《自然》（20240725出版）一周論文導讀

編譯|未玖

Nature,  25 July 2024, VOL 631, ISSUE 8022

《自然》2024年7月25日，第631卷，8022期

材料科學Materials Science

Plasticity in single-crystalline Mg₃Bi₂ thermoelectric material

單晶Mg3Bi2熱電材料的塑性

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

▲鏈接：

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

▲摘要：

大多數最先進的熱電材料是無機半導體。由于定向共價鍵，其通常在室溫下表現出有限的塑性，例如，拉伸應變小于5%。

研究組發現單晶Mg₃Bi₂在沿（0001）平面（即ab平面）施加張力時顯示出高達100%的室溫拉伸應變。該值比傳統熱電材料至少高出一個數量級，且優于許多以類似結構結晶的金屬。

實驗結果表明，變形的Mg₃Bi₂中存在滑移帶和位錯，表明位錯滑動是塑性變形的微觀機制。化學鍵分析揭示了多個低滑移勢壘能面，表明Mg₃Bi₂中存在多種滑移體系。此外，滑移過程中連續的動態鍵合防止了原子面解理，從而保持了較大的塑性變形。

重要的是，碲摻雜的單晶Mg₃Bi2在室溫下沿ab平面顯示出約55μW/cm·K²平方的功率因數和約0.65的品質系數，優于現有的延展性熱電材料。

▲ 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.

工程學Engineering

3D printable elastomers with exceptional strength and toughness

具有非凡強度和韌性的3D打印彈性體

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

▲鏈接：

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

▲摘要：

三維（3D）打印已成為一種頗具吸引力的制造技術，因其在實現幾何復雜的可定制產品方面具有非凡的自由度。然而，其大規模生產的潛力受到低制造效率（打印速度）和產品質量不足（機械性能）的阻礙。

光聚合物超快速3D打印技術的最新進展緩解了制造效率的問題，但典型打印聚合物的機械性能仍遠遠落后于傳統加工技術。這是因為打印要求限制了分子設計實現高機械性能的潛力。

研究組報道了一種3D光打印樹脂化學策略，其產生的彈性體抗拉強度為94.6 MPa，韌性為310.4 MJ m^-3，兩者均遠遠超過任何3D打印彈性體。從機理上講，這通過打印聚合物中的動態共價鍵來實現，同時還實現了網絡拓撲結構重構。

這有助于形成分層氫鍵（特別是酰胺氫鍵）、微相分離和互穿結構，從而協同促進卓越的機械性能。該工作為使用3D打印進行大規模制造提供了一個更光明的前景。

▲ 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模型在使用遞歸生成數據進行訓練時會崩潰

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

▲鏈接：

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

▲摘要：

穩定擴散徹底改變了描述性文本的圖像創建。GPT-2、GPT-3.5和GPT-4在各種語言任務中表現優異。ChatGPT向公眾推廣了這種語言模型。現在很明顯，大語言模型（LLMs）等生成式人工智能（AI）將長期存在，并將極大地改變在線文本和圖像的生態系統。

研究組考慮一旦LLM貢獻了在線找到的大量文本，GPT-{n}可能會發生什么。他們發現，在訓練中濫用模型生成的內容會導致生成的模型出現不可逆轉的缺陷，即原始內容分布的尾部會消失。

研究組將這種效應稱為“模型崩潰”，并表明其可能發生在LLM以及變分自編碼器（VAEs）和高斯混合模型（GMMs）中。他們在該現象背后構建了理論直覺，并描繪了其在所有學習生成模型中的普遍性。

研究組表明，若要維持從網絡上抓取的大規模數據進行訓練的益處，就必須認真對待這個問題。事實上，在從互聯網抓取的數據中存在LLM生成內容的情況下，收集到的關于人類與系統間真實交互數據的價值將越來越高。

▲ 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.

化學Chemistry

Unlocking carbene reactivity by metallaphotoredox α-elimination

金屬光氧化還原α-消除解鎖卡賓反應性

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

▲鏈接：

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

▲摘要：

駕馭高能中間體的能力對合成化學非常重要，它使復雜分子的構建成為可能，并推動合成領域的進步。沿著這些路線，卡賓和類卡賓中間體頗具吸引力，但往往是未知的高能中間體。獲取金屬卡賓中間體的經典方法是利用雙電子化學形成碳-金屬鍵。然而，由于試劑安全問題，這些方法通常被禁止，限制了其在合成中的廣泛應用。

從機理上講，可避免這些缺陷的卡賓中間體的替代方法涉及兩個單電子步驟：自由基加到金屬中形成最初的碳-金屬鍵，然后氧化還原促進α-消除，產生所需的金屬卡賓中間體。

研究組通過金屬光氧化還原平臺實現了這一策略，利用鐵卡賓的反應性，使用易于獲得的化學原料作為自由基來源，并從六種先前未開發的離去基團中進行α-消除。這些發現可實現豐富且穩定的羧酸、氨基酸和醇類進行環丙烷化，并將σ鍵插入到N-H、S-H和P-H鍵中，從而為卡賓介導的化學多樣化挑戰提供了一個通用解決方案。

▲ 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.

地球科學Earth Science

Global atmospheric methane uptake by upland tree woody surfaces

高地樹木木質表面對全球大氣甲烷的吸收

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

▲鏈接：

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

▲摘要：

甲烷是一種重要的溫室氣體，但樹木在甲烷預算中的作用仍不確定。雖然已有研究表明濕地和一些高地樹木可在莖干部釋放土壤來源的甲烷，但也有人認為高地樹木可作為大氣甲烷的凈匯。

課題組研究了熱帶、溫帶高地和北方森林樹木的原位木質表面甲烷交換。結果發現，木質表面的甲烷吸收，特別是在森林地面以上約2米處，可以主導樹木的凈生態系統貢獻，導致樹木的甲烷凈匯。木質表面室空氣中甲烷的穩定碳同位素測量和提取木芯的過程水平研究結果與甲烷營養一致，表明微生物介導的甲烷在木質表面和組織中減少。

通過應用陸地激光掃描衍生的異速測量來量化全球森林樹木木質表面積，初步估計樹木可能貢獻24.6~49.9 Tg的全球大氣甲烷吸收。該研究結果表明，熱帶和溫帶森林保護和再造林的氣候效益可能比此前假設的要大。

▲ 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

橄欖巖超低氧逸度記錄的深、熱、古熔融事件

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

▲鏈接：

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

▲摘要：

脊橄欖巖記錄的對流上地幔氧逸度（f_O2）變化幅度超過4個數量級。盡管人們較多關注驅動構造背景間地幔f_O2變化的機制以及現代巖石與古地幔衍生巖石之間的f_O2比較，但對現代洋中脊背景下橄欖巖記錄的f_O2高變異性來源知之甚少。

研究組報道了蓋克爾海脊和東太平洋海隆（EPR）橄欖巖的巖石學和地球化學，包括16次新的f_O2高精度測定。蓋克爾海脊的難熔橄欖巖記錄的f_O2比地幔平均值低4個數量級以上。通過熱力學和礦物分配模型，研究組發現，在高位溫（Tp）下，從石榴石區開始，持續到尖晶石區，可產生向超低f_O2的偏移，這些條件符合古科馬提巖而非現代玄武巖的形成過程。

這并不意味著太古代環境對流上地幔的鐵/亞鐵比率低于今天，也不意味著熱點石榴石區的現代熔融形成的巖漿減少。相反，這表明古老、難熔、超還原的地幔“筏”繼續在現代地幔中循環，而對現代脊火山活動貢獻甚微。

▲ 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.