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Crystallization kinetics of Ni51Mn36Sn13 free-standing alloy thin films
Vacuum 130 (2016) 124e129
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Crystallization kinetics of Ni51Mn36Sn13 free-standing alloy thin films Z.H. Wang, E.J. Guo, C.L. Tan*, X.H. Tian* College of Materials Science and Engineering, Harbin University of Science and Technology, Harbin, 150080, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2016 Received in revised form 5 May 2016 Accepted 7 May 2016 Available online 9 May 2016
The crystallization kinetics of the Ni51Mn36Sn13 free-standing alloy thin films is investigated by differential scanning calorimetry (DSC) in the mode of non-isothermal and isothermal annealing. The DSC curves have been analyzed in terms of activation energy and kinetic model. It is found that all the DSC curves have a single exothermic peak which is asymmetrical, with a leading edge and a long high temperature tail. The activation energy from an amorphous state to crystallization of Ni51Mn36Sn13 freestanding alloy thin films is found to be 59 kJ/mol by Avrami’s method and 54 kJ/mol by Kissinger’s method. The DSC curves fitting procedure reveals that the crystallization of Ni51Mn36Sn13 free-standing alloy thin films follows the JMA-like kinetic model with the Avrami exponent varying from 1.17 to 1.73 under different temperatures, which indicates diffusion-controlled growth with the crystallization temperature. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Magnetic shape memory alloy Free-standing thin films Crystallization kinetics Thermal analysis
1. Introduction NieMneX (X]In, Sn, Sb) based ferromagnetic shape memory alloys (FSMAs) have been systematically investigated over the past decade [1e3]. In FSMAs, shape memory effect can be controlled not only by stress and temperature, but also by magnetic field [4e8]. Up to now, the applications of NieMneSn alloy thin films wide spread to many fields, such as magnetic refrigerant, actuators, sensors, micro-electro-mechanical systems (MEMS), etc. Usually, NieMneSn alloy thin films used as device of MEMS are deposited on silicon or silicon oxide substrate by magnetron sputtering system and other sputtering methods. Prepared NieMneSn alloy thin films by sputtering methods on unheated substrates are amorphous [9e11]. The amorphous thin films after crystallization exhibit excellent shape memory effect, which make it a good candidate material in MEMS device. Therefore, the crystallization behavior, such as the crystallization temperature and its activation energy, is very important for the fabrication process of NieMneSn alloy thin films. How to obtain an amorphous of NieMneSn alloy thin film by magnetron sputtering method, and the thin films possess the characteristic of lower crystallization temperature and activation energy, which make it a good candidate material for the wide application of the magnetic field controlled shape memory alloy.
* Corresponding authors. E-mail addresses: [email protected] (C.L. Tan), [email protected]. cn (X.H. Tian). http://dx.doi.org/10.1016/j.vacuum.2016.05.007 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
For this purpose, it is necessary to investigate crystallization kinetics of our thin film. Several crystallization kinetics of amorphous thin films for possessing expected properties have been reported by many researchers. Lei et al. [12] revealed the crystallization kinetics of TixeNi1x thin films. It is found that the effective activation energy and frequency factor of the crystallization are affected by Ni content and heating rate. Wang et al. [13] studied the crystallization nucleation and growth rates of NieTi shape memory alloy thin films. Ainissa et al. [14] investigated the nucleation and growth process of NieTi alloy thin films. However, so far, the crystallization nucleation and growth rates of NieMneSn thin films are still unclear. As above mentioned, ferromagnetic shape memory alloy thin films are directly deposited on substrates, such as Silicon, Silicon oxide and MgO(001). For substrate-attached ferromagnetic shape memory alloy thin films, crystallization processes are influenced not only by homogenous nucleation inside thin films, but also by heterogeneous nucleation occurring on interface between thin films and substrates. Chen et al. [15] studied the crystallization kinetics of TieNi free-standing thin films for the first time. It is found that the substrate gives the thin film a constrained force and affects the crystallization behavior of the amorphous TieNi thin films. Lei et al. [16,17] investigated the effect of substrate on crystallization kinetics of TieNi thin films. These works have shown that the substrate has an important role on the crystallization behavior of TieNi thin films. TieNi films have been prepared by DC magnetron sputtering on Si (100) and poly-Si/Si(100) substrates to
Z.H. Wang et al. / Vacuum 130 (2016) 124e129
elucidate the influence of the substrate on the crystallization of these films. The relationship between the intermediate layer of poly-Si and the crystallization behavior, as well as the final structure of the deposited films, was discussed [18]. Jetta et al. [19] investigated the effect of crystallization temperature of sputtered NieMneGa magnetic shape memory alloy thin films. To the best of our knowledge, there is no previous report in the literature concerning the study of the crystallization kinetics of amorphous NieMneSn free-standing thin films. In the current work, we successfully obtained the amorphous NieMneSn free-standing alloy thin films, and the crystallization kinetics of the Ni51Mn36Sn13 free-standing alloy thin films is investigated by differential scanning calorimetric (DSC) in the mode of non-isothermal and isothermal annealing for the first time. It is found that all the DSC curves have a single exothermic peak which is asymmetrical, with a leading edge and a long high temperature tail, and the crystallization temperature and activation energy of the Ni51Mn36Sn13 free-standing alloy thin films are lower than the similar and related materials. These results provide guidance for NieMneSn alloy thin films design and processing techniques.
2. Experimental The photoresist thin film is deposited on silicon (100) substrate by spinning, and amorphous Ni51Mn36Sn13 thin films are deposited on photoresist/silicon substrate by high vacuum direct current (DC) magnetron sputtering system. Later, the photoresist under the amorphous Ni51Mn36Sn13 thin films was removed with acetone. The elemental composition of the freestanding thin films was determined by energy dispersive X-ray spectrometry (EDS). Non-isothermal and isothermal crystallization of amorphous Ni51Mn36Sn13 free-standing thin films samples are carried out on diamond differential scanning calorimetry (Diamond DSC, PerkineElmer). For non-isothermal annealing, a set of DSC scans are recorded at heating rates of 10 K/min, 20 K/min, 30 K/min, 40 K/min and 60 K/min, respectively. For isothermal annealing, the amorphous samples were first heated to a fixed temperature (below crystalline temperature) with 300 K/min, and then held for a certain period of time until fully crystalline state was achieved. The temperatures of isothermal crystallization are 305, 315, 321, 325 and 330 K, respectively. The amorphous Ni51Mn36Sn13 free-standing thin films after the crystallization annealing are determined with a Rigaku D/max-rb rotating anode X-ray diffraction meter (XRD) using Cu Ka radiation at room temperature.
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3. Results and discussion Fig. 1(a) shows the XRD patterns of the Ni51Mn36Sn13 freestanding thin films crystallized at non-isothermal process. It is found that the Ni51Mn36Sn13 free-standing thin films were indexed to be a mixture of L21 cubic structure (austenite) as the A(220), A(222), A(422) peaks [20,21] and four-layered orthorhombic (4O) structure (martensite) as the 4O(023) peak. In present work no significant difference is found in these XRD patterns, but the intensity of the diffraction peaks of the Ni51Mn36Sn13 free-standing thin films phase increases with increasing heating rate. This characteristic comes from the fact that the heating rate of the Ni51Mn36Sn13 free-standing thin films increases continuously during the course of non-isothermal process [19]. The higher the heating rate, the more the temperature can be accumulated for crystallization processes, and the nucleation and growth of crystallization process are obviously. Hence, the A(220) peak intensity increases with the increase of heating rate. Fig. 1(b) shows the XRD patterns of the Ni51Mn36Sn13 free-standing thin films crystallized at isothermal process. It can be seen that the same crystallographic phase of the thin films were observed in different isothermal crystallization processes. This phenomenon implies that the higher of isothermal crystallization temperature has a shorter incubation time [15], and it is good for the nucleation and growth in crystallization process. Thus, It is obvious that the A(220), A(422) and 4O(023) peaks intensity increases with the increase of isothermal crystallization temperature. Fig. 2 shows the DSC curves obtained from the Ni51Mn36Sn13 free-standing thin films at heating rates of 10, 20, 30, 40 and 60 K/ min, respectively. All the DSC curves have a single exothermic peak related to the crystallization process, and the peak temperatures are significantly shifted to higher temperatures with increasing heating rate. This phenomenon indicates that the crystallization behaviors in a marked kinetic nature [15,17,23]. The values of the detected amorphous transition temperature Tg (which is defined as the temperature corresponding to the intersection point of the tangents to the portions adjoining the transition elbow in the DSC curves) and the crystallization peak temperature Tp for the different heating rates of samples are given in Table 1. Wuttig et al. [24] and Vishnoi et al. [25] indicates that the film-on-Si substrate of Ni50Mn30Ga20 sputtering film and the film-on-Si substrate of Ni50Mn35Sn15 sputtering film annealed at 673 K can develop the martensitic transformation, and them crystallization temperature is approximately 673 K, and the films deposited of temperature below 673 K are amorphous. However, from our results it is clear that the Ni51Mn36Sn13 free-standing thin film has been crystallized, when the crystallization temperature is 305 K for the first time. This
Fig. 1. XRD patterns of the Ni51Mn36Sn13 free-standing thin films. (a) non-isothermal crystallization at the heating rates of 10, 20, 30, 40 and 60 K/min, (b) isothermal crystallization at the temperature of 305, 315, 321, 325 and 330 K.
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Fig. 3. Plot of lnðb=Tp2 Þ vs. 1000/Tp for the free-standing Ni51Mn36Sn13 thin films.
Fig. 2. DSC curves with different heating rates for the Ni51Mn36Sn13 free-standing thin films.
phenomenon implies that the Ni51Mn36Sn13 free-standing thin films possess the characteristic of lower crystallization temperature, and it should be more conducive to magnetic field controlled the happening of the martensite phase transformation. The effective activation energy of crystallization Ec can be determined by the Kissinger equations [26]:
. ln b Tp2 ¼ Ec =RTp þ constants
(1)
whereb is the heating rate, R is the gas constant, Tp is the crystallization peak temperature. Thus, the activation energy Ec for crystallization of the Ni51Mn36Sn13 free-standing thin films can be derived from the slope of plotting lnðb=Tp2 Þ vs. 1/Tp. A straight line is obtained by plotting lnðb=Tp2 Þ vs. 1/Tp as indicated in Fig. 3. Thus, the activation energy Ec for crystallization of the Ni51Mn36Sn13 freestanding thin films can be derived from the slope of the straight line. From the calculation by the Kissinger’s equations, the apparent activation energy Ec of the Ni51Mn36Sn13 free-standing thin films is 54 ± 2 kJ/mol for non-isothermal crystallization. We fond that the crystallization temperature Tp and crystallization activation energy Ec of TieNi alloy thin films and NieMneGa alloy thin films are much higher than those of NieMneSn alloy thin films, say 374 kJ/ mol and 723 K, respectively, for Ti45.6Ni54.4 alloy thin films [15], and 234 kJ/mol and 694 K, respectively, for Ni51.45Mn25.30Ga23.25 alloy thin films [22], and 54 kJ/mol and 305 K, respectively, for Ni51Mn36Sn13 free-standing alloy thin film. This effect could be due to the difference of the material system and the influence of crystallization behavior. Fig. 4 shows the isothermal DSC curves obtained from the Ni51Mn36Sn13 free-standing thin films at a heating temperature of 305, 315, 321, 325 and 330 K, respectively. In all DSC curves, a single
exothermic peak is investigated after passing a certain incubation time. The incubation time of the isothermal crystallization process is decreasing with increasing isothermal temperature, indicating that the crystallization process is influenced by the isothermal temperature [15]. To reveal the mechanism of nucleation and growth in crystallization process, the value of x(t) can be calculated in each sampling time point and the results are plotted in Fig. 5. The crystallized fraction thin films x(t) is proportional to the fractional area of peak area. The curve of Fig. 5 displays a similar sigmoidal transformation curves for the crystallized fraction as a function of time, and the inset show in figures is incubation time for different annealing temperature [14e17]. An important point should be noted that the linear range of the crystallized volume fraction increase without the influence of the substrate. Furthermore, the isothermal kinetics of the transformation can be analyzed in terms of JohnsoneMehleAvrami (JMA) equation [27]. In this model, the crystallized fraction x, as a function of time t, is expressed as:
Table 1 Values of Tg and Tp determined at different heating rates for the Ni51Mn36Sn13 freestanding thin films. Temperature (K)
Tg Tp
Heating rates (K/min) 10
20
30
40
60
293 305
303 315
310 321
317 325
324 330
Fig. 4. Isothermal DSC curves for the Ni51Mn36Sn13 free-standing thin films at different temperatures.
Z.H. Wang et al. / Vacuum 130 (2016) 124e129
Fig. 5. The crystallized fraction as a function of time with annealing temperatures 305, 315, 321, 325 and 330 k for the Ni51Mn36Sn13 free-standing thin films. The inset time show in figures is incubation time for different annealing temperature.
ln½lnð1=ð1 xÞÞ ¼ n ln k þ n lnðt tÞ
(2)
Where t is the incubation time, n is the Avrami exponent which reflects the characteristics of nucleation and growth during crystallization process, k is the reaction rate which is function of annealing temperature by assuming an Arrhenius correspondence:
E k ¼ A exp A RT
(3)
Here A is a constant and EA is the activation energy for isothermal crystallization process. Fig. 6 shows the plot of ln½lnð1=ð1 xÞÞ again lnðt tÞ at each annealing temperatures in Fig. 5 for the Ni51Mn36Sn13 free-standing thin films. The data for ln½lnð1=ð1 xÞÞ were approximates to straight lines. The value of Avrami exponent n can be calculated from the slope in Fig. 6. The data for 10 < x < 80% are almost on a straight line, but it just has a
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slight deviation in the linearity or rather a decrease of the initial slope. This deviation could be attributed to the non-steady nucleation in the final stages of crystallization [23]. For the Ni51Mn36Sn13 free-standing thin films, the Avrami exponent n and the reaction constant k at different temperatures are listed in Table 2. From Table 2, the value of n ranges from 1.17 to 1.73 for the Ni51Mn36Sn13 free-standing thin films indicating that the crystallization process is two dimension diffusion-controlled growths [14e17]. It should be noted that the nucleation mechanisms of amorphous thin films are different under different isothermal crystallization temperatures. Fig. 7 shows the activation energy EA of the isothermal crystallization process of the amorphous Ni51Mn36Sn13 free-standing thin films. The activation energy can be obtained from the slope of line ln K against 1000/T. From the calculation by the JMA equations, the activation energy of the thin films is 59 ± 2 kJ/mol for isothermal crystallization. It should be noted that the activation energy of the isothermal crystallization process of the amorphous Ni51Mn36Sn13 free-standing thin films is lower than the TieNi free-standing thin films (385 kJ/mol) [15]. It is well known that the value of the activation energy is affected by the isothermal crystallization processes, and crystallization processes are categorized according to the n value, since the reaction order corresponds to the phase transformation mechanism. Considering diffusion limited processes, n < 1.5 indicates one dimensional grain growth from the nuclei. For 1.5 < n < 2.5 indicates the crystallization process in which grain growth occurs with nucleation. For n > 2.5, nucleation rate increases with the progress of grain growth [28]. For the TieNi free-standing thin films, the value of n are 2.5, 2.65, 2.7, and 3 for isothermal annealing temperatures of 800, 805, 815 and 820 K respectively. It indicates that during the crystallization process of the TieNi free-standing thin films, the nucleation rate increases with the progress of grain growth. This phenomenon implies that the atoms have more energy to overcome the activation energy barrier to nucleate at the beginning of crystallization. Thus, the nucleation rate of the TieNi free-standing thin films increases with the progress of grain growth at the higher isothermal annealing temperature. However, for Ni51Mn36Sn13 free-standing thin films, the value of n are 1.17, 1.30, 1.41, 1.51 and 1.73 for isothermal annealing temperatures of 305, 315, 321, 325 and 330 K respectively. It indicates that the crystallization process is two dimension diffusion-controlled growths [14e17]. It should be noted that the nucleation mechanisms of amorphous thin films are different under different isothermal crystallization temperatures. For the Ni51Mn36Sn13 free-standing thin films, at a lower isothermal annealing temperature, the activation energy barrier is lower to nucleate at the beginning of crystallization. Thus, an n value of 2 implies a constant nucleation rate and a constant crystal growth rate during the crystallized process. According to the above analysis, we suggest that the amorphous Ni51Mn36Sn13 free-standing thin films showed relatively low value of activation energy. By the way, the value of Ec obtained by the Kissinger for heating rates from 10 to 60 K/min (54 kJ/mol) is very close to that obtained from JMA equation (59 kJ/mol) for the isothermal DSC measurements. We suggest that the amorphous Ni51Mn36Sn13
Table 2 Kinetic parameters of the Ni51Mn36Sn13 free-standing thin films during isothermal annealing. Isothermal annealing temperature (K)
Fig. 6. Avrami plots for the isothermal crystallization of the Ni51Mn36Sn13 freestanding thin films at different temperatures.
Incubation time (min) Avrami exponent (n)
305
315
321
325
330
0.115 1.17
0.098 1.30
0.068 1.41
0.058 1.51
0.037 1.73
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Z.H. Wang et al. / Vacuum 130 (2016) 124e129
temperatures have distinct effect on crystallization process for the Ni51Mn36Sn13 free-standing thin films. 4. Conclusions In summary, the crystallization kinetics of the Ni51Mn36Sn13 free-standing alloy thin films is investigated by the non-isothermal and isothermal crystallization by DSC analysis and XRD analysis for the first time. The following results are obtained:
Fig. 7. Plot of ln K vs. 1000/Tp for the Ni51Mn36Sn13 free-standing thin films.
free-standing thin films obtained by differential scanning calorimetry (DSC) has a low crystallization temperature and activation energy, and it is a good candidate material for the wide application of the magnetic field controlled shape memory alloy. Calka and Radlinski [29] have proposed an alternative method of analysis of the isothermal DSC results of amorphous solid crystallization, which efficiently gives the local value of n(x) with volume fraction x. That allows us to study the crystallization behavior as a function of time. The local Avrami exponent n(x) is defined as:
(1) The amorphous Ni51Mn36Sn13 free-standing alloy thin films have been successfully fabricated by high vacuum direct current (DC) magnetron sputtering system. (2) The non-isothermal and isothermal annealing of crystallization of the Ni51Mn36Sn13 free-standing thin films are indexed to be a mixture of L21 cubic structure and fourlayered orthorhombic (4O) structure. (3) The non-isothermal and isothermal crystallization of the activation energies calculated are Ec ¼ 54 kJ/mol and EA ¼ 59 kJ/mol, respectively. The amorphous Ni51Mn36Sn13 free-standing thin films obtained by differential scanning calorimetry (DSC) has a lower crystallization temperature and activation energy, and it is a good candidate material for the wide application of the magnetic field controlled shape memory alloy. (4) Based on JohnsoneMehleAvrami (JMA) equation, it is found that the crystallization of the Ni51Mn36Sn13 free-standing thin films is governed by diffusion-controlled two-dimensional growth of particles nucleated at constant rate. Acknowledgment
d ln½ lnð1 xÞ nðxÞ ¼ d lnðt tÞ
(4)
Fig. 8 shows the value of local Avrami exponent n(x) at different temperature with crystallized transformation fraction for the Ni51Mn36Sn13 free-standing thin films. It is found that crystallization process has slight influence on the all local Avrami exponent n(x) of the Ni51Mn36Sn13 free-standing thin films, which approximately show horizontal line while the crystallized fraction in the range 10 < x < 80. Moreover, it is found that isothermal
Fig. 8. Local Avrami exponent n(x) plots as a function of crystallized fraction x derived from JMA equation.
The authors acknowledge the supports of National Natural Science Foundation of China (Grant Nos. 51471064 and 51301054); the Program for New Century Excellent Talents (Grant No.1253-NCET009); and Program for Youth Academic Backbone in Heilongjiang Provincial University (Grant No. 1251G022). References [1] Y. Sutou, Y. Imano, N. Koeda, T. Omori, R. Kainuma, K. Ishida, K. Oikawa, Appl. Phys. Lett. 85 (2004) 4358. [2] T. Krenke, M. Acet, E.F. Wassermann, X. Moya, L. Manosa, A. Planes, Phys. Rev. B 72 (2005) 014412. [3] A. Planes, L. Manosa, M. Acet, J. Phys. Condens. Matter 21 (2009) 233201. [4] N. Teichert, A. Auge, E. Yüzüak, I. Dincer, Y. Elerman, B. Krumme, H. Wende, O. Yildirim, K. Potzger, A. Hütten, Acta Mater. 86 (2015) 279. [5] T. Krenke, E. Duman, M. Acet, X. Moya, L. Manosa, A. Planes, J. Appl. Phys. 102 (2007) 033903. [6] I. Dincer, E. Yüzüak, Y. Elerman, J. Alloys Compd. 509 (2011) 794. [7] R. Vishnoi, D. Kaur, J. Appl. Phys. 107 (2010) 103907. ska, Y.V. Kudryavtsev, A. Szlaferek, J. Magn. Magn. Mater. [8] J. Dubowik, I. Goscian 310 (2007) 2773. [9] R. Vishnoi, R. Singhal, D. Kaur, J. Nanoparticle Res. 13 (2011) 3975. [10] R. Vishnoi, D. Kaur, Surf. Coat. Technol. 204 (2010) 3773. [11] E. Yüzüak, I. Dincer, Y. Elerman, A. Auge, N. Teicher, A. Hütten, Appl. Phys. Lett. 103 (2013) 222403. [12] Y.C. Lei, H.J. Zhao, W. Cai, X. An, L.X. Gao, Phys. B 405 (2010) 947. [13] X. Wang, J.J. Vlassak, Scr. Mater. 54 (2006) 925. [14] A.G. Ramirez, H. Ni, H.-J. Lee, Mater. Sci. Eng. A Struct. A 438e440 (2006) 703. [15] J.Z. Chen, S.K. Wu, J. Thin Solid Films 339 (1999) 194. [16] Y.C. Lei, H.J. Zhao, W. Cai, L. Wu, X. Gao, X. An, Vacuum 84 (2010) 1138. [17] Y.C. Lei, W. Cai, X. An, L.X. Gao, Non Cryst. Solids 354 (2008) 4573. [18] R.M.S. Martins, F.M. Braz Fernandes, R.J.C. Silva, L. Pereira, P.R. Gordo, M.J.P. Maneira, M. Beckers, A. Mücklich, N. Schell, et al., Appl. Phys. A 83 (2006) 139e145. [19] N. Jetta, N. Ozdemir, S. Rios, D. Bufford, I. Karaman, X. Zhang, et al., Thin Solid Films 520 (2012) 3433e3439. [20] H.X. Zheng, W. Wang, S.C. Xue, Q.J. Zhai, J. Frenzel, Z.P. Luo, Acta Mater. 61 (2013) 4648. [21] H.X. Zheng, W. Wang, D.Z. Wu, S.C. Xue, Q.J. Zhai, J. Frenzel, Z.P. Luo, Intermetallics 36 (2013) 90.
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S.K. Wu, K.H. Tseng, J.Y. Wang, Thin Solid Films 408 (2002) 316e320. Z.Z. Yuan, X.D. Chen, B.X. Wang, Z.J. Chen, J. Alloys Compd. 399 (2005) 166. M. Wuttig, C. Craciunescu, J. Li, Mater. Trans. JIM 41 (2000) 933. Ritu Vishnoi, Davinder Kaur, Surf. Coat. Technol. 204 (2010) 3773e3782.
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Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Crystallization kinetics of Ni51Mn36Sn13 free-standing alloy thin films Z.H. Wang, E.J. Guo, C.L. Tan*, X.H. Tian* College of Materials Science and Engineering, Harbin University of Science and Technology, Harbin, 150080, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2016 Received in revised form 5 May 2016 Accepted 7 May 2016 Available online 9 May 2016
The crystallization kinetics of the Ni51Mn36Sn13 free-standing alloy thin films is investigated by differential scanning calorimetry (DSC) in the mode of non-isothermal and isothermal annealing. The DSC curves have been analyzed in terms of activation energy and kinetic model. It is found that all the DSC curves have a single exothermic peak which is asymmetrical, with a leading edge and a long high temperature tail. The activation energy from an amorphous state to crystallization of Ni51Mn36Sn13 freestanding alloy thin films is found to be 59 kJ/mol by Avrami’s method and 54 kJ/mol by Kissinger’s method. The DSC curves fitting procedure reveals that the crystallization of Ni51Mn36Sn13 free-standing alloy thin films follows the JMA-like kinetic model with the Avrami exponent varying from 1.17 to 1.73 under different temperatures, which indicates diffusion-controlled growth with the crystallization temperature. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Magnetic shape memory alloy Free-standing thin films Crystallization kinetics Thermal analysis
1. Introduction NieMneX (X]In, Sn, Sb) based ferromagnetic shape memory alloys (FSMAs) have been systematically investigated over the past decade [1e3]. In FSMAs, shape memory effect can be controlled not only by stress and temperature, but also by magnetic field [4e8]. Up to now, the applications of NieMneSn alloy thin films wide spread to many fields, such as magnetic refrigerant, actuators, sensors, micro-electro-mechanical systems (MEMS), etc. Usually, NieMneSn alloy thin films used as device of MEMS are deposited on silicon or silicon oxide substrate by magnetron sputtering system and other sputtering methods. Prepared NieMneSn alloy thin films by sputtering methods on unheated substrates are amorphous [9e11]. The amorphous thin films after crystallization exhibit excellent shape memory effect, which make it a good candidate material in MEMS device. Therefore, the crystallization behavior, such as the crystallization temperature and its activation energy, is very important for the fabrication process of NieMneSn alloy thin films. How to obtain an amorphous of NieMneSn alloy thin film by magnetron sputtering method, and the thin films possess the characteristic of lower crystallization temperature and activation energy, which make it a good candidate material for the wide application of the magnetic field controlled shape memory alloy.
* Corresponding authors. E-mail addresses: [email protected] (C.L. Tan), [email protected]. cn (X.H. Tian). http://dx.doi.org/10.1016/j.vacuum.2016.05.007 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
For this purpose, it is necessary to investigate crystallization kinetics of our thin film. Several crystallization kinetics of amorphous thin films for possessing expected properties have been reported by many researchers. Lei et al. [12] revealed the crystallization kinetics of TixeNi1x thin films. It is found that the effective activation energy and frequency factor of the crystallization are affected by Ni content and heating rate. Wang et al. [13] studied the crystallization nucleation and growth rates of NieTi shape memory alloy thin films. Ainissa et al. [14] investigated the nucleation and growth process of NieTi alloy thin films. However, so far, the crystallization nucleation and growth rates of NieMneSn thin films are still unclear. As above mentioned, ferromagnetic shape memory alloy thin films are directly deposited on substrates, such as Silicon, Silicon oxide and MgO(001). For substrate-attached ferromagnetic shape memory alloy thin films, crystallization processes are influenced not only by homogenous nucleation inside thin films, but also by heterogeneous nucleation occurring on interface between thin films and substrates. Chen et al. [15] studied the crystallization kinetics of TieNi free-standing thin films for the first time. It is found that the substrate gives the thin film a constrained force and affects the crystallization behavior of the amorphous TieNi thin films. Lei et al. [16,17] investigated the effect of substrate on crystallization kinetics of TieNi thin films. These works have shown that the substrate has an important role on the crystallization behavior of TieNi thin films. TieNi films have been prepared by DC magnetron sputtering on Si (100) and poly-Si/Si(100) substrates to
Z.H. Wang et al. / Vacuum 130 (2016) 124e129
elucidate the influence of the substrate on the crystallization of these films. The relationship between the intermediate layer of poly-Si and the crystallization behavior, as well as the final structure of the deposited films, was discussed [18]. Jetta et al. [19] investigated the effect of crystallization temperature of sputtered NieMneGa magnetic shape memory alloy thin films. To the best of our knowledge, there is no previous report in the literature concerning the study of the crystallization kinetics of amorphous NieMneSn free-standing thin films. In the current work, we successfully obtained the amorphous NieMneSn free-standing alloy thin films, and the crystallization kinetics of the Ni51Mn36Sn13 free-standing alloy thin films is investigated by differential scanning calorimetric (DSC) in the mode of non-isothermal and isothermal annealing for the first time. It is found that all the DSC curves have a single exothermic peak which is asymmetrical, with a leading edge and a long high temperature tail, and the crystallization temperature and activation energy of the Ni51Mn36Sn13 free-standing alloy thin films are lower than the similar and related materials. These results provide guidance for NieMneSn alloy thin films design and processing techniques.
2. Experimental The photoresist thin film is deposited on silicon (100) substrate by spinning, and amorphous Ni51Mn36Sn13 thin films are deposited on photoresist/silicon substrate by high vacuum direct current (DC) magnetron sputtering system. Later, the photoresist under the amorphous Ni51Mn36Sn13 thin films was removed with acetone. The elemental composition of the freestanding thin films was determined by energy dispersive X-ray spectrometry (EDS). Non-isothermal and isothermal crystallization of amorphous Ni51Mn36Sn13 free-standing thin films samples are carried out on diamond differential scanning calorimetry (Diamond DSC, PerkineElmer). For non-isothermal annealing, a set of DSC scans are recorded at heating rates of 10 K/min, 20 K/min, 30 K/min, 40 K/min and 60 K/min, respectively. For isothermal annealing, the amorphous samples were first heated to a fixed temperature (below crystalline temperature) with 300 K/min, and then held for a certain period of time until fully crystalline state was achieved. The temperatures of isothermal crystallization are 305, 315, 321, 325 and 330 K, respectively. The amorphous Ni51Mn36Sn13 free-standing thin films after the crystallization annealing are determined with a Rigaku D/max-rb rotating anode X-ray diffraction meter (XRD) using Cu Ka radiation at room temperature.
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3. Results and discussion Fig. 1(a) shows the XRD patterns of the Ni51Mn36Sn13 freestanding thin films crystallized at non-isothermal process. It is found that the Ni51Mn36Sn13 free-standing thin films were indexed to be a mixture of L21 cubic structure (austenite) as the A(220), A(222), A(422) peaks [20,21] and four-layered orthorhombic (4O) structure (martensite) as the 4O(023) peak. In present work no significant difference is found in these XRD patterns, but the intensity of the diffraction peaks of the Ni51Mn36Sn13 free-standing thin films phase increases with increasing heating rate. This characteristic comes from the fact that the heating rate of the Ni51Mn36Sn13 free-standing thin films increases continuously during the course of non-isothermal process [19]. The higher the heating rate, the more the temperature can be accumulated for crystallization processes, and the nucleation and growth of crystallization process are obviously. Hence, the A(220) peak intensity increases with the increase of heating rate. Fig. 1(b) shows the XRD patterns of the Ni51Mn36Sn13 free-standing thin films crystallized at isothermal process. It can be seen that the same crystallographic phase of the thin films were observed in different isothermal crystallization processes. This phenomenon implies that the higher of isothermal crystallization temperature has a shorter incubation time [15], and it is good for the nucleation and growth in crystallization process. Thus, It is obvious that the A(220), A(422) and 4O(023) peaks intensity increases with the increase of isothermal crystallization temperature. Fig. 2 shows the DSC curves obtained from the Ni51Mn36Sn13 free-standing thin films at heating rates of 10, 20, 30, 40 and 60 K/ min, respectively. All the DSC curves have a single exothermic peak related to the crystallization process, and the peak temperatures are significantly shifted to higher temperatures with increasing heating rate. This phenomenon indicates that the crystallization behaviors in a marked kinetic nature [15,17,23]. The values of the detected amorphous transition temperature Tg (which is defined as the temperature corresponding to the intersection point of the tangents to the portions adjoining the transition elbow in the DSC curves) and the crystallization peak temperature Tp for the different heating rates of samples are given in Table 1. Wuttig et al. [24] and Vishnoi et al. [25] indicates that the film-on-Si substrate of Ni50Mn30Ga20 sputtering film and the film-on-Si substrate of Ni50Mn35Sn15 sputtering film annealed at 673 K can develop the martensitic transformation, and them crystallization temperature is approximately 673 K, and the films deposited of temperature below 673 K are amorphous. However, from our results it is clear that the Ni51Mn36Sn13 free-standing thin film has been crystallized, when the crystallization temperature is 305 K for the first time. This
Fig. 1. XRD patterns of the Ni51Mn36Sn13 free-standing thin films. (a) non-isothermal crystallization at the heating rates of 10, 20, 30, 40 and 60 K/min, (b) isothermal crystallization at the temperature of 305, 315, 321, 325 and 330 K.
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Fig. 3. Plot of lnðb=Tp2 Þ vs. 1000/Tp for the free-standing Ni51Mn36Sn13 thin films.
Fig. 2. DSC curves with different heating rates for the Ni51Mn36Sn13 free-standing thin films.
phenomenon implies that the Ni51Mn36Sn13 free-standing thin films possess the characteristic of lower crystallization temperature, and it should be more conducive to magnetic field controlled the happening of the martensite phase transformation. The effective activation energy of crystallization Ec can be determined by the Kissinger equations [26]:
. ln b Tp2 ¼ Ec =RTp þ constants
(1)
whereb is the heating rate, R is the gas constant, Tp is the crystallization peak temperature. Thus, the activation energy Ec for crystallization of the Ni51Mn36Sn13 free-standing thin films can be derived from the slope of plotting lnðb=Tp2 Þ vs. 1/Tp. A straight line is obtained by plotting lnðb=Tp2 Þ vs. 1/Tp as indicated in Fig. 3. Thus, the activation energy Ec for crystallization of the Ni51Mn36Sn13 freestanding thin films can be derived from the slope of the straight line. From the calculation by the Kissinger’s equations, the apparent activation energy Ec of the Ni51Mn36Sn13 free-standing thin films is 54 ± 2 kJ/mol for non-isothermal crystallization. We fond that the crystallization temperature Tp and crystallization activation energy Ec of TieNi alloy thin films and NieMneGa alloy thin films are much higher than those of NieMneSn alloy thin films, say 374 kJ/ mol and 723 K, respectively, for Ti45.6Ni54.4 alloy thin films [15], and 234 kJ/mol and 694 K, respectively, for Ni51.45Mn25.30Ga23.25 alloy thin films [22], and 54 kJ/mol and 305 K, respectively, for Ni51Mn36Sn13 free-standing alloy thin film. This effect could be due to the difference of the material system and the influence of crystallization behavior. Fig. 4 shows the isothermal DSC curves obtained from the Ni51Mn36Sn13 free-standing thin films at a heating temperature of 305, 315, 321, 325 and 330 K, respectively. In all DSC curves, a single
exothermic peak is investigated after passing a certain incubation time. The incubation time of the isothermal crystallization process is decreasing with increasing isothermal temperature, indicating that the crystallization process is influenced by the isothermal temperature [15]. To reveal the mechanism of nucleation and growth in crystallization process, the value of x(t) can be calculated in each sampling time point and the results are plotted in Fig. 5. The crystallized fraction thin films x(t) is proportional to the fractional area of peak area. The curve of Fig. 5 displays a similar sigmoidal transformation curves for the crystallized fraction as a function of time, and the inset show in figures is incubation time for different annealing temperature [14e17]. An important point should be noted that the linear range of the crystallized volume fraction increase without the influence of the substrate. Furthermore, the isothermal kinetics of the transformation can be analyzed in terms of JohnsoneMehleAvrami (JMA) equation [27]. In this model, the crystallized fraction x, as a function of time t, is expressed as:
Table 1 Values of Tg and Tp determined at different heating rates for the Ni51Mn36Sn13 freestanding thin films. Temperature (K)
Tg Tp
Heating rates (K/min) 10
20
30
40
60
293 305
303 315
310 321
317 325
324 330
Fig. 4. Isothermal DSC curves for the Ni51Mn36Sn13 free-standing thin films at different temperatures.
Z.H. Wang et al. / Vacuum 130 (2016) 124e129
Fig. 5. The crystallized fraction as a function of time with annealing temperatures 305, 315, 321, 325 and 330 k for the Ni51Mn36Sn13 free-standing thin films. The inset time show in figures is incubation time for different annealing temperature.
ln½lnð1=ð1 xÞÞ ¼ n ln k þ n lnðt tÞ
(2)
Where t is the incubation time, n is the Avrami exponent which reflects the characteristics of nucleation and growth during crystallization process, k is the reaction rate which is function of annealing temperature by assuming an Arrhenius correspondence:
E k ¼ A exp A RT
(3)
Here A is a constant and EA is the activation energy for isothermal crystallization process. Fig. 6 shows the plot of ln½lnð1=ð1 xÞÞ again lnðt tÞ at each annealing temperatures in Fig. 5 for the Ni51Mn36Sn13 free-standing thin films. The data for ln½lnð1=ð1 xÞÞ were approximates to straight lines. The value of Avrami exponent n can be calculated from the slope in Fig. 6. The data for 10 < x < 80% are almost on a straight line, but it just has a
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slight deviation in the linearity or rather a decrease of the initial slope. This deviation could be attributed to the non-steady nucleation in the final stages of crystallization [23]. For the Ni51Mn36Sn13 free-standing thin films, the Avrami exponent n and the reaction constant k at different temperatures are listed in Table 2. From Table 2, the value of n ranges from 1.17 to 1.73 for the Ni51Mn36Sn13 free-standing thin films indicating that the crystallization process is two dimension diffusion-controlled growths [14e17]. It should be noted that the nucleation mechanisms of amorphous thin films are different under different isothermal crystallization temperatures. Fig. 7 shows the activation energy EA of the isothermal crystallization process of the amorphous Ni51Mn36Sn13 free-standing thin films. The activation energy can be obtained from the slope of line ln K against 1000/T. From the calculation by the JMA equations, the activation energy of the thin films is 59 ± 2 kJ/mol for isothermal crystallization. It should be noted that the activation energy of the isothermal crystallization process of the amorphous Ni51Mn36Sn13 free-standing thin films is lower than the TieNi free-standing thin films (385 kJ/mol) [15]. It is well known that the value of the activation energy is affected by the isothermal crystallization processes, and crystallization processes are categorized according to the n value, since the reaction order corresponds to the phase transformation mechanism. Considering diffusion limited processes, n < 1.5 indicates one dimensional grain growth from the nuclei. For 1.5 < n < 2.5 indicates the crystallization process in which grain growth occurs with nucleation. For n > 2.5, nucleation rate increases with the progress of grain growth [28]. For the TieNi free-standing thin films, the value of n are 2.5, 2.65, 2.7, and 3 for isothermal annealing temperatures of 800, 805, 815 and 820 K respectively. It indicates that during the crystallization process of the TieNi free-standing thin films, the nucleation rate increases with the progress of grain growth. This phenomenon implies that the atoms have more energy to overcome the activation energy barrier to nucleate at the beginning of crystallization. Thus, the nucleation rate of the TieNi free-standing thin films increases with the progress of grain growth at the higher isothermal annealing temperature. However, for Ni51Mn36Sn13 free-standing thin films, the value of n are 1.17, 1.30, 1.41, 1.51 and 1.73 for isothermal annealing temperatures of 305, 315, 321, 325 and 330 K respectively. It indicates that the crystallization process is two dimension diffusion-controlled growths [14e17]. It should be noted that the nucleation mechanisms of amorphous thin films are different under different isothermal crystallization temperatures. For the Ni51Mn36Sn13 free-standing thin films, at a lower isothermal annealing temperature, the activation energy barrier is lower to nucleate at the beginning of crystallization. Thus, an n value of 2 implies a constant nucleation rate and a constant crystal growth rate during the crystallized process. According to the above analysis, we suggest that the amorphous Ni51Mn36Sn13 free-standing thin films showed relatively low value of activation energy. By the way, the value of Ec obtained by the Kissinger for heating rates from 10 to 60 K/min (54 kJ/mol) is very close to that obtained from JMA equation (59 kJ/mol) for the isothermal DSC measurements. We suggest that the amorphous Ni51Mn36Sn13
Table 2 Kinetic parameters of the Ni51Mn36Sn13 free-standing thin films during isothermal annealing. Isothermal annealing temperature (K)
Fig. 6. Avrami plots for the isothermal crystallization of the Ni51Mn36Sn13 freestanding thin films at different temperatures.
Incubation time (min) Avrami exponent (n)
305
315
321
325
330
0.115 1.17
0.098 1.30
0.068 1.41
0.058 1.51
0.037 1.73
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temperatures have distinct effect on crystallization process for the Ni51Mn36Sn13 free-standing thin films. 4. Conclusions In summary, the crystallization kinetics of the Ni51Mn36Sn13 free-standing alloy thin films is investigated by the non-isothermal and isothermal crystallization by DSC analysis and XRD analysis for the first time. The following results are obtained:
Fig. 7. Plot of ln K vs. 1000/Tp for the Ni51Mn36Sn13 free-standing thin films.
free-standing thin films obtained by differential scanning calorimetry (DSC) has a low crystallization temperature and activation energy, and it is a good candidate material for the wide application of the magnetic field controlled shape memory alloy. Calka and Radlinski [29] have proposed an alternative method of analysis of the isothermal DSC results of amorphous solid crystallization, which efficiently gives the local value of n(x) with volume fraction x. That allows us to study the crystallization behavior as a function of time. The local Avrami exponent n(x) is defined as:
(1) The amorphous Ni51Mn36Sn13 free-standing alloy thin films have been successfully fabricated by high vacuum direct current (DC) magnetron sputtering system. (2) The non-isothermal and isothermal annealing of crystallization of the Ni51Mn36Sn13 free-standing thin films are indexed to be a mixture of L21 cubic structure and fourlayered orthorhombic (4O) structure. (3) The non-isothermal and isothermal crystallization of the activation energies calculated are Ec ¼ 54 kJ/mol and EA ¼ 59 kJ/mol, respectively. The amorphous Ni51Mn36Sn13 free-standing thin films obtained by differential scanning calorimetry (DSC) has a lower crystallization temperature and activation energy, and it is a good candidate material for the wide application of the magnetic field controlled shape memory alloy. (4) Based on JohnsoneMehleAvrami (JMA) equation, it is found that the crystallization of the Ni51Mn36Sn13 free-standing thin films is governed by diffusion-controlled two-dimensional growth of particles nucleated at constant rate. Acknowledgment
d ln½ lnð1 xÞ nðxÞ ¼ d lnðt tÞ
(4)
Fig. 8 shows the value of local Avrami exponent n(x) at different temperature with crystallized transformation fraction for the Ni51Mn36Sn13 free-standing thin films. It is found that crystallization process has slight influence on the all local Avrami exponent n(x) of the Ni51Mn36Sn13 free-standing thin films, which approximately show horizontal line while the crystallized fraction in the range 10 < x < 80. Moreover, it is found that isothermal
Fig. 8. Local Avrami exponent n(x) plots as a function of crystallized fraction x derived from JMA equation.
The authors acknowledge the supports of National Natural Science Foundation of China (Grant Nos. 51471064 and 51301054); the Program for New Century Excellent Talents (Grant No.1253-NCET009); and Program for Youth Academic Backbone in Heilongjiang Provincial University (Grant No. 1251G022). References [1] Y. Sutou, Y. Imano, N. Koeda, T. Omori, R. Kainuma, K. Ishida, K. Oikawa, Appl. Phys. Lett. 85 (2004) 4358. [2] T. Krenke, M. Acet, E.F. Wassermann, X. Moya, L. Manosa, A. Planes, Phys. Rev. B 72 (2005) 014412. [3] A. Planes, L. Manosa, M. Acet, J. Phys. Condens. Matter 21 (2009) 233201. [4] N. Teichert, A. Auge, E. Yüzüak, I. Dincer, Y. Elerman, B. Krumme, H. Wende, O. Yildirim, K. Potzger, A. Hütten, Acta Mater. 86 (2015) 279. [5] T. Krenke, E. Duman, M. Acet, X. Moya, L. Manosa, A. Planes, J. Appl. Phys. 102 (2007) 033903. [6] I. Dincer, E. Yüzüak, Y. Elerman, J. Alloys Compd. 509 (2011) 794. [7] R. Vishnoi, D. Kaur, J. Appl. Phys. 107 (2010) 103907. ska, Y.V. Kudryavtsev, A. Szlaferek, J. Magn. Magn. Mater. [8] J. Dubowik, I. Goscian 310 (2007) 2773. [9] R. Vishnoi, R. Singhal, D. Kaur, J. Nanoparticle Res. 13 (2011) 3975. [10] R. Vishnoi, D. Kaur, Surf. Coat. Technol. 204 (2010) 3773. [11] E. Yüzüak, I. Dincer, Y. Elerman, A. Auge, N. Teicher, A. Hütten, Appl. Phys. Lett. 103 (2013) 222403. [12] Y.C. Lei, H.J. Zhao, W. Cai, X. An, L.X. Gao, Phys. B 405 (2010) 947. [13] X. Wang, J.J. Vlassak, Scr. Mater. 54 (2006) 925. [14] A.G. Ramirez, H. Ni, H.-J. Lee, Mater. Sci. Eng. A Struct. A 438e440 (2006) 703. [15] J.Z. Chen, S.K. Wu, J. Thin Solid Films 339 (1999) 194. [16] Y.C. Lei, H.J. Zhao, W. Cai, L. Wu, X. Gao, X. An, Vacuum 84 (2010) 1138. [17] Y.C. Lei, W. Cai, X. An, L.X. Gao, Non Cryst. Solids 354 (2008) 4573. [18] R.M.S. Martins, F.M. Braz Fernandes, R.J.C. Silva, L. Pereira, P.R. Gordo, M.J.P. Maneira, M. Beckers, A. Mücklich, N. Schell, et al., Appl. Phys. A 83 (2006) 139e145. [19] N. Jetta, N. Ozdemir, S. Rios, D. Bufford, I. Karaman, X. Zhang, et al., Thin Solid Films 520 (2012) 3433e3439. [20] H.X. Zheng, W. Wang, S.C. Xue, Q.J. Zhai, J. Frenzel, Z.P. Luo, Acta Mater. 61 (2013) 4648. [21] H.X. Zheng, W. Wang, D.Z. Wu, S.C. Xue, Q.J. Zhai, J. Frenzel, Z.P. Luo, Intermetallics 36 (2013) 90.
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