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Nanocomposite cerium oxide polymer matching layers with adjustable acoustic impedance between 4 MRayl and 7 MRayl
Ultrasonics 50 (2010) 363–366
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Nanocomposite cerium oxide polymer matching layers with adjustable acoustic impedance between 4 MRayl and 7 MRayl Frank Tiefensee a,*, Carsten Becker-Willinger b, Gisela Heppe b, Petra Herbeck-Engel b, Anette Jakob a a b
Fraunhofer–Institute for Biomedical Engineering Ensheimer Strasse 48, 66386 St. Ingbert, Germany Leibniz–Institut für Neue Materialien Campus D2 2, 66123 Saarbrücken, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 27 April 2009 Received in revised form 21 August 2009 Accepted 23 August 2009 Available online 1 September 2009 Keywords: Matching layer Nanotechnology Ultrasound Acoustic impedance high frequency
A new class of materials for ultrasonic matching layers is presented. The materials consist of nanoscale cerium oxide particles in an epoxy functionalized organic inorganic hybrid polymer matrix. The cerium oxide agglomerates to particles with 20 nm diameters. The content of particles in the polymer matrix could be increased to 75 wt.% which corresponds to 37 vol.%. The most technical important piezoelectrical ceramics have an acoustic impedance of about 30 MRayl, to improve coupling into water or biological tissue with an acoustic impedance of about 1.5 MRayl a matching layer should have an acoustic impedance of about 6.8 MRayl. With a filling degree of 75 wt.% the new composite material reaches an acoustic impedance of 7 MRayl. The materials are synthesized by a hydrolytic condensation combined with polymerization. This way of synthesis allows the use of organic solvents to adjust the viscosity of the sol and the application of different coating techniques. Ultrasound transducers (100 MHz) were built to test the new matching layers and an increase of the voltage signal amplitude of about 100% could be detected. Ó 2009 Elsevier B.V. All rights reserved.
thickness and k the wave number. With the condition of Formula (3) and d = (2n + 1) k/4 the transmission coefficient becomes T = 1.
1. Introduction The positive effect of matching layers is widely known in ultrasound technology. The transmission through an interface between two media depends on the difference of their acoustic impedances. The acoustic impedance of a material Z is defined as the product of the velocity of sound in the material with its density. The transmission coefficient of the intensity of an ultrasound wave at an interface between two media with the acoustic impedances Z1 and Z2 is given with Formula (1).
T¼
4 Z1 Z2
ð1Þ
ðZ 1 þ Z 2 Þ2
with Z1 and Z2 in [MRayl], Z = v q with v in [m/s] and q in [kg/m3]. With acoustic impedances of 30 MRayl for a piezoelectric ceramic and 1.5 MRayl for water, the transmission coefficient in (1) is about 0.18. According to Sutilov [1] the transmission coefficient of the pressure of a sound wave through a layer, e.g. a matching layer, with parallel surfaces is given with Formula (2).
T¼
4 Z 2 =Z 1 2
ðZ 2 =Z 1 þ 1Þ2 ðZ 22 =Z 2ML 1Þ ðZ 2ML =Z 21 1Þ sin kd
ð2Þ
with Z1 and Z2 acoustic impedances of the media in front and behind the layer, ZML the acoustic impedance of the layer, d the layer * Corresponding author. Tel.: +49 0 6894/980 270; fax: +49 0 6894/980 234. E-mail address: [email protected] (F. Tiefensee). 0041-624X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2009.08.012
Z ML ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z1 Z2
ð3Þ
The acoustic impedance of a matching layer ZML has to be the geometrical average of the acoustic impedance of the materials in front and behind of the matching layer, Z1 and Z2, and the thickness of the matching layer has to correspond to a quarter of the wavelength in its material. For instance, the transmission of ultrasound from a piezoelectric PZT ceramic into water can be increased with a matching layer, that has an acoustic impedance of 6.8 MRayl. New ultrasound applications in medical diagnosis in ophthalmology, dermatology and vascular wall diagnosis use frequencies up to 100 MHz, because high frequencies lead to smaller wavelengths and allow a better spatial resolution. The state of the art to produce matching layers with adjustable acoustic impedance is to mix microscale powders in a polymer and to adjust the acoustic impedance by the content of powder. In most cases the powder consists of aluminum oxide, Al2O3. The frequency range of these matching layers is limited by the use of microscale particles, because they lead to a high damping at frequencies higher than 10 MHz because of scattering. A possible solution of this problem is to reduce the size of the particles. But with the decrease of the particle diameter the surface energy in a defined volume unit increases and with the surface energy the tendency of the particles to agglomerate increases. The agglomeration of the particles would destroy the effect of small particles to reduce scattering. In [2] the
364
F. Tiefensee et al. / Ultrasonics 50 (2010) 363–366
use of nanoparticles for matching layers is described, but as the authors have used the relatively light Al2O3 (q = 3.94 g/cm3) particles compared with the CeO2 particles (q = 7.13 g/cm3) of the work at hand, the maximal acoustic impedance was 4.8 MRayl. With the synthesis described in [3–5] on wet chemical nanocomposite formation, coating systems with a high degree of filling and dispersed particles could be obtained in the past. In the paragraphs below a synthesis is shown to produce nanocomposite polymers with high filling degrees and dispersed nanoparticles, a method is described to measure the acoustic impedance at thin layers and the verification of the new matching layers with 100 MHz transducer is given. 2. Synthesis of the material The acoustic impedance of a material is the product of the density of the material and the velocity of sound in the material. A first estimation of the needed density of the matching layer material has used the fact, that in the predominant number of polymers the velocity of sound is between 2000 m/s and 3000 m/s and the assumption, that the velocity of sound does not change with the filling degree. To reach the technical important acoustic impedance of 6.8 MRayl for coupling from most piezoelectric ceramics into water, the density of the matching layer should have values between 2.0 g/cm3 and 3.0 g/cm3. As it is technically difficult to reach high filling degrees with nanopowder, it is desirable to operate with a filler, which permits low filling degrees. For this reason nanoparticles of cerium oxide with 7.1 g/cm3 were chosen for the further work. The chosen particles are commercially available and have average diameters between 10 nm and 15 nm. To avoid agglomeration of the particles, their surface was modified prior to use with trioxadecanic acid. 10 wt.% trioxadecanic acid was mixed with the cerium oxide particles. An aqueous solution of the surface modified nanoparticles was mixed with epoxy silane and zirconium alkoxide in order to form the nanocomposite system. The components are 3-glycidoxypropyltriethoxysilane, CASNo [2602-34-8] from EVONIK, and zirconium butylate, Zr(OC4H9)4. These reactants build an epoxy functionalized organic inorganic hybrid polymer. The optimal curing temperature for the synthesis was 160 °C. Lower curing temperatures are possible but it is difficult to produce solid layers at temperatures lower than 120 °C. Zirconium butylate was used as a catalyst for the synthesis of the inorganic network. The synthesis is described in Fig. 1. Fig. 2 shows
4 mol 3-glycidoxypropyltriethoxysilane + 1 mol zirconium butylate + 4 mol glacial acetic acid
hydrolysis for 12 h with aqueous dispersion of CeO2 agitation for 12 h
partial removal of the solvent by distillation
spin coating
thermal curing at 160°C, 3 h heating, 1 h hold, 6 h cooling Fig. 1. Synthesis of the matching layer materials.
Fig. 2. TEM picture of the matrix with cerium oxide particles (10 wt.% CeO2 solid content), preparation: microtome cutting.
a TEM picture of a layer with 10 wt.% cerium oxide particles. The CeO2 agglomerates to particles with diameters of about 20 nm. The particles are homogeneously distributed in the matrix material and this homogeneous dispersion of particles leads to constant material properties like density. As shown later, the velocity of sound in the material is about 2100 m/s. At 100 MHz the wavelength is 21 lm. Compared to the wavelength the agglomerates do not cause scattering. For TEM investigation a sample of lower particle content (10 wt.%) has been chosen because in the case of high particle contents (34–75 wt.%) the number of the 20 nm agglomerates per volume is too high to distinguish one aggregate from the other. Materials with different filling degrees between 34 wt.% and 75 wt.%, which corresponds to 9–37 vol.% respectively, have been produced. With these materials thin layers were coated on glass substrates by spin coating. The layers had an uniform thickness between 1 lm and 10 lm. 3. Measurement of the acoustic impedance As it is difficult to measure the velocity of sound in a thin layer of several microns, the acoustic impedance was measured by measuring the reflection coefficient of the layers. The measurements were calibrated with a known material. The reference was sapphire with a velocity of sound of 11,150 m/s and a density of 3.98 g/cm3 [6]. The reflection coefficient of the intensity of a sound wave at the interface between two media is
R¼
I1 ðZ 2 Z 1 Þ2 ¼ I0 ðZ 2 þ Z 1 Þ2
ð4Þ
I0 is the intensity of the emitted signal, I1 the intensity of the reflected signal, Z1 the acoustic impedance of the media in which the ultrasound propagates, here water at room temperature, and Z2 the acoustic impedance of the media at which the ultrasound is reflected. The quotient of the reflection coefficient of a matching layer and the reflection coefficient of sapphire is
I1 ðZ ML Z 1 Þ2 ðZ S þ Z 1 Þ2 ¼ I2 ðZ ML þ Z 1 Þ2 ðZ S Z 1 Þ2
ð5Þ
F. Tiefensee et al. / Ultrasonics 50 (2010) 363–366
365
Fig. 3. V(z)-scan for a sample filled with 34 wt.% of cerium oxide.
With I1 intensity of the reflected signal from a matching layer, I2 intensity of the reflected signal from a sapphire plate, ZML the acoustic impedance of the matching layer, ZS the acoustic impedance of sapphire and Z1 the acoustic impedance of a coupling media. ZML is the only unknown variable in Eq. (5). A scanning ultrasound microscope, which was developed at Fraunhofer Institute for Biomedical Engineering, IBMT, was used to measure the reflection coefficient. The ultrasound microscope was finished with a focused 200 MHz transducer with a focus length of 500 lm. The ultrasound beam was always focused on the surface of the layer by using a V(z)-scan. This method means, that the transducer is slowly moved perpendicular to the surface and when the maximal signal amplitude is received, the transducer is focused on the surface. The signal amplitude of the focus was used for further calculations. An example for a V(z)-scan is shown in Fig. 3. The ultrasound beam is focused at a time of flight of about 1100 ns. The ultrasound needs this time to propagate to the surface and back. The measurement equipment also allowed to distinguish the signal reflected at the interface between water and matching layer from the signal reflected at the interface between matching layer and substrate. The second signal travelled two times through the matching layer. Thus the attenuation of the matching layer material could be evaluated. The thicknesses of the matching layers were measured with a profilometer.
4. Results In Fig. 4 the results of the measurements are shown. The acoustic impedance can be varied continuously between 4 MRayl and 7 MRayl. The acoustic impedance is a linear function of the density with a slope of 2100 m/s. This velocity corresponds to the velocity of sound in the unfilled polymer. The evaluated attenuation of all matching layer materials was 0.5 dB/lm. To verify the functionality of the new material 100 MHz ultrasound transducers were coated with the new matching layer. The transducers were fabricated with methods of microtechnology,
Fig. 5. 16 16 ultrasound transducers on a silicon wafer.
that have been developed at IBMT and are published in [7]. With photolithography and physical vapour deposition 16 16 round piezoelectric zinc oxide pads were deposited on a silicon wafer. The piezoelectric pads had two gold electrodes. As the bottom electrode on the silicon had a larger diameter than the zinc oxide pad with the upper electrode, the single pads could be contacted with a coaxial probe. Fig. 5 shows a silicon wafer with 16 16 ultrasound transducer on one side. The opposite side of the silicon wafer was coated with the nanocomposite matching layer by spin coating. The ultrasound waves are generated with the zinc oxide layer and propagate through the 500 lm thick silicon wafer before they couple into water. For this reason the matching layer was calculated for the transition from silicon to water: The velocity of sound in silicon is 8900 m/ s, the density of silicon is 2.33 g/cm3. With Formula (3) the acoustic impedance of a matching layer should be 5.57 MRayl and its thickness should be k/4 = 5.25 lm for a 100 MHz transducer. The transducers were separated with a micro dice, electrically contacted with a coaxial probe and sealed against water. The transducers were positioned to an aluminum oxide reflector and the reflected signals were measured with and without matching layer. As shown in Figs. 6 and 7 the voltage amplitude with matching layer was about 100% higher than without matching layer. This result can be verified with a simple calculation. With Formula (1) and the acoustic impedances of silicon and water, the transmission coefficient is 0.25. With Formula (2), ZML = 5.57 MRayl ± 10%, d = 5.2 lm ± 10% and k = 21 lm, the minimal transmission coefficient with a matching layer is 0.87. The attenuation of the matching layer material should be maximal 3 dB and reduces the transmission to 0.60. The evaluated transmission with a matching layer is surely 100% higher, than the transmission with-
8.000 7.000
Z [MRayl]
6.000 5.000 4.000 3.000 2.000 1.000 0.000 0
0.5
1
1.5
2
2.5
3
3
density [g/cm ] Fig. 4. The acoustic impedance of cerium oxide in an epoxy functionalized organic inorganic hybrid polymer matrix.
Fig. 6. Voltage amplitude of a 100 MHz signal without matching layer.
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F. Tiefensee et al. / Ultrasonics 50 (2010) 363–366
range for coupling from the piezoelectric ceramic PZT into water. The linear slope in Fig. 4 is 2100 m/s and corresponds to the velocity of sound in the unfilled polymer. This result suggests, that scattering does not play a role in the material because the velocity is not influenced by the content of particles. As the material is synthesized via a wet chemical approach, solvents can be used to adjust the viscosity. The matching layers were produced with a spin coating process and the thickness could be varied between 1 lm and 10 lm. The functionality of the material could be shown with 100 MHz transducers. The matching layers lead to an increase of the voltage amplitude of 100%. Fig. 7. Voltage amplitude of a 100 MHz signal with matching layer.
out a matching layer and measurement results within this range can be expected. 5. Conclusions New nanocomposite materials for matching layers are presented. The material consists of cerium oxide nanoparticles in an epoxy functionalized organic inorganic hybrid polymer. By the content of particles the acoustic impedance can be adjusted to values between 4 MRayl and 7 MRayl, which is a technical important
References [1] Vladimir A. Sutilov, Physik des Ultraschalls, Springer Verlag, 1984, pp. 177. [2] Q.F. Zhou, J.H. Cha, Y. Huang, R. Zhang, W. Cao, J.M. Cannata, K.K. Shung, Nanocomposite matching layers for high frequency ultrasound transducers, In: IEEE Ultrasonics Symposium 3–6 October 2006, pp. 2365–2368. [3] L. Spanhel, E. Arpac, R. Nass, H. Schmidt, EP 0607213 B1, INM gGmbH, 09.10.1992. [4] E. Arpac, H. Krug, P. Müller, P. Oliviera, H. Schmidt, S. Sepeur, B. Werner, WO 98/ 51747 A1, INM gGmbH, 13.05.1997. [5] E. Arpac et. al., WO 99/52964, INM gGmbH, 09.04.1998. [6] A. Briggs, Acoustic Microscopy, Calderon Press, Oxford, 1992, p. 103. [7] A. Jakob, E.C. Weiss, T. Knoll, F. Bauerfeld, J. Herrmann, R. Lemor, Silicon based GHz acoustic lenses for time resolved acoustic microscopy, In: IEEE Ultrasonics Symposium 28–31 October 2007, pp. 1605–1608.
Contents lists available at ScienceDirect
Ultrasonics journal homepage: www.elsevier.com/locate/ultras
Nanocomposite cerium oxide polymer matching layers with adjustable acoustic impedance between 4 MRayl and 7 MRayl Frank Tiefensee a,*, Carsten Becker-Willinger b, Gisela Heppe b, Petra Herbeck-Engel b, Anette Jakob a a b
Fraunhofer–Institute for Biomedical Engineering Ensheimer Strasse 48, 66386 St. Ingbert, Germany Leibniz–Institut für Neue Materialien Campus D2 2, 66123 Saarbrücken, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 27 April 2009 Received in revised form 21 August 2009 Accepted 23 August 2009 Available online 1 September 2009 Keywords: Matching layer Nanotechnology Ultrasound Acoustic impedance high frequency
A new class of materials for ultrasonic matching layers is presented. The materials consist of nanoscale cerium oxide particles in an epoxy functionalized organic inorganic hybrid polymer matrix. The cerium oxide agglomerates to particles with 20 nm diameters. The content of particles in the polymer matrix could be increased to 75 wt.% which corresponds to 37 vol.%. The most technical important piezoelectrical ceramics have an acoustic impedance of about 30 MRayl, to improve coupling into water or biological tissue with an acoustic impedance of about 1.5 MRayl a matching layer should have an acoustic impedance of about 6.8 MRayl. With a filling degree of 75 wt.% the new composite material reaches an acoustic impedance of 7 MRayl. The materials are synthesized by a hydrolytic condensation combined with polymerization. This way of synthesis allows the use of organic solvents to adjust the viscosity of the sol and the application of different coating techniques. Ultrasound transducers (100 MHz) were built to test the new matching layers and an increase of the voltage signal amplitude of about 100% could be detected. Ó 2009 Elsevier B.V. All rights reserved.
thickness and k the wave number. With the condition of Formula (3) and d = (2n + 1) k/4 the transmission coefficient becomes T = 1.
1. Introduction The positive effect of matching layers is widely known in ultrasound technology. The transmission through an interface between two media depends on the difference of their acoustic impedances. The acoustic impedance of a material Z is defined as the product of the velocity of sound in the material with its density. The transmission coefficient of the intensity of an ultrasound wave at an interface between two media with the acoustic impedances Z1 and Z2 is given with Formula (1).
T¼
4 Z1 Z2
ð1Þ
ðZ 1 þ Z 2 Þ2
with Z1 and Z2 in [MRayl], Z = v q with v in [m/s] and q in [kg/m3]. With acoustic impedances of 30 MRayl for a piezoelectric ceramic and 1.5 MRayl for water, the transmission coefficient in (1) is about 0.18. According to Sutilov [1] the transmission coefficient of the pressure of a sound wave through a layer, e.g. a matching layer, with parallel surfaces is given with Formula (2).
T¼
4 Z 2 =Z 1 2
ðZ 2 =Z 1 þ 1Þ2 ðZ 22 =Z 2ML 1Þ ðZ 2ML =Z 21 1Þ sin kd
ð2Þ
with Z1 and Z2 acoustic impedances of the media in front and behind the layer, ZML the acoustic impedance of the layer, d the layer * Corresponding author. Tel.: +49 0 6894/980 270; fax: +49 0 6894/980 234. E-mail address: [email protected] (F. Tiefensee). 0041-624X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ultras.2009.08.012
Z ML ¼
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Z1 Z2
ð3Þ
The acoustic impedance of a matching layer ZML has to be the geometrical average of the acoustic impedance of the materials in front and behind of the matching layer, Z1 and Z2, and the thickness of the matching layer has to correspond to a quarter of the wavelength in its material. For instance, the transmission of ultrasound from a piezoelectric PZT ceramic into water can be increased with a matching layer, that has an acoustic impedance of 6.8 MRayl. New ultrasound applications in medical diagnosis in ophthalmology, dermatology and vascular wall diagnosis use frequencies up to 100 MHz, because high frequencies lead to smaller wavelengths and allow a better spatial resolution. The state of the art to produce matching layers with adjustable acoustic impedance is to mix microscale powders in a polymer and to adjust the acoustic impedance by the content of powder. In most cases the powder consists of aluminum oxide, Al2O3. The frequency range of these matching layers is limited by the use of microscale particles, because they lead to a high damping at frequencies higher than 10 MHz because of scattering. A possible solution of this problem is to reduce the size of the particles. But with the decrease of the particle diameter the surface energy in a defined volume unit increases and with the surface energy the tendency of the particles to agglomerate increases. The agglomeration of the particles would destroy the effect of small particles to reduce scattering. In [2] the
364
F. Tiefensee et al. / Ultrasonics 50 (2010) 363–366
use of nanoparticles for matching layers is described, but as the authors have used the relatively light Al2O3 (q = 3.94 g/cm3) particles compared with the CeO2 particles (q = 7.13 g/cm3) of the work at hand, the maximal acoustic impedance was 4.8 MRayl. With the synthesis described in [3–5] on wet chemical nanocomposite formation, coating systems with a high degree of filling and dispersed particles could be obtained in the past. In the paragraphs below a synthesis is shown to produce nanocomposite polymers with high filling degrees and dispersed nanoparticles, a method is described to measure the acoustic impedance at thin layers and the verification of the new matching layers with 100 MHz transducer is given. 2. Synthesis of the material The acoustic impedance of a material is the product of the density of the material and the velocity of sound in the material. A first estimation of the needed density of the matching layer material has used the fact, that in the predominant number of polymers the velocity of sound is between 2000 m/s and 3000 m/s and the assumption, that the velocity of sound does not change with the filling degree. To reach the technical important acoustic impedance of 6.8 MRayl for coupling from most piezoelectric ceramics into water, the density of the matching layer should have values between 2.0 g/cm3 and 3.0 g/cm3. As it is technically difficult to reach high filling degrees with nanopowder, it is desirable to operate with a filler, which permits low filling degrees. For this reason nanoparticles of cerium oxide with 7.1 g/cm3 were chosen for the further work. The chosen particles are commercially available and have average diameters between 10 nm and 15 nm. To avoid agglomeration of the particles, their surface was modified prior to use with trioxadecanic acid. 10 wt.% trioxadecanic acid was mixed with the cerium oxide particles. An aqueous solution of the surface modified nanoparticles was mixed with epoxy silane and zirconium alkoxide in order to form the nanocomposite system. The components are 3-glycidoxypropyltriethoxysilane, CASNo [2602-34-8] from EVONIK, and zirconium butylate, Zr(OC4H9)4. These reactants build an epoxy functionalized organic inorganic hybrid polymer. The optimal curing temperature for the synthesis was 160 °C. Lower curing temperatures are possible but it is difficult to produce solid layers at temperatures lower than 120 °C. Zirconium butylate was used as a catalyst for the synthesis of the inorganic network. The synthesis is described in Fig. 1. Fig. 2 shows
4 mol 3-glycidoxypropyltriethoxysilane + 1 mol zirconium butylate + 4 mol glacial acetic acid
hydrolysis for 12 h with aqueous dispersion of CeO2 agitation for 12 h
partial removal of the solvent by distillation
spin coating
thermal curing at 160°C, 3 h heating, 1 h hold, 6 h cooling Fig. 1. Synthesis of the matching layer materials.
Fig. 2. TEM picture of the matrix with cerium oxide particles (10 wt.% CeO2 solid content), preparation: microtome cutting.
a TEM picture of a layer with 10 wt.% cerium oxide particles. The CeO2 agglomerates to particles with diameters of about 20 nm. The particles are homogeneously distributed in the matrix material and this homogeneous dispersion of particles leads to constant material properties like density. As shown later, the velocity of sound in the material is about 2100 m/s. At 100 MHz the wavelength is 21 lm. Compared to the wavelength the agglomerates do not cause scattering. For TEM investigation a sample of lower particle content (10 wt.%) has been chosen because in the case of high particle contents (34–75 wt.%) the number of the 20 nm agglomerates per volume is too high to distinguish one aggregate from the other. Materials with different filling degrees between 34 wt.% and 75 wt.%, which corresponds to 9–37 vol.% respectively, have been produced. With these materials thin layers were coated on glass substrates by spin coating. The layers had an uniform thickness between 1 lm and 10 lm. 3. Measurement of the acoustic impedance As it is difficult to measure the velocity of sound in a thin layer of several microns, the acoustic impedance was measured by measuring the reflection coefficient of the layers. The measurements were calibrated with a known material. The reference was sapphire with a velocity of sound of 11,150 m/s and a density of 3.98 g/cm3 [6]. The reflection coefficient of the intensity of a sound wave at the interface between two media is
R¼
I1 ðZ 2 Z 1 Þ2 ¼ I0 ðZ 2 þ Z 1 Þ2
ð4Þ
I0 is the intensity of the emitted signal, I1 the intensity of the reflected signal, Z1 the acoustic impedance of the media in which the ultrasound propagates, here water at room temperature, and Z2 the acoustic impedance of the media at which the ultrasound is reflected. The quotient of the reflection coefficient of a matching layer and the reflection coefficient of sapphire is
I1 ðZ ML Z 1 Þ2 ðZ S þ Z 1 Þ2 ¼ I2 ðZ ML þ Z 1 Þ2 ðZ S Z 1 Þ2
ð5Þ
F. Tiefensee et al. / Ultrasonics 50 (2010) 363–366
365
Fig. 3. V(z)-scan for a sample filled with 34 wt.% of cerium oxide.
With I1 intensity of the reflected signal from a matching layer, I2 intensity of the reflected signal from a sapphire plate, ZML the acoustic impedance of the matching layer, ZS the acoustic impedance of sapphire and Z1 the acoustic impedance of a coupling media. ZML is the only unknown variable in Eq. (5). A scanning ultrasound microscope, which was developed at Fraunhofer Institute for Biomedical Engineering, IBMT, was used to measure the reflection coefficient. The ultrasound microscope was finished with a focused 200 MHz transducer with a focus length of 500 lm. The ultrasound beam was always focused on the surface of the layer by using a V(z)-scan. This method means, that the transducer is slowly moved perpendicular to the surface and when the maximal signal amplitude is received, the transducer is focused on the surface. The signal amplitude of the focus was used for further calculations. An example for a V(z)-scan is shown in Fig. 3. The ultrasound beam is focused at a time of flight of about 1100 ns. The ultrasound needs this time to propagate to the surface and back. The measurement equipment also allowed to distinguish the signal reflected at the interface between water and matching layer from the signal reflected at the interface between matching layer and substrate. The second signal travelled two times through the matching layer. Thus the attenuation of the matching layer material could be evaluated. The thicknesses of the matching layers were measured with a profilometer.
4. Results In Fig. 4 the results of the measurements are shown. The acoustic impedance can be varied continuously between 4 MRayl and 7 MRayl. The acoustic impedance is a linear function of the density with a slope of 2100 m/s. This velocity corresponds to the velocity of sound in the unfilled polymer. The evaluated attenuation of all matching layer materials was 0.5 dB/lm. To verify the functionality of the new material 100 MHz ultrasound transducers were coated with the new matching layer. The transducers were fabricated with methods of microtechnology,
Fig. 5. 16 16 ultrasound transducers on a silicon wafer.
that have been developed at IBMT and are published in [7]. With photolithography and physical vapour deposition 16 16 round piezoelectric zinc oxide pads were deposited on a silicon wafer. The piezoelectric pads had two gold electrodes. As the bottom electrode on the silicon had a larger diameter than the zinc oxide pad with the upper electrode, the single pads could be contacted with a coaxial probe. Fig. 5 shows a silicon wafer with 16 16 ultrasound transducer on one side. The opposite side of the silicon wafer was coated with the nanocomposite matching layer by spin coating. The ultrasound waves are generated with the zinc oxide layer and propagate through the 500 lm thick silicon wafer before they couple into water. For this reason the matching layer was calculated for the transition from silicon to water: The velocity of sound in silicon is 8900 m/ s, the density of silicon is 2.33 g/cm3. With Formula (3) the acoustic impedance of a matching layer should be 5.57 MRayl and its thickness should be k/4 = 5.25 lm for a 100 MHz transducer. The transducers were separated with a micro dice, electrically contacted with a coaxial probe and sealed against water. The transducers were positioned to an aluminum oxide reflector and the reflected signals were measured with and without matching layer. As shown in Figs. 6 and 7 the voltage amplitude with matching layer was about 100% higher than without matching layer. This result can be verified with a simple calculation. With Formula (1) and the acoustic impedances of silicon and water, the transmission coefficient is 0.25. With Formula (2), ZML = 5.57 MRayl ± 10%, d = 5.2 lm ± 10% and k = 21 lm, the minimal transmission coefficient with a matching layer is 0.87. The attenuation of the matching layer material should be maximal 3 dB and reduces the transmission to 0.60. The evaluated transmission with a matching layer is surely 100% higher, than the transmission with-
8.000 7.000
Z [MRayl]
6.000 5.000 4.000 3.000 2.000 1.000 0.000 0
0.5
1
1.5
2
2.5
3
3
density [g/cm ] Fig. 4. The acoustic impedance of cerium oxide in an epoxy functionalized organic inorganic hybrid polymer matrix.
Fig. 6. Voltage amplitude of a 100 MHz signal without matching layer.
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F. Tiefensee et al. / Ultrasonics 50 (2010) 363–366
range for coupling from the piezoelectric ceramic PZT into water. The linear slope in Fig. 4 is 2100 m/s and corresponds to the velocity of sound in the unfilled polymer. This result suggests, that scattering does not play a role in the material because the velocity is not influenced by the content of particles. As the material is synthesized via a wet chemical approach, solvents can be used to adjust the viscosity. The matching layers were produced with a spin coating process and the thickness could be varied between 1 lm and 10 lm. The functionality of the material could be shown with 100 MHz transducers. The matching layers lead to an increase of the voltage amplitude of 100%. Fig. 7. Voltage amplitude of a 100 MHz signal with matching layer.
out a matching layer and measurement results within this range can be expected. 5. Conclusions New nanocomposite materials for matching layers are presented. The material consists of cerium oxide nanoparticles in an epoxy functionalized organic inorganic hybrid polymer. By the content of particles the acoustic impedance can be adjusted to values between 4 MRayl and 7 MRayl, which is a technical important
References [1] Vladimir A. Sutilov, Physik des Ultraschalls, Springer Verlag, 1984, pp. 177. [2] Q.F. Zhou, J.H. Cha, Y. Huang, R. Zhang, W. Cao, J.M. Cannata, K.K. Shung, Nanocomposite matching layers for high frequency ultrasound transducers, In: IEEE Ultrasonics Symposium 3–6 October 2006, pp. 2365–2368. [3] L. Spanhel, E. Arpac, R. Nass, H. Schmidt, EP 0607213 B1, INM gGmbH, 09.10.1992. [4] E. Arpac, H. Krug, P. Müller, P. Oliviera, H. Schmidt, S. Sepeur, B. Werner, WO 98/ 51747 A1, INM gGmbH, 13.05.1997. [5] E. Arpac et. al., WO 99/52964, INM gGmbH, 09.04.1998. [6] A. Briggs, Acoustic Microscopy, Calderon Press, Oxford, 1992, p. 103. [7] A. Jakob, E.C. Weiss, T. Knoll, F. Bauerfeld, J. Herrmann, R. Lemor, Silicon based GHz acoustic lenses for time resolved acoustic microscopy, In: IEEE Ultrasonics Symposium 28–31 October 2007, pp. 1605–1608.