作者:未玖 来源:科学网微信公众号 发布时间:2024/3/2 20:34:22
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《自然》(20240229出版)一周论文导读

 

编译 | 未玖

Nature, 29 February 2024, VOL 626, ISSUE 8001

《自然》2024年2月29日,第626卷,8001期

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天文学Astronomy

Most of the photons that reionized the Universe came from dwarf galaxies

使宇宙再电离的大多数光子来自矮星系

▲ 作者:Hakim Atek, Ivo Labbé, Lukas J. Furtak, Iryna Chemerynska, Seiji Fujimoto, David J. Setton, et al.

▲ 链接:

https://www.nature.com/articles/s41586-024-07043-6

▲ 摘要:

宇宙再电离是宇宙大爆炸后约600~800 Myr从中性氢到电离等离子体的主要相变,但人们对确定驱动宇宙再电离的来源一直存在争议。一些模型表明,类星体的高电离发射率和逃逸分数(fesc)支持其在推动宇宙再电离中发挥作用。

另一些人提出,明亮星系的高fesc值会产生足够的电离辐射来驱动这一过程。最后,一些研究表明,当与电离效率和fesc的恒星质量依赖模型结合时,微弱星系的数量密度可以有效控制宇宙再电离。然而迄今为止,由于低质量星系极其微弱,尚未对其进行全面的光谱研究。

研究组报道了在再电离时期对8个超微弱星系(在一个非常小的场中)的分析,绝对星等在MUV≈?17等和?15等之间(低至0.005L?)。结果发现,在宇宙的前10亿年间,微弱星系产生了log[ξion?(Hz?erg?1)]= 25.80±0.14的电离光子,比通常假设的值高出4倍。

如果这个场代表了微弱星系的大规模分布,那么电离光子的速率就超过了再电离所需的速率,即使fesc为5%数量级时亦是如此。

▲ Abstract:

The identification of sources driving cosmic reionization, a major phase transition from neutral hydrogen to ionized plasma around 600–800?Myr after the Big Bang, has been a matter of debate. Some models suggest that high ionizing emissivity and escape fractions (fesc) from quasars support their role in driving cosmic reionization. Others propose that the high fesc values from bright galaxies generate sufficient ionizing radiation to drive this process. Finally, a few studies suggest that the number density of faint galaxies, when combined with a stellar-mass-dependent model of ionizing efficiency and fesc, can effectively dominate cosmic reionization. However, so far, comprehensive spectroscopic studies of low-mass galaxies have not been done because of their extreme faintness. Here we report an analysis of eight ultra-faint galaxies (in a very small field) during the epoch of reionization with absolute magnitudes between MUV?≈??17?mag and ?15?mag (down to 0.005L?). We find that faint galaxies during the first thousand million years of the Universe produce ionizing photons with log[ξion?(Hz?erg?1)]= 25.80±0.14, a factor of 4 higher than commonly assumed values. If this field is representative of the large-scale distribution of faint galaxies, the rate of ionizing photons exceeds that needed for reionization, even for escape fractions of the order of 5%.

材料科学Materials Science

High fatigue resistance in a titanium alloy via near-void-free 3D printing

通过几乎无气孔的3D打印工艺实现钛合金的高抗疲劳性

▲ 作者:Zhan Qu, Zhenjun Zhang, Rui Liu, Ling Xu, Yining Zhang, Xiaotao Li, et al.

▲ 链接:

https://www.nature.com/articles/s41586-024-07048-1

▲ 摘要:

3D打印(即结构材料的增材制造,AM)的优势因其差强人意的疲劳性能而受到严重损害。通常,较差的疲劳性能似乎因当前印刷工艺过程中产生气孔所造成。因此,研究组提出疑问,消除这些气孔是否可为显著提高无孔AM(Net-AM)合金的抗疲劳性提供可行的解决方案?

通过了解相变和晶粒生长的异步性,研究组进一步开发Net-AM工艺技术,成功地在Ti-6Al-4V钛合金中重建了几乎无气孔的AM微结构。他们确定了这种AM微结构的抗疲劳性,并表明其约1 GPa的高疲劳极限超越了所有AM和锻造钛合金以及其他金属材料的抗疲劳性能。

研究结果证实了Net-AM微结构具有高抗疲劳性能,以及AM工艺在生产具有最大疲劳强度结构部件方面的潜在优势,这有利于AM技术在工程领域的进一步应用。

▲ Abstract:

The advantage of 3D printing—that is, additive manufacturing (AM) of structural materials—has been severely compromised by their disappointing fatigue properties. Commonly, poor fatigue properties appear to result from the presence of microvoids induced by current printing process procedures. Accordingly, the question that we pose is whether the elimination of such microvoids can provide a feasible solution for marked enhancement of the fatigue resistance of void-free AM (Net-AM) alloys. Here we successfully rebuild an approximate void-free AM microstructure in Ti-6Al-4V titanium alloy by development of a Net-AM processing technique through an understanding of the asynchronism of phase transformation and grain growth. We identify the fatigue resistance of such AM microstructures and show that they lead to a high fatigue limit of around 1?GPa, exceeding the fatigue resistance of all AM and forged titanium alloys as well as that of other metallic materials. We confirm the high fatigue resistance of Net-AM microstructures and the potential advantages of AM processing in the production of structural components with maximum fatigue strength, which is beneficial for further application of AM technologies in engineering fields.

Site-specific reactivity of stepped Pt surfaces driven by stress release

应力释放驱动台阶型Pt表面的位点特异性反应性

▲ 作者:Guangdong Liu, Arthur J. Shih, Huiqiu Deng, Kasinath Ojha, Xiaoting Chen, Mingchuan Luo, et al.

▲ 链接:

https://www.nature.com/articles/s41586-024-07090-z

▲ 摘要:

多相催化剂被广泛用于促进化学反应。虽然已知化学反应通常发生在催化剂表面,但只有特定的表面位点具有高催化活性。因此,确定活性位点并使其最大限度地发挥作用是催化研究的核心,其中经典模型是根据不同的表面构型(如平台面和台阶)对活性位点进行分类。

然而,这种简单的分类往往会导致催化剂活性预测的数量级误差和活性位点的定性不确定,从而限制了催化剂设计的机会。

以台阶型Pt(111)表面和电化学氧还原反应(ORR)为例,研究组证明了较大误差和不确定性的根本原因是简化了分类,忽略了由表面应力释放驱动的原子位点特异性反应性。具体而言,各台阶的表面应力释放引入了不均匀的应变场,压缩率高达5.5%,导致具有相同局部配位的平台面原子具有不同的电子结构和反应性,并导致原子位点特异性的ORR活性增强。

对于台阶边缘两侧的平台面原子,其活性增强程度比平台面中间的原子高出50倍,这实现了通过改变平台面宽度或控制外部应力来控制ORR反应性。因此,上述协同作用的发现为催化活性原子位点的基本认识和多相催化剂的设计原理提供了新视角。

▲ Abstract:

Heterogeneous catalysts are widely used to promote chemical reactions. Although it is known that chemical reactions usually happen on catalyst surfaces, only specific surface sites have high catalytic activity. Thus, identifying active sites and maximizing their presence lies at the heart of catalysis research, in which the classic model is to categorize active sites in terms of distinct surface motifs, such as terraces and steps. However, such a simple categorization often leads to orders of magnitude errors in catalyst activity predictions and qualitative uncertainties of active sites, thus limiting opportunities for catalyst design. Here, using stepped Pt(111) surfaces and the electrochemical oxygen reduction reaction (ORR) as examples, we demonstrate that the root cause of larger errors and uncertainties is a simplified categorization that overlooks atomic site-specific reactivity driven by surface stress release. Specifically, surface stress release at steps introduces inhomogeneous strain fields, with up to 5.5% compression, leading to distinct electronic structures and reactivity for terrace atoms with identical local coordination, and resulting in atomic site-specific enhancement of ORR activity. For the terrace atoms flanking both sides of the step edge, the enhancement is up to 50 times higher than that of the atoms in the middle of the terrace, which permits control of ORR reactivity by either varying terrace widths or controlling external stress. Thus, the discovery of the above synergy provides a new perspective for both fundamental understanding of catalytically active atomic sites and design principles of heterogeneous catalysts.

Supramolecular polymers form tactoids through liquid–liquid phase separation

超分子聚合物通过液-液相分离形成类晶簇

▲ 作者:Hailin Fu, Jingyi Huang, Joost J. B. van der Tol, Lu Su, Yuyang Wang, Swayandipta Dey, et al.

▲ 链接:

https://www.nature.com/articles/s41586-024-07034-7

▲ 摘要:

生物聚合物的液—液相分离(LLPS)最近被证明在具有多种生物功能的无膜细胞器形成中起着核心作用。LLPS和大分子凝聚之间的相互作用是持续研究的一部分。合成的超分子聚合物是大分子的非共价拥挤物,但鲜有报道其发生LLPS。

研究组表明,通过合成组分的超分子聚合获得的连续生长的原纤维,通过熵驱动的途径负责相分离成高度各向异性的水性液滴(类晶簇)。受右旋糖苷浓度调节的拥挤环境不仅影响超分子聚合动力学,还影响LLPS的性质,包括相分离动力学、形态、内部有序、流动性和最终类晶簇的力学性能。

此外,基底—液和液—液界面被证明能够加速超分子聚合物的LLPS,可产生无数的三维有序结构,包括表面高度有序的微米长的类晶簇阵列。几种超分子聚合物证明了超分子聚合控制新兴形态的普遍性和诸多可能性,开辟了一个从稳定LLPS的高度结构化水溶液到纳米级软物质的新物质领域。

▲ Abstract:

Liquid–liquid phase separation (LLPS) of biopolymers has recently been shown to play a central role in the formation of membraneless organelles with a multitude of biological functions. The interplay between LLPS and macromolecular condensation is part of continuing studies. Synthetic supramolecular polymers are the non-covalent equivalent of macromolecules but they are not reported to undergo LLPS yet. Here we show that continuously growing fibrils, obtained from supramolecular polymerizations of synthetic components, are responsible for phase separation into highly anisotropic aqueous liquid droplets (tactoids) by means of an entropy-driven pathway. The crowding environment, regulated by dextran concentration, affects not only the kinetics of supramolecular polymerizations but also the properties of LLPS, including phase-separation kinetics, morphology, internal order, fluidity and mechanical properties of the final tactoids. In addition, substrate–liquid and liquid–liquid interfaces proved capable of accelerating LLPS of supramolecular polymers, allowing the generation of a myriad of three-dimensional-ordered structures, including highly ordered arrays of micrometre-long tactoids at surfaces. The generality and many possibilities of supramolecular polymerizations to control emerging morphologies are demonstrated with several supramolecular polymers, opening up a new field of matter ranging from highly structured aqueous solutions by means of stabilized LLPS to nanoscopic soft matter.

化学Chemistry

Identifying general reaction conditions by bandit optimization

用bandit优化法确定通用反应条件

▲ 作者:Jason Y. Wang (王亿珩), Jason M. Stevens, Stavros K. Kariofillis, Mai-Jan Tom, Dung L. Golden, Jun Li, et al.

▲ 链接:

https://www.nature.com/articles/s41586-024-07021-y

▲ 摘要:

目前亟需普遍适用于各种底物的反应条件,特别是在制药和化学工业中。尽管有许多方法可用于评估所开发条件的普遍适用性,但在优化过程中有效发现这些条件的通用方法却很少。

研究组报道了强化学习bandit优化模型的设计、实现和应用,通过有效的条件采样和实验反馈的评估来识别普遍适用的条件。现有数据集的性能基准统计显示,该模型识别通用条件的准确性很高,与模仿最先进优化法的基线相比提高了31%。

研究组对钯催化的咪唑C-H芳基化反应、苯胺酰胺偶联反应和苯酚烷基化反应进行了实验研究,以评估bandit优化模型在实践中的应用情况和功能。在调查了不到15%的专家设计的反应空间后,研究确定了上述三种情况下最普遍适用但尚未进行充分研究的反应条件。

▲ Abstract:

Reaction conditions that are generally applicable to a wide variety of substrates are highly desired, especially in the pharmaceutical and chemical industries. Although many approaches are available to evaluate the general applicability of developed conditions, a universal approach to efficiently discover these conditions during optimizations is rare. Here we report the design, implementation and application of reinforcement learning bandit optimization models to identify generally applicable conditions by efficient condition sampling and evaluation of experimental feedback. Performance benchmarking on existing datasets statistically showed high accuracies for identifying general conditions, with up to 31% improvement over baselines that mimic state-of-the-art optimization approaches. A palladium-catalysed imidazole C–H arylation reaction, an aniline amide coupling reaction and a phenol alkylation reaction were investigated experimentally to evaluate use cases and functionalities of the bandit optimization model in practice. In all three cases, the reaction conditions that were most generally applicable yet not well studied for the respective reaction were identified after surveying less than 15% of the expert-designed reaction space.

社会学Sociology

Online images amplify gender bias

网络图像放大了性别偏见

▲ 作者:Douglas Guilbeault, Solène Delecourt, Tasker Hull, Bhargav Srinivasa Desikan, Mark Chu & Ethan Nadler

▲ 链接:

https://www.nature.com/articles/s41586-024-07068-x

▲ 摘要:

每年,人们花在阅读上的时间越来越少,而花在观看网络激增图像上的时间越来越多。每天有数百万人下载来自谷歌和维基百科等平台的图片,还有数百万人通过Instagram和TikTok等社交媒体进行互动,主要为交换视觉内容。与此同时,新闻机构和数字广告商越来越多地通过使用图像在网上吸引人们的注意力,人们对图像的处理相比文本速度更快、更含蓄、更容易记住。

研究组表明,网络图像的兴起大大加剧了性别偏见,在统计普遍性和心理影响上均是如此。他们从谷歌、维基百科和互联网电影数据库(IMDb)的100多万张图片以及这些平台的数十亿个单词中,研究了3495个社会类别(如“护士”或“银行家”)的性别关联。

结果发现,在两性的社会类别中,性别偏见在图像中始终比在文本中更普遍。同时,有记录的女性网络代表性不足,这种不足在图像中比在文本、公众舆论和美国人口普查数据中要严重得多。最后,一项具有全国代表性的预注册实验表明,在谷歌上搜索职业图片而非文字描述会放大参与者信念中的性别偏见。

解决这种大规模转向视觉传播的社会影响,对于发展一个公平包容的互联网未来至关重要。

▲ Abstract:

Each year, people spend less time reading and more time viewing images, which are proliferating online. Images from platforms such as Google and Wikipedia are downloaded by millions every day, and millions more are interacting through social media, such as Instagram and TikTok, that primarily consist of exchanging visual content. In parallel, news agencies and digital advertisers are increasingly capturing attention online through the use of images, which people process more quickly, implicitly and memorably than text. Here we show that the rise of images online significantly exacerbates gender bias, both in its statistical prevalence and its psychological impact. We examine the gender associations of 3,495 social categories (such as ‘nurse’ or ‘banker’) in more than one million images from Google, Wikipedia and Internet Movie Database (IMDb), and in billions of words from these platforms. We find that gender bias is consistently more prevalent in images than text for both female- and male-typed categories. We also show that the documented underrepresentation of women online is substantially worse in images than in text, public opinion and US census data. Finally, we conducted a nationally representative, preregistered experiment that shows that googling for images rather than textual descriptions of occupations amplifies gender bias in participants’ beliefs. Addressing the societal effect of this large-scale shift towards visual communication will be essential for developing a fair and inclusive future for the internet.

 
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