s o f the etchants was 0.5, 1, 2 and 5 % a n d the etching times were 0.5, 1.0, 3.0, 4.0, 5.0, 7.5 and 10.0 min. Two e t c h a n t mixtures, containing N B S purified reagents H F , HC1, H N O 3, a n d tripledistilled water at volume ratios of 1 : 2 : 3 : 6 a n d 1 : 2 : 3 : 1 1 8 were also used at etching times of 0.5, 1.0 and 7.5 rain. After etching, an aliquot from each b a t c h of etched microspheres was placed o n a scanning electron microscope (SEM) stub a n d s h a d o w e d with gold. The S E M microscopy, using a n A m r a y 1600 T, was performed at the Geology D e p a r t m e n t of the University of Missouri a n d examples o f the p o r o u s microspheres are penetrated in the p h o t o m i c r o g r a p h s in Fig. 1.
Uranium determination Epoxy resin mounts, 25 m m in dia., were prepared by coating a sheet of lead, 10 cm x 10 cm a n d 2 m m thick, with a thin film of epoxy resin o n t o which Bakelite ring forms, 2 5 . 4 m m O D x 2 2 . 2 m m ID, were placed. T h e n 25 mg of microspheres were placed within the ring forms followed by a layer of epoxy resin to cover the microspheres. After the resin h a d set, the m o u n t s were removed from the forms a n d polished to expose p o r t i o n s o f the e m b e d d e d microspheres. Next, muscovite mica track detectors were placed over the m o u n t e d microspheres. This combin a t i o n was placed in a polyethylene bag which was hermetically v a c u u m sealed to ensure intimate contact between detector and m o u n t . Glass s t a n d a r d wafers, N B S S R M 612 a n d 614, with k n o w n u r a n i u m c o n c e n t r a t i o n (Carpenter, 1972 a n d N B S Certificate S t a n d a r d Reference Material 610-616, 1973) of
P R O D U C T I O N OF M I C R O S P H E R E S BY N U C L E A R T R A C K T E C H N I Q U E
291
Table 2. Uranium concentration determined in the microspheres
Sample number SB-lt SB-5t SB-7t SRM-614-3:~ SRM-614-1:~ SRM-612-2§ SRM-612-3:~
Irradiation time (s) 300 75 300 75 300 150 75
Average track density (track cm-2) * 5.507 x 1.202 × 2.104 x 2.731 x 1.079 x 2.096 × 1.005 x
103 10 3
103 103 104 105 105
Uranium concentration (ppb)* 161.9 141.4 61.9 834.5 824.1 37.35 × 1 0 3 35.8 X 1 0 3
*Measurement precision _+3%. tMicrospheres with assumed 235U isotope abundance of 0.7205. :~SRM-614 glasses and SRM-612-3 glass used as an unknown with 235U isotope abundance of 0.2792 and 0.2392, respectively. §SRM-612 used as standard with 235Uisotope abundance of 0.2392.
37.38ppm and 0.823 ppm and 235U atom percent abundances of 0.2392 and 0.2792, respectively, were also covered with mica detectors and sealed in the same manner as the epoxy mounts. Then both the mounts and standard glasses were placed in an irradiation container and irradiated in the NBSR RT-4 pneumatic tube for 300s at a neutron fluence rate of 1.33 x 1013ncm-2s -l. Following the irradiation, residual radioactivity was allowed to decay for 3 weeks before the mica detectors were separated. Then the detectors were etched in 48% H F for 45min at 22°C, rinsed in distilled water, and air dried. The etched detectors were then mounted on microscope slides for track counting with the aid of an optical transmission light microscope at a magnification of 1200 × . The tracks in both detectors, standard glass and microspheres, were counted under the pre-established conditions of either a minimum of at least 1000 tracks or 50 random fields of view. The random fields of view were of an area of 2.54 x 10 -4 cm 2. An example of the track densities obtained from the counting of the detectors is given in Table 2. R ES UL T S AND D I S C U S S I O N The uranium concentration of the microspheres was determined from the mica detectors which covered the epoxy mounts. The results are given in Table 2 and indicate that the uranium distribution in the microspheres was inhomogeneous. As a result of this inhomogeneity in the trace uranium concentration, which varied from 62 ppb to 162 ppb, there was a variation in the surface track density of the porosity of the microspheres. This variation is evident in the SEM photomicrographs of Fig. 1, which shows a random sampling of one of the irradiated batches. Track density, or porosity, is controlled by both U concentration and neutron fluence (the total number of neutrons per cm2). As the neutron fluence increases, the induced fissioning of 235U will increase, The range of the fission fragment will also affect the surface track density or the porosity because the
track is created by the finite distance the fragment traverses as it comes to rest. This finite distance limits the depth from which pore-producing fragments can be generated. If the distance travelled is too short for the fragment to cross the surface of the microsphere or if the latent track is so situated that it is not reached when the bulk material is removed during etching, then an etchable track will not be created. Etching conditions were varied to observe how they affect pore size. Pore diameters were found to be controlled by both the concentration of the etchant and the etching time. The conditions used and the resultant pore size ranges are given in Table 1. The pores generated in this study covered a wide range from 0.41 to 16/~m and there is evidence that the range can be even greater. Bean and co-workers made pores as small as 30 A in mica detectors (Fleischer et al., 1975; Bean et al., 1970). Pores smaller than those found in this study could be made by using milder etchant concentrations and/or shorter etching times. Larger pores could be made by using stronger etchants and/or longer etching times. Figure 2 illustrates that at constant etching times pore size will vary as etchant concentration is changed. CONCLUSION The nuclear track technique can be successfully used to form pores in soda lime glass microspheres. Pore density depends on the uranium concentration and on the neutron fluence exposure. If the 235U concentration is insufficient to produce the desired pore density an alternate means of overcoming this limitation would be to suspend the microspheres in, or adsorb onto their surfaces, a medium with an elevated uranium concentration. In addition, the pore sizes can be regulated by choosing the appropriate etchant concentration and etching time. These porous microspheres can be stabilized for chromatographic use or can be used as an inert carrier for the immobilization of enzymes, catalysts, or drugs that are useful as chemical reactors or for
292
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localized medical treatment. Their a d v a n t a g e over p o r o u s microspheres p r o d u c e d by n o n - i r r a d i a t e d techniques is that the pores are interconnected rather t h a n superficially located at the surface. This total porosity is preferred over surface porosity because it m a k e s the a n c h o r i n g of the organic c o m p o u n d s more
stable and more accessible. In addition, when used in a pressurized flow-through system, these totally porous microspheres should provide a m i n i m u m build-up of back-pressure. Finally, devoid of i m m o bilized organic c o m p o u n d s , these materials also can be used as molecular sieves.
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Bean C. P., Doyle M. V. and Entine G. (1970) Etching of submicron pores in irradiated mica. J. Appl. Phys. 41, 1454. Carpenter B. S. (1972) Determination of trace concentration of boron and uranium in glass by the nuclear track technique. Analyt. Chem. 44, 600. Deckman H. W., Halpern G. M. and Dunsmuir J. G. (1983) Method for filling hollow shells with gas for use as laser fusion targets. U.S. Patent 4,380,855. Eaton D. L. (1974a) Porous glass chromatographic support. U.S. Patent 3,792,987. Eaton D. L. (1974b) Porous glass chromatographic support. U.S. Patent 3,790,475. Elmer T. H. and Tischer R. E. (1974) Porous glass support for automotive emission control. U.S. Patent 3,804,647. Fleischer R. L., Price P. B. and Walker R. M. (1975) Nuclear Tracks in Solids: Principles and Applications, University of California Press. Haller W. (1970) Material and method for performing steric separations. U.S. Patent 3,549,524. Hammel J. J. and Allersma T. (1974) Stable crush resistant microporous catalyst support. U.S. Patent 3,843,341. National Bureau of Standards, Certificate Standard Reference Material 610 through 616, Trace Elements in Glass (1973). Pretorius V. and Schieke J. D. (1979) Stationary phase surface for chromatography. U.S. Patent 4,169,790.