Samsung has recalled its new Galaxy chú ý 7 điện thoại cảm ứng after reports it can overheat & even explode. The company said it "conducted a thorough investigation and found a battery cell issue." (UPDATE: It has now stopped production of the phone.)
There have been dozens of reported cases và 2.5 million phones have been produced. While that figure is relatively high (batteries typically fail at a rate of 1 in tens of millions), it"s still far from common.
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"Battery failures are exceedingly rare," says Donal Finegan, a chemical engineer at University College London. "Any kind of fault does garner a lot of truyền thông attention & can really affect the reputation of a product that relies on the battery."
Like many rechargeable devices, phones use lithium-ion cells. But what makes these batteries great at powering gadgets also makes them vulnerable lớn catching fire, says Finegan. "They are so energy-dense and can operate under such high power that they can combust in a particularly catastrophic way."
Overheating is obviously driven by temperature rise. This can be due to lớn the environment, such as a hot car in summer, or through heat transferred khổng lồ a battery from another component inside the phone. Heating can also begin within a battery itself, which is what"s behind the "battery cell issue" in Samsung"s lưu ý 7.
Samsung Galaxy note 7 (Image: Samsung)
One cause of combustion is a problem with the "battery management system" that monitors the electrical current và normally tells a cpu inside the phone khổng lồ stop the current once a battery is fully charged. If either the system or cpu is faulty, a battery can enter a state of "overcharge".
"The battery can continue to lớn charge and can become even more unstable and eventually just burst into flames itself, without any kind of external heating," Finegan explains.
Phones don"t contain fans or the liquid cooling mechanism you find in a gaming PC or electric vehicle, so heat must radiate out into the surroundings. If that doesn"t happen, heat is generated faster than it can be dissipated or lost.
When a battery reaches about 100ºC (200ºF), its materials start to lớn break down, triggering a chemical chain reaction that releases its own energy. This accelerates the warming và leads to lớn a snowball effect -- a process called "thermal runaway".
"The snowball effect happens so fast that, within a second, the entire cell goes from being intact to lớn being completely destroyed," says Finegan.
So what triggers the catastrophic failure that causes explosions?
What triggers thermal runaway inside a battery?
The changes that allow heat to spread through a lithium-ion cell were recently revealed by Donal Finegan & colleagues in Paul Shearing"s lab at University College London (UCL).
Researchers traditionally rely on X-ray photos to examine a battery before & after failure, but Shearing"s team have simulated a range of operating conditions -- up khổng lồ an abnormal 300ºC (over 500ºF) -- and used thermal imaging to see what happens during thermal runaway. The UCL team also employed high-frequency X-rays to lớn create 3 chiều images of the aftermath.
Lithium-ion cells are made-up of multiple layers, each consisting of a positive và a negative electrode, separated by an insulator. During charging, lithium ions flow from positive to lớn negative, and ions return khổng lồ the positive electrode as the battery drains.
When materials inside a cell move around, the electrically insulating layer that separates electrodes can tear, causing a short-circuit that creates a spark. "Even the tiniest spark inside a battery is enough to lớn initiate the thermal runaway process," says Finegan.
Lithium ions are carried by an electrolyte solution, which is a volatile liquid. Finegan"s X-ray snapshots reveal that when heated, electrolytes produce gas bubbles that causes a cell to thua thảm structural integrity & can create the short circuit. His videos show that heat spreads across each electrode layer and between layers until the entire battery gets too hot.
The thin, rectangular battery inside a phone has repeating layers of electrodes, which resembles the pages of a notepad, whereas layers in cylindrical cell (like a standard AA battery) are rolled-up lượt thích a newspaper. Thermal runaway is similar in both arrangements.
Rectangular batteries are more prone to failure compared to cylindrical cells because the latter usually have a central tư vấn that helps prevent internal layers from deforming, as well as a "rupture disc" that will cut-off the current when stretched. Phones don"t have such secondary fail-safes because small kích thước is a desirable feature.
Batteries aren"t as safe as they could be, partly because scientists must cater to the demands of manufacturers, who prefer high performance -- but less stable -- cells. "They usually try khổng lồ use the most highly energy-dense material, so we have khổng lồ keep up with that," Finegan points out. "It"s not as simple as saying, "Just use a safer material"."
The UCL team is currently working with NASA to develop devices that deliberately cause a short circuit at a known location. This has enabled the researchers lớn build a maps that predicts how heat spreads through a cell. "This map of propagation allowed us to then suggest why, for example, one battery explodes and bursts its contents out into the neighboring area," says Finegan.
Chemical engineering could help the phone industry reduce the risk of battery failure in future. "Hopefully this information will be used by the industry lớn try & improve their designs & to prevent the worst-case scenario of the really most catastrophic failures," says Finegan. "That"s the aim of what we"re doing right now."
I"m a science communicator và award-winning journalist with a PhD in evolutionary biology. I specialise in explaining scientific concepts that appear in popular culture and mainly write about health, nature and technology. I spent several years at bbc Science Focus magazine, running the features section and writing about everything from gay genes & internet memes to lớn the science of death & origin of life. I"ve also contributed lớn Scientific American and Men"s Health. My latest book is "50 Biology Ideas You Really Need to Know".