Magnetic properties in neutron irradiated graphites

1. Magnetic properties in neutron irradiated graphites D. Pontiroli, A. Goffredi, M. Belli, F. Giaferrari, M. Mazzani, M. Riccò Università degli Studi di Parma and CNISM, Via G. Usberti 7/a, 43100 Parma (Italy) Daniele Pontiroli –Università degli Studi di Parma

2. Aim of the work If magnetism in graphite is induced by defects, neutrons could represent a good alternative way to protons B. T. Kelly, IAEA, Wien 2000 Two different strategies: • Direct neutron bombardment of graphite compounds (powders) at high energy (~1 MeV) • Indirect irradiation by fission of atoms (Boron) incorporated into graphite (powder) with neutrons Daniele Pontiroli –Università degli Studi di Parma

3. 99% if Eneutr = 1 MeV! Direct irradiation with neutrons • If Eneutr < 60 eV, neutrons do not produce any defect in graphite • If 60 eV <Eneutr< 120 eV, neutrons produce only primary defects • If Eneutr> 120 eV, neutrons produce also secondary defects Daniele Pontiroli –Università degli Studi di Parma

4. Irradiation of boron enriched graphite Two different typologies of compounds: • Molecules containing boron intercalated in graphite • Boron compounds and graphite composites (NaBH4) The defect generation is due to the breaking of 10B nuclei induced by epithermal neutrons (~1eV) Daniele Pontiroli –Università degli Studi di Parma

5. The L.E.N.A. Laboratory (Pavia, IT) T.R.I.G.A. MARK II nuclear reactor, 250 KW (central channel) Bi-modal spectrum: peaks at: • 1.71012 n/cm2 sec at 1 eV • 0.81012 n/cm2 sec at 1 MeV Daniele Pontiroli –Università degli Studi di Parma

6. Intercalated and exfoliated graphites • Exfoliation of K-EtOH intercalated graphite: KC8 + CH3CH2OH  8C + KOCH2CH3 + ½ H2 If the planes of graphite are well outdistanced the approximation of graphene-like compound could be maintained even in the bulk phase heating at 1050° C • Production of other intercalated graphite (KCx exposed to NH3 vapours, RW-A graphite exposed to SO3 vapours) Daniele Pontiroli –Università degli Studi di Parma

7. Sample Preparation Pristine: • RW-A pristine graphite Intercalated graphites: • KC24 + NH3 • RW-A graphite + SO3 Exfoliated graphite: • KC8/KC24 + EtOH, then exfoliated With boron: • KC24 + borazine • KC24 + BCl3 • Boron composites: RW-A + NaBH4 at 1, 20, 50% We prepared the following samples (powder): Daniele Pontiroli –Università degli Studi di Parma

8. Sample Preparation Two different sample holders: • Quartz vial (the sample always remained in the same vessel) • Aluminium cylinder (better than Teflon®) Irradiation time: 6 h (central channel) I ) Defects induced directly by n Eneutr = 1 MeV Thomson-Wright formula Doseneutr = 1.73e16 n/cm2 1 Cdis every  44100 at. (3 1018 def/cm3) 1 def. in a cube of 35 at. edge (~5 nm) Daniele Pontiroli –Università degli Studi di Parma

9. Sample Preparation Two different sample holders: • Quartz vial (the sample always remained in the same vessel) • Aluminium cylinder (better than Teflon®) Irradiation time: 6 h (central channel) II ) Defects induced by B fission Eneutr = 1 eV Optimal doping:5 – 20%B in weight, in order to maximise the penetration depth (lambda) and the reaction rate Daniele Pontiroli –Università degli Studi di Parma

10. 0 Before n irrad. After n irrad. Magnetic Moment (emu/g) -0.1 -0.2 0 50 100 150 200 250 300 Temperature (K) Pristine RW-A graphite After irradiation, a clear onset of Pauli paramagnetism was observed. No FM was detected down to 10 K Before n irradiation 0.1 After n irradiation 0 Magnetic Moment (emu/g) -0.1 -0.2 -40 -20 0 20 40 Applied Field (kOe) Daniele Pontiroli –Università degli Studi di Parma

11. Magnetic Results Daniele Pontiroli –Università degli Studi di Parma

12. 20 10 Ms (emu) 0 -10 -20 -10 -5 0 5 10 Applied field (KOe) Magnetic Results Magnetic SQUID measurements (Quantum Design MPMS) Graphite + SO3 • Background subtraction • Measure of the same vial before and after the treatment • Ms = [M(after)-M(before)]s • Sensitivity:  5 emu Data Fit Ms = 2(1)10-5 emu Daniele Pontiroli –Università degli Studi di Parma

13. 0 B 1% B 20% 0.02 -0.05 B 50% B 1% 0 B 20% B 50% -0.02 -0.1 FIT  (emu/g) -0.04 Magnetic Moment (emu/g) -0.06 -0.15 -0.08 -0.1 -0.2 0 100 200 300 -0.12 Temperature (K) -0.14 0 10 20 30 40 50 Applied Field (kOe) Magnetic Results Magnetic SQUID measurements (Quantum Design MPMS) Boron composites • Presence of Curie (and Pauli?) paramagnetism • Extrapolated FM signal: 1% = -3.310-4 emu/g 20% = 8.110-4 emu/g 50% = 5.4 10-4 emu/g • No clear onset of FM signal Daniele Pontiroli –Università degli Studi di Parma

14. Conclusions • Irradiated pristine RW-A graphite (powder) displays the onset of Pauli paramagnetism • In-plane defects produced by neutrons does not induce clear FM Future developements • New investigations on the still not clear presence of FM magnetism into intercalated precursors (intrinsic effect?) • New irradiation of boron composites with smaller grain size Daniele Pontiroli –Università degli Studi di Parma

15. Thanks for your attention! Daniele Pontiroli –Università degli Studi di Parma

16. Daniele Pontiroli –Università degli Studi di Parma

17. Daniele Pontiroli –Università degli Studi di Parma

18. Montecarlo Calculation for Boron Composites The particle range and the number of event for 7Li and 4He in graphite and NaBH4 was symulated with SRIM2003 Daniele Pontiroli –Università degli Studi di Parma