Local Modification of Speed of Sound in Lithium Alumino-Silicate

These results permit the volumetric patterning of delay lines by laser direct write techniques for generating complex profile ultrasonic wave patterns...
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Local Modification of Speed of Sound in Lithium Alumino-Silicate Glass/Ceramic Material by Pulsed Laser Irradiation and Thermal Processing Y. Kim and H. Helvajian* Physical Sciences Laboratories, The Aerospace Corporation, Los Angeles, California 90009, United States ABSTRACT: Glass and glass/ceramics are now used in modern devices with increasing frequency. A list of the notable material properties commonly will not include a capability to guide ultrasonic waves. The photosensitive glass ceramics (PSGCs), an old invention with recent technological rebirth, may enable this capability. The speed of sound (SoS) has been measured at an ultrasonic frequency (75 MHz) in a commercially available PSGC material. The measurements are made using a pulse echo time-of-flight (TOF) technique as a function of UV laser exposure and thermal processing. The measured increase in the SoS correlates with the density of crystalline matter present, which can be metered by controlling the exposure dose. For the Li2SiO3 crystalline phase, the results show the shear (transverse) wave mode velocity can be increased by 4.8% relative to an unexposed area where no crystalline matter exists. The maximum change in velocity for the longitudinal (compressional) wave mode is only 2%. However, by altering the thermal processing protocols to grow the high temperature Li2Si2O5 crystalline phase, the measured change in the SoS increases to 11% and 9%, respectively. These results permit the volumetric patterning of delay lines by laser direct write techniques for generating complex profile ultrasonic wave patterns. Moreover, by patterned 3D shaping (i.e., photostructuring), ultrasound energy can be harnessed and utilized to advantage.

I. INTRODUCTION With the advent of photonics, microelectromechanical systems (MEMS), and its derivative MOEMS (micro opto electromechanical system), there has been a sustained worldwide effort to build integrated systems with ever increasing complexity. The emergence of MEMS was realized by extending the material processing techniques that were commonly used in semiconductor manufacturing to add additional functional properties to silicon through a stepwise process of patterned structuring. The approach has proved credible. Over the past three decades an increasing number of integrated systems have been developed where the subunits are cofabricated on a common substrate. The key idea that originated this approach and all the subsequent worldwide developments was the work of Petersen1 who was first to explore the use of silicon material, not as an electronic substrate but as a mechanical substrate; therefore, by extending the range of a common process (i.e., chemical etching), a seemingly irrelevant material property was utilized to derive new capabilities (i.e., structural) not commonly associated with silicon. That seminal work provides the impetus for this work on functionalizing glass/ceramic materials for ultrasonic applications. Glass/ceramics are a material class where the material properties can be precisely engineered in manufacturing through compositional control.2,3 Consequently, these materials are ubiquitous in applications that range from the medical to © XXXX American Chemical Society

aerospace systems, from photonics to RF microwave communications, from nuclear reactors to automotive, and from entertainment to biological implants. In nearly all these applications, the engineered material has properties that are globally the same. In a class of structural ceramics that are photosensitive, it has been shown that material property changes can be administered on a local scale, and this allows for the cofabrication of devices on a common substrate.4−8 The photosensitive glass ceramic (PSGC) materials offer this capability through control of the exposure dose during patterning. The metering of the dose can be either by graylevel mask lithography or by laser direct write processing. In both cases there is an effect on the crystalline growth process during a subsequent thermal processing step. For a particular photosensitive aluminosilicate PSGC material there is evidence that the chemical solubility in acid could be altered,9,10 the optical transparency varied,11−13 optical index changed,14−16 the mechanical strength tuned,17−19 phase state change irreversibly,20,21 and the phase state reversibly changed.22 All of these property changes could be locally induced via patterning. Many of these material property changes are related Special Issue: Curt Wittig Festschrift Received: March 31, 2013 Revised: August 13, 2013

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to Ag°, which enhances atomic mobility and enables agglomeration to form nanometer size clusters (eq 3).

to the density of crystalline matter grown within the amorphous glass. Moreover, there is evidence that with pulsed UV laser exposures it is also possible to affect the growth of a particular crystalline phase.23 While most research has focused on the properties of the crystalline matter with regards to its chemical solubility, optical transparency, and structural stiffness, we present initial experimental results in this article on the change in the ultrasonic wave velocity as a result of changes in the crystalline density. Applications and use of ultrasonics are on the increase. Some modern digital information displays, where a touch screen is the interactive media, use ultrasonic waves to sense touch;24−26 ultrasound sensors are also finding increasing use in collision avoidance systems, which will be a critical part of modern robots,27,28 and may become useful in future gesture sensing devices (e.g., Microsoft Kinect sensor).29 There is also experimental evidence of ultrasonic waves affecting gas phase chemical catalysis on surfaces.30,31 Sound travels between 4 and 6 km/s in glass/ceramic composite matrixes with the compressional waves being typically faster than the transverse or shear waves. An intriguing aspect of ultrasonic frequencies (>10 MHz) is that the wavelength is on the scale that lends to easy patterning via standard lithography or laser direct write processing. For a nondispersing material, the wavelength, Λ, the speed of sound, υ, and frequency, ν, can be related by the following equation (υ = Λν).32 Consequently, an ultrasonic wave at 100 MHz has an approximate propagation wavelength of 50 μm (for υ = 5 km/s). If the speed of sound in glass/ceramics could be controllably varied, then it would be possible to fabricate delay lines that locally functionalize the material for practical technological use such as cancellation by destructive interference, excitation by constructive interference (e.g., cavitation/ microbubble formation for fluidic applications) and focusing by wave guiding. These functionalized devices could be integrated with ultrasonic transducer drivers to form a complete instrument. Acoustic transducers at ultrasonic frequencies are commercially available. In addition, it is also possible to generate it using pulsed laser irradiation below the ablation threshold and at laser power levels where the irradiated material response is thermoelastic.33 In this article we present experimental results for the controllable increase of acoustic velocity in a glass/ceramic material that has been transformed from the amorphous phase by UV pulsed laser irradiation and subsequent heating. We show evidence of a velocity increase in both the compressional and shear waves. The velocity of sound is measured by ultrasonic time-of-flight technique that uses a pulsed piezoelectric transducer at a driving frequency of 100 MHz. The present study utilizes the PSGC Foturan, which is manufactured by the Schott Corporation (Mainz, Germany) and is optically transparent from the near UV to deep red wavelengths. Foturan is an alkali-aluminosilicate glass and consists primarily of silica (SiO2, 75−85 wt %) along with various stabilizing oxide admixtures, such as Li2O (7−11 wt %), K2O and Al2O3 (3−6 wt %), Na2O (1−2 wt %), ZnO ( ΔυL) cannot be explained by the large aspect ratio of the crystal structure. Perhaps the larger change observed in the shear velocity (i.e., ΔυS) is just representative of the increased chemical interaction that form structures (i.e., crystals, the formation and lengthening), and these structures do support shear waves. An analogy might be that of water. In its liquid form, it does not support shear waves, but as it freezes and crystals form, shear waves can propagate. While the measured results for ΔυS and ΔυL do have practical use, a larger Δυ (ΔυS or ΔυL) would expand the possible number of applications. The lithium metasilicate (Li2SiO3) crystal under higher baking temperatures (>700 °C) dissolves to form a lithium disilicate (Li2Si2O5) phase. The lithium disilicate (LDS) is a layered structure as opposed to dendritic38 and should pack with better efficiency. The LMS phase, however, is soluble in HF acid, while the LDS phase is not. Consequently, structures formed in the LDS phase cannot easily be removed, and the surrounding unexposed amorphous glass typically forms various phases of quartz. An experiment was conducted to test the prediction for a higher speed of sound when the LDS phase is grown. One would expect not only larger ΔυS and ΔυL values but also the change in velocity to be numerically closer because of the layered structure and less crystalline anisotropy. The results of a sample that received saturation exposure and baked to form the LDS phase are

and L wave modes could be linked to the dendritic crystalline structure of the LMS phase. Figure 9 shows a transmission

Figure 9. TEM of UV exposed and thermally processed Foturan showing the dendritic crystalline structure of lithium metasilicate.

electron microscopy (TEM) image of a thermally processed sample showing the dendritic crystalline structure of the LMS. Following a normal bake process protocol, the crystals grow to form a randomly oriented 3D skeletal framework where the overall length approaches 1 μm. However, the diameter of the rod-shaped segments; only grow to an approximate width of 91 nm, a factor of 10 less.9 The Li2SiO3 crystal has an orthorhombic structure (space group Cmc21)35 with lattice constants (a,b,c) = (9.396, 4.396, 4.661 Å), respectively.36 From the calculated values for the elastic tensor, Cij, for Li2SiO3 (9 for an orthorhombic system), the derived velocity-elastic modulus formulas37 and the fact that the diagonal coefficients of the tensor matrix are larger (factor of 2−3) than the off-diagonal components (Cii > Cij where i is not equal to j) and υ2 = Cij/ρ

Figure 10. Speed of sound results from a LDS sample: (a) the raw TOF 2D data of the % increase in υL mode; the data in the yellow cross-section is presented in (b) in units of % increase where the ΔυL ≈ 9.2%. (c) The raw TOF 2D data for the υS mode increase; the data in the yellow crosssection is shown in (d) where the ΔυS ≈ 11.3%. G

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shown in Figure 10. The change in the speed of sound ΔυS and ΔυL increase to over 11% and 9% for the shear and longitudinal modes, respectively, and as expected, the difference between the two modes is closer in comparison to that measured for the LMS crystalline form.

(2) McMillan, P. Glass Ceramics, 2nd ed.; Academic Press: New York, 1979. (3) Vogel, W.; Kreidl, N. J.; Lense, E. Chemistry of Glass; American Ceramic Society Press: Westerville, OH, 1985. (4) Stookey, S. Machining of Photosensitive Glass. Ind. Eng. Chem. 1953, 45, 115−118. (5) Berezhnoi, A. L. Glass-Ceramics and Photo-Sitalls; Plenum Press: New York, 1970. (6) Hansen, W. W.; Janson, S. W.; Helvajian, H. Direct-Write UV Laser Microfabrication of 3D Structures in Lithium Aluminosilicate Glass. Proc. SPIE 1997, 2991, 104−112. (7) Talkenberg, M.; Kreutz, E.-W.; Horn, A.; Jacquorie, M.; Poprawe, R. UV Laser Radiation-Induced Modifications and Microstructuring of Glass. Proc. SPIE 2002, 4637, 258−269. (8) Livingston, F.; Adams, P.; Helvajian, H. Examination of the LaserInduced Variations in the Chemical Etch Rate of a Photosensitive Glass Ceramic. Appl. Phys. A: Mater. Sci. Process. 2007, 89, 97−107. (9) Livingston, F. E.; Helvajian, H. Photophysical Processes that Lead to Ablation-Free Microfabrication in Glass-Ceramic Materials. In 3D Laser Microfabrication: Principles and Applications; Misawa, H., Juodkazis, S.; Eds.; Wiley-VCH Verlag Press: Berlin, Germany, 2006; pp 287−339. (10) Sugioka, K.; Cheng, Y.; Masuda, M.; Midorikawa, K.; Shihoyama, K. Fabrication of Microreactors in Photostructurable Glass by 3D Femtosecond Laser Direct Write. Proc. SPIE 2004, 5339, 205−213. (11) Lui, L. Y.; Fuqua, P. D.; Helvajian, H. Measurement of the Critical UV Dose in Lamp Exposure of a Photostructurable GlassCeramic. Report No: ATR-2001(8260)-1; The Aerospace Corporation Press: El Segundo, CA, 2001. (12) Livingston, F. E.; Adams, P. M.; Helvajian, H. Active PhotoPhysical Processes in the Pulsed UV Nanosecond Laser Exposure of Photostructurable Glass Ceramic Materials. Proc. SPIE 2004, 5652, 44−50. (13) Cheng, Y.; Sugioka, K.; Masuda, M.; Shihoyama, K.; Toyoda, K.; Midorikawa, K. Three-Dimensional Micro-Optical Components Embedded in Foturan Glass by a Femtosecond Laser. Proc. SPIE 2003, 5063, 103−107. (14) Ho, S.; Cheng, Y.; Herman, P. R.; Sugioka, K.; Midorikawa, K. Direct Ultrafast Laser Writing of Buried Waveguides in Foturan Glass. Conference on Lasers and Electro-Optics, 2004; paper CTHD6. (15) An, R.; Li, Y.; Liu, D.; Dou, Y.; Qi, F.; Yang, H.; Gong, Q. Optical Waveguide Writing Inside Foturan glass with Femtosecond Laser Pulses. Appl. Phys. A: Mater. Sci. Process. 2007, 86, 343−346. (16) Li, Z.; Low, D.; Ho, M.; Lim, G.; Moh, K. Fabrication of Waveguides in Foturan by Femtosecond Laser. J. Laser Appl. 2006, 18, 320−324. (17) Huang, A.; Hansen, W. W.; Janson, S. W.; Helvajian, H. Development of a 100 gm Class Inspector Satellite Using Photostructurable Glass/Ceramic Materials. Proc. SPIE 2002, 4637, 297− 304. (18) Stillman, J. Three-Dimensional Microfabrication with LaserPatterned Photostructurable Glass. Ph.D. Thesis, University of California, Los Angeles, CA, 2008 (19) Stillman, J.; Judy, J.; Helvajian, H. Laser Alteration of the Mechanical Properties of Photostructurable Glass-Ceramic. Proc. SPIE 2008, 6879, 68790E−1. (20) Miyamoto, I.; Cvecek, K.; Okamoto, Y.; Schmidt, M.; Helvajian, H. Characteristics of Laser Absorption and Welding in FOTURAN Glass by Ultrashort Laser Pulses. Opt. Express 2011, 19, 22961−22973. (21) Fisette, B.; Busque, F.; Degorce, J.-Y.; Meunier, M. ThreeDimensional Crystallization Inside Photosensitive Glasses by Focused Femtosecond Laser. Appl. Phys. Lett. 2006, 88, 091104−091104−3. (22) Veiko, V.; Kieu, Q.; Nikonorov, N.; Skiba, P. On the Reversibility of Laser-Induced Phase-Structure Modification of GlassCeramics. J. Laser Micro/Nanoeng. 2006, 1, 149−154. (23) Livingston, F. E.; Helvajian, H. Laser Processing Architecture for Improved Material Processing. In Laser Processing of Materials; Schaaf,

V. CONCLUSIONS An experiment has been conducted to show that by laser direct write exposure processing it is possible to change the speed of sound properties of a commercially available photostructurable glass/ceramic. By using a 355 nm pulsed UV laser, a change in the shear mode velocity of up to 4.8% has been achieved when the LMS crystalline phase is grown. The change in the longitudinal mode velocity is 2%. Both values have practical use for controlling ultrasonic waves especially at high frequencies (>50 MHz). The material transformation process that induces the formation of the LMS phase can be used to develop devices where the crystalline phase is patterned to form ultrasonic delay lines, and by chemical etching, sections of the patterned/ exposed areas can be selectively removed. Furthermore, by altering the protocol of the crystalline growth process, we have shown that a different phase of the lithium silicate could also be formed that show velocity changes (i.e., ΔυS, ΔυL) of over 11% and 9%, respectively. Finally, by implementing laser amplitude modulation and the use of specific pulse sequence profiles, it becomes possible to pattern both the LMS and LDS phases on the same sample23 and thereby allow sections to be removed by chemical etching and sections to remain for controlling the propagation of ultrasonic waves. It also becomes feasible to engineer the propagation of mechanical force through a material where Poisson’s ratio can be varied locally. The current experimental focus is on the trapped photoelectron defect (eq 1) and whether its state could be altered by a subsequent laser pulse to affect the density of crystalline matter that would grow. If feasible, then it would be possible to write and erase the material transformation changes.

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AUTHOR INFORMATION

Corresponding Author

*(H.H.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for support from The Aerospace Corporation’s Independent Research and Development program. One of us (H.H.) also wishes to thank The Air Force Office of Scientific Research (Dr. Howard Schlossberg) for financial support to complete this work. Finally and sincerely, H.H. thanks Professor Curt Wittig for his inspirational mentorship, guidance, and support during his graduate studies, now many years ago. One lesson learned from the Wittig Laboratories can be best elucidated through an old saying. “Iron rusts from disuse; stagnant water loses its purity and in cold weather becomes frozen; even so does inaction sap the vigor of the mind” (da Vinci, L. Note-Books., Vol. Codice Atlantico 289 v.c.).

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