Could Advanced Neutron Gauging Improve Quality
Control in Li-ion Battery Manufacturing Processes?
John P Barton Feb 10 2013
In the manufacture of Li-ion batteries, long sandwich strips are assembled with a polymer separator between the metal anode and cathode plates. Faults in the separator layer could lead during subsequent usage to short circuits developing internally in the cell. Advanced neutron gauging, during manufacture, may complement other methods of quality control. For example, depending on the energies of neutrons and energy sensitivity of the detectors, the hydrogen in the separator may provide greater sensitivity to faults than complementary techniques, such as gamma ray on-line gauging. In this paper the question is asked: could neutron radiologists be of assistance to those responsible for aviation safety reviews or others working to allow the Boeing 787 aircraft to resume programs following the battery incidents of Jan 7 and Jan 16 2013?
Background information in a previous article “Li-ion Burned Batteries: Neutron Tests Options?” includes a description of the batteries, graphics and related references. [1-2].
On Feb 7 2013 The US National Transportation Safety Board (NTSB), investigating the Jan 7 fire on a Japan Airlines plane parked at Boston Airport, gave preliminary findings at a news briefing. Investigators, NTSB explained, had so far ruled out two possible causes: a mechanical shock or an electrical shock from outside the battery.
NTSB stated recorded data from the plane showed the voltage of the 8 cell battery unexpectedly dropped from 32 volts to 28 volts, indicating a short circuit in one 4 volt cell. Other indications point to an event started by internal short circuiting in a single cell, followed by “thermal runaway” and overheating that propagated to adjacent cells.
The NTSB Chairwoman, Ms. D. Hersman, is reported to have stated “We have not yet identified what the cause of the short circuit is. We are looking at the design of the battery, at the manufacturing, and we are also looking at the cell charging. There are a lot of things we are still looking at.” She is also reported to have said that investigators have a long road ahead to flesh out all of the possible hazards, and that the NTSB plan to provide an interim report of findings in 30 days.
The US Federal Aviation Authority administrator, Mr. M. Huerta, in a related statement, said the agency’s review would be data driven, and they will take any action necessary to further ensure safety. Boeing Company officials, also in related statements, said they regard the NTSB findings as narrowing the likely source of the problem to within the battery itself. They also recognize that it will take a combination of changes to restore confidence in the battery system.
The NTSB was critical of the 2007 conclusions of Boeing and the FAA that the batteries would have a probability of smoke of only one in 10 million flight hours and negligible probability of fire, and observers remarked that this criticism would put pressure on the FAA to “be tougher” on Boeing than it was before.
Reduction of risks of manufacturing faults in the separator.
A battery consultant has proposed that faults in the manufacturing process affecting the separating material between the two electrode sheets could keep the electric current from flowing uniformly, and create hot spots that could set off overheating. Problems could be contamination by metal shavings or folds and wrinkles in the separator materials.
It is suggested in this article below that advanced neutron gauging techniques, if not already incorporated in the manufacturing process quality control, could be considered by the safety approval authorities as one way to reduce the risks exposed by the incidents.
The basic principal of neutron gauging would be to pass the metal- polymer-metal sandwich through a screening gauge, with a neutron source on one side and a detector on the other to monitor for changes in the transmitted signal. The evaluation would start with a review of published reports on neutron gauging and online analysis for quality control during manufacturing or similar processes. [3-8]. the review would also include consideration of neutron source alternatives. While accelerators or radioactive isotopic sources are both available, the steady output from the latter would be a positive factor.. Of the several alternatives, three types, Americium-Curium-Beryllium, and Californium 252 deserve early evaluation. [9-10]. Also Antimony Beryllium could provide neutrons of different energies. Speed of on-line screening will be one consideration. A single cell contains 3 sandwich strips, each about 10 meters long. A battery of 8 cells therefore requires screening a total sandwich area of about 2,400,000 square centimeters. Types of neutron detector to be considered should include microchannel plates that collect data from many pixels simultaneously . Although individually small those presently available from Nova Scientific are abutable on three sides.
A simple experimental series of tests would be an essential step to investigate the degree to which the relatively neutron attenuating materials, Lithium and Hydrogen, effect the transmitted signal, compared with the relatively neutron transparent materials, copper, aluminum and carbon. Considerations would be given to variations in neutron source energy (and various moderators and filters), and choices of neutron detectors with different energy sensitivities. Statistical and noise considerations would be part of evaluations of overall sensitivity to the range of faults of possible concern in the battery manufacturing process.
The consideration of advanced neutron gauging methods for improved quality control of battery manufacturing process would be compared with the previous quality control processes, and any other improved methods. The plans should be to complete the evaluation within a few weeks. Such rapid results should be achievable if done in one of the many centers already equipped for such work with radiation sources.
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