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Abstract
We report the successful picosecond laser welding of AlSi and YAG. This material combination is of significant interest to the field of laser design and construction. Parameter maps are presented that demonstrate the impact of pulse energy and focal position on the resultant weld. Weld performance relevant to industrial applications is measured, i.e., shear strength, process yield, and absolute thermal resistance are presented.
Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. INTRODUCTION
Significant and sustained research has been applied in recent years to the development of ultrafast laser welding of dissimilar materials. The process advantages of high speed, precision, and extremely low thermal damage, without any requirement for bonding interlayers, provide an attractive solution to dissimilar material bonding in a range of industrial applications [1–3].
The process depends on use of a tightly focused ultrafast laser that can create the plasma that drives the welding process while limiting the extent of thermally affected material to few 10’s or 100’s of µm around a weld seam. Careful positioning of the intense focus of the laser beam can create a plasma from both materials, surrounded by a miniature melt volume. As this highly localized and confined plasma and melt cool, a bond forms between the two materials [2,3].
Although the majority of data published on this technique to date has concentrated on the use of femtosecond laser pulses [2,4–8], there has been growing interest in the use of picosecond laser systems [9–15]. In this study, we demonstrate welding of YAG crystals to AlSi, a common heatsink material for Nd:YAG lasers. We also present a systematic study of two key parameters of the welding process as a step toward a reliable, industrial-capable process, paying particular attention to the thermal contact of the bond, which is highly relevant to using this process for manufacturing lasers.
2. MATERIALS
Following unpublished initial proof-of-principle experiments that demonstrated the feasibility of welding Nd:YAG to AlSi, our detailed study used undoped YAG discs of 5 mm diameter and 2 mm thickness to provide a large number (100’s) of samples at reasonable cost. The 5 mm diameter faces were polished to optical quality. Early proof-of-principle work showed no significant deviation in the behavior of the welding process in Nd:YAG compared to undoped YAG. Hence undoped YAG was used for statistical testing on cost grounds; and the results are therefore expected to be relevant to Nd:YAG.
Sandvik Osprey CE8F AlSi discs of 10 mm diameter, 2 mm thickness were used as the substrate material. This was measured with an Alicona surface profiler and found to have a surface roughness of 400 nm (Sa).
All parts were individually cleaned in nine parts de-ionized water and one part Decon Neutracon non-corrosive cleaning agent in an ultrasonic bath for 5 min before being wiped clean with a lint free lens tissue. This was followed by a further 10 min in an ultrasonic bath in de-ionized water, and subsequently dried with a jet of compressed nitrogen. The components were finally inspected by eye for dirt or residue on the bonding face before being wrapped in a lint free cleanroom cloth and stored until required. Prior to being placed in the welding apparatus, the samples were wiped with a lint free lens tissue soaked in methanol and blow dried with ionized nitrogen. The thermal properties of the materials used, including a thermal interface material [Panasonic Soft–PGS (pyrolytic graphite sheet), compressible type], are summarized in Table 1.
Table 1. Comparison of Room Temperature Thermal Properties of the Materials Used in This Study
Fig. 1. Welding clamping setup used. A holder is used to center the YAG onto the welding aperture and AlSi, and a pneumatic piston applies pressure to the base plate supporting the AlSi to bring the YAG and AlSi into close contact.
Fig. 2. Basic cut-bar concept for measuring YAG on AlSi discs with the calculation of bar thermal gradients and sample temperature drop $\Delta T$.
3. EXPERIMENTAL SETUP
A. Welding
The experimental setup and procedure used for welding are similar to those used in our previously reported work [14] with only a minor alteration to the clamping system to accommodate changes to available material sizes. The YAG and AlSi discs are placed in a clamping arrangement as shown in Fig. 1 with a holder used to center the YAG on the welding aperture and the AlSi. Pressure is then applied to the metal base plate supporting the AlSi to bring the two surfaces into close contact using a pneumatic piston arrangement to apply between 0.6 and ${6.5}\;{\rm{N}.\rm{mm}}^2$. A picosecond pulsed laser is then used to create a 2.5 mm diameter weld spiral with inner radius of 0.2 mm and pitch of 0.156 mm by translating the sample using an Aerotech ANT95XY nanometer precision stage. All experiments were carried out using a 5.9 ps, 1030 nm, 400 kHz Trumpf TruMicro $5 \times 50$ laser. The focusing optic is a 20 mm focal length plano–convex lens AR coated for 600–1050 nm, creating a calculated focal spot diameter in air of 2.6 µm (NA 0.25). However, spherical aberrations are to be expected to result in an actual spot size larger than this.
B. Thermal Resistance Measurement
Measurements of thermal resistances are obtained through a steady state method based on the ASTM D5470 standard [20]. This is a common method of evaluating thermal resistance [21] and is adapted here for a comparative evaluation of the thermal performance of a YAG:AlSi laser bond.
An in-house cut-bar apparatus, designed to measure the absolute thermal resistance of samples in ${\rm{K}.{\rm{W}}^{- 1}}$ over a range of loads up to 300 N, is used. The basic design is presented in Fig. 2 as well as the calculation of the temperature gradient of the two bars and temperature drop across the sample. A calibrated load is applied to the top of the column, and heat flows from the heater to the cooler, setting up a steady state thermal gradient across the conductive bars and sample.
The heat flow through the sample and its resistance are then calculated using the steady state heat Eq. (1) along with Eqs. (2) and (3) under a range of applied loads, which are converted to pressures considering the sample area. $Q$ is heat flow, $k$ is thermal conductivity of the bar, $A$ is the contact cross-section area, $\frac{{dT}}{{dz}}$ is the temperature gradient down the bar (top and bottom), ${{\Delta}}T$ is the temperature drop across the sample, and $R$ is absolute thermal resistance in ${\rm{K}.{\rm{W}}^{- 1}}$:
where ${{i}} = {{1}}$ corresponds to the top bar, ${{i}} = {{2}}$ the bottom bar, and ${{i}} = {{3}}$ the sample.C. Cut-Bar Design
Figure 3 shows an image of the cut-bar apparatus used. Calibrated load is applied from the top with an actuator and measured by a load cell nested in the baseplate. Steady heat is supplied by resistors attached to the top bar and is removed through conduction cooling of the baseplate.
A thermally insulating block is placed above the resistors to ensure heat flows toward the sample. Temperatures are measured using thermocouples, while a plastic cover (not shown) is placed over the apparatus during experiments to minimize heat loss through convection. To assist with achieving good contact with the rods and the YAG/AlSi interface, PGSs are used on either side of the sample.
Data are taken starting at a low load, which is increased after each measurement up to the desired maximum. The sample is considered to be at steady state once the temperatures vary by less than $0.1^\circ\rm C$ over a period of 2 min, in line with ASTM D5470 [20]. The coolant water fluctuates by less than $0.05^\circ \rm C$, which is well within the tolerance recommended by ASTM D5470. Also, according to the standard, samples are measured at a recommended temperature of around $50^\circ\rm C$. Using these criteria, the cut-bar provided consistent results over repeated readings.
It is worth noting that ASTM D5470 states that the bar cross section must match that of the sample; however, this is less relevant for a comparative study of highly conductive samples with non-uniform geometry [22]. We found that the conductive nature of the samples means that heat flow is not drastically inhibited, and a comparative conclusion on the thermal resistance of the interface between YAG and AlSi can still be drawn if the sample geometry is unchanged. The sample geometry is shown in Fig. 4. The interface was either: (i) a weld; (ii) PGS thermal interface material; or (iii) direct contact (no interface material).
Fig. 4. Model of YAG/AlSi sample stack used in the cut-bar apparatus. The interface used was the weld, PGS thermal interface material, with no interlayer for the three comparative tests conducted.
D. Shear Testing
The final stage of the analysis is a destructive shear test to determine the strength of the welds. This was carried out using a specially designed rig, shown in Fig. 5. The welded sample is placed in the recessed slot of the bottom part of the rig with the AlSi substrate facing down. The upper part of the rig is placed on top taking care to place the YAG into the recessed slot without applying any sideways force. Then the rig is bolted to firmly hold it in place while mounting into the shear testing system; the bolts are then released and the two halves of the rig pulled apart using an Instron 30 kN tensile testing rig while monitoring the force until the weld fails.
4. RESULTS
A. Welding Parameter Map
The first stage of this study involved generating a map of the viable welding parameters that succeed at bonding the YAG to the AlSi. A successful weld is defined as one that remains bonded after 24 h, such as the welded sample shown in Fig. 6(A). Furthermore, a minimum shear strength of 7 N was set to ensure the weld meets real-world bonding application requirements; bonds that did not reach this strength were also considered failures. Two samples were created for each parameter combination as shown in Fig. 7.
Fig. 6. (A) Image of a successfully welded YAG to AlSi. (B) Microscope bright-field image of a variable focus spiral weld used to narrow down the possible parameter space (details in text).
Fig. 7. Parameter map showing all investigated parameters. Two samples were created per parameter combination tested except for the focal planes of 0 µm, ${-}{{125}}\;{{\unicode{x00B5}{\rm m}}}$, and ${-}{{250}}\;{{\unicode{x00B5}{\rm m}}}$ with an average power of 4.25 W where five samples were created for each combination.
To narrow down the potential parameter space to be investigated, variable focus welds were used to generate a 2.5 mm diameter weld spiral where the focal plane varies from 400 µm above the YAG-AlSi interface (defined as ${+}{{400}}\;{{\unicode{x00B5}{\rm m}}}$) to 400 µm below the interface (${-}{{400}}\;{{\unicode{x00B5}{\rm m}}}$) for several average powers. By examining the resultant weld track under a microscope, it was possible to determine the range of focal positions for which a good weld was obtained in each case, as indicated in Fig. 6(B). This information was used to select the ranges of focal position and laser power for closer investigation. The resulting parameter map is shown in Fig. 7.
A laser average power of 4.25 W (at 400 kHz) was selected as being best, and five samples were welded for each of three different focal positions: 0 µm, ${-}{{125}}\;{\rm{\unicode{x00B5}{\rm m}}}$, and ${-}{{250}}\;{{\unicode{x00B5}{\rm m}}}$. These samples were shear tested, and the resultant cumulative probability distribution for each set is plotted in Fig. 8. This plot indicates that the best sets were produced using the 0 µm focal position, with a mean shear strength of ${{18}}\;{{\pm}}\;{{5}}\;{\rm{N}.\rm{m}}{{\rm{m}}^{- 2}}$.
Fig. 8. Cumulative probability distribution of welded sample sets for 0 µm, ${-}{{125}}\;{{\unicode{x00B5}{\rm m}}}$, and ${-}{{250}}\;{{\unicode{x00B5}{\rm m}}}$ with an average power of 4.25 W.
B. Thermal Resistivity
Given the results reported above, measurements of absolute thermal resistance were carried out for welded samples using an average power of 4.25 W, 0 µm focal plane. This was achieved using the cut-bar method in a comparative test as described above. Two comparative sample types were used: the first was a YAG and AlSi disc in direct contact, and the second a layer of PGS between a YAG and AlSi disc. These are referred to as YAG:AlSi (not welded) and YAG:PGS:AlSi (not welded), respectively and represent different conditions of thermal contact between YAG and AlSi, which can also be compared to calculated thermal resistance of adhesive interface materials The welded samples are referred to as YAG:AlSi (welded). The undoped YAG, PGS, and AlSi were also investigated separately to determine the absolute thermal resistance of the materials used. This comparative study involved three samples for each material or sample configuration tested.
Contact thermal resistance is pressure dependent, with increasing applied pressure improving the thermal contact at the material interface. This dependence is a two-term power series in nature. This relationship is expressed in Eq. (4) [23,24], where the ${{x}}$ is the pressure, with ${{a}}{{{x}}^b}$ representing the pressure dependent contact resistance and $c$ representing the resistance of the bulk material (see Fig. 9), and ${{R}}$ is the thermal resistance:
Fig. 9. Example of the pressure dependent thermal resistance of a 2.5 cm diameter Panasonic compressible graphite sheet (PGS) disc [19]. The measurement uncertainty in the thermal resistance was determined by taking the measured temperature variation over the 2 min measurement window and evaluating the resulting change in thermal resistance.
The bulk material has a constant resistance regardless of applied pressure, as determined by the material properties. The contact resistance is pressure dependent, as increasing pressure deforms the material surfaces at the interface to increase conformity of the surfaces, which improves thermal contact. At a critical pressure, further deformation of the interface is not possible, and thus the curve levels off. Using the cut-bar setup to measure thermal resistance at several applied pressures up to ${{16}}\;{\rm{N}.\rm{mm}}^2$ and fitting the data for all three samples of each type to a two-term power series curve, it is possible to obtain the thermal resistance of each sample type. It was observed that there was considerable variation in the absolute thermal resistance among the three welded samples tested. Hence, these were treated as having a different absolute thermal resistance as opposed to being combined into a single data set. The Soft-PGS was used as a thermal interface material at the upper and lower interfaces of each sample to ensure good thermal contact, and the results of this analysis are shown in Fig. 10. The error bars were determined by propagating the temperature variation over the course of the 2 min measurement window, and the uncertainty in the sample area through calculation of the absolute thermal resistance described in Eqs. (1 )–(3). The temperature fluctuation varied from sample to sample and was measured by recording the temperature once per second throughout each measurement but was kept below 0.1°C.
Fig. 10. Absolute thermal resistance as a function of applied pressure for Soft-PGS, YAG, AlSi, Soft-PGS between YAG and AlSi (YAG:PGS:AlSi); direct contact (no interface material) between the YAG and AlSi (YAG:AlSi not welded); and three welded YAG to AlSi samples (YAG:AlSi welded). Each measured material/material combination was sandwiched top and bottom with Soft-PGS to ensure good thermal contact with the instrument.
It can be observed from Fig. 10 that the three welded samples have a higher absolute thermal resistance compared to the YAG:AlSi (not welded) and YAG:PGS:AlSi (not welded). It should be noted that the weld area (from the 2.5 mm diameter weld spiral) was substantially less than the contact area (5 mm diameter YAG disc) used in these calculations. Future work would be required to investigate the effect of weld diameter, which was kept constant in this initial study to determine the optimum parameters for weld success and strength. The measured material, contact, and total absolute thermal resistance calculated from the two-term power series curve fits are listed in Table 2. These values were obtained from fitting the data in Fig. 10 using Eq. (4); see Fig. 9.
From Table 2, it is shown that the measured total absolute thermal resistances for all three welded samples are slightly above the values for YAG:AlSi (not welded), while YAG:PGS:AlSi had the lowest value. Of particular interest is the contact resistance of the interface between YAG and AlSi in the three configurations, which were analyzed further by treating the samples as resistors in series; see Fig. 11.
Fig. 11. Thermal resistance model of an example composite sample. The total configuration thermal resistance can be treated as resistors in series where each component material has a bulk resistance $R$ and a contact resistance $R_{C}$.
Fig. 12. Normalized calculated curves for the absolute thermal resistance of the interfaces of YAG:AlSi (welded) (three samples), YAG:AlSi (not welded) and YAG:PGS:AlSi (not welded). The dashed line represents the specified absolute thermal resistance of Avantor CV2–2946 thermally conductive adhesive with a minimum bond thickness of 50 µm and area of ${19.6}\;{\rm{mm}}^2$.
The measured absolute thermal resistances of the bulk YAG and AlSi were subtracted from the YAG:AlSi (not welded), YAG:PGS:AlSi (not welded), and YAG:AlSi (welded) curves. The absolute thermal resistance of the PGS thermal interface material was not subtracted, as this was shown to be negligible and below the measurement threshold of the cut-bar setup used [25]. To account for convection losses that will differ between samples types (due to higher resistance increasing parasitic loss in the setup), the curves were normalized to the mean YAG:PGS:AlSi (not welded), as this should have a thermal resistance at the interface approaching zero in this setup.
Furthermore, as ultrashort pulse laser welding of YAG to AlSi is intended to replace currently used adhesives, such as Avantor CV2-2946 thermally conductive adhesive, it is important to give context to our results. To this end, the manufacturer stated that properties of this adhesive were used to model an idealized bond of 50 µm minimum thickness [26] with a bond equal to the surface area of the 5 mm diameter YAG disc. The resulting curves are shown in Fig. 12 where the adhesive thermal resistance is represented as a dashed line. This value is used as context only, as it cannot be directly compared to our results owing to the comparative nature of this measurement.
From Fig. 12, it can be observed that the ultrashort pulse laser welded samples have an absolute thermal resistance in the interface that does not depend on applied pressure, and that is substantially lower than an idealized bond using a state-of-the-art thermally conductive adhesive. There is, however, a noticeable variation in the thermal resistance of the three welded samples, ranging from ${\sim}{{1}}$ to ${{2}}\;{\rm{K}.{\rm{W}}^{- 1}}$. The cause of this variation is uncertain and should be studied further. The thermal resistance of the interface for the YAG:AlSi (not welded) sample was quite low, and indicates good contact between the polished YAG face and the low surface roughness (400 nm Sa) AlSi face, which improves as the applied pressure is increased.
Finally, it is important to note that Avantor CV2-2946 has a specified cured tensile strength of ${2.8}\;{\rm{N}.\rm{m}}{{\rm{m}}^{- 2}}$, whereas ultrashort pulse laser welded YAG:AlSi was demonstrated to have a shear strength of ${{18}}\;{{\pm}}\;{{5}}\;{\rm{N}.\rm{m}}{{\rm{m}}^{- 2}}$, considerably higher than the stated value for the adhesive.
5. CONCLUSION
In conclusion, ultrashort pulse laser welding of YAG to aluminum silicon alloy AlSi CE8F has been demonstrated, which opens the door to welding Nd:YAG laser crystals to a heatsink material with a matched thermal expansion coefficient, thus avoiding additional interface layers.
In this work, we have determined the ideal laser processing parameters considering weld success and strength of the bonds (4.25 W, 5.9 ps, 1030 nm, 400 kHz, 2.6 µm spot diameter) located at the material interface (0 µm offset). The samples were 5 mm diameter undoped YAG discs of 2 mm thickness, bonded to AlSi with surface roughness of 400 nm (Sa). The laser welded bonds had a mean shear strength of ${{18}}\;{{\pm}}\;{{5}}\;{\rm{N}.\rm{m}}{{\rm{m}}^{- 2}}$ for the 2.5 mm diameter spiral spot weld generated in the center of the samples. This value compares favorably to the specified cured tensile strength of ${2.8}\;{\rm{N}.\rm{m}}{{\rm{m}}^{- 2}}$ for Avantor CV2-2946 thermally conductive adhesive typically used in this application.
Furthermore, the absolute thermal resistances of the ultrafast laser welded bonds were measured for the first time, using a cut-bar apparatus following the ASTM D5470 standard, and directly compared to an unwelded sample and a sample with Panasonic Soft-PGS thermal interface material compressed between YAG and AlSi. The thermal resistance of the bond interface was further analyzed and normalized to the ideal low resistance of the PGS thermal interface material. In this analysis, the laser welded interfaces had thermal resistances, independent of applied pressure, in the range of ${\sim}{{1}}$ to ${{2}}\;{\rm{K}.{\rm{W}}^{- 1}}$ for the different YAG samples. This is substantially lower than the value for CV2-2946 adhesive, but slightly higher than the sample that was not welded (no interface material), which had a pressure-dependent thermal resistance of ${{0}.\rm{65 - 0}.{5}}\;{\rm{K}.{\rm{W}}^{- 1}}$ more than the ideal interface.
6. FUTURE WORK
Although the optimum laser processing parameters for weld success and weld strength have been determined, it was shown that the laser bond has a noticeable variation in thermal resistance from sample to sample. The authors postulate that this may be due to the thermally affected material (10’s to 100’s of µm) around the weld seam, or a generated air gap between YAG and AlSi outside the weld region; both effects could vary from sample to sample. These could be investigated by analyzing the variance in material composition within the weld microstructure, as well as attempting laser bonds with different weld patterns and line lengths (varying weld area) to explore its impact on the absolute thermal resistance of the welded samples. Welding through a thermal interface layer such as PGS or commonly used indium may also be possible and could potentially reduce the variation in thermal resistance from weld to weld, while still avoiding pressure dependent thermal resistance. Furthermore, considering results from welding other material combinations [27,28], the magnitude and distribution of stress induced birefringence associated with the weld should be investigated, as this could have an impact on the design of Nd:YAG lasers utilizing this as new bonding method.
Funding
Leonardo MW Ltd.; Engineering and Physical Sciences Research Council (EP/R511948/1, EP/T517999/1).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
REFERENCES
1. A. W. Y. Tan and F. E. H. Tay, “Localized laser assisted eutectic bonding of quartz and silicon by Nd:YAG pulsed-laser,” Sens. Actuators A Phys. 120, 550–561 (2005). [CrossRef]
2. H. Huang, L.-M. Yang, and J. Liu, “Ultrashort pulsed fiber laser welding and sealing of transparent materials,” Appl. Opt. 51, 2979–2986 (2012). [CrossRef]
3. W. Watanabe, T. Tamaki, and K. Itoh, “Ultrashort laser welding and joining,” in Femtosecond Laser Micromachining, O. Roberto, C. Giulio, and R. Roberta, eds. (Springer-Verlag, 2012), pp. 467–477.
4. D. Hélie, M. Bégin, F. Lacroix, and R. Vallée, “Reinforced direct bonding of optical materials by femtosecond laser welding,” Appl. Opt. 51, 2098–2106 (2012). [CrossRef]
5. A. Horn, I. Mingareev, A. Werth, M. Kachel, and U. Brenk, “Investigations on ultrafast welding of glass–glass and glass–silicon,” Appl. Phys. 93, 171–175 (2008). [CrossRef]
6. S. Richter, S. Döring, A. Tünnermann, and S. Nolte, “Bonding of glass with femtosecond laser pulses at high repetition rates,” Appl. Phys. 103, 257–261 (2011). [CrossRef]
7. K. Sugioka, M. Iida, H. Takai, and K. Micorikawa, “Efficient microwelding of glass substrates by ultrafast laser irradiation using a double-pulse train,” Opt. Lett. 36, 2734–2736 (2011). [CrossRef]
8. G. Zhang and G. Cheng, “Direct welding of glass and metal by 1 kHz femtosecond laser pulses,” Appl. Opt. 54, 8957–8961 (2015). [CrossRef]
9. I. Alexeev, K. Cvecek, C. Schmidt, I. Miyamoto, T. Frick, and M. Schmidt, “Characterization of shear strength and bonding energy of laser produced welding seams in glass,” J. Laser Micro Nanoeng. 7, 279–283 (2012). [CrossRef]
10. I. Miyamoto, K. Cvecek, Y. Okamoto, and M. Schmidt, “Novel fusion welding technology of glass using ultrashort pulse lasers,” Phys. Procedia 5, 483–493 (2010). [CrossRef]
11. R. Carter, J. Chen, J. D. Shephard, R. R. Thomson, and D. P. Hand, “Picosecond laser welding similar and dissimilar materials,” Appl. Opt. 53, 4233–4238 (2014). [CrossRef]
12. R. M. Carter, M. Troughton, J. Chen, I. Elder, R. R. Thomson, R. A. Lamb, M. J. D. Esser, and D. P. Hand, “Picosecond laser welding of optical to metal components,” Proc. SPIE 9736, 973615 (2016). [CrossRef]
13. O. P. Ciuca, R. M. Carter, P. B. Prangnell, and D. P. Hand, “Characterisation of weld zone reactions in dissimilar glass-to-aluminium pulsed picosecond laser welds,” Mater. Charact. 120, 53–62 (2016). [CrossRef]
14. R. M. Carter, M. Troughton, J. Chen, I. Elder, R. R. Thomson, M. J. D. Esser, R. A. Lamb, and D. P. Hand, “Towards industrial ultrafast laser microwelding: SiO2 and BK7 to aluminum alloy,” Appl. Opt. 56, 4873–4881 (2017). [CrossRef]
15. R. Carter, P. Morawska, S. Hann, M. J. D. Esser, and D. P. Hand, “High yield direct fusion welding of glass and metal,” Abstract from Lasers in Manufacturing, Munich, Germany, 2019.
16. Sandvik Materials Technology UK, “Osprey CE alloys for thermal management,” http://smt.sandvik.com/globalassets/global/downloads/products_downloads/ce_alloys/ce_alloys.pdf.
17. “Yttrium Aluminium Garnet (YAG) optical material,” https://www.crystran.co.uk/optical-materials/yttrium-aluminium-garnet-yag.
18. Northrop Grumman, “Neodymium: yttrium aluminum garnet—Nd:YAG,” http://www.northropgrumman.com/BusinessVentures/SYNOPTICS/Products/LaserCrystals/Documents/pageDocs/Nd-YAG.pdf.
19. “Thermal interface sheet, graphite, 400 W/m·K, 118 × 53 mm 0.2 mm,” EYGS0512ZLGE, RS Components, https://uk.rs-online.com/web/p/thermal-pads/1359660/.
20. ASTM International, Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials (2006).
21. D. Zhao, X. Qian, X. Gu, S. A. Jajja, and R. Yang, “Measurement techniques for thermal conductivity and interfacial thermal conductance of bulk and thin film materials,” J. Electron. Packag. 138, 040802 (2016). [CrossRef]
22. D. R. Thompson, S. R. Rao, and B. A. Cola, “A stepped-bar apparatus for thermal resistance measurements,” J. Electron. Packag. Trans. ASME 135, 1–9 (2013). [CrossRef]
23. I. Savija, J. R. Culham, M. M. Yovanovich, and E. E. Marotta, “Review of thermal conductance models for joints incorporating enhancement materials,” J. Thermophys. Heat Transf. 17, 43–52 (2003). [CrossRef]
24. S. Vandevelde, A. Daidié, and M. Sartor, “Use of 1D mechanical and thermal models to predetermine the heat transferable by a thermal interface material layer in space applications,” Proc. Inst. Mech. Eng. C 234, 3459–3473 (2020). [CrossRef]
25. R. J. Warzoha, A. N. Smith, and M. Harris, “Maximum resolution of a probe-based, steady-state thermal interface material characterization instrument,” J. Electron. Packag. Trans. ASME 139, 1–8 (2017). [CrossRef]
26. CV-2946, “Thermally conductive, controlled volatility silicone,” Avantor, https://www.avantorsciences.com/nusil/en/product/CV-2946/thermally-conductive-controlled-volatility-silicone.
27. S. N. Hann, A. Dzipalski, R. Carter, I. Elder, R. A. Lamb, M. J. D. Esser, and D. P. Hand, “Measurement of stress induced birefringence of direct laser bonded BK7 to aluminium,” in Optical Design and Fabrication Congress (2021).
28. S. Hann, N. Macleod, P. Morawska, R. Carter, I. Elder, R. A. Lamb, M. J. D. Esser, and D. P. Hand, “Stress induced birefringence of glass-to-metal ultrashort pulse welded components,” in 7th Industrial Laser Applications Symposium (2021).
