编译|未玖

Nature,  25 July 2024, VOL 631, ISSUE 8022

《自然》2024年7月25日，第631卷，8022期

材料科学Materials Science

Plasticity in single-crystalline Mg₃Bi₂ thermoelectric material

单晶Mg3Bi2热电材料的塑性

▲ 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

▲摘要：

大多数最先进的热电材料是无机半导体。由于定向共价键，其通常在室温下表现出有限的塑性，例如，拉伸应变小于5%。

研究组发现单晶Mg₃Bi₂在沿（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。该值比传统热电材料至少高出一个数量级，且优于许多以类似结构结晶的金属。

实验结果表明，变形的Mg₃Bi₂中存在滑移带和位错，表明位错滑动是塑性变形的微观机制。化学键分析揭示了多个低滑移势垒能面，表明Mg₃Bi₂中存在多种滑移体系。此外，滑移过程中连续的动态键合防止了原子面解理，从而保持了较大的塑性变形。

重要的是，碲掺杂的单晶Mg₃Bi2在室温下沿ab平面显示出约55μW/cm·K²平方的功率因数和约0.65的品质系数，优于现有的延展性热电材料。

▲ 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.

工程学Engineering

3D printable elastomers with exceptional strength and toughness

具有非凡强度和韧性的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

▲摘要：

三维（3D）打印已成为一种颇具吸引力的制造技术，因其在实现几何复杂的可定制产品方面具有非凡的自由度。然而，其大规模生产的潜力受到低制造效率（打印速度）和产品质量不足（机械性能）的阻碍。

光聚合物超快速3D打印技术的最新进展缓解了制造效率的问题，但典型打印聚合物的机械性能仍远远落后于传统加工技术。这是因为打印要求限制了分子设计实现高机械性能的潜力。

研究组报道了一种3D光打印树脂化学策略，其产生的弹性体抗拉强度为94.6 MPa，韧性为310.4 MJ m^-3，两者均远远超过任何3D打印弹性体。从机理上讲，这通过打印聚合物中的动态共价键来实现，同时还实现了网络拓扑结构重构。

这有助于形成分层氢键（特别是酰胺氢键）、微相分离和互穿结构，从而协同促进卓越的机械性能。该工作为使用3D打印进行大规模制造提供了一个更光明的前景。

▲ 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模型在使用递归生成数据进行训练时会崩溃

▲ 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

▲摘要：

稳定扩散彻底改变了描述性文本的图像创建。GPT-2、GPT-3.5和GPT-4在各种语言任务中表现优异。ChatGPT向公众推广了这种语言模型。现在很明显，大语言模型（LLMs）等生成式人工智能（AI）将长期存在，并将极大地改变在线文本和图像的生态系统。

研究组考虑一旦LLM贡献了在线找到的大量文本，GPT-{n}可能会发生什么。他们发现，在训练中滥用模型生成的内容会导致生成的模型出现不可逆转的缺陷，即原始内容分布的尾部会消失。

研究组将这种效应称为“模型崩溃”，并表明其可能发生在LLM以及变分自编码器（VAEs）和高斯混合模型（GMMs）中。他们在该现象背后构建了理论直觉，并描绘了其在所有学习生成模型中的普遍性。

研究组表明，若要维持从网络上抓取的大规模数据进行训练的益处，就必须认真对待这个问题。事实上，在从互联网抓取的数据中存在LLM生成内容的情况下，收集到的关于人类与系统间真实交互数据的价值将越来越高。

▲ 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 α-elimination

金属光氧化还原α-消除解锁卡宾反应性

▲ 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

▲摘要：

驾驭高能中间体的能力对合成化学非常重要，它使复杂分子的构建成为可能，并推动合成领域的进步。沿着这些路线，卡宾和类卡宾中间体颇具吸引力，但往往是未知的高能中间体。获取金属卡宾中间体的经典方法是利用双电子化学形成碳-金属键。然而，由于试剂安全问题，这些方法通常被禁止，限制了其在合成中的广泛应用。

从机理上讲，可避免这些缺陷的卡宾中间体的替代方法涉及两个单电子步骤：自由基加到金属中形成最初的碳-金属键，然后氧化还原促进α-消除，产生所需的金属卡宾中间体。

研究组通过金属光氧化还原平台实现了这一策略，利用铁卡宾的反应性，使用易于获得的化学原料作为自由基来源，并从六种先前未开发的离去基团中进行α-消除。这些发现可实现丰富且稳定的羧酸、氨基酸和醇类进行环丙烷化，并将σ键插入到N-H、S-H和P-H键中，从而为卡宾介导的化学多样化挑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 α-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 α-elimination from six classes of previously underexploited leaving groups. These discoveries permit cyclopropanation and σ-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

高地树木木质表面对全球大气甲烷的吸收

▲ 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

▲摘要：

甲烷是一种重要的温室气体，但树木在甲烷预算中的作用仍不确定。虽然已有研究表明湿地和一些高地树木可在茎干部释放土壤来源的甲烷，但也有人认为高地树木可作为大气甲烷的净汇。

课题组研究了热带、温带高地和北方森林树木的原位木质表面甲烷交换。结果发现，木质表面的甲烷吸收，特别是在森林地面以上约2米处，可以主导树木的净生态系统贡献，导致树木的甲烷净汇。木质表面室空气中甲烷的稳定碳同位素测量和提取木芯的过程水平研究结果与甲烷营养一致，表明微生物介导的甲烷在木质表面和组织中减少。

通过应用陆地激光扫描衍生的异速测量来量化全球森林树木木质表面积，初步估计树木可能贡献24.6~49.9 Tg的全球大气甲烷吸收。该研究结果表明，热带和温带森林保护和再造林的气候效益可能比此前假设的要大。

▲ 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录的深、热、古熔融事件

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

▲链接：

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

▲摘要：

脊橄榄岩记录的对流上地幔氧逸度（f_O2）变化幅度超过4个数量级。尽管人们较多关注驱动构造背景间地幔f_O2变化的机制以及现代岩石与古地幔衍生岩石之间的f_O2比较，但对现代洋中脊背景下橄榄岩记录的f_O2高变异性来源知之甚少。

研究组报道了盖克尔海脊和东太平洋海隆（EPR）橄榄岩的岩石学和地球化学，包括16次新的f_O2高精度测定。盖克尔海脊的难熔橄榄岩记录的f_O2比地幔平均值低4个数量级以上。通过热力学和矿物分配模型，研究组发现，在高位温（Tp）下，从石榴石区开始，持续到尖晶石区，可产生向超低f_O2的偏移，这些条件符合古科马提岩而非现代玄武岩的形成过程。

这并不意味着太古代环境对流上地幔的铁/亚铁比率低于今天，也不意味着热点石榴石区的现代熔融形成的岩浆减少。相反，这表明古老、难熔、超还原的地幔“筏”继续在现代地幔中循环，而对现代脊火山活动贡献甚微。

▲ Abstract：

The oxygen fugacity (f_O2) 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 f_O2 between tectonic settings and to comparisons of f_O2 between modern rocks and ancient-mantle-derived rocks, comparatively little has been done to understand the origins of the high variability in f_O2 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 f_O2. Refractory peridotites from the Gakkel Ridge record f_O2 more than four orders of magnitude below the mantle average. With thermodynamic and mineral partitioning modelling, we show that excursions to ultralow f_O2 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.