Abstract—The reflectance spectra R(λ) within 200nm- 2500nm

wavelength range of the two nanocrystalline bulk metal thulium(Tm) samples were studied. The reflectivities R(λ) of Sample 1 and Sample 2 reach their minima 2.383% at 256nm and 3.439% at 214nm respectively. These minima correspond to the plasma oscillations in nanocrystalline bulk metal Tm.

I. INTRODUCTION AND BACKGROUND EING one of the heavy rare earth metals, thulium

(Tm), which has a very low content in the earth, has been widely used in different fields of science and technology.

For example, the fiber laser, especially in the case of S band fiber amplifier, takes advantage of the rich energy level structures of thulium ion, which is important for the exploitation of a potential communication resources frequency space1,2; thulium doped crystals can be used for quantum information storage3, etc.. The research of thulium involves in many aspects, including optical properties of thulium film4,5,6, correlation between the spin disorder resistivity and crystal structure of thulium thin Films7, the single crystal elastic constants of thulium as a function of temperature and magnetic field8, etc.. However, the study of nanocrystalline bulk thulium metal is rare. This paper is aimed at providing reflectance spectrum and plasma oscillation frequency within 200nm-2500nm wavelength range of two nanocrystalline bulk thulium samples, which were prepared by spark plasma sintering, and revealing the electronic properties thereof.

II. EXPERIMENT

Two samples were prepared by spark plasma sintering (SPS) technique and using equipment model is SPS-5.40MK-IV/ET. Bulk nanocrystalline Tm sample 1 was prepared by high energy ball milling and subsequent fast consolidation. The coarse-grained Tm metal having a high purity of 99.99wt% and a mean grain size of 1000nm was crushed into powders in a high energy vibrating mill under protection of pure argon. The milled powders were collected into a tungsten-carbon mold and fast consolidated at 573K under 500 MPa by the SPS technique. The mean size of sample 1 was 30nm. Sample 2 was prepared by part of the thus obtained samples being subsequently subjected to annealing at 773K. The mean size of sample 2 was 100nm.

The reflectance spectra R(λ) within 200nm-2500nm wavelength range of the two samples were measured by UV spectrophotometer (UV-VIS-NIR) with corresponding energy of 0.5-6 eV.

III. RESULTS

The experimental curves of reflectance R(λ) spectrum within the wavelength range of 200nm-2500nm for the two samples are shown in Fig. 1 and Fig. 2. It was found that the reflectance R(λ) of nanocrystalline bulk Tm is lower than 50% in this wavelength range. For bulk metal materials, the transmissivity equals to zero. That is to say there are visible metallic surface absorbance and scatter of the Tm samples. The reflectivity R(λ) linearly increases with wavelength λ increasing in the wavelength domain of 240nm~2500nm.

The protuberances located at 840nm of the two samples correspond to the change of light source and has nothing to do with the materials. Meanwhile, the reflectivity has a minimum R(λ) value as found between 220nm-240nm. The reflectivity R(λ) of Sample 1, the nanocrystalline bulk metal Tm sintered at 573K, reaches its minimum value R(λ)=2.534% at 240nm, and

Liu Fengyana Hou Bihuia Yue Ming b Hao Jincheng b aCollege of Applied Science, Beijing University of Technology, Beijing 100124, P.R.China

b The Key Laboratory of Advanced Functional Materials, Ministry of Education, and college of Materials

Science and engineering, Beijing University of Technology, Beijing 100124, P.R.China

The reflectance and electrical properties of nanocrystalline metal Tm

B

the reflectivity of Sample 2, the nanocrystalline bulk metal Tm sintered at 573K and then annealed at 773K, arrives at its minimum value R(λ)=3.815% at 232nm. These two wavelength values correspond to the ultraviolet transmission limits of the two nanocrystalline bulk metal Tm samples. The ultraviolet transmission limits of nanocrystalline bulk metal Tm are similar to those of alkali metals (Li 155nm, Na 210nm, K 315nm, Rb 340nm)9. However, the reflectance R(λ) of nanocrystalline bulk metal Tm is very low in lower frequency band which should approach 100% theoretically for typical metal. That means part of photon energy was absorbed and changed into Joule heat. We suppose that those minimum values R(λ) correspond to the plasma oscillations in nanocrystalline bulk metal Tm. The number density n of metallic free electrons is a very important physical quantity for metal material because it is only one independent parameter in the free electron theory for metal, and many other physical parameters can be derived from n such as Fermi energy, Fermi temperature and so on. The number density n can be calculated from the measured values of plasma oscillations frequency ωp or Hall coefficient R(H). The plasma oscillations frequency ωp locates at the lowest point of reflectance R(λ) spectrum.

The relationship of the plasma oscillation frequency ωp, the number density n of free electrons and effective mass m* of electrons is given by

22

*0

pnem

ωε

=

Where e is the charge of electron and ε0 is vacuum permeability. Here we calculate the number density n of free electrons with rest mass instead of effective mass of electrons.

2(573) 0 28 3

573 2 1.94 10 / (1)p mn m

eω ε

= = ×2

(773) 0 28 3773 2 2.074 10 / (2)p mn m

eω ε

= = ×

The values of number density n of free electrons of nanocrystalline bulk metal Tm are same as those of alkali metals in the order of magnitude, showing the Tm still has a relatively strong metal property. Metal Tm has a hexagonal close packed structure at room temperature, with lattice constants of a=b=0.354nm and c=0.556nm9. The outer-shell electron configuration of Tm atom is 4f136s2. Usually its ion is trivalent in compound, and bivalent in some cases.

Let a parallelepiped with a rhombus basal surface be a primitive cell, and each primitive cell has two atoms. Considering such an example that each atom of metal state simple substance loses two 6s electrons, the number density of free electrons can be worked out in this way,

28 32 6.65 10 / (3)Zn mV×= = ×

Where “2” is each primitive cell has two atoms, Z stands for the electron number each atom losing, V is the volume of a primitive cell. Comparing the n value from expression (3) with those of expressions (1) and (2) in which the plasma oscillation frequency ωp is from reflectance R(λ) experiment, the calculated ones have the same magnitude order as the values from experiment, while the theoretical value of density of free electrons, based on the metal electron theory, is somewhat higher than n573 and n773 values calculated from the ωp.

Additionally, we measured the resistivities of Tm samples at room temperature. The two resistivities are about 90×10-6μΩ·cm for sample 1 and 67×10-6μΩ·cm for sample 2. The resistivity of sample 1 is larger than that of sample 2, and the number density n of free electrons is smaller than that of samle 2. The reason why the resistivities of two samples are different is the increasing of the disorder degree for sample 1. The mean grain size of sample 1 is about 30nm, smaller than 100nm of sample 2. The smaller the mean grain size is, the more seriously the lattice structure was destroyed, and the larger the resistivity is.

Comparing the n value from expression (3) with those of expressions (1) and (2), it can also be seen that not all of 6s2 valence electrons in the nanocystalline metal Tm are free electrons, instead part of them may turn into itinerant electron, decreasing the mobility. It can be calculated easily that the number of free electron in each Tm atom is less than one, only 0.6.

IV. CONCLUSION

The reflectance spectra within 200nm-2500nm wavelength range of the two samples prepared by spark plasma sintering were measured by UV spectrophotometer(UV-VIS-NIR). The reflectivities have minima between 220nm-240nm, correspond to the plasma oscillations in nanocrystalline bulk metals Tm. The number density n of free electrons of nanocrystalline bulk metal Tm samples is about 2×1028/m3, suggesting semimetal property of Tm, which is correspond to the resistivity of the samples.

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