Thermal management issues impact the performance and reliability of high-power electronics, such as laser diodes. Liquid cooling through complex flow passage design (e.g. microchannels) at the heat source is the state-of-the-art solution. Unfortunately these microchannel coolers can have very short operating lives due to current leakage, erosion and corrosion effects. High purity de-ionized water (DIW) is used to minimize current leakage. But DIW attacks metal parts accelerating the corrosion process. Erosion further damages the flow channels under continuous high speed liquid flow. For example, the standard lifetime of microchannel coolers for laser diodes is 6 months.
Advanced Cooling Technologies, Inc. has developed an Applied Nanoscale Corrosion Erosion Resistant (ANCERTM) coating to protect the heat transfer surfaces in copper heat exchangers used with De-Ionized Water (DIW) as the coolant. The coating, applied by vapor deposition techniques, is both inert (to protect from corrosion) and hard (to protect from erosion). The ANCERTM coating is applied at nanometer thickness and therefore does not hinder the thermal or hydraulic performance of copper heat exchangers, even as flow channels approach the micro-scale. Additionally the coating provides corrosion protection in DIW coolant with a wide range of pH and Dissolved Oxygen (DO), thereby reducing the strict water quality control requirements for DIW systems.
A copper plate was coated with the ANCER™ coating and the ACT logo was masked in the center of the plate. When removed, the mask revealed an uncoated bare copper logo while the remainder of the sample was protected by the ANCER™ coating. The copper plate was then heated from ambient to 400 ˚C in open atmosphere. As the copper sample increased in temperature, the uncoated bare copper logo thermally oxidized, reacting with oxygen in the atmosphere to create copper oxide. The formation of the copper oxide changed the color of the uncoated copper, as shown in the video. The ANCER™ coating protected the remainder of the copper sample from thermal oxidation, which retained its original color.
Figure 1. Gold plating and ANCER™ coating were tested side by side in a long term flow loop which simulated the laser diodes cooling condition and also accelerated the corrosion by 10 times. The gold plating, which is the current industry standard for corrosion and erosion protection of copper microchannel coolers, lost its function after 500 hours; whereas the ANCER™ coating showed superior and reliable protection throughout the 1000 hours.
The vapor deposition coating technique ensures thin, uniform coating of complex geometries that are inaccessible for gold plating. This pin-hole free coating provides superior corrosion resistance. As can be seen in Fig 1., corrosion rate is two orders of magnitude lower for the ANCER™ coating compared to gold plating over a wide range of pH values and water qualities. Life testing in high velocity flows (up to 4.8 m/s) and thermal cycling testing (30°C – 75°C) also demonstrated its high erosion-corrosion resistance and durability. Finally, the dielectric property and nanoscale thickness of this coating makes it an ideal electrical insulator without hindering the thermal path from source to evaporator.
Figure 2. After 30 minutes at 300 ºC, the bare copper sample experienced significant thermal oxidation indicated by the dark color, while the sample with ANCER™ coating was unaffected.
Copper heat exchangers utilizing high velocity, high purity, de-ionized water (DIW) coolants undergo degradation from erosion-corrosion, also known as flow assisted corrosion. Under this degradation mechanism, the passive, native oxide layer of copper is removed by the erosion forces of the high velocity DIW, thereby exposing the active, underlying copper substrate, as shown in Figure 3.
Figure 3. The erosion/corrosion mechanism removes the passive native oxide layer which exposes the underlying metal substrate to corrosive DIW, thus accelerating the corrosion of the copper heat exchanger.
The native oxide layer protects the copper from corrosion damage, thus removing the passive layer makes the underlying copper more susceptible to corrosion damage. Over time, the erosion-corrosion mechanism degrades heat transfer surfaces and consequently hinders the thermal performance of the heat exchanger until it ultimately needs replacing. The current method to extend the lifetime of copper heat exchangers in DIW cooling systems is to apply a nickel and gold plating on the internal features of the heat exchanger or control the DIW properties in a tight pH and dissolved oxygen (DO) operating range. However, as heat transfer surfaces approach the micro-scale, nickel and gold plating techniques can no longer provide uniformity presenting either pin-holes or clogging of the micro-scale flow passages, as shown in Figure 4.
Figure 4. Plugging can occur in fine channels when attempting to apply the protective gold plating.
Furthermore, controlling the DIW properties requires costly monitoring and control hardware to maintain the DIW coolant within the desired operating range, and unforeseen failures can occur in the event that these systems fail. To evaluate the level of passivation provided to copper heat exchangers in DIW systems, ACT conducted corrosion evaluations using electrochemical techniques to evaluate the corrosion rate of uncoated and ANCERTM coated copper in 0.3 MΩ·cm DIW in pH ranging from 6 to 9 and DO ranging from 0.5 to 10 ppm. The results are shown in the copper release maps (contour plots of corrosion rate against pH and DO) below; see Figure 5.
Figure 5. Copper release maps for uncoated and ANCERTM coated samples. ANCERTM coatings provide a one to two order of magnitude reduction in corrosion rate compared to the baseline copper sample. Tight control of the pH and dissolved oxygen is no longer required, significantly reducing operating costs.
The copper release maps in Figure 3 demonstrate that the ANCERTM coating provides a one to two order of magnitude reduction in corrosion rate compared to the baseline copper sample. Additionally, the copper release maps exhibit the dependence of the corrosion rate of the baseline copper sample on the DIW coolant conditions and the consistent corrosion rate of the ANCERTM coated sample across a wide range of pH and DO. Thus, the ANCERTM coating provides corrosion protection across a wide range of DIW coolant pH and DO and reduces DIW control requirements and provides protection in the event of failure of DIW coolant conditions systems.
The erosion-corrosion protection in extended exposure to high purity DIW was evaluated by measuring the corrosion rate using electrochemical techniques at predetermined time points throughout a 1,000 hour exposure to 0.3 MΩ·cm, pH 6, DIW with a linear velocity of 3.8 and 4.8 m/s for bare copper, nickel and gold plated, and ANCERTM coated copper samples.
Figure 6. Corrosion rate of ANCERTM coated copper, nickel and gold plated copper, and baseline uncoated copper in 0.3 MΩ·cm DIW with a velocity of 3.8 and 4.8 m/s. The consistent and reduced corrosion rate of the ANCERTM coated copper samples not only indicates the ANCERTM coating provides passivation to the uncoated copper, but is also resistant to the erosive conditions of high velocity DIW. Conversely, while the industry standard nickel and gold plating provided an initial reduction in corrosion rate, erosion forces degraded the nickel and gold plating thus increasing the corrosion rate and reducing the effectiveness of the nickel and gold plating layers.
As shown in Figure 6, the corrosion rate of the ANCERTM coated copper sample exhibited the lowest corrosion rate. Furthermore, this corrosion rate was consistent throughout the 1,000 hour duration and between the 3.8 and 4.8 m/s linear velocities, thus indicating protection from erosion. However, the nickel and gold plated sample exhibited an increase in corrosion rate throughout the duration of the 1,000 hour test and exhibited a larger increase in corrosion rate at higher flow rates indicating damage from erosion. Thus, the ANCERTM coating exhibited long term protection in erosion and corrosion.
The thermal and hydraulic performance of copper heat exchangers used in laser diode thermal management was evaluated by measuring the pressure loss and thermal resistance of ANCERTM coated and uncoated copper heat exchangers. As shown in the plots below, the pressure loss and thermal resistance of ANCERTM coated and uncoated heat exchangers were equivalent demonstrating that the coating does not impeded the thermal or hydraulic performance of the heat exchanger.
Figure 7. Heat exchanger thermal resistance and pressure drops are unaffected by the nanometer thin ANCER™ coating.
The ANCERTM coated and uncoated copper heat exchangers used in laser diode thermal management were evaluated for 200 hours in high velocity, 0.3 MΩ·cm, pH 6 DIW and the pressure coefficient of the respective coolers was monitored throughout the duration of the test, as shown in Figure 8.
Figure 8. Pressure coefficient. A deviation from 1 indicates either plugging (increase in coefficient) or erosion (decrease in coefficient, as the microchannel erodes, increasing the effective diameter. Note that the pressure drops of the ANCER™ coated samples remain constant indicating that the hydraulic diameter of the microchannel was consistent throughout the duration of the test.
The deviation in pressure coefficient from 1 indicates a change in the hydraulic diameter of the flow channels within the heat exchanger. As shown in the figure above, the ANCERTM coated samples exhibited a pressure coefficient stabilized at 1 throughout the duration of the 200 hour test, demonstrating protection from erosion/corrosion degradation, whereas the uncoated bare copper heat exchangers exhibited deviations in pressure coefficient that indicate a change in hydraulic diameter of the heat exchanger, and no protection. For a detailed description of test conditions and results of the ANCERTM coating in copper heat exchangers for laser diode thermal management, see ACT’s manuscript entitled A Corrosion and Erosion Protection Coating for Complex Microchannel Coolers used in High Power Laser Diodes presented at ITherm 2014.
Figure 3. Benefits of ACT’s ANCER™ Applied NanoScale Corrosion Erosion Resistant Coatings