S. R. Djukić, R. Lj. Simeunović, A. M. Maričić* ) Technical Faculty Čačak, Svetog Save 65, Čačak, Serbia and Montenegro

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1 Science of Sintering, 7 (005) 1-5 DOI: 10.98/SOS0501D UDK 59.1:55.1 Influence of Structural Changes on Electrical and Magnetic Properties of the Co 84 Fe 5, Si 8,5 B, Amorphous Alloy S. R. Djukić, R. Lj. Simeunović, A. M. Maričić* ) Technical Faculty Čačak, Svetog Save 65, 000 Čačak, Serbia and Montenegro Abstract: The crystallization process of the Co 84 Fe 5. Si 8.5 B. amorphous alloy examined by differential scanning calorimetry (DSC) exhibits three exothermal steps at T cr1 =649 K, T cr =800 K, and T cr =88 K. The rate constants of the first relaxation process (determined at 598 K and 6 K) are k 1 = s and k = s and the corresponding activation energy E a1 =6. kj/mol. The data for the relaxation process before the second crystallization step (determined at 68 K and 71 K) are k = s and k 4 = s and the corresponding activation energy E a =60.0 kj/mol. The process of structural relaxation in non-isothermal and isothermal conditions was studied by analysis of the results of measurements of the thermo electromotive force (TEMF). From the change of the temperature coefficient of TEMF that follows each annealing process, the relative electronic state density changes at the Fermi level were determined: N 1 /N =5.45%, N /N =5.76%, N /N =7.57% and N 4 /N =9.85%. Keywords: Amorphous alloys-metallic glasses, Crystallization, Thermo electromotive force. 1. Introduction Physical features of a metal amorphous alloy are irreversibly changed in the crystallization process. This has been the subject of our research for several years [1-5]. The crystallization process is usually studied under conditions of non-isothermal cooling at temperatures slightly below the crystallization temperature. A thorough investigation of the kinetic features of amorphous alloys shows a connection between the physical nature of the anomalous behavior of electronic state density at Fermi level, thermal conductivity, heat capacitance and electric resistivity on one hand and structural inhomogenities in these materials on the other [6-8]. Two competitive processes take place during annealing of amorphous alloys, at temperatures up to 100 K lower than the crystallization temperature: on one hand, the free volume decreases, which lowers the rate of diffusion mass transport, and on the other hand, arrangement processes bring the alloy closer to the crystallized state increasing its readiness for crystallization. In this paper the relative change of free electron density during heating of metallic *) Corresponding author: marec@tfc.kg.ac.yu

2 S.R. Djukić et al./science of Sintering, 7 (005) 1-5 glass (MG) Co 84 Fe 5. Si 8.5 B. has been used to study electric and magnetic properties.. Experimental A crystalline precursor with nominal Co 84 Fe 5. Si 8.5 B. composition was prepared by arc melting using high-purity elements. Rapidly solidified ribbons approximately 0 µm thick were prepared by the melt-spinning method. The crystallization process was examined by differential scanning calorimetry (DSC) at a heating rate of 0 K/min. Electrical resistance was measured by a four probe method using mechanical contacts. The mechanical junction of the investigated alloy and copper conductor was constructed for measuring the thermo electromotive force (TEMF). Measurements of TEMF and the temperature dependence of the electric resistance were performed using a double channel, voltage signal writer with a sensitivity of 1 µv in an argon atmosphere.. Result and discission DSC traces of Co 84 Fe 5. Si 8.5 B. amorphous alloy shown on Fig. 1 exhibit three steps of the crystallization process with peak temperatures T cr1 = K, T cr =800.7 K, and T cr =88.8 K. Co 84 Fe 5, Si 8,5 B, 0 K/min. DSC (mw) T (K) Fig. 1 DSC trace for the Co 84 Fe 5. Si 8.5 B. amorphous alloy. The process of structural relaxation of the Co 84 Fe 5. Si 8.5 B. amorphous alloy was studied by analyzing the results of measurements of the thermo electromotive force (TEMF). For this purpose a mechanical junction of the investigated alloy and a copper conductor was constructed and heated in non-isothermal and isothermal conditions. Isothermal measurements were done at temperatures T 1 =598 K, T =6 K, T =65 K, and T 4 =68 K and T 5 =71 K with 00 s duration. Changes of the TEMF coefficient (α) were recorded after every thermal treatment (Fig. ). This coefficient is dependent on the difference of the free electron concentration N(E F ) on the Fermi level (E F ): / ( N / ( E ) / F N ( E F ) )) h α = (1) 1 m 8π e

3 S.R. Djukić et al./science of Sintering, 6 (004) 1-5 where h is the Planks constant, m e is the electron mass, N(E F ) 1 is the free electron concentration in copper and N(E F ) is the free electron concentration in an amorphous alloy.,5,0 ε, (mv) 1,5 1,0 0,5 0, T, (K) Fig. The dependence of TEMF on temperature for the Co 84 Fe 5. Si 8.5 B. amorphous alloy: - as-cast sample, and after isothermal annealing at: K, -6 K, +-65 K and x-68 K. At the junction of materials, with different Fermi levels, electrons move from the material with a higher Fermi level to the material with a lower Fermi level due to the thermodynamic tendency of equalizing Fermi levels in these two materials. Therefore, the internal potential difference is a consequence of the difference in the concentration of electron gasses of conducting materials in contact. In the first approximation copper as a crystalline material is in a stable state, i.e. the N(E F ) 1 free electron concentration in copper is constant, and the changes of the TEMF coefficient are due to the changes of N(E F ), i.e. free electron concentration in the amorphous alloy. From equation (1) values of the relative change in the free electron density of the investigated amorphous alloy are calculated as: N 1 /N =5.45%, N /N =5.76%, N /N =7.57% and N 4 /N =9.85%. Experimentally observed isothermal dependencies of TEMF of the Cu-MG thermocouple on time at temperatures T 1 =598 K, T =6 K, T =65 K, and T 4 =68 K and T 5 =71 K are linear (see Fig. ), i.e. one can see a linear relationship: ε ( τ ) = ε 0 kτ () where ε(τ)- is the TEMF during isothermal annealing, k-is a constant rate of the process of structural relaxation on an appropriate temperature and ε O is the TEMF at the beginning of isothermal annealing, i.e. at τ=0 s. This value is proportional to the temperature of isothermal annealing and inversely proportional to the sample-heating rate to the given temperature. From the linear slopes presented in Fig. structural relaxation rate constants for the relaxation process before the first crystallization peak k 1 = s and k = s were determined. For the relaxation process before the second crystallization process rate constants are k = s and k 4 = s. As the dependence ln k on 1/T is linear, the slopes enable calculation of the values of activation energies of these two processes by equation: () ln k E a = R (1/ T ) where R is the gaseous constant.

4 4 S.R. Djukić et al./science of Sintering, 7 (005) 1-5,6,5,4, T 1 =598 K T =6 K T =65 K T 4 =68 K T 5 =71 K ε, mv,,1,0 1,9 1,8 τ, (s) Fig. Isothermal dependence of TEMF on Cu-MG thermocouple of the Co 84 Fe 5. Si 8.5 B. amorphous alloy. Obtained values of activation energies are E a1=6. kj/mol and E a =60.0 kj/mol. Thus, measurement of the TEMF can be used for investigation of the structural state of the amorphous metal alloy and the changes of the TEMF coefficient can be used to evaluate the degree of system homogeneity. 4. Conclusion The Co 84 Fe 5. Si 8.5 B. amorphous alloy exhibits three steps in the crystallization process with peak temperatures T cr1 = K, T cr =800.7 K, and T cr =88.8 K. The process of structural transformation is followed with the increase of free electron concentration on the Fermi level. Calculated values of relative change in the free electron density are: N 1 /N =5.45%, N /N =5.76% before the first crystallization peak and N /N =7.57% and N 4 /N =9.85% before the second crystallization process. Therefore, the rapid increase of conductivity during crystallization is affected by the increase of the free electron concentration on the Fermi level. 5. References 1. M.V. Šušić, A.M. Maričić, N.S. Mitrović, S. Đukić and D. Stojanović, Science of Sintering Vol.8 Spec. Issue, (1996) p A. M. Maričić and M. M. Ristić, Science of Sintering Vol.8 Spec. Issue, (1996) p A. M. Maričić, M.V. Šušić, and M. M. Ristić, J. Serb. Chem. Soc. 6(8) (1997) p S. Đukić and N. Mitrović, Science of Sintering Vol.0() Spec. Issue, (1998), p N. Mitrović, S. Đukić and S. Đurić, IEEE Transaction on Magnetics MAG-6(5) (000), p V.E.Egoruskin, N.V. Melnikova, Metalofizika, T.10, No1, (1988) p L.A. Jacobson and J. McKittrik, Rapid Solidification Processing, Elsevier, K. Suzuki, H. Fudzimori and K. Hasamoto, Amorfnye metally, Metallurgiya, Moskva 1987 (in Russian).

5 S.R. Djukić et al./science of Sintering, 6 (004) Резюме: Кристаллизация аморфного сплава Co 84 Fe 5. Si 8.5 B. происходит при T cr1 =648,98 K, T cr =800,7 K, и T cr =88,8 K. Процесс структурного преобразования сопроввождается ростом концентрации электронов на уровне Ферми. Рассчитанные значения относительного изменениа плотности свободных электронов до первого максимума кристаллизации N 1 /N =5,45%, N /N =5,76% и N /N =7,57%, N 4 /N =9,85% до второго максимума кристаллизации. Скорый рост проводимости в течение процесса кристаллизации результат роста концентрации свободных электронов на уровне Ферми. Ключевые слова: Аморфныие сплав металлические стекла, кристаллизация ТЭМС. Садржај: Кристализација аморфне легуре Co 84 Fe 5. Si 8.5 B. одиграва се на T cr1 =648,98 K, T cr =800,7 K, и T cr =88,8 K. Процес структурне трансформације прати пораст концентрације електрона на Фермијевом нивоу. Прорачунате вредности релативне промене густине слободних електрона су N 1 /N =5,45%, N /N =5,76% пре првог пика кристализације и N /N =7,57% и N 4 /N =9,85% пре другог пика кристализације. Брзи пораст проводљивости током кристализације последица је пораста концентрације слободних електрона на Фермијевом нивоу. Кључне речи: Аморне легуре металлна стакла, кистализација, термоелектромоторна сила.