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Selective response of Nitinol substrate using Bragg reflector structures
Vacuum 84 (2010) 422–425
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Short communication
Selective response of Nitinol substrate using Bragg reflector structures O. Carton a, b, S. Zaidi b, F. Lamarque b, M. Lejeune a, * a b
Universite´ Picardie Jules Verne, Laboratoire de Physique de la Matie`re Condense´e EA2081, France Universite´ de Technologie de Compie`gne, Laboratoire Roberval, UMR 6253, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2008 Received in revised form 4 August 2009 Accepted 13 August 2009
The aim of this work is to elaborate low stress dichroic filters intended for the control of shape memory alloys deformation (SMA). This deformation has been performed by the use of laser sources (658 nm and 785 nm, for this preliminary study) in order to heat different samples of SMA. The two dichroic filters have been elaborated using a Bragg reflector structure with the choice of the materials discussed considering the superelastic behaviour of the substrate. The presented structure was made up of the stack of two materials, the amorphous hydrogenated silicon (a-Si:H) and the amorphous polymethacrylic acid (a-pMAA), deposited on glass substrate with the order: (a-Si:H a-pAM)3 a-Si:H. Finally the dichroic filters have been tested with success: the deformation of shape memory alloy has been observed and the filters show a high stability towards laser irradiation. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Plasma deposition Magnetron sputtering PECVD Optical filter Bragg reflector
1. Introduction Deformable materials have found a large utility in daily life and the use of these materials has been increased significantly in high technology devices. In these devices, the deformable materials provide the basic actuation mechanism whereas specific operations can be performed by other grafted materials [1,2]. However, the deposition of an active coating on a deformable substrate introduces two difficulties: firstly, the deposition technique should not distort the substrate properties and secondly, the deformation of the substrate should not modify the properties of the coating. Among the different deformable substrate, Nitinol (nickel–titanium alloy), is commonly used as substrate for mechanical or biomedical applications. It is a shape memory alloy having superelasticity or shape memory behaviour. Nitinol can be highly deformed (8%) and then can get back to its initial shape after a heating phase. It is also important to find a way to heat, without contact, one selected Nitinol sample among several others because in case of contact heating (as in case of electrical heating) the functionality of Nitinol structure can be seriously affected by electrical wires. As shown in a previous study [3], laser beam seems to be an interesting choice to heat efficiently the Nitinol sample to induce phase transformation. In order to independently deform different samples of Nitinol, laser sources with different wavelengths have been used in this study.
* Corresponding author. LPMC 33 rue St Leu F-80000 Amiens, France. Tel.: þ33 3 2282 7628; fax: þ33 3 22 82 7891. E-mail address: [email protected] (M. Lejeune). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.08.002
The selectivity in between different Nitinol samples can be performed by covering each Nitinol sample with an optical filter transmitting one specific laser wavelength and reflecting the others. In this particular work, we will study the possibility to heat and distort different Nitinol samples independently by using laser sources as a heating system and by using optical filters on glass substrate to achieve the selective response of the Nitinol samples. After validation, the optical filters will be directly elaborated on Nitinol substrates. 2. Design selection and experimental procedure 2.1. Term of reference Following the study concerning the heating of Nitinol samples by laser irradiation [3], we started the work on optical filters to be able to select two laser wavelengths 785 nm (infrared) and 658 nm (red). The technical constraints have imposed some conditions about the elaboration of dichroic filters (transmission of one wavelength and reflection of the other one): low thickness of the filter in order to minimize the weight supported by the SMA, low internal stress in the structure to limit the mechanical action of the filter on the SMA, high elasticity in order to preserve a good adhesion when the SMA is strongly deformed, low light absorption in order to transmit the maximum of energy.
O. Carton et al. / Vacuum 84 (2010) 422–425
423
Fig. 1. Simulation of red pass filter deposited on a glass substrate transmittance (grey line), reflectance (black line), and absorbance (black filled).
These conditions are not necessary for the filters deposited on glass substrate but will be necessary when the optical filter will be transferred on Nitinol sample directly, that is why we have chosen to apply these specifications in this preliminary work. 2.2. Design, materials choice and deposition techniques In the large panel of techniques for elaboration of optical filters we have chosen a distributed Bragg reflector structure. This structure is a periodic stack of bilayers. By varying the refractive index and the thickness of the layers, the phase of the incident light changes as a function of wavelength. Thus, it is possible to transmit a selected wavelength (phase difference ¼ 0) and to reflect the others (phase difference ¼ p). Each optical layer thickness corresponds to one quarter of the wavelength for which the mirror is designed. The choice of the material constituting the dichroic filter has been guided by the expected mechanical properties of the dichroic filters. The high refractive index material (M1) was the amorphous hydrogenated silicon (a-Si:H). a-Si:H films show a low elasticity related to a high Young’s modulus value: 160 GPa [4]. This mechanical property can be compensated by the choice of the second material. Indeed, it has been shown that the elastic behaviour of a multilayer structure is dominated by the material with the lowest Young’s modulus [5]. In this way, a polymeric film (the Young’s moduli of polymers are around 1 GPa [6]) has been opted for the second material (M2). Dichroic filters have been elaborated using a combination of two plasma techniques. Amorphous hydrogenated silicon is deposited by magnetron sputtering of a silicon target by 13.56 MHz radiofrequency plasma of Ar/H2 (10% H2) at room temperature. The polymeric layers have been deposited by inductive plasma enhanced chemical deposition (PECVD) from a pure methacrylic acid vapor source (purity 99.9% Alfa Aesar). The work pressure was 0.05 Pa and the power is transmitted to the plasma via a 6-rings coil from a RF 13.56 MHz continuous generator. 2.3. Coating characterization Optical properties (transmission, reflection and absorption) of the deposited films have been measured using Varian Cary 5E spectrophotometer in the UV–visible–near infrared range (400–4000 nm). Computer simulations, using COating DEsigner (CODE) software [7] program have been performed in order to
Fig. 2. Refractive index and extinction coefficient of a-Si:H films as a function of the plasma parameters.
choose the design of the filters. For the data process, transmittance, reflectance and absorbance are determined using two models: the O’Leary–Johnson–Lim interband transition model (OJL2) [8] or the Tauc–Lorentz model [9]. These two models are well adapted to the study of amorphous or disordered materials selected previously [10]. The middle-infrared spectra in transmission mode were obtained using a Bruker Vector 22 Fourier transform spectrometer (FTIR). This equipment and the FTIR data process are described in details in Ref. [11]. 3. Results and discussion 3.1. Computer simulation In Fig. 1, we can observe the simulated transmittance, reflectance and absorbance curves (with a normal incidence) of the red pass filter deposited on a glass substrate. This filter and the infrared pass filter (not shown in this letter) have been elaborated by the stacking of 7 layers: 3 bilayers constituting by two materials (M1 and M2) with a respective refractive index at a wavelength of 1 mm of 2.5 and 1.5, and one M1 top layer. The difference between the red pass filter (RP) and the infrared pass filter (IP) is due to the thickness of the layers: 34 nm M1 and 210 nm M2 for RP and 30 nm M1 and 140 nm M2 for IP. The amount of layers constituting the filter has been determined both by the quality of the selectivity of the filtered wavelength and by the light transmission rate necessary for
424
O. Carton et al. / Vacuum 84 (2010) 422–425
Fig. 3. FTIR spectra of a-pMAA films deposited by inductive PECVD.
the heating of a Nitinol sample. For the selected filtering, we have found a transmission rate of 90% at 658 nm – 10% at 785 nm and 15% at 658 nm – 95% at 785 nm for the RP and IP filter respectively. For the red pass filter, the transmission peak is centered around 658 nm with a width at half maximum of 50 nm. This last value is good enough to precisely select and distinguish the two laser wavelengths. For the two filters the absorbance in the wavelength of the laser is very weak (<3%). 3.2. a-Si:H deposition A brief study of the optical parameters of a-Si:H thin film as a function of the deposition parameters has been performed. In Fig. 2a and b, we can observe the variations of the refractive index (n) as a function of the RF power and pressure. As shown in Fig. 2a, the refractive index increases with RF power at a constant work pressure (1.9 Pa) and we can observe that for a constant RF power (80 W), an increase of the work pressure of Ar/H2 mixture induces a soft decrease of the refractive index. The results of this study suggest the deposition of films with the power of 80 W and a pressure of 3.7 Pa in order to obtain a material close to the material predicted by the simulation. Moreover, we observe in Fig. 2b, an increase in the extinction coefficient (k) with the increase of the power. This high k-value signifies a strong light absorption and therefore a loss of energy transmitted by the film. Thus high power and/or low pressure of deposition are not the convenient parameters for the deposition light transmitting materials. The evolution of the extinction coefficient and the refractive index as a function of the pressure and the power has been observed and studied intensively [12–14]. With plasma parameters of this study, the atomic structure is still amorphous, but an increase of plasma energy induces a better connectivity of the atomic network leading to an increase of the refractive index and a decrease of the extinction coefficient. The effect of the work pressure is linked to the incorporation of the hydrogen atoms in the atomic network that favours the formation of Si–Si bonds. 3.3. Polymethacrylic acid deposition As shown by the FTIR spectra drawn in Fig. 3, the films have not a crystalline structure but are amorphous or highly disordered. The
Fig. 4. Experimental (full line) and simulated (dash line) transmittance (black) and reflectance (grey) of IP (a) and RP filters (b) deposited on glass substrates.
decrease of the 1730 cm1 peak attributed to C]O bonds with the increase of the power can be explained by a decrease of functional terminal bonds and an increase of the interconnectivity of the carbon framework. This behaviour as a function of the power has already been observed and explained for polymers deposited by plasma techniques [11]. This amorphous structure can be a boon for the aimed application because firstly we are not interested by the chemical functionalization of these films and secondly the amorphous polymers usually show a more elastic behaviour than the crystalline polymers [15]. The refractive index of amorphous polymethacrylic acid (a-pMAA) films varied continuously from 1.49 to 1.6 with the increase of the power from 3 W to 50 W, whereas the extinction coefficient is constant with a null value. Due to the low value of the refractive index, we have chosen to deposit the film with a RF power of 3 W. 3.4. Bragg’s reflector With the two materials selected, the dichroic filters have been deposited using the Bragg reflector structure: glass substrate – (30 nm a-Si:H 140 nm a-pMAA)3 – 30 nm a-Si:H and glass substrate – (34 nm a-Si:H 210 nm a-pMAA)3 – 34 nm a-Si:H for the IP filter and RP filter respectively. The transmittance and the reflectance were measured in the spectral range from 350 nm to
O. Carton et al. / Vacuum 84 (2010) 422–425
425
Fig. 5. Infrared filter (IP) and red filter (RP) stacked to deformed Nitinol samples, with laser sources off (a), infrared laser on (b), and laser on (c).
1000 nm (Fig. 4) using a Cary 5E spectrophotometer. The experimental and simulated transmission rates were found to be very close. The reflectance curves ensure that the non-transmitted light is reflected and not absorbed by the filter. The reflectance measurements show a reflectance ration of 10% at 785 nm and 5% at 658 nm for the IP and RP filters respectively. We note that the sum of the reflectance and the transmittance are close to one suggesting that the absorbance of the filters is very weak. These absorbance low values are crucial in order to avoid the heat of the filter and the consecutive heating of the support by the heat diffusion. Finally, the internal stress of the structure has been calculated using the Stoney formula, as explained in Ref. [16]. After deposition of the 7 layers, the internal stress value is lower than 10 MPa, this value is very low suggesting a compensation of stress between the a-Si:H layer and the a-pMAA layer. This behaviour is in good agreement with the elastic properties of the polymeric film that can adapt the stress induced by the silicon layer. At this step, we can conclude that on glass, we are able to elaborate some selective optical filters using Bragg reflector structure.
suggesting that the two filters can be exposed to 130 mW laser sources without modification in their filter properties.
3.5. Effective light filtering and SMA deformation
This work and the PhD Thesis of Olivier Carton are supported by the MICHROB project kindly funded by the Conseil Re´gional de Picardie and the European Social Fund. S. Zaidi wants to thank Govt. of Pakistan especially the Higher Education Commission of Pakistan and Govt. of France to select him for the overseas PhD scholarship in France.
Fig. 5 shows the experimental results of the deformation in the Nitinol samples by using the elaborated filters explained above and the laser light as heating source. Firstly, two Nitinol samples (4 4 mm 100 mm) were deformed at the ambient condition i.e. at martensite phase. Two laser sources 785 nm and 658 nm (130 mW) were positioned over the Nitinol samples (SMA‘‘1’’ and SMA‘‘2’’). The RP and IP filters were placed between the laser sources and the Nitinol samples: the RP and IP filter covered SMA‘‘1’’ and SMA‘‘2’’ respectively (Fig. 5a). With the infrared laser source ‘‘on’’ and the red laser ‘‘off’’, phase transformation in SMA‘‘2’’ was observed whereas no phase change was observed in SMA‘‘1’’ (Fig. 5b). Then, the infrared laser source was switched off, and laser source was switched on, causing the phase transformation in SMA‘‘1’’ (Fig. 5c). The same experiment has been performed in reserve order i.e. illuminating the red laser first and then the infrared laser. In this way, phase transformation in SMA‘‘1’’ has been observed before the phase transformation in SMA‘‘2’’. The response time for the completion of the phase transformation was 8.5 s. We have verified that this time response was due to the size of the Nitinol samples and not to an energy absorption by the filter by performing the same experiment using 2 2 mm 100 mm Nitinol samples. With these small samples we have observed the same transformations but with time response of 2.5 s [17]. After experiments, transmission measurements have been performed on the dichroic filters in order to test their stabilities to laser lighting. No significant change had been observed
4. Conclusions In summary, a selective response of Nitinol samples has been investigated by using optical filters and by heating the samples with laser sources. The selection has been performed using a Bragg reflector structure composed of the stack of seven layers: amorphous hydrogenated silicon and polymeric layers. The presented filters show some interesting properties such as a no stressed structure, low optical absorption, a quality of filtering adapted to the set application and a good stability to laser light. With the validation of this principle, we will continue the use of Bragg’s structure and the next step will be to deposit directly the filter on the Nitinol sample. Acknowledgments
References [1] Bellouard Y. Mater Sci Eng A 2008;481–482:582–9. [2] Bansiddhi A, Sargeant TD, Stupp SI, Dunand DC. Acta Biomater 2008;4:773–82. [3] Zaidi S, Lamarque F, Favergeon J, Carton O, Prelle C. Proc of Mecatronics, 2008, paper 143. [4] Boucinha M, Brogueira P, Chu V, Conde JP. Appl Phys Lett 2000;77(No. 6): 907–9. [5] Teo EHT, Chua DHC, Tay BK. Diamond Relat Mater 2007;16:1882–6. [6] Wakabayashi K, Register RA. Polymer 2006;47(No. 8):2874–83. [7] Theiss W. Hard and software for optical spectroscopy, Dr. Bernhard-Klein-Str. 110, 52078 Aachen, Germany, www.mtheiss.com. [8] O’Leary SK, Johnson SR, Lim PK. J Appl Phys 1997;82(No. 7):3334–40. [9] Laidani N, Bartali R, Gottardi G, Anderle M, Cheyssac P. J Phys Condens Matter 2008;20:015216. [10] Jellison GE, Modine FA. Appl Phys Lett 1996;69(No. 3):371–3. [11] Lejeune M, Bretagnol F, Ceccone G, Colpo P, Rossi F. Surf Coat Technol 2006;200:5902–7. [12] Andu´jar LJ, Kasaneva J, Serra J, Canillas A, Roch C, Morenza JL, et al. Sens Actuators A 1993;37–38:733–6. [13] Fukaya K, Tabata A, Mizutani T. Vacuum 2004;74:561–5. [14] Hossain M, Abu-Safe M, Haseem H, Brown W. J Non-Cryst Solids 2006;352: 18–23. [15] Wakabayashi K, Register RA. Polymer 2005;46:8838–45. [16] Lejeune M, Benlahsen M. Diamond Relat Mater 2008;17:29–35. [17] Zaidi S, Lamarque F, Prelle C, Favergeon J, Carton O, Lejeune M, et al. Proc of Actuator, 2008; ISBN 3-933339-10-3:398-401.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Short communication
Selective response of Nitinol substrate using Bragg reflector structures O. Carton a, b, S. Zaidi b, F. Lamarque b, M. Lejeune a, * a b
Universite´ Picardie Jules Verne, Laboratoire de Physique de la Matie`re Condense´e EA2081, France Universite´ de Technologie de Compie`gne, Laboratoire Roberval, UMR 6253, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 September 2008 Received in revised form 4 August 2009 Accepted 13 August 2009
The aim of this work is to elaborate low stress dichroic filters intended for the control of shape memory alloys deformation (SMA). This deformation has been performed by the use of laser sources (658 nm and 785 nm, for this preliminary study) in order to heat different samples of SMA. The two dichroic filters have been elaborated using a Bragg reflector structure with the choice of the materials discussed considering the superelastic behaviour of the substrate. The presented structure was made up of the stack of two materials, the amorphous hydrogenated silicon (a-Si:H) and the amorphous polymethacrylic acid (a-pMAA), deposited on glass substrate with the order: (a-Si:H a-pAM)3 a-Si:H. Finally the dichroic filters have been tested with success: the deformation of shape memory alloy has been observed and the filters show a high stability towards laser irradiation. Ó 2009 Elsevier Ltd. All rights reserved.
Keywords: Plasma deposition Magnetron sputtering PECVD Optical filter Bragg reflector
1. Introduction Deformable materials have found a large utility in daily life and the use of these materials has been increased significantly in high technology devices. In these devices, the deformable materials provide the basic actuation mechanism whereas specific operations can be performed by other grafted materials [1,2]. However, the deposition of an active coating on a deformable substrate introduces two difficulties: firstly, the deposition technique should not distort the substrate properties and secondly, the deformation of the substrate should not modify the properties of the coating. Among the different deformable substrate, Nitinol (nickel–titanium alloy), is commonly used as substrate for mechanical or biomedical applications. It is a shape memory alloy having superelasticity or shape memory behaviour. Nitinol can be highly deformed (8%) and then can get back to its initial shape after a heating phase. It is also important to find a way to heat, without contact, one selected Nitinol sample among several others because in case of contact heating (as in case of electrical heating) the functionality of Nitinol structure can be seriously affected by electrical wires. As shown in a previous study [3], laser beam seems to be an interesting choice to heat efficiently the Nitinol sample to induce phase transformation. In order to independently deform different samples of Nitinol, laser sources with different wavelengths have been used in this study.
* Corresponding author. LPMC 33 rue St Leu F-80000 Amiens, France. Tel.: þ33 3 2282 7628; fax: þ33 3 22 82 7891. E-mail address: [email protected] (M. Lejeune). 0042-207X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2009.08.002
The selectivity in between different Nitinol samples can be performed by covering each Nitinol sample with an optical filter transmitting one specific laser wavelength and reflecting the others. In this particular work, we will study the possibility to heat and distort different Nitinol samples independently by using laser sources as a heating system and by using optical filters on glass substrate to achieve the selective response of the Nitinol samples. After validation, the optical filters will be directly elaborated on Nitinol substrates. 2. Design selection and experimental procedure 2.1. Term of reference Following the study concerning the heating of Nitinol samples by laser irradiation [3], we started the work on optical filters to be able to select two laser wavelengths 785 nm (infrared) and 658 nm (red). The technical constraints have imposed some conditions about the elaboration of dichroic filters (transmission of one wavelength and reflection of the other one): low thickness of the filter in order to minimize the weight supported by the SMA, low internal stress in the structure to limit the mechanical action of the filter on the SMA, high elasticity in order to preserve a good adhesion when the SMA is strongly deformed, low light absorption in order to transmit the maximum of energy.
O. Carton et al. / Vacuum 84 (2010) 422–425
423
Fig. 1. Simulation of red pass filter deposited on a glass substrate transmittance (grey line), reflectance (black line), and absorbance (black filled).
These conditions are not necessary for the filters deposited on glass substrate but will be necessary when the optical filter will be transferred on Nitinol sample directly, that is why we have chosen to apply these specifications in this preliminary work. 2.2. Design, materials choice and deposition techniques In the large panel of techniques for elaboration of optical filters we have chosen a distributed Bragg reflector structure. This structure is a periodic stack of bilayers. By varying the refractive index and the thickness of the layers, the phase of the incident light changes as a function of wavelength. Thus, it is possible to transmit a selected wavelength (phase difference ¼ 0) and to reflect the others (phase difference ¼ p). Each optical layer thickness corresponds to one quarter of the wavelength for which the mirror is designed. The choice of the material constituting the dichroic filter has been guided by the expected mechanical properties of the dichroic filters. The high refractive index material (M1) was the amorphous hydrogenated silicon (a-Si:H). a-Si:H films show a low elasticity related to a high Young’s modulus value: 160 GPa [4]. This mechanical property can be compensated by the choice of the second material. Indeed, it has been shown that the elastic behaviour of a multilayer structure is dominated by the material with the lowest Young’s modulus [5]. In this way, a polymeric film (the Young’s moduli of polymers are around 1 GPa [6]) has been opted for the second material (M2). Dichroic filters have been elaborated using a combination of two plasma techniques. Amorphous hydrogenated silicon is deposited by magnetron sputtering of a silicon target by 13.56 MHz radiofrequency plasma of Ar/H2 (10% H2) at room temperature. The polymeric layers have been deposited by inductive plasma enhanced chemical deposition (PECVD) from a pure methacrylic acid vapor source (purity 99.9% Alfa Aesar). The work pressure was 0.05 Pa and the power is transmitted to the plasma via a 6-rings coil from a RF 13.56 MHz continuous generator. 2.3. Coating characterization Optical properties (transmission, reflection and absorption) of the deposited films have been measured using Varian Cary 5E spectrophotometer in the UV–visible–near infrared range (400–4000 nm). Computer simulations, using COating DEsigner (CODE) software [7] program have been performed in order to
Fig. 2. Refractive index and extinction coefficient of a-Si:H films as a function of the plasma parameters.
choose the design of the filters. For the data process, transmittance, reflectance and absorbance are determined using two models: the O’Leary–Johnson–Lim interband transition model (OJL2) [8] or the Tauc–Lorentz model [9]. These two models are well adapted to the study of amorphous or disordered materials selected previously [10]. The middle-infrared spectra in transmission mode were obtained using a Bruker Vector 22 Fourier transform spectrometer (FTIR). This equipment and the FTIR data process are described in details in Ref. [11]. 3. Results and discussion 3.1. Computer simulation In Fig. 1, we can observe the simulated transmittance, reflectance and absorbance curves (with a normal incidence) of the red pass filter deposited on a glass substrate. This filter and the infrared pass filter (not shown in this letter) have been elaborated by the stacking of 7 layers: 3 bilayers constituting by two materials (M1 and M2) with a respective refractive index at a wavelength of 1 mm of 2.5 and 1.5, and one M1 top layer. The difference between the red pass filter (RP) and the infrared pass filter (IP) is due to the thickness of the layers: 34 nm M1 and 210 nm M2 for RP and 30 nm M1 and 140 nm M2 for IP. The amount of layers constituting the filter has been determined both by the quality of the selectivity of the filtered wavelength and by the light transmission rate necessary for
424
O. Carton et al. / Vacuum 84 (2010) 422–425
Fig. 3. FTIR spectra of a-pMAA films deposited by inductive PECVD.
the heating of a Nitinol sample. For the selected filtering, we have found a transmission rate of 90% at 658 nm – 10% at 785 nm and 15% at 658 nm – 95% at 785 nm for the RP and IP filter respectively. For the red pass filter, the transmission peak is centered around 658 nm with a width at half maximum of 50 nm. This last value is good enough to precisely select and distinguish the two laser wavelengths. For the two filters the absorbance in the wavelength of the laser is very weak (<3%). 3.2. a-Si:H deposition A brief study of the optical parameters of a-Si:H thin film as a function of the deposition parameters has been performed. In Fig. 2a and b, we can observe the variations of the refractive index (n) as a function of the RF power and pressure. As shown in Fig. 2a, the refractive index increases with RF power at a constant work pressure (1.9 Pa) and we can observe that for a constant RF power (80 W), an increase of the work pressure of Ar/H2 mixture induces a soft decrease of the refractive index. The results of this study suggest the deposition of films with the power of 80 W and a pressure of 3.7 Pa in order to obtain a material close to the material predicted by the simulation. Moreover, we observe in Fig. 2b, an increase in the extinction coefficient (k) with the increase of the power. This high k-value signifies a strong light absorption and therefore a loss of energy transmitted by the film. Thus high power and/or low pressure of deposition are not the convenient parameters for the deposition light transmitting materials. The evolution of the extinction coefficient and the refractive index as a function of the pressure and the power has been observed and studied intensively [12–14]. With plasma parameters of this study, the atomic structure is still amorphous, but an increase of plasma energy induces a better connectivity of the atomic network leading to an increase of the refractive index and a decrease of the extinction coefficient. The effect of the work pressure is linked to the incorporation of the hydrogen atoms in the atomic network that favours the formation of Si–Si bonds. 3.3. Polymethacrylic acid deposition As shown by the FTIR spectra drawn in Fig. 3, the films have not a crystalline structure but are amorphous or highly disordered. The
Fig. 4. Experimental (full line) and simulated (dash line) transmittance (black) and reflectance (grey) of IP (a) and RP filters (b) deposited on glass substrates.
decrease of the 1730 cm1 peak attributed to C]O bonds with the increase of the power can be explained by a decrease of functional terminal bonds and an increase of the interconnectivity of the carbon framework. This behaviour as a function of the power has already been observed and explained for polymers deposited by plasma techniques [11]. This amorphous structure can be a boon for the aimed application because firstly we are not interested by the chemical functionalization of these films and secondly the amorphous polymers usually show a more elastic behaviour than the crystalline polymers [15]. The refractive index of amorphous polymethacrylic acid (a-pMAA) films varied continuously from 1.49 to 1.6 with the increase of the power from 3 W to 50 W, whereas the extinction coefficient is constant with a null value. Due to the low value of the refractive index, we have chosen to deposit the film with a RF power of 3 W. 3.4. Bragg’s reflector With the two materials selected, the dichroic filters have been deposited using the Bragg reflector structure: glass substrate – (30 nm a-Si:H 140 nm a-pMAA)3 – 30 nm a-Si:H and glass substrate – (34 nm a-Si:H 210 nm a-pMAA)3 – 34 nm a-Si:H for the IP filter and RP filter respectively. The transmittance and the reflectance were measured in the spectral range from 350 nm to
O. Carton et al. / Vacuum 84 (2010) 422–425
425
Fig. 5. Infrared filter (IP) and red filter (RP) stacked to deformed Nitinol samples, with laser sources off (a), infrared laser on (b), and laser on (c).
1000 nm (Fig. 4) using a Cary 5E spectrophotometer. The experimental and simulated transmission rates were found to be very close. The reflectance curves ensure that the non-transmitted light is reflected and not absorbed by the filter. The reflectance measurements show a reflectance ration of 10% at 785 nm and 5% at 658 nm for the IP and RP filters respectively. We note that the sum of the reflectance and the transmittance are close to one suggesting that the absorbance of the filters is very weak. These absorbance low values are crucial in order to avoid the heat of the filter and the consecutive heating of the support by the heat diffusion. Finally, the internal stress of the structure has been calculated using the Stoney formula, as explained in Ref. [16]. After deposition of the 7 layers, the internal stress value is lower than 10 MPa, this value is very low suggesting a compensation of stress between the a-Si:H layer and the a-pMAA layer. This behaviour is in good agreement with the elastic properties of the polymeric film that can adapt the stress induced by the silicon layer. At this step, we can conclude that on glass, we are able to elaborate some selective optical filters using Bragg reflector structure.
suggesting that the two filters can be exposed to 130 mW laser sources without modification in their filter properties.
3.5. Effective light filtering and SMA deformation
This work and the PhD Thesis of Olivier Carton are supported by the MICHROB project kindly funded by the Conseil Re´gional de Picardie and the European Social Fund. S. Zaidi wants to thank Govt. of Pakistan especially the Higher Education Commission of Pakistan and Govt. of France to select him for the overseas PhD scholarship in France.
Fig. 5 shows the experimental results of the deformation in the Nitinol samples by using the elaborated filters explained above and the laser light as heating source. Firstly, two Nitinol samples (4 4 mm 100 mm) were deformed at the ambient condition i.e. at martensite phase. Two laser sources 785 nm and 658 nm (130 mW) were positioned over the Nitinol samples (SMA‘‘1’’ and SMA‘‘2’’). The RP and IP filters were placed between the laser sources and the Nitinol samples: the RP and IP filter covered SMA‘‘1’’ and SMA‘‘2’’ respectively (Fig. 5a). With the infrared laser source ‘‘on’’ and the red laser ‘‘off’’, phase transformation in SMA‘‘2’’ was observed whereas no phase change was observed in SMA‘‘1’’ (Fig. 5b). Then, the infrared laser source was switched off, and laser source was switched on, causing the phase transformation in SMA‘‘1’’ (Fig. 5c). The same experiment has been performed in reserve order i.e. illuminating the red laser first and then the infrared laser. In this way, phase transformation in SMA‘‘1’’ has been observed before the phase transformation in SMA‘‘2’’. The response time for the completion of the phase transformation was 8.5 s. We have verified that this time response was due to the size of the Nitinol samples and not to an energy absorption by the filter by performing the same experiment using 2 2 mm 100 mm Nitinol samples. With these small samples we have observed the same transformations but with time response of 2.5 s [17]. After experiments, transmission measurements have been performed on the dichroic filters in order to test their stabilities to laser lighting. No significant change had been observed
4. Conclusions In summary, a selective response of Nitinol samples has been investigated by using optical filters and by heating the samples with laser sources. The selection has been performed using a Bragg reflector structure composed of the stack of seven layers: amorphous hydrogenated silicon and polymeric layers. The presented filters show some interesting properties such as a no stressed structure, low optical absorption, a quality of filtering adapted to the set application and a good stability to laser light. With the validation of this principle, we will continue the use of Bragg’s structure and the next step will be to deposit directly the filter on the Nitinol sample. Acknowledgments
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