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Interviewing the developers
Analytical Instruments Design Department, Development and Design Division
Hitachi High-Tech Science
In an effort to address global warming, Japan’s government has committed to achieving zero emissions—zero overall emission of greenhouse gases—by 2050, and this goal has heightened expectations for more widespread adoption of fuel-cell vehicles, which boast outstanding environmental performance and do not emit CO2 or environmentally harmful substances. Toyota first launched its Mirai fuel-cell vehicle in December 2014, and released a new Mirai last December. The new Mirai improves on the performance of previous models in multiple areas, including major upgrades to the fuel cell itself.
Fuel cells generate power by using catalysis and other mechanisms to induce chemical reactions between hydrogen and oxygen within the cell. However, because the voltage produced by individual cells is rather low, in practice multiple cells are stacked atop each other to yield practical voltages.
One strategy used in the new Mirai to improve the efficiency of the fuel-cell stack (FC stack) is to reduce the thickness of electrolyte films; by using films thinner than those found in conventional fuel cells, the Mirai achieves enhanced proton conductivity. However, reducing film thickness increases the relative impact of metal contaminants—trace quantities of iron or other specific elements—that make their way into the interior of fuel cells during the manufacturing process; this makes fuel cells less robust against chemical degradation of electrolyte films, shortening their useful lifetime. To guarantee the quality of the FC stack, cells containing these metal contaminants must be excluded from production.
A powerful tool for this quality-assurance task is the X-ray diagnostic technology offered by Hitachi High-Tech Science. By incorporating Hitachi High-Tech Science’s diagnostic technology into production lines for the Mirai’s FC stack, Toyota gained the ability to test every unit via inline testing, enabling mass production of high-quality FC stacks.
What sorts of technical innovations were needed to realize this novel quality-control system, so crucial for successfully mass-producing fuel-cell vehicles? We decided to ask an expert.
At Hitachi High-Tech Science, back in 2013 we had already developed the EA8000, an X-ray particle contaminant analyzer for detection and elemental analysis of metal contaminants in lithium-ion batteries—a setting in which contaminants cause problems similar to those seen in fuel cells.
The EA8000 is capable of detecting metal contaminants with particle sizes of 20 μm and larger—in a sample with dimensions of 250×200 mm—in just a few minutes. By applying image-processing techniques to the entire area of transmission X-ray images captured at high speeds, the instrument is able to pinpoint the locations of contaminants automatically, after which it performs elemental analysis via X-ray fluorescence spectroscopy. At the time the EA8000 was introduced, conventional X-ray fluorescence analyzers required as long as 10 hours to complete a contaminant analysis. In contrast, our hybrid method combined X-ray fluorescence analysis with transmission X-rays—the same technology used for medical imaging—and added technical innovations, such as a new detector and an X-ray irradiation system employing X-ray condensers to achieve small, high-brightness beams, to enable detection of metal contaminants within the interior of samples at speeds 100 times faster than previous systems. Toyota’s fuel-cell manufacturing groups were users of the EA8000, and this formed the thread that led eventually to Toyota’s incorporation of the Mirai into production lines for inline testing.
Figure 1. From contaminant detection to elemental analysis: the EA8000.
There was a proposal from Toyota sometime around December 2018; at the same time they were working on customizations to enable incorporating the system into their production lines.
A first issue was that instruments used for inline testing needed to operate at higher testing speeds than conventional instruments.
This spurred us to limit the system’s analytical capabilities to just X-ray transmission imaging; this decision, together with the new X-ray camera we had been developing to accelerate the diagnostic process, allowed us to achieve the necessary precision and speed.
Using the system for inline testing also required switching to a new X-ray wavelength regime to improve sensitivity to specific types of metal contaminants. The test apparatus, which incorporated transport systems for inline testing, needed to comply immediately with the equipment specifications required by Toyota’s manufacturing plants; an equipment manufacturer with copious experience was brought in to assist in building the system.
This equipment manufacturer had never before worked with X-ray test equipment, and expressed some concerns at the prospect, but apparently plunged enthusiastically into the construction of a prototype that met the detailed specifications of Hitachi High-Tech Science.
With all three companies working as a team to develop the inline-testing system, we succeeded in building a prototype within just six months of the original proposal.
“During the construction process, we received numerous observations and requests from Toyota, as well as advice and suggestions from folks in many different groups. Improving the design in accordance with Toyota’s requests wasn’t easy, but I think we had a strong sense throughout the project that the back-and-forth was helping us achieve a higher level of perfection, and ultimately we came to respect each other quite a bit. With Hitachi, Toyota, and our equipment-manufacturer partner working as a three-way team to build the system, we got to enjoy the pleasure of building something as a team—on top of which, of course, was the sheer joy one feels as an engineer working at the absolute frontier of cutting-edge, innovative manufacturing technology." (Takahara)
Once the prototype was installed, we turned to developing techniques for estimating the surface area of metal contaminants. In the original approach to X-ray testing, contaminants were detected as shadows, appearing as black spots containing lighter and darker regions; thus, only in-plane information could be obtained. However, quality assurance for fuel cells required determining not only the type and in-plane size of contaminants, but also their surface area—that is, three-dimensional geometric information.
In high-resolution X-ray transmission images captured by Hitachi High-Tech Science instruments, contaminants with greater thickness absorb correspondingly larger quantities of X-ray energy—thus producing darker regions in images. On the other hand, we had observed that contaminants of the same thickness but different in-plane sizes exhibited different light-dark shading patterns. This is due to light diffraction phenomena—and furnished a key hint that inspired the development of our latest diagnostic technology.
We received a proposal from Toyota: Might it be possible to estimate surface area from the shading of black spots? To look for correlations between black-spot shading and the surface area of metal contaminants, we prepared contaminant samples and conducted a series of trial experiments shared between Hitachi and Toyota. Using a focused ion beam (FIB) apparatus developed by Hitachi High-Tech Science, Toyota told us they had fabricated multiple contaminant samples with tiny differences in thickness, as required for validation experiments.
The results largely agreed with expectations, so we used these samples in the actual test system. As a result, we succeeded in designing a system capable of estimating the three-dimensional size of contaminants—in a matter of seconds—from the shading patterns of black spots.
Also, this test system was designed to allow end users themselves to replace X-ray sources. In all previous instruments it had simply been assumed that the task of replacing X-ray sources would be carried out by service personnel from Hitachi High-Tech Science; however, such an approach is clearly inadequate for a system intended to be used 24 hours a day at manufacturing plants. This novel requirement spurred us to choose the shape of the X-ray source to make it easier to replace—and to develop an automated warm-up mechanism enabling the system to calibrate newly replaced X-ray sources automatically. User-friendly design improvements such as these are valuable for us at Hitachi High-Tech Service as well, as they streamline the task of servicing our own instruments, and we are currently considering similar adaptations for overseas customers.
Figure 2: Using the EA8000 to characterize contaminant sizes and abundances.
In this case we were applying our technology to inline testing of fuel cells, but the need for higher speeds is even more urgent for inline testing of lithium-ion batteries. Of course, the high frequency of erroneous or excessive detections remains an industry-wide problem.
Currently, at Hitachi High-Tech Science we are developing new X-ray cameras to accelerate the diagnostic process; we expect the new cameras to be around 10 times faster than those we have today. To address the problems of erroneous and excessive detection, we are continuing to develop technologies—including wavelength tuning and elemental analysis via X-ray fluorescence spectroscopy—for detecting metal contaminants with greater precision.
“I really feel we have an obligation to meet the two key needs of our customers: faster testing and higher first-pass yield. Achieving these objectives involves some tradeoffs, and at present there are still many problems associated with erroneous or excessive detection. Going forward, we’re committed to doing everything we can to develop technologies capable of addressing these challenges.”
“Last Sunday, for the first time, I saw a TV commercial for the new Mirai. Ordinarily I would grab the remote and change the channel when commercials come on, but this commercial had me riveted. It only lasted about thirty seconds, but it made me realize how much Hitachi High-Tech Science’s X-ray diagnostic technology had helped ensure that this innovation would see the light of day—and it warmed my heart to think that we and so many other scientific-instrument makers had squeezed out every last drop of ingenuity we could muster to offer the world a path toward a low-carbon future.” (Takahara)
This article was reported by the SI NEWS editorial office and written by Ai Iiyama.
Rapid X-ray imaging technology ensures high-quality fuel cells for electric vehicles
Two major forces in Japanese manufacturing came together to solve the problem of particulate contamination of fuel cells for electric vehicles.
（ SI NEWS Featured article, collaboration with Nature Research Custom Media ）
Rapid Detection and Element Identification of Fine Metal Particles for Underpinning Battery Quality —X-ray Particle Contaminant Analyzer EA8000—
Along with measures for reducing the load on the environment, development of fuel cells and electric vehicles using rechargeable batteries has proceeded with the aim of reducing CO2 emissions. For the LIBs, fuel cells, and other components used for this purpose, greater safety and quality is required particularly in automotive applications. Metal particle contamination in batteries is one cause of degrading reliability and quality, making this an important aspect of quality control. To satisfy this new demand, Hitachi High-Tech Science has developed X-ray technology and equipment to detect and identify fine metal particles.
（Hitachi Review Vol. 65 (2016), No. 7）