- Email: [email protected]
Development of nuclear micro-battery with solid tritium source
ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 1234–1238
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Development of nuclear micro-battery with solid tritium source Sook-Kyung Lee a,, Soon-Hwan Son a, KwangSin Kim a, Jong-Wan Park b, Hun Lim c, Jae-Min Lee d, Eun-Su Chung d a
Nuclear Power Laboratory, Korea Electric Power Research Institute, 65 Munji-Ro, Yuseong-Gu, Daejeon, Republic of Korea Technical Application Team, National Nanofab Center, 53-3 Eoeun-Dong, Yuseong-Gu, Daejeon, Republic of Korea Control Technology Research Institute, Samchang Enterprise Co., Ltd., 974-1 Goyeon-Ri, Woongchon-Myon, Ulju-Gun, Ulsan, Republic of Korea d Radiation Environment Team, EnEsys Co., Ltd., 328 Guam-Dong, Yuseong-Gu, Daejeon, Republic of Korea b c
a r t i c l e in f o
a b s t r a c t
Keywords: Tritium Nuclear battery Titanium tritide Betavoltaic
A micro-battery powered by tritium is being developed to utilize tritium produced from the Wolsong Tritium Removal Facility. The 3D p–n junction device has been designed and fabricated for energy conversion. Titanium tritide is adopted to increase tritium density and safety. Sub micron films or nanopowders of titanium tritide is applied on silicon semiconductor device to reduce the self absorption of beta rays. Until now protium has been used instead of tritium for safety. Hydrogen was absorbed up to atomic ratio of 1.3 and 1.7 in titanium powders and films, respectively. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Since 2007, tritium is produced from Wolsong Tritium Removal Facility located in Wolsong CANDU Nuclear Power Plant site in Korea. Tritium is a radioactive isotope of hydrogen with similar chemical and physical properties to hydrogen. Tritium is a pure beta ray source with 0–18.6 keV (5.7 keV in average) energies and 12.323 years of a half-life. The fact that tritium is a pure beta source and emits the low energy beta rays makes it very attractive to be used in the commercial product without worrying much about radiation hazards. To utilize the tritium as a resource a nuclear battery using tritium is being developed. The tritium battery utilizes these beta rays as energy source to produce electricity. The principle of tritium battery is very similar to solar cell batteries. The solar cell uses visible light as the energy source while the tritium battery uses beta ray—flux of electrons as the energy source. The mechanism inside the semiconductor to produce electron–hole pair is similar for both batteries. Previous tritium batteries used tritium gas as the energy source (Braun et al., 1973). However, gaseous tritium has several problems such as low tritium density (i.e. low beta ray density) and leakage. To address these problems, the solid state tritium source was considered in this project. Metal hydrides are the good candidate for the solid state tritium source. Tritiated amorphous silicon can be used but its tritium storage capacity is smaller than metal tritides (Kosteski et al., 2005). Table 1 shows properties of some hydrogen storage metals. Metal’s stabilities, the equilibrium
Corresponding author. Tel.: +82 42 865 5562; fax: +82 42 865 5504.
E-mail address: [email protected] (S.K. Lee). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.02.064
pressure ðP H2 Þ, desorption temperature (Tdes), the metal’s activation temperature (Tact) and pressure (Pact), and the manufacturing processes of semiconductor were considered to choose the metal. Titanium and zirconium were good candidates. Titanium was chosen over zirconium due to its higher hydrogen storage capacity. Fig. 1 shows the manufacturing procedures of the tritium micro-batteries. The manufacturing procedures consist of the design and manufacturing of semiconductors, manufacturing of solid tritium sources, and sealing and packaging batteries. One problem with the titanium tritide is its relatively high beta ray self-absorption. Due to low energy of beta rays emitted from tritium, beta rays tend to be absorbed by storage material. To reduce the self-absorption, the thickness of titanium tritide should be minimized. Considering the self-absorption, the efficiency of the beta ray was assumed 16% for 500 nm titanium tritide films (Kherani and Shmayda, 1992). It is clear that thicker film absorbs more beta rays. However, too thin film contains too small amount of tritium such that the advantage of having higher tritium density compare to tritium gas is lost. Thus, we opted to use films with 500 nm thickness and powders with 100 nm or smaller in diameter. With powders, we made titanium tritide layers of about 500 nm thick on the p–n junction surface. For 500 nm thick titanium tritide films, the effective beta ray energy is more than 500 times higher than that of tritium gas at STP. The advantages of large storage capacity, higher concentration and safer storage of tritium make metal tritide batteries superior to gaseous tritium batteries. This paper introduces the development of manufacturing methods of tritium micro-batteries with the solid tritium source.
ARTICLE IN PRESS S.K. Lee et al. / Applied Radiation and Isotopes 67 (2009) 1234–1238
2. Materials and methods
Ar
Tritium cannot be used freely in the experiments. Since it is radioactive, there is a license limit to the tritium amount which can be used in the lab. We replaced tritium with protium in the experiments whenever it is possible. Otherwise, the properties of beta rays of tritium were simulated with visible light and electron beam from a scanning electron microscope (SEM). 2.1. Tritiation
1235
H2
O2/moisture trap
Two types of titanium—powder and film were used as the solid tritium source. Fine powders are better because self-absorption is less and the powder can be coated more uniformly. Commercially available nano-powders from Aldrich and Nano Technology were used for powder type source. Titanium films were coated directly on the p–n junction surface by sputtering. Fig. 2 shows the schematic diagram of solid tritium source manufacturing system. The solid tritium source manufacturing apparatus activates metal powders and films for tritiation and measures the amount of absorbed tritium gas quantitatively as
Vacuum system
P
T
Metering tank Table 1 Properties of hydrogen storage metals. Metal
Capacity
P H2 (atm)
Tdes (1C)
Tact (1C)
Pact (atm)
Remarks
Mg
7.66
1 106
325
10
Pd
0.72
0.0082
N/A
N/A
Ti Zr
3.98 2.16
4 1020 6.4 1028
279 287 85 147 643 881
400–600 400–600
1 1
Ca
4.76
o10
1,050
4300
1
U
1.25
1.4 1013
432
250
1
Tdes too low PH2 too high Tdes too low Too expensive Adopted Lower capacity than Ti Chemically unstable Nuclear material
25
holder Heating system
Fig. 2. The Solid tritium source manufacturing apparatus.
Design of semiconductor
Manufacturing of tritium powder source
Manufacturing of slurry
Sample
Manufacturing of
Test of characteristics of semiconductor
semiconductor
Ti sputtering on semiconductor
Coating of slurry on semiconductor
Injection of tritium intoTi film
Sealing and packaging of tritium battery
Test of tritium battery
Fig. 1. Manufacturing flowchart of tritium micro-batteries.
ARTICLE IN PRESS 1236
S.K. Lee et al. / Applied Radiation and Isotopes 67 (2009) 1234–1238
well as tritiates the metals. The tritium handling system stores tritium gas in uranium beds, delivers tritium gas to the solid tritium source manufacturing apparatus, and recovers unused tritium gas from the solid tritium source manufacturing apparatus. The extent of tritiation was measured quantitatively with elastic recoil detection (ERD) method for films and with P-V-T measurements for powders. 2.2. Slurry preparation Since the powders are very tricky to handle, it is not easy to combine the titanium tritide powder with the p–n junction. Thus, the powder was made into slurry by adding solvent before it was applied on the surface of the device. To make the slurry stable appropriate dispersants were added to the slurry. Polar and nonpolar solvents such as pentane, hexane, acetone, and acetyl acetone, ethylene glycol, iso-propyl alcohol were tested. Materials containing both hydrophilic and hydrophobic radicals such as polysilane, oxalic acid, poly-methyl methacrylate (PMMA), and poly acrylic acid sodium salt were tested as dispersants. The micro-pipette was used to apply the slurry on the device. Since some of the devices were as small as 2 mm 2 mm, the optical microscope with closed circuit camera was used while the slurry was applied on the cell. 2.3. Design of p–n junction device Silicon was chosen for semiconductor material considering the energy level of beta rays from tritium. The maximum energy of beta rays from tritium is only 18.6 keV while the energy level of beta ray which can damage the silicon is 140–200 keV (Olsen, 1973). P-type single crystal silicon wafer was used as the starting
material for the p–n junction device. As mentioned earlier, the solid tritium source increases the density of tritium on the cell. To increase the tritium density further, the surface area is increased by etching micron sized holes on the surface of the device. After the etching, phosphorus was implanted to form n-type layer on the device to make diode type. Fig. 3 shows the cross-sectional schemes of p–n junction type semiconductor for tritium micro-battery and the optical microscopic pictures of the semiconductors. The prototype design (a) shown in Fig. 3 was improved and revised. The revised design (b) has larger surface area by increasing the well depth and the electrode material was changed from aluminum to gold for better conductivity and solderability. When the devices with film type source were made, titanium was sputtered on the junction area after the implantation. For the devices for the powder type source, this titanium sputtering step was omitted. The next step is to sputter the electrode material on the p–n junction to complete the device manufacturing. Aluminum was used as the electrode material at first. Later it was changed to gold since wires cannot be soldered onto aluminum electrodes. 2.4. Test of the p–n junction device Since tritium cannot be used freely, two alternative methods were used to test the characteristics of the p–n junction devices. Firstly, the output current and voltage were measured when the device is exposed to visible light. Since the devices have very similar operating principle and structure to solar cells, the cells react to visible lights even though they were not optimized for sun light. Thus, visible light was used only to test whether the cells had the characteristics of diodes or not. Secondly, a SEM was used to examine the cells. The electron beam of the SEM simulates beta
Ti/Al SiO2
N+
PN junction area
P+ P-type Silicon NPrototype
SiO2
N-
N+ P-substrate P+ Ti/Au Revision 1
Fig. 3. The cross sectional schemes of p–n junction type semiconductor for tritium micro-batteries and their optical microscopic pictures (top view).
ARTICLE IN PRESS S.K. Lee et al. / Applied Radiation and Isotopes 67 (2009) 1234–1238
rays from tritium. While electron beam of the SEM has a single energy level at a time, beta rays from tritium have a unique spectrum of energy levels. For now, experiments were done at 6 keV, which is close to the average beta ray energy of (tritium) 5.7 keV. In the future the energy level of electron beam will be changed from 1 to 18.6 keV and the beta ray spectrum will be simulated closely. The flux of electron beam was measured with Faraday cup and controlled with spot size of the beam.
3. Results and discussion 3.1. Tritiation Fig. 4 shows the ERD analysis of the hydride films. The manufactured titanium hydride showed maximum atomic ratios of 1.7(TiT1.7) for films. Oxide layer of 100–250 nm existed on the surface of the film and the hydrogen content was increasing along the depth of the film. The heating temperature was 500 1C in Fig. 4(a). We also tried 600 1C in Fig. 4(b) but the temperature was too high that the surface showed cracks and peel-offs. Contrary to the films which needed to be heated to absorb hydrogen, powders could absorb hydrogen at room temperature after the activation due to their large surface area. Repeated cooling and heating to 500 1C in hydrogen atmosphere did not show improvements in hydrogenation. The manufactured titanium hydride powder showed maximum atomic ratios of 1.3(TiT1.3) as shown in Table 2. 3.2. Coating of titanium hydride slurry on p–n junction device
acetone, the result is somewhat better having the thinner coating layer as shown in Fig. 5(b). However, the powder tended to sag to the bottom and showed the uncoated areas at the top. When dispersant was changed to poly acrylic acid sodium salt from polysilane and PMMA mixture, the result was best: showing thin and even layer on all the surfaces as shown in Fig. 5(c). The same result was obtained when the slurry was applied on different shape of hole as shown in Fig. 5(d) even though some coarse particles were present at the bottom of the holes due to uneven particle size distribution in the slurry. Poly acrylic acid sodium salt has both polar and non-polar properties. It seemed that non-polar radicals were adsorbed onto titanium hydride particle surface. The ends of the adsorbed sodium salt polymer chain were slightly charged, which prevents the tangling of adsorbed polymers. Thus, poly acrylic acid sodium salt could be an excellent candidate for dispersant in our experiments.
3.3. Beta ray simulation Since the production of semiconductor cell has not been completed, commercial solar cell was used in beta ray simulation experiments. The flux of electron beam was controlled to match the spectrum of beta ray emitted from titanium plate (Kherani and Shmayda, 1992). Fig. 6 illustrates two curves: (a) estimated spectrum of beta ray from TiT2 plate (Kherani and Shmayda, 1992), (b) energy level and spot size of SEM electron beam to simulate beta ray spectrum from 500 nm thick TiT2 plate. These data will be used in the future to simulate beta ray closely. The output current from solar cell showed linear relationship to the electron beam flux at all energy levels up to 16 keV. Table 2 H2/Ti atomic ratio of titanium hydride powder. No.
Closed thermal cycle
1 2 3 4 5 6
1.36 1.30 1.32 1.29 1.31 1.29
Mean
1.31
2
2
1.8
1.8
1.6
exp3_H
1.6
1.4
exp3_O
1.4
exp8_H
1.2
exp4_H
1.2
exp8_O
1
exp9_H
0.8
exp9_O
0.6
exp10_H
0.4
exp10_O
1
exp4_O
0.8
exp5_H
0.6
exp5_O
0.4
exp6_H
0.2
exp6_O
0
exp7_H 0
500 1000 Depth (nm)
exp7_O
TiTx (600°C)
TiTx (500°C)
Various polar and non-polar solvents were tested for the preparation of slurries. It was proved that solvents having oxygen with unshared electrons, such as acetone showed the best result. Also the boiling point of the solvents should not be too high so that it can be removed without affecting the tritide and the silicon cell, and should not be too low so that it can maintain stable slurry composition. As a result, acetone and acetyl acetone were chosen for slurry preparation. Dispersants which worked well with the solvents were chosen. Fig. 5 shows some SEM photos of powder coated cell surfaces after solvents were removed. Fig. 5(a) is the result of slurry with acetone and dispersant of polysilane and PMMA mixture. The mixed dispersant was recommended by a former research (Hurysz, 1998). It shows porous and uneven distribution of powders. When the solvent was changed to acetyl
1237
exp11_H
0.2 0
exp11_O 0
500 1000 Depth (nm)
Fig. 4. Depth profile of hydrade films: (a) heating at 500 1C and (b) heating at 600 1C.
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S.K. Lee et al. / Applied Radiation and Isotopes 67 (2009) 1234–1238
Fig. 5. Photos of coated cell surface: (a) acetone, polysilane and PMMA, (b) acetyl acetone, polysilane and PMMA, (c) acetyl acetone, poly acrylic acid sodium salt and (d) acetyl acetone, poly acrylic acid sodium salt.
Energy Spectrum
4
500nm 200nm 100nm
3
Tritium Spot Size 32 Spot Size 33 Spot Size 34 Spot Size 35 Spot Size 36 Spot Size 37 Spot Size 38 Spot Size 39 Spot Size 40
3
2.5 2 1.5 1
2.5 2 1.5 1
0.5 0
Energy Spectrum
3.5
Flux (pA)
Electron Flux (pA/cm2)
3.5
0.5 0 2 4 6 8 10 12 14 16 Electron Energy (keV)
0
0 2 4
6 8 10 12 14 16 Energy (keV)
Fig. 6. Comparison of beta ray spectrum and its simulation with SEM: (a) estimated beta ray spectrum TiT2 plate and (b) energy level and spot size to simulate the spectrum from 500 nm thick TiT2 plate.
4. Conclusion
Acknowledgments
To make a tritium battery with p–n junction device and solid tritium source, the conditions and method to manufacture titanium hydride was developed. The activation was performed at 500 1C and film was harder than powder to hydrogenate. Maximum atomic ratios of 1.3 and 1.7 were obtained for powders and films, respectively. Further optimization is required to enhance the atomic ratio. So far, slurry preparation comprising acetyl acetone as solvent and poly acrylic acid sodium salt as dispersant showed the best result. The optimum composition of the slurries is to be found through further research. Applying slurry on the cell by hand with the micro-pipette was simple. Semiconductor p–n junction device is still under development to be optimized for tritium beta rays. The technique to simulate beta rays with a scanning electron microscope was developed and will be used to test the cells.
This work has been financially supported by the Ministry of Commercial, Industry and Energy of Korea, Grant 06NG05. The authors would like to give thanks to the Electric Power Industry Technology Evaluation and Planning (ETEP) of Korea, too. References Braun, J., Fermvik, L., Stenback, A., 1973. Theory and performance of a tritium battery for the microwatt range. Journal of Physics E: Scientific Instruments 6, 727–731. Hurysz, K.M., 1998. The processing of titanium hydride powders into uniform hollow spheres. Master’s Thesis of Georgia Institute of Technology, pp. 56–61. Kherani, N.P., Shmayda, W.T., 1992. Electron flux at the surface of titanium tritide films. Fusion Technology 21, 334–339. Kosteski, T., Kherani, N.P., Shmayda, W.T., Costea, S., Zukotynski, S., 2005. Nuclear batteries using tritium and thin film hydrogenated amorphous silicon. Fusion Science and Technology 48, 700–703. Olsen, L.C., 1973. Betavoltaic energy conversion. Energy Conversion 13, 117–127.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Development of nuclear micro-battery with solid tritium source Sook-Kyung Lee a,, Soon-Hwan Son a, KwangSin Kim a, Jong-Wan Park b, Hun Lim c, Jae-Min Lee d, Eun-Su Chung d a
Nuclear Power Laboratory, Korea Electric Power Research Institute, 65 Munji-Ro, Yuseong-Gu, Daejeon, Republic of Korea Technical Application Team, National Nanofab Center, 53-3 Eoeun-Dong, Yuseong-Gu, Daejeon, Republic of Korea Control Technology Research Institute, Samchang Enterprise Co., Ltd., 974-1 Goyeon-Ri, Woongchon-Myon, Ulju-Gun, Ulsan, Republic of Korea d Radiation Environment Team, EnEsys Co., Ltd., 328 Guam-Dong, Yuseong-Gu, Daejeon, Republic of Korea b c
a r t i c l e in f o
a b s t r a c t
Keywords: Tritium Nuclear battery Titanium tritide Betavoltaic
A micro-battery powered by tritium is being developed to utilize tritium produced from the Wolsong Tritium Removal Facility. The 3D p–n junction device has been designed and fabricated for energy conversion. Titanium tritide is adopted to increase tritium density and safety. Sub micron films or nanopowders of titanium tritide is applied on silicon semiconductor device to reduce the self absorption of beta rays. Until now protium has been used instead of tritium for safety. Hydrogen was absorbed up to atomic ratio of 1.3 and 1.7 in titanium powders and films, respectively. & 2009 Elsevier Ltd. All rights reserved.
1. Introduction Since 2007, tritium is produced from Wolsong Tritium Removal Facility located in Wolsong CANDU Nuclear Power Plant site in Korea. Tritium is a radioactive isotope of hydrogen with similar chemical and physical properties to hydrogen. Tritium is a pure beta ray source with 0–18.6 keV (5.7 keV in average) energies and 12.323 years of a half-life. The fact that tritium is a pure beta source and emits the low energy beta rays makes it very attractive to be used in the commercial product without worrying much about radiation hazards. To utilize the tritium as a resource a nuclear battery using tritium is being developed. The tritium battery utilizes these beta rays as energy source to produce electricity. The principle of tritium battery is very similar to solar cell batteries. The solar cell uses visible light as the energy source while the tritium battery uses beta ray—flux of electrons as the energy source. The mechanism inside the semiconductor to produce electron–hole pair is similar for both batteries. Previous tritium batteries used tritium gas as the energy source (Braun et al., 1973). However, gaseous tritium has several problems such as low tritium density (i.e. low beta ray density) and leakage. To address these problems, the solid state tritium source was considered in this project. Metal hydrides are the good candidate for the solid state tritium source. Tritiated amorphous silicon can be used but its tritium storage capacity is smaller than metal tritides (Kosteski et al., 2005). Table 1 shows properties of some hydrogen storage metals. Metal’s stabilities, the equilibrium
Corresponding author. Tel.: +82 42 865 5562; fax: +82 42 865 5504.
E-mail address: [email protected] (S.K. Lee). 0969-8043/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.02.064
pressure ðP H2 Þ, desorption temperature (Tdes), the metal’s activation temperature (Tact) and pressure (Pact), and the manufacturing processes of semiconductor were considered to choose the metal. Titanium and zirconium were good candidates. Titanium was chosen over zirconium due to its higher hydrogen storage capacity. Fig. 1 shows the manufacturing procedures of the tritium micro-batteries. The manufacturing procedures consist of the design and manufacturing of semiconductors, manufacturing of solid tritium sources, and sealing and packaging batteries. One problem with the titanium tritide is its relatively high beta ray self-absorption. Due to low energy of beta rays emitted from tritium, beta rays tend to be absorbed by storage material. To reduce the self-absorption, the thickness of titanium tritide should be minimized. Considering the self-absorption, the efficiency of the beta ray was assumed 16% for 500 nm titanium tritide films (Kherani and Shmayda, 1992). It is clear that thicker film absorbs more beta rays. However, too thin film contains too small amount of tritium such that the advantage of having higher tritium density compare to tritium gas is lost. Thus, we opted to use films with 500 nm thickness and powders with 100 nm or smaller in diameter. With powders, we made titanium tritide layers of about 500 nm thick on the p–n junction surface. For 500 nm thick titanium tritide films, the effective beta ray energy is more than 500 times higher than that of tritium gas at STP. The advantages of large storage capacity, higher concentration and safer storage of tritium make metal tritide batteries superior to gaseous tritium batteries. This paper introduces the development of manufacturing methods of tritium micro-batteries with the solid tritium source.
ARTICLE IN PRESS S.K. Lee et al. / Applied Radiation and Isotopes 67 (2009) 1234–1238
2. Materials and methods
Ar
Tritium cannot be used freely in the experiments. Since it is radioactive, there is a license limit to the tritium amount which can be used in the lab. We replaced tritium with protium in the experiments whenever it is possible. Otherwise, the properties of beta rays of tritium were simulated with visible light and electron beam from a scanning electron microscope (SEM). 2.1. Tritiation
1235
H2
O2/moisture trap
Two types of titanium—powder and film were used as the solid tritium source. Fine powders are better because self-absorption is less and the powder can be coated more uniformly. Commercially available nano-powders from Aldrich and Nano Technology were used for powder type source. Titanium films were coated directly on the p–n junction surface by sputtering. Fig. 2 shows the schematic diagram of solid tritium source manufacturing system. The solid tritium source manufacturing apparatus activates metal powders and films for tritiation and measures the amount of absorbed tritium gas quantitatively as
Vacuum system
P
T
Metering tank Table 1 Properties of hydrogen storage metals. Metal
Capacity
P H2 (atm)
Tdes (1C)
Tact (1C)
Pact (atm)
Remarks
Mg
7.66
1 106
325
10
Pd
0.72
0.0082
N/A
N/A
Ti Zr
3.98 2.16
4 1020 6.4 1028
279 287 85 147 643 881
400–600 400–600
1 1
Ca
4.76
o10
1,050
4300
1
U
1.25
1.4 1013
432
250
1
Tdes too low PH2 too high Tdes too low Too expensive Adopted Lower capacity than Ti Chemically unstable Nuclear material
25
holder Heating system
Fig. 2. The Solid tritium source manufacturing apparatus.
Design of semiconductor
Manufacturing of tritium powder source
Manufacturing of slurry
Sample
Manufacturing of
Test of characteristics of semiconductor
semiconductor
Ti sputtering on semiconductor
Coating of slurry on semiconductor
Injection of tritium intoTi film
Sealing and packaging of tritium battery
Test of tritium battery
Fig. 1. Manufacturing flowchart of tritium micro-batteries.
ARTICLE IN PRESS 1236
S.K. Lee et al. / Applied Radiation and Isotopes 67 (2009) 1234–1238
well as tritiates the metals. The tritium handling system stores tritium gas in uranium beds, delivers tritium gas to the solid tritium source manufacturing apparatus, and recovers unused tritium gas from the solid tritium source manufacturing apparatus. The extent of tritiation was measured quantitatively with elastic recoil detection (ERD) method for films and with P-V-T measurements for powders. 2.2. Slurry preparation Since the powders are very tricky to handle, it is not easy to combine the titanium tritide powder with the p–n junction. Thus, the powder was made into slurry by adding solvent before it was applied on the surface of the device. To make the slurry stable appropriate dispersants were added to the slurry. Polar and nonpolar solvents such as pentane, hexane, acetone, and acetyl acetone, ethylene glycol, iso-propyl alcohol were tested. Materials containing both hydrophilic and hydrophobic radicals such as polysilane, oxalic acid, poly-methyl methacrylate (PMMA), and poly acrylic acid sodium salt were tested as dispersants. The micro-pipette was used to apply the slurry on the device. Since some of the devices were as small as 2 mm 2 mm, the optical microscope with closed circuit camera was used while the slurry was applied on the cell. 2.3. Design of p–n junction device Silicon was chosen for semiconductor material considering the energy level of beta rays from tritium. The maximum energy of beta rays from tritium is only 18.6 keV while the energy level of beta ray which can damage the silicon is 140–200 keV (Olsen, 1973). P-type single crystal silicon wafer was used as the starting
material for the p–n junction device. As mentioned earlier, the solid tritium source increases the density of tritium on the cell. To increase the tritium density further, the surface area is increased by etching micron sized holes on the surface of the device. After the etching, phosphorus was implanted to form n-type layer on the device to make diode type. Fig. 3 shows the cross-sectional schemes of p–n junction type semiconductor for tritium micro-battery and the optical microscopic pictures of the semiconductors. The prototype design (a) shown in Fig. 3 was improved and revised. The revised design (b) has larger surface area by increasing the well depth and the electrode material was changed from aluminum to gold for better conductivity and solderability. When the devices with film type source were made, titanium was sputtered on the junction area after the implantation. For the devices for the powder type source, this titanium sputtering step was omitted. The next step is to sputter the electrode material on the p–n junction to complete the device manufacturing. Aluminum was used as the electrode material at first. Later it was changed to gold since wires cannot be soldered onto aluminum electrodes. 2.4. Test of the p–n junction device Since tritium cannot be used freely, two alternative methods were used to test the characteristics of the p–n junction devices. Firstly, the output current and voltage were measured when the device is exposed to visible light. Since the devices have very similar operating principle and structure to solar cells, the cells react to visible lights even though they were not optimized for sun light. Thus, visible light was used only to test whether the cells had the characteristics of diodes or not. Secondly, a SEM was used to examine the cells. The electron beam of the SEM simulates beta
Ti/Al SiO2
N+
PN junction area
P+ P-type Silicon NPrototype
SiO2
N-
N+ P-substrate P+ Ti/Au Revision 1
Fig. 3. The cross sectional schemes of p–n junction type semiconductor for tritium micro-batteries and their optical microscopic pictures (top view).
ARTICLE IN PRESS S.K. Lee et al. / Applied Radiation and Isotopes 67 (2009) 1234–1238
rays from tritium. While electron beam of the SEM has a single energy level at a time, beta rays from tritium have a unique spectrum of energy levels. For now, experiments were done at 6 keV, which is close to the average beta ray energy of (tritium) 5.7 keV. In the future the energy level of electron beam will be changed from 1 to 18.6 keV and the beta ray spectrum will be simulated closely. The flux of electron beam was measured with Faraday cup and controlled with spot size of the beam.
3. Results and discussion 3.1. Tritiation Fig. 4 shows the ERD analysis of the hydride films. The manufactured titanium hydride showed maximum atomic ratios of 1.7(TiT1.7) for films. Oxide layer of 100–250 nm existed on the surface of the film and the hydrogen content was increasing along the depth of the film. The heating temperature was 500 1C in Fig. 4(a). We also tried 600 1C in Fig. 4(b) but the temperature was too high that the surface showed cracks and peel-offs. Contrary to the films which needed to be heated to absorb hydrogen, powders could absorb hydrogen at room temperature after the activation due to their large surface area. Repeated cooling and heating to 500 1C in hydrogen atmosphere did not show improvements in hydrogenation. The manufactured titanium hydride powder showed maximum atomic ratios of 1.3(TiT1.3) as shown in Table 2. 3.2. Coating of titanium hydride slurry on p–n junction device
acetone, the result is somewhat better having the thinner coating layer as shown in Fig. 5(b). However, the powder tended to sag to the bottom and showed the uncoated areas at the top. When dispersant was changed to poly acrylic acid sodium salt from polysilane and PMMA mixture, the result was best: showing thin and even layer on all the surfaces as shown in Fig. 5(c). The same result was obtained when the slurry was applied on different shape of hole as shown in Fig. 5(d) even though some coarse particles were present at the bottom of the holes due to uneven particle size distribution in the slurry. Poly acrylic acid sodium salt has both polar and non-polar properties. It seemed that non-polar radicals were adsorbed onto titanium hydride particle surface. The ends of the adsorbed sodium salt polymer chain were slightly charged, which prevents the tangling of adsorbed polymers. Thus, poly acrylic acid sodium salt could be an excellent candidate for dispersant in our experiments.
3.3. Beta ray simulation Since the production of semiconductor cell has not been completed, commercial solar cell was used in beta ray simulation experiments. The flux of electron beam was controlled to match the spectrum of beta ray emitted from titanium plate (Kherani and Shmayda, 1992). Fig. 6 illustrates two curves: (a) estimated spectrum of beta ray from TiT2 plate (Kherani and Shmayda, 1992), (b) energy level and spot size of SEM electron beam to simulate beta ray spectrum from 500 nm thick TiT2 plate. These data will be used in the future to simulate beta ray closely. The output current from solar cell showed linear relationship to the electron beam flux at all energy levels up to 16 keV. Table 2 H2/Ti atomic ratio of titanium hydride powder. No.
Closed thermal cycle
1 2 3 4 5 6
1.36 1.30 1.32 1.29 1.31 1.29
Mean
1.31
2
2
1.8
1.8
1.6
exp3_H
1.6
1.4
exp3_O
1.4
exp8_H
1.2
exp4_H
1.2
exp8_O
1
exp9_H
0.8
exp9_O
0.6
exp10_H
0.4
exp10_O
1
exp4_O
0.8
exp5_H
0.6
exp5_O
0.4
exp6_H
0.2
exp6_O
0
exp7_H 0
500 1000 Depth (nm)
exp7_O
TiTx (600°C)
TiTx (500°C)
Various polar and non-polar solvents were tested for the preparation of slurries. It was proved that solvents having oxygen with unshared electrons, such as acetone showed the best result. Also the boiling point of the solvents should not be too high so that it can be removed without affecting the tritide and the silicon cell, and should not be too low so that it can maintain stable slurry composition. As a result, acetone and acetyl acetone were chosen for slurry preparation. Dispersants which worked well with the solvents were chosen. Fig. 5 shows some SEM photos of powder coated cell surfaces after solvents were removed. Fig. 5(a) is the result of slurry with acetone and dispersant of polysilane and PMMA mixture. The mixed dispersant was recommended by a former research (Hurysz, 1998). It shows porous and uneven distribution of powders. When the solvent was changed to acetyl
1237
exp11_H
0.2 0
exp11_O 0
500 1000 Depth (nm)
Fig. 4. Depth profile of hydrade films: (a) heating at 500 1C and (b) heating at 600 1C.
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Fig. 5. Photos of coated cell surface: (a) acetone, polysilane and PMMA, (b) acetyl acetone, polysilane and PMMA, (c) acetyl acetone, poly acrylic acid sodium salt and (d) acetyl acetone, poly acrylic acid sodium salt.
Energy Spectrum
4
500nm 200nm 100nm
3
Tritium Spot Size 32 Spot Size 33 Spot Size 34 Spot Size 35 Spot Size 36 Spot Size 37 Spot Size 38 Spot Size 39 Spot Size 40
3
2.5 2 1.5 1
2.5 2 1.5 1
0.5 0
Energy Spectrum
3.5
Flux (pA)
Electron Flux (pA/cm2)
3.5
0.5 0 2 4 6 8 10 12 14 16 Electron Energy (keV)
0
0 2 4
6 8 10 12 14 16 Energy (keV)
Fig. 6. Comparison of beta ray spectrum and its simulation with SEM: (a) estimated beta ray spectrum TiT2 plate and (b) energy level and spot size to simulate the spectrum from 500 nm thick TiT2 plate.
4. Conclusion
Acknowledgments
To make a tritium battery with p–n junction device and solid tritium source, the conditions and method to manufacture titanium hydride was developed. The activation was performed at 500 1C and film was harder than powder to hydrogenate. Maximum atomic ratios of 1.3 and 1.7 were obtained for powders and films, respectively. Further optimization is required to enhance the atomic ratio. So far, slurry preparation comprising acetyl acetone as solvent and poly acrylic acid sodium salt as dispersant showed the best result. The optimum composition of the slurries is to be found through further research. Applying slurry on the cell by hand with the micro-pipette was simple. Semiconductor p–n junction device is still under development to be optimized for tritium beta rays. The technique to simulate beta rays with a scanning electron microscope was developed and will be used to test the cells.
This work has been financially supported by the Ministry of Commercial, Industry and Energy of Korea, Grant 06NG05. The authors would like to give thanks to the Electric Power Industry Technology Evaluation and Planning (ETEP) of Korea, too. References Braun, J., Fermvik, L., Stenback, A., 1973. Theory and performance of a tritium battery for the microwatt range. Journal of Physics E: Scientific Instruments 6, 727–731. Hurysz, K.M., 1998. The processing of titanium hydride powders into uniform hollow spheres. Master’s Thesis of Georgia Institute of Technology, pp. 56–61. Kherani, N.P., Shmayda, W.T., 1992. Electron flux at the surface of titanium tritide films. Fusion Technology 21, 334–339. Kosteski, T., Kherani, N.P., Shmayda, W.T., Costea, S., Zukotynski, S., 2005. Nuclear batteries using tritium and thin film hydrogenated amorphous silicon. Fusion Science and Technology 48, 700–703. Olsen, L.C., 1973. Betavoltaic energy conversion. Energy Conversion 13, 117–127.