1. CBEE Fabrication of Microchannel Devices Via Diffusion Bonding and Transient Liquid Phase Bonding Purpose: To reduce the device size of microfluidic devices and heat exchanges to achieve higher efficiencies and better portability. Present Diffusion Bonding Method : Diffusion bonding and transient liquid phase bonding require pressure and heat Figures 1 and 2. The vacuum hot press can apply several tons force and up to 1200 °C to a stack of shims. Purpose of applying pressure and heat: Promote contact Increase diffusion speed The temperature must remain below the melting point of 316 stainless steel. Materials: Stainless Steel Shims 50 μm and 100 μm thick patterned with microchannels. Pressure distribution plates Shims coated with nickel boron or nickel phosphorus Vacuum Hot Press Optical Microscope Future Transient Liquid Phase Bonding The present work is on un-coated shims. Each stack was arranged to maximize the vacuum hot press working time. Table 1 illustrates the parameters used for this experiment. Ramp up rate (°C/min)Shim span (μm) Dwell temperature (°C)Dwell pressure (psi) Dwell duration (minutes) Run 12400980100060 Run 28800980100060 Run 38400980100060 Run 42800980100060 Table 1: Diffusion bonding design of experiment used to maximize the information from each run. The two parameters varied per run are the temperature ramp rate and the shim span, which is the distance between channels on the shim. Figure 1: schematic of vacuum hot press illustrating basic configuration of shims and microchannels to bond the device. Q is the heat delivered to the system and F is the force applied by the press. Figure 2: The vacuum hot press used to apply heat and pressure simultaneously to bond the shims. Figure 3: Diffusion bonded stainless steel shims with channel deformation. Diffusion bonded shims at 1800 psi and 980 °C. The deformation is caused by too much pressure during bonding or cutting and polishing. Previous Work: Present Work: The shims are stacked to maximize information from each run. The stack is composed of four distribution plates and two thicknesses of shim, 50 μm and 100 μm as shown in Figure 4, which will be cut using a wire EDM and inspected. 10 shims 50 μm thick 10 shims 100 μm thick 5 shims 100 μm thick 5 shims 50 μm thick End blocks Figure 4: An example of the shim stacks used to maximize data collected per run. In preparation for future work on this project, modeling has been done for the interlayer thickness vs dwell time, Figure 6. Transient liquid phase (TLP) bonding requires an interlayer between two parent metal shims. Using boron or phosphorus in a nickel interlayer will lower the melting point of the nickel hastening the bonding process, Figure 5. Figure 5: The TLP bonding process. The interlayer melts, suppressant diffuses, and the bond solidifies Figure 6: Interlayer thickness vs dwell time in the vacuum hot press governed by the equations shown. D is the diffusion coefficient, t f is the dwell time, C αL is the critical concentration, C o is the initial concentration, and C m is the concentration at the interface of melting point suppressant in the interlayer. Thinner interlayers correspond to lower dwell times. Higher temperatures reduce dwell time due to the exponential effect of the temperature on diffusion coefficient. Goals of diffusion bonding: 1.Minimize channel deformation. 2.Maximize the ratio of good channels to total channels. 3.Minimize bonding temperature and pressure, and therefore costs. 4.Maximize bonding efficiency. 1.Interlayer melts 2. Melting point suppressant diffuses into the parent metal 3. The concentration of suppressant decreases 4. Interlayer solidifies 5. Bond homogenizes Special thanks to Steve Leith. Todd Miller, Jack Rundel, Danielle Clair, and Phillip Harding for all of their help and support. Vacuum Chamber and furnace Pressure Ram