Integrated Microreactor System for Gas-Phase Catalytic Reactions. 2

Aug 23, 2007 - Integrated Microreactor System for Gas-Phase Catalytic Reactions. 2. ... details on the design, testing, and integration of packaging...
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Integrated Microreactor System for Gas-Phase Catalytic Reactions. 2. Microreactor Packaging and Testing D. J. Quiram and K. F. Jensen Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

M. A. Schmidt Department of Electrical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

P. L. Mills,* J. F. Ryley, M. D. Wetzel, and D. J. Kraus DuPont Company, Central Research & DeVelopment, Experimental Station, Wilmington, Delaware 19880-0304

An integrated packaging system is developed for the multilayer laminate gas-phase microreactor die whose design and fabrication was described in Part 1 of this series. A commercial plastic socket used for integrated circuit testing was adapted so the reactor chip could be easily installed while maintaining consistent alignment with all electrical contacts. A heated fluidics interface was developed that connects the nonmetallic feed and product gas ports on the microreactor chip to metal tubing. Thermal experiments and 3-D finite-element heat transfer simulations of the combined socket-fluidics assembly showed that the plastic reactor socket could be safely operated up to 250 °C. Other tests showed that the microreactor heaters were capable of achieving membrane temperatures in excess of 600 °C. Step-response tests demonstrated that temperature changes of ca. 100 °C could be achieved in less than 10 ms. Testing of the electrical leads on the reactor chip verified that the device resistance on a single reactor chip was uniform within a few percentage points. The packaged system developed here is used in Part 3 of this series to create a modular reactor board for incorporation into an integrated process system. 1. Introduction Packaging of microelectromechanical (MEMS) systems is generally concerned with the science and technology of connecting circuit-level devices within an appropriate operating environment so that electrical signals can be transmitted within the design specifications both to and from an external process for subsequent data processing, process control, storage, or visualization.1,2 Packaging techniques are the major factors that ultimately determine the reliability and long-term stability of MEMS-based systems3 and account for >75% of the final device cost.1 Packaging design must generally prevent failures to delicate microfabricated structures, circuits, electrical interconnects, and other subcomponents due to external forces and related factors, such as environmental effects or induced thermomechanical stresses, while allowing the devices to be periodically calibrated and tested to assess operational validity.4 It is generally agreed that packaging technology for MEMS has lagged behind device development, which has consequently extended the time required for commercialization.5 Additional details on the design, testing, and integration of packaging technologies are available in various monographs and the references cited therein5,6 as well as the proceedings of dedicated symposia.7,8 By comparison, the literature on packaging of emerging nanoelectromechanical (NEMS) systems is essentially nonexistent so that new challenges can be expected in the future as these are translated from research toward the commercial stage.9 Packaging within the realm of microreactor technology (MRT)10 can be analyzed on several levels that can be * Corresponding author. Current address: Department of Chemical and Natural Gas Engineering, Texas A&M UniversitysKingsville, Kingsville, TX 78362-8202. E-mail: [email protected]. Tel.: (+1) (361) 593-4827. Fax: (+1) (361) 593-2106.

differentiated using length-scales. The smallest length-scale for packaging, assuming that the molecular level is not included, occurs on the length-scale of the materials and subcomponents used to fabricate microreactors11 and other process devices, such as valves, mixers, heat exchangers, pumps, and separators.12 Conversely, the largest length-scale in terms of current technology occurs for modular microplants where various microdevices are assembled to create a system for research, process development, or for small-scale manufacturing.13-17 In many respects, the packaging challenges encountered in MRT, especially on the component or subcomponent scale, are the same or similar to those mentioned above for MEMS. However, some additional complications are introduced because of the presence of chemical, biochemical, polymerization, or other types of reactions. Both the reaction kinetics and process fluids encountered in these reactions translate into certain process requirements in terms of fluid handling, materials of construction, temperature, pressure, process analytics, process automation, and design for safe operation.18 These requirements ultimately manifest themselves in terms of process components and a systems design that must function as an integrated system through appropriate packaging. A review of packaging concepts in terms of fluidic and electrical interconnects between selected devices, or between devices to user-accessible outputs, is provided in several recent monographs on microprocess engineering.19,20 However, two recent special issues dedicated to microreaction technology21,22 did not contain any reports that specifically addressed the broader issues of MRT packaging, which suggests that many opportunities exist for advances on this particular aspect. A few recent articles on packaging that have relevance to MRT are mentioned here to provide some perspective on several key issues. Recent work on development of a hermetic gas-tight

10.1021/ie070108o CCC: $37.00 © 2007 American Chemical Society Published on Web 08/23/2007

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microreactor fabricated from silicon carbide for high-temperature operation showed that a major packaging issue was connecting the ceramic microdevice to metal fluid transfer lines and then sealing the ceramic components within a metal housing.23 This example points to the important packaging issue of connecting components consisting of different materials of construction so they can be safely operated at elevated process conditions over an extended period of time. In another recent report, the analysis of both the thermal and thermo-mechanical stresses that occur in a multilayer complex radio frequency (RF) device shows that detailed 3-D finite element simulation models can rigorously quantify these issues on a local level, thereby permitting application of a rational design approach.24 This type of indepth engineering analysis generally allows optimization of subcomponents before prototypes are fabricated, which translates into obvious benefits from both timing and cost perspectives. A similar approach is used in this paper to examine thermal behavior of the microreactor package. In Part 1 of this series, the design and fabrication of a multilaminate, chiplike microreactor for gas-phase catalyzed reactions was described that contained two parallel reactor channels with multiple heaters and temperature sensors.25 The application required that all electrical and fluidic connections between the microreactor and the other system components employ packaging methods that were straightforward to use while maintaining process robustness. The objectives of the packaging method design included the following four key aspects: (1) small, compact size with minimal system footprint; (2) thermal and chemical compatibility with the reactor operating conditions; (3) capability for fast and straightforward reactor replacement; and (4) robust fluidic and electrical connections. Point 1 was mandated by the need to demonstrate simultaneous operation of multiple microreactors. Points 2 and 4 were driven by the reaction requirements, since the microreactor and any supporting components in contact with process streams had to be heated to temperatures that exceeded the dew points of the feed and product stream compositions, thereby preventing any condensation from the gas phase. In addition, all wetted parts and other components that were in direct contact with process streams had to be fabricated from materials of construction that were both chemically inert and resistant to corrosion. Point 3 also had particular importance since previous experience showed that the microreactor chip itself was typically the weakest component in the system and subject to a higher failure rate than other system components, especially during the prototyping stage. For this reason, a simplified methodology was required that facilitated microreactor removal and replacement. Similarly, the electrical and fluidic connections had to be designed so that straightforward tools and methods could be used during assembly, disassembly, and troubleshooting operations. The primary objective of this work is to describe the development of novel microreactor packaging hardware that satisfies the above operational requirements. Particular attention is focused on electrical and microfluidic connections, fabrication methods, and choices for the component materials of construction. Another objective is to outline the test methods that were used to evaluate the performance of the microreactor packaged system after fabrication was completed, but prior to exposing the assembly to actual process fluids and the reaction environment. This latter step and the corresponding results are particularly important since the microreactor and the packaging methodology are prototypes that are largely based upon various electronic components and subassemblies. These must all

operate according to the design specifications before they can be successfully incorporated into the remaining system-level unit. 2. Microreactor Packaging System 2.1. Microreactor Packaging Hardware. Previous microreactors and microreactor packing schemes developed at both MIT and DuPont could not be used since the electrical and fluidic interfaces required significant effort to implement. In addition, the microreactors previously developed at DuPont did not contain imbedded process elements needed for implementation of local temperature control in a process gas or reaction zone. Complicating aspects were mainly attributed to unreliable and fragile electrical and fluidic connections, as well as excessive footprint size. To simplify the microreactor packaging problem while improving process robustness, the following options were considered: (i) a spring contact probe-based fixture; (ii) Known Good Die (KGD) socket technology; (iii) a Tape Automated Bonding-based fixture (TAB); and (iv) a parallel plate PC board fixture. Ultimately, the microreactor packaging problem was solved using the DieMate Known Good Die socket manufactured by Texas Instruments.26 The packaging fixtures based on spring contact probes and TAB tape were not selected because of the likelihood of lengthy fabrication time requirements if the prototype was not successful. In addition, the TAB tape fixture would have been considerably more expensive than the other options. The PC board overlay system was not used because of the uncertainty in making electrical connections. The KGD socket technology met nearly all of the required technical design requirements and was the most economical when compared to other methods. In addition, it was concluded that the DieMate socket could be readily modified for development of the required microfluidic connections, namely, introduction of reactant feed gases and removal of the hot product gas mixture. Photographs of the Texas Instruments DieMate Known Good Die socket used for the final microreactor packaging scheme are shown in Figure 1. The particular DieMate socket chosen for the microreactor scale-up system was the 110 pin version (Texas Instruments, Dallas, TX, P/N CBC110 004 K). Versions with a larger and smaller number of pins were available, but this design was best suited for this particular application. 2.2. DieMate Electrical Interconnect Design. As shown in Figure 1a, the DieMate socket holds the die in place and generates electrical contact using a row of pins on each side of the socket. The pins are forced into contact with the die by four springs that are placed on each corner of the socket. These springs are visible in the side view shown in Figure 1b. The springs force the upper piece of the socket away from the lower socket body. This force is redirected by the pins to press on the die in the socket. The latching force of the pins on the die is adjustable from 0.5 to 12 lbf, which is more than sufficient to form a gas-seal using O-rings. By pressing on the top piece of the socket, the springs are compressed downward and the pins of the socket are lifted off and away from the microreactor die. This action allows for easy replacement of the microreactor. The bond pads of the microreactor die and the pins of the DieMate socket are self-aligning in this system. Figure 2 shows that the socket contains an outer lip or ledge beneath the electrical contact pins. The ledge, which is a distance of 1.1 mm below the inner socket lip, provides a flat surface for holding the microreactor die. The die is carefully cut from a silicon wafer for a clearance fit inside the socket so that it is flush with the walls of the ledges. When a die is placed in the socket, the tight tolerance of the chip design to the socket

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Figure 1. Texas Instruments DieMate Known Good Die socket used for microreactor packaging in the scale-up system.

Figure 2. Rendered drawing of the DieMate socket showing the resting location of the microreactor die.

ensures proper alignment of the DieMate pins with the microreactor bond pads. The thickness of the microreactor die is ca. 1.3 mm, which results in a moderate force from the pins to the die. Since only 56 electrical connections were needed for the scale-up microreactor die, the bond pad layout takes advantage of the excess number of pins of the socket. Every other pin in the socket was used by the microreactor. This created a better tolerance between the bond pad-to-pin alignment, with a bond pad pitch of 0.8 mm. More importantly, it allows a larger lead width, which reduces the electrical resistance in the leads to the microheaters and microtemperature sensors. In the case of the microheaters, high resistance in the leads causes unnecessary power dissipation over the bulk silicon of the microreactor die. For the microtemperature sensors, a larger resistance outside of the resistive temperature device (RTD) structure generates a greater inaccuracy in the temperature measurement.

Figure 3 is a rendered version of the scale-up microreactor die that shows the metallization on the die. Note that a large amount of the silicon area is covered by the lead lines. However, some of the leads on the die are still narrow. This was done to make the resistances in all the leads be approximately the same. This also simplified the design of the electrical circuits controlling the microheaters and microtemperature sensors. The 56 electrical leads from the DieMate socket are connected to the external sensing and driving circuitry for the microheaters and microtemperature sensors by mounting the socket on a custom-designed PC board. The connections are straightforward using the DieMate’s pins. This mounting concept was first tested on a prototype multilayer PC board, which is shown in Figure 4. 2.3. DieMate Fluidic Interconnect Design. Since the DieMate socket was designed for use with integrated circuits, additional modifications were required to accommodate the

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Figure 3. Rendered top view of the scale-up microreactor die. Metallization is shown in gray, silicon substrate is shown in blue, and exposed membrane is shown in yellow.

Figure 4. Prototype PC board used to test the scale-up microreactor.

fluidic interconnections. The solid bottom of the socket was removed using conventional machining techniques so that a gas manifold could be placed underneath the microreactor die. A photograph of the DieMate manifold is shown in Figure 5. The final microreactor packaging scheme is shown in Figure 6. The DieMate manifold extends up into the socket where it forms a seal with the Pyrex bottom of the microreactor die using Kalrez O-rings. The O-rings were 9/64 in. o.d. by 3/64 in. i.d. (DuPontDow Elastomers, Newark, DE, P/N AS-568A K#002). The sealing force is created by the pins pressing on the top of the microreactor and also by the socket head cap screws pushing the DieMate manifold upward into the socket. Inspection of Figure 1a shown earlier suggests that it is not obvious how the DieMate manifold is held in position inside the socket. The bottom of the DieMate socket was altered as shown in Figure 7. The bottom layer is almost completely removed except for two areas that contain socket head cap screws that hold the DieMate socket together. The DieMate manifold piece mates with a T-shaped metal piece (this piece

is subsequently referred to as the DieMate manifold nut plate) around these two remaining areas of the socket bottom layer, as illustrated in Figure 8. Socket head cap screws that extend through the DieMate manifold and into the DieMate manifold nut plate are used to pull these pieces closer together. This effectively raises the DieMate manifold into the socket and forces the top of the manifold to come into contact with the bottom of the microreactor. 2.4. DieMate Manifold Heater Design. The DieMate manifold also functioned as the microreactor die heater since it had four drilled-through holes to accommodate 1/8 in. o.d. heating cartridges. The heating cartridges each provided 15 W of power using a 48 Vdc power supply. The heating cartridges were custom-manufactured (Technical Industrial Products, P/N ITPHEA001)27 and had an overall length of 1 in. The temperature of the DieMate manifold was monitored using two commercially available 100 Ω platinum resistive temperature devices (RTDs). One of the RTDs in the manifold had a twowire configuration (Technical Industrial Products, P/N RTD15-

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Figure 5. DieMate manifold used for fluidic connections and microreactor heating.

Figure 6. Exploded view of the DieMate packaging assembly.

100L-SS116-1-PVC24-12-2), and the other had a four-wire configuration (Technical Industrial Products, P/N RTD15-100LSS116-1-PVC28-12-4). The CAL temperature controllers could only be configured for two-wire RTDs, but the National Instruments (NI) analog input cards were configured for fourwire RTDs to improve the measurement accuracy. Both types were 1/16 in. o.d. and had an overall length of 1 in. Figure 5 shows the placement of the heating cartridges and RTDs in the manifold. 2.5. DieMate Manifold Fabrication. The DieMate manifold was fabricated using type 316L stainless steel using conventional machining techniques. Although the thermal conductivity of stainless steel is not as high as some other metals, it is

chemically compatible with the components that are expected to be present in the reaction mixture. It is also suitable for operation at 200 °C, which is the maximum manifold temperature required when the microreactor is used for reaction studies. The DieMate manifold nut plate was also machined from type 316L stainless steel. The material of construction is not as critical for this component since reaction gases do not have direct contact with it. To prevent galling and seizing of the socket head cap screws in the DieMate manifold nut plate, the screws are coated with Swak, which is a commercial thread lubricant (Cajon, P/N MS-PTS-6).28 Figure 9 shows key details of how the fluidic transport tubing is routed and attached to the manifold. A pair of 1/16 in. o.d. × 0.030 in. i.d. feed and product gas transfer lines are attached to the DieMate manifold. The lines are constructed of type 316L stainless steel tubing obtained from Valco Instruments.29 The feed and product gas transfer lines were connected to the DieMate manifold using silver solder. The latter material was used for joining instead of conventional welding because of the difficulties associated with welding 1/16 in. stainless steel tubing. Although silver is an excellent oxidation catalyst, the small amount of silver surface area that might contact the feed and product gases was not expected to generate a significant reactive site. Corrosion of the silver solder joints or the solder acting catalytically was also not observed during the experiments. 2.6. DieMate Manifold Insulation. The DieMate manifold is insulated to improve thermal uniformity and to prevent exposure of the DieMate socket to excess temperature. (The DieMate socket maximum temperature specification is 150 °C.) Figure 10 shows the insulated DieMate manifold before attachment of the 1/16 in. transfer lines. The insulation is ceramic fiber strip, which was obtained from a commercial source (McMaster-Carr, Cat. No. 87575K87).30 The fiber strip is 1 in. wide and has a thickness of 1/8 in. Its specified thermal conductivity is 0.38 (BTU‚in)/(h‚ft2‚°F), and its maximum operating temperature is 2300 °F.

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Figure 8. Cross section showing how the DieMate manifold is held in the DieMate socket. Note that the DieMate manifold nut plate is tapped for the socket head cap screws, but the DieMate manifold is not.

Figure 7. Rendered views of the DieMate socket showing the unaltered socket (top) and the altered socket (bottom). The unnecessary pins have been removed in both views.

3. Testing Procedures The microreactor die that was described in Part 1 and the associated microreactor packaging was subjected to a series of bench performance tests prior to being used under actual reaction conditions. The particular experiments discussed here include: (1) the electrical testing of the microreactor and the modified DieMate socket; (2) the determination of the operating temperature range of the DieMate manifold mounted in a DieMate socket; and (3) the determination of the integrity of the gas seals of the DieMate manifold with the microreactor chip. Microreactor testing under reaction conditions is described in Part 4 of this series. 3.1. Electrical Testing of the Microreactor and DieMate Socket. Electrical testing of the connection of the DieMate socket with the microreactor die was essential to ensure that it

Figure 9. DieMate manifold piping assembly.

operated properly when installed in the scaled-up system. Figure 4 shows the prototype PC board with a mounted DieMate socket that was custom-built for this testing (Phillipsburg Electronics, Inc.).31 This microreactor PC board was used to connect the microreactor heater and temperature sensor structures with their appropriate driver circuitry. This circuitry was also custom-built for this specific application. Testing of the microreactor structures thus served a dual purpose of verifying their functionality along with the functionality of the driver and sensor circuit boards. These boards were prototypes of the circuits to be used in the scale-up system. Electrical testing was performed using both the first and final generation scale-up microreactors. Each had a dedicated microreactor PC board for testing since the bond pad pattern was different in the two microreactor designs. The testing setup used for this experiment is shown in Figure 11. A computer running LabVIEW v5.1 from National Instruments (NI) was used to

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Figure 10. Insulated DieMate manifold: top isometric view (left) and top view (right).

Figure 11. Setup used for initial electrical and flow testing of the scale-up microreactor and DieMate socket.

Figure 12. Data acquisition, sensor, and driver electronics for the initial electrical and flow testing of the scale-up microreactor and DieMate socket.

monitor and control the testing through a NI SCXI-1001 chassis with two NI SCXI-1141 analog input modules and a NI PCI6713 analog output board (National Instruments Corporation).32 Figure 12 shows the SCXI chassis along with the microreactor circuit boards used in this experiment.

The testing procedure consisted of two aspects: (1) measuring the resistances of the microreactor heaters and temperature sensors at room temperature, which verified the ability of the microreactor heaters to achieve temperatures in excess of 200 °C; and (2) determining the dynamic temperature response

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Figure 13. LabVIEW graphical user interface for microreactor testing.

of the microreactor heaters. The resistances of the microreactor device structures were measured to assess uniformity across the device. This would allow the electrical circuitry to be adjusted, if necessary, for optimal operation of the scaled-up system. The final scale-up microreactor was designed to have approximately the same measured resistance for all of the temperature sensors on a die and for all of the heaters on a die. The measured resistance refers to the resistance measurement of the structure taken at the microreactor bond pads. The scale-up microreactor design did not incorporate features for four-point resistance measurements, so the lead resistance to the structure is added to its resistance. This uniform measured resistance feature was not incorporated into the first generation Y-microreactor. The microreactor heaters had to generate temperatures in excess of 200 °C without breakage of the reactor membrane or induced failure due to electromigration. This temperature level was considered a minimum specification since the oxidation of ammonia, which is one of the test reactions implemented in Part 4 of this series, required a membrane temperature of approximately 200 °C to produce measurable conversions. The design of both generations of scale-up microreactors utilized simulations to predict the microreactor heater temperature at which electromigration would occur. This temperature was chosen to be ca. 600 °C, but only data from previous microreactor experiments was available to predict when electromigration might actually occur. Furthermore, testing at various temperature levels was clearly needed to determine the membrane stability, since this was the most likely failure mode. Dynamic testing of the microreactor heater was also performed to determine its response time to step changes in heater power. Consequently, the data acquisition and control electronics had to be chosen accordingly. This response data was later used to develop temperature control algorithms for the microreactor. In addition, this testing was useful in demonstrating another safety feature of these microreactors, namely, that high membrane temperatures could be quickly reduced to lower levels.

The resistance measurements were performed with a Tektronix DMM916 multimeter and the NI data acquisition system described previously. The microreactor temperature sensors operate through a resistance measurement performed by the sensor circuitry. The NI equipment allowed data to be rapidly collected. A multimeter was needed to measure the resistances of the heater structures, since the sensor circuits do not measure this directly. In addition, the multimeter was used to tune and verify the accuracy of the temperature sensor circuits and the data-acquisition equipment. The temperature sensor circuits contain an adjustable potentiometer that is tuned to improve their accuracy. The NI data-acquisition system was used to verify that a microreactor membrane temperature of 200 °C could be achieved. A LabVIEW Virtual Instrument (VI) was created so the operator could adjust the power voltage to the microreactor heaters and monitor the temperature sensor resistances. Figure 13 shows the process automation panel for this VI. This same VI was also used for the dynamic heater testing. Besides the ability to manually change heater set points through the user interface, it is also capable of automatically performing step changes in the heater voltage set points and recording the resulting temperature sensor response. Because of the fast thermal response of the microreactors, data was recorded at a rate of 4 kHz for these dynamic tests. The equipment was capable of reading data at faster rates, but the above rate was sufficient to capture the dynamic behavior of the heaters. The LabVIEW VI automated this testing protocol by reading in a data file containing the desired step changes and then performing these step changes at a user-defined time interval. 3.2. DieMate Socket Thermal Stability Testing. The purpose of this test was to demonstrate stability of the DieMate socket at the maximum operating temperature of the DieMate manifold, which was 200 °C. The maximum operating temperature of the DieMate socket was specified as 150 °C by the manufacturer. For this reason, it was necessary to determine the maximum temperature of the DieMate manifold to prevent

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Figure 14. Top (top) and bottom (bottom) views of the PC board used for temperature testing of the DieMate manifold and socket.

potential adverse effects of thermal energy on both the performance and material properties of the DieMate socket. The lowest operating temperature of the manifold was determined by the dew point temperature of the reaction mixture, the latter of which contained water vapor as one of the components. One of the prototype commercial PC boards (Phillipsburg Electronics, Easton, PA) was used for this test to represent the actual PC boards in the scale-up system. The DieMate socket on the PC board was modified as described earlier to accommodate the DieMate manifold. Figure 14 shows both the top and bottom views of this board. A microreactor die with broken membranes was placed in the socket during testing to measure the temperature on the top surface of the die with a thermocouple. The DieMate manifold was placed into the DieMate socket with the reactor die mounted on top. Four heating cartridges, which were wired in parallel, were used to heat the manifold. Specifications for these cartridges were given previously. Two Type J 1/16 in. o.d. thermocouples, one for control and another for over-temperature sensing, were used to monitor the temperature of the DieMate manifold. A CAL 3200 temperature controller was used to adjust the power to the four heating cartridges in the DieMate manifold (CAL Control, Inc.).33 A second CAL 3200 temperature controller was used for interlocking in the event of an over-temperature excursion. An additional Type J 0.020 in. o.d. thermocouple was used to measure the surface temperature of the microreactor die. It was not physically attached to the die, but temperature measurements were collected by manually holding the tip of the thermocouple against the bottom of the reaction channels. The DieMate

manifold was wrapped in insulation, as shown above in Figure 10, to mimic how the manifold would be used in the scale-up system. The testing procedure was initially based upon increasing the temperature of the DieMate manifold from an initial temperature of 45 °C to a final temperature of 150 °C at increments of 5 to 15 °C. At each intermediate temperature, the manifold was allowed to stabilize for about 1/2 h. When 150 °C was reached, longer-term stability testing was performed by increasing the temperature beyond this level using increments of 25 °C with the manifold remaining at a given temperature for a period of at least 2 h. The manifold was then cooled and removed from the socket to visually examine the socket for thermal damage. 3.3. Fluidic Testing of the Microreactor Packaging. The O-rings that sealed the DieMate manifold to the microreactor die were tested for fluidic integrity at room temperature and at 200 °C. The flow testing setup was shown earlier in Figure 11. Helium was used as the test gas. The gas flow rate to each of the microreactor channels was controlled using a gas flow manifold consisting of a Tylan mass flow controller (MFC) (Millipore Corp.),34 a 0-25 psig GO back-pressure regulator (BPR) (GO Regulator, Inc.),35 and a pressure gauge to monitor the pressure after the MFC. In addition, a two-stage pressure regulator with gauges was connected to the valve of the helium cylinder. Testing was performed by establishing a flow rate of 50 mL/ min (standard temperature and pressure, STP) of helium through one of the microreactor channels. This flow rate was chosen since it was the maximum anticipated operational flow rate for a given microreactor. Helium was used as the test gas since it

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average resistance (ohms)

resistance variabilitya (ohms)

group 1, heaters group 2, heaters group 3, heaters group 1, temperature sensors group 2, temperature sensors group 3, temperature sensors

512 482 477 1700 1590 1650

10 11 18 100 130 170

a The standard deviation of the sample is used to determine the variability in the structure resistance.

has a large difference in thermal conductivity versus air. An Alltech Associates digital bubble-flow meter was used to measure the gas flow rate.36 After the flow rate was established and the DieMate manifold had stabilized at the set-point temperature for 1/2 h, a commercial gas leak detector (GowMac, model 21-250)37 was used to check for helium leaks between the microreactor gas seals and the DieMate manifold. This particular leak detector operates by using a thermal conductivity sensor and can detect helium leak rates as low as 1.0 × 10-5 mL/s. 4. Results and Discussion A summary of the results for the testing procedures described above is given in this section. In general, test results concerning microreactor performance are only given for the microreactors with the ACT silicon nitride film, since these were the only ones used for testing of the scale-up system. These reactors are divided into three groups where the microreactors for each group are distinguished by time interval over which platinum was deposited during the fabrication process. 4.1. Resistances of the Microreactor Structures. Table 1 summarizes the results of the resistance testing according to reactor group and also as whole devices. A total of 11 reactor dies were used for these determinations. Room-temperature resistances of the microreactor heaters and temperature sensors show that the device resistances were quite uniform on a single die. The variability of the structure resistances, using the sample standard deviation as its measure, revealed that the heaters had an average variability per die of