质料科学Materials Science

Plasticity in single-crystalline Mg3Bi2 thermoelectric material

单晶Mg3Bi2热电质料的塑性

▲ 作者：Peng Zhao, Wenhua Xue, Yue Zhang, Shizhen Zhi, Xiaojing Ma, Jiamin Qiu, et al.

▲链接：

https://www.nature.com/articles/s41586-024-07621-8

▲择要：

年夜大都开始进的热电质料是有机半导体。因为定向共价键，其凡是于室温下体现出有限的塑性，例如，拉伸应变小在5%。

研究组发明单晶Mg3Bi2于沿（0001）平面（即ab平面）施加张力时显示出高达100%的室温拉伸应变。该值比传统热电质料至少超出跨越一个数目级，且优在很多以近似布局结晶的金属。

试验成果注解，变形的Mg3Bi2中存于滑移带以及位错，注解位错滑动是塑性变形的微不雅机制。化学键阐发展现了多个低滑移势垒能面，注解Mg3Bi2中存于多种滑移系统。此外，滑移历程中持续的动态键合避免了原子面解理，从而连结了较年夜的塑性变形。

主要的是，碲掺杂的单晶Mg3Bi2于室温下沿ab平面显示出约55 W/cm K2平方的功率因数以及约0.65的质量系数，优在现有的延展性热电质料。

▲ Abstract：

Most of the state-of-the-art thermoelectric materials are inorganic semiconductors. Owing to the directional covalent bonding, they usually show limited plasticity at room temperature, for example, with a tensile strain of less than five per cent. Here we discover that single-crystalline Mg3Bi2 shows a room-temperature tensile strain of up to 100 per cent when the tension is applied along the (0001) plane (that is, the ab plane). Such a value is at least one order of magnitude higher than that of traditional thermoelectric materials and outperforms many metals that crystallize in a similar structure. Experimentally, slip bands and dislocations are identified in the deformed Mg3Bi2, indicating the gliding of dislocations as the microscopic mechanism of plastic deformation. Analysis of chemical bonding reveals multiple planes with low slipping barrier energy, suggesting the existence of several slip systems in Mg3Bi2. In addition, continuous dynamic bonding during the slipping process prevents the cleavage of the atomic plane, thus sustaining a large plastic deformation. Importantly, the tellurium-doped single-crystalline Mg3Bi2 shows a power factor of about 55 microwatts per centimetre per kelvin squared and a figure of merit of about 0.65 at room temperature along the ab plane, which outperforms the existing ductile thermoelectric materials.

项目学Engineering

3D printable elastomers with exceptional strength and toughness

具备特殊强度以及韧性的3D打印弹性体

▲ 作者：Zizheng Fang, Hongfeng Mu, Zhuo Sun, Kaihang Zhang, Anyang Zhang, Jiada Chen, et al.

▲链接：

https://www.nature.com/articles/s41586-024-07588-6

▲择要：

三维（3D）打印已经成为一种颇具吸引力的打造技能，因其于实现几何繁杂的可定制产物方面具备特殊的自由度。然而，其年夜范围出产的潜力遭到低打造效率（打印速率）以及产物品质有余（机械机能）的拦阻。

光聚合物超倏地3D打印技能的最新进展减缓了打造效率的问题，但典型打印聚合物的机械机能仍远远掉队在传统加工技能。这是由于打印要求限定了份子设计实现高机械机能的潜力。

研究组报导了一种3D光打印树脂化学计谋，其孕育发生的弹性体抗拉强度为94.6 MPa，韧性为310.4 MJ m-3，二者均远远跨越任何3D打印弹性体。从机理上讲，这经由过程打印聚合物中的动态共价键来实现，同时还实现了收集拓扑布局重构。

这有助在造成分层氢键（出格是酰胺氢键）、微相分散以及互穿布局，从而协同促成卓着的机械机能。该事情为使用3D打印举行年夜范围打造提供了一个更光亮的远景。

▲ Abstract：

Three-dimensional (3D) printing has emerged as an attractive manufacturing technique because of its exceptional freedom in accessing geometrically complex customizable products. Its potential for mass manufacturing, however, is hampered by its low manufacturing efficiency (print speed) and insufficient product quality (mechanical properties). Recent progresses in ultra-fast 3D printing of photo-polymers have alleviated the issue of manufacturing efficiency, but the mechanical performance of typical printed polymers still falls far behind what is achievable with conventional processing techniques. This is because of the printing requirements that restrict the molecular design towards achieving high mechanical performance. Here we report a 3D photo-printable resin chemistry that yields an elastomer with tensile strength of 94.6 MPa and toughness of 310.4 MJ m-3, both of which far exceed that of any 3D printed elastomer. Mechanistically, this is achieved by the dynamic covalent bonds in the printed polymer that allow network topological reconfiguration. This facilitates the formation of hierarchical hydrogen bonds (in particular, amide hydrogen bonds), micro-phase separation and interpenetration architecture, which contribute synergistically to superior mechanical performance. Our work suggests a brighter future for mass manufacturing using 3D printing.

人工智能Artificial Intelligence

AI models collapse when trained on recursively generated data

AI模子于使用递归天生数据举行练习时会瓦解

▲ 作者：Ilia Shumailov, Zakhar Shumaylov, Yiren Zhao, Nicolas Papernot, Ross Anderson Yarin Gal

▲链接：

https://www.nature.com/articles/s41586-024-07566-y

▲择要：

不变散布完全转变了描写性文本的图象创立。GPT-二、GPT-3.5以及GPT-4于各类言语使命中体现优秀。ChatGPT向公家推广了这类言语模子。此刻很较着，年夜言语模子（LLMs）等天生式人工智能（AI）将持久存于，并将极年夜地转变于线文本以及图象的生态体系。

研究组思量一旦LLM孝敬了于线找到的年夜量文本，GPT-{n}可能会发生甚么。他们发明，于练习中滥用模子天生的内容会致使天生的模子呈现不成逆转的缺陷，即原始内容漫衍的尾部会消散。

研究组将这类效应称为 模子瓦解 ，并注解其可能发生于LLM和变分自编码器（VAEs）以及高斯混淆模子（GMMs）中。他们于该征象暗地里构建了理论直觉，并描绘了其于所有进修天生模子中的遍及性。

研究组注解，若要维持从收集上抓取的年夜范围数据举行练习的益处，就必需当真看待这个问题。事实上，于从互联网抓取的数据中存于LLM天生内容的环境下，网络到的关在人类与体系间真实交互数据的价值将愈来愈高。

▲ Abstract：

Stable diffusion revolutionized image creation from descriptive text. GPT-2, GPT-3(.5) and GPT-4 demonstrated high performance across a variety of language tasks. ChatGPT introduced such language models to the public. It is now clear that generative artificial intelligence (AI) such as large language models (LLMs) is here to stay and will substantially change the ecosystem of online text and images. Here we consider what may happen to GPT-{n} once LLMs contribute much of the text found online. We find that indiscriminate use of model-generated content in training causes irreversible defects in the resulting models, in which tails of the original content distribution disappear. We refer to this effect as model collapse and show that it can occur in LLMs as well as in variational autoencoders (VAEs) and Gaussian mixture models (GMMs). We build theoretical intuition behind the phenomenon and portray its ubiquity among all learned generative models. We demonstrate that it must be taken seriously if we are to sustain the benefits of training from large-scale data scraped from the web. Indeed, the value of data collected about genuine human interactions with systems will be increasingly valuable in the presence of LLM-generated content in data crawled from the Internet.

化学Chemistry

Unlocking carbene reactivity by metallaphotoredox -elimination

金属光氧化还原 -消弭解锁卡宾反映性

▲ 作者：Benjamin T. Boyle, Nathan W. Dow, Christopher B. Kelly, Marian C. Bryan David W. C. MacMillan

▲链接：

https://www.nature.com/articles/s41586-024-07628-1

▲择要：

操作把持高能中间体的威力对于合成化学很是主要，它使繁杂份子的构建成为可能，并鞭策合成范畴的前进。沿着这些线路，卡宾以及类卡宾中间体颇具吸引力，但每每是未知的高能中间体。获取金属卡宾中间体的经典要领是哄骗双电子化学造成碳-金属键。然而，因为试剂保险问题，这些要领凡是被克制，限定了其于合成中的广泛运用。

从机理上讲，可防止这些缺陷的卡宾中间体的替换要领触及两个单电子步调：自由基加到金属中造成最初的碳-金属键，然后氧化还原促成 -消弭，孕育发生所需的金属卡宾中间体。

研究组经由过程金属光氧化还原平台实现了这一计谋，哄骗铁卡宾的反映性，使用易在得到的化学原料作为自由基来历，并从六种先前未开发的拜别基团中举行 -消弭。这些发明可实现富厚且不变的羧酸、氨基酸以及醇类举行环丙烷化，并将 键插入到N-H、S-H以及P-H键中，从而为卡宾介导的化学多样化应战提供了一个通用解决方案。

▲ Abstract：

The ability to tame high-energy intermediates is important for synthetic chemistry, enabling the construction of complex molecules and propelling advances in the field of synthesis. Along these lines, carbenes and carbenoid intermediates are particularly attractive, but often unknown, high-energy intermediates. Classical methods to access metal carbene intermediates exploit two-electron chemistry to form the carbon metal bond. However, these methods are usually prohibitive because of reagent safety concerns, limiting their broad implementation in synthesis. Mechanistically, an alternative approach to carbene intermediates that could circumvent these pitfalls would involve two single-electron steps: radical addition to metal to forge the initial carbon metal bond followed by redox-promoted -elimination to yield the desired metal carbene intermediate. Here we realize this strategy through a metallaphotoredox platform that exploits iron carbene reactivity using readily available chemical feedstocks as radical sources and -elimination from six classes of previously underexploited leaving groups. These discoveries permit cyclopropanation and -bond insertion into N H, S H and P H bonds from abundant and bench-stable carboxylic acids, amino acids and alcohols, thereby providing a general solution to the challenge of carbene-mediated chemical diversification.

地球科学Earth Science

Global atmospheric methane uptake by upland tree woody surfaces

高地树木木质外貌对于全世界年夜气甲烷的接收

▲ 作者：Vincent Gauci, Sunitha Rao Pangala, Alexander Shenkin, Josep Barba, David Bastviken, Viviane Figueiredo, et al.

▲链接：

https://www.nature.com/articles/s41586-024-07592-w

▲择要：

甲烷是一种主要的温室气体，但树木于甲烷预算中的作用仍不确定。虽然已经有研究注解湿地以及一些高地树木可于茎干部开释泥土来历的甲烷，但也有人以为高地树木可作为年夜气甲烷的净汇。

课题组研究了热带、温带高地以及北方丛林树木的原位木质外貌甲烷互换。成果发明，木质外貌的甲烷接收，出格是于丛林地面以上约2米处，可以主导树木的净生态体系孝敬，致使树木的甲烷净汇。木质外貌室空气中甲烷的不变碳同位素丈量以及提取木芯的历程程度研究成果与甲烷养分一致，注解微生物介导的甲烷于木质外貌以及构造中削减。

经由过程运用陆地激光扫描衍生的异速丈量来量化全世界丛林树木木质外貌积，开端预计树木可能孝敬24.6~49.9 Tg的全世界年夜气甲烷接收。该研究成果注解，热带以及温带丛林掩护以及再造林的天气效益可能比此前假定的要年夜。

▲ Abstract：

Methane is an important greenhouse gas, but the role of trees in the methane budget remains uncertain. Although it has been shown that wetland and some upland trees can emit soil-derived methane at the stem base, it has also been suggested that upland trees can serve as a net sink for atmospheric methane. Here we examine in situ woody surface methane exchange of upland tropical, temperate and boreal forest trees. We find that methane uptake on woody surfaces, in particular at and above about 2m above the forest floor, can dominate the net ecosystem contribution of trees, resulting in a net tree methane sink. Stable carbon isotope measurement of methane in woody surface chamber air and process-level investigations on extracted wood cores are consistent with methanotrophy, suggesting a microbially mediated drawdown of methane on and in tree woody surfaces and tissues. By applying terrestrial laser scanning-derived allometry to quantify global forest tree woody surface area, a preliminary first estimate suggests that trees may contribute 24.6 49.9 Tg of atmospheric methane uptake globally. Our findings indicate that the climate benefits of tropical and temperate forest protection and reforestation may be greater than previously assumed.

Deep, hot, ancient melting recorded by ultralow oxygen fugacity in peridotites

橄榄岩超低氧逸度记载的深、热、古熔融事务

▲ 作者：Suzanne K. Birner, Elizabeth Cottrell, Fred A. Davis Jessica M. Warren

▲链接：

https://www.nature.com/articles/s41586-024-07603-w

▲择要：

脊橄榄岩记载的对于流上地幔氧逸度（fO2）变迁幅度跨越4个数目级。只管人们较多存眷驱动组织配景间地幔fO2变迁的机制和现代岩石与古地幔衍生岩石之间的fO2比力，但对于现代洋中脊配景下橄榄岩记载的fO2高变同性来历知之甚少。

研究组报导了盖克尔海脊以及东承平洋海隆（EPR）橄榄岩的岩石学以及地球化学，包孕16次新的fO2高精度测定。盖克尔海脊的难熔橄榄岩记载的fO2比地幔平均值低4个数目级以上。经由过程热力学以及矿物分配模子，研究组发明，于高位温（Tp）下，从石榴石区最先，连续到尖晶石区，可孕育发生向超低fO2的偏移，这些前提切合古科马提岩而非现代玄武岩的造成历程。

这其实不象征着旷古代情况对于流上地幔的铁/亚铁比率低在昨天，也不料味着热门石榴石区的现代熔融造成的岩浆削减。相反，这注解陈旧、难熔、超还原的地幔 筏 继承于现代地幔中轮回，而对于现代脊火山勾当孝敬甚微。

▲ Abstract：

The oxygen fugacity (fO2) of convecting upper mantle recorded by ridge peridotites varies by more than four orders of magnitude. Although much attention has been given to mechanisms that drive variations in mantle fO2 between tectonic settings and to comparisons of fO2 between modern rocks and ancient-mantle-derived rocks, comparatively little has been done to understand the origins of the high variability in fO2 recorded by peridotites from modern mid-ocean ridge settings. Here we report the petrography and geochemistry of peridotites from the Gakkel Ridge and East Pacific Rise (EPR), including 16 new high-precision determinations of fO2. Refractory peridotites from the Gakkel Ridge record fO2 more than four orders of magnitude below the mantle average. With thermodynamic and mineral partitioning modelling, we show that excursions to ultralow fO2 can be produced by large degrees of melting at high potential temperature (Tp), beginning in the garnet field and continuing into the spinel field conditions met during the generation of ancient komatiites but not modern basalts. This does not mean that ambient convecting upper mantle had a lower ferric to ferrous ratio in Archaean times than today nor that modern melting in the garnet field at hotspots produce reduced magmas. Instead, it implies that rafts of ancient, refractory, ultrareduced mantle continue to circulate in the modern mantle while contributing little to modern ridge volcanism.

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