全文获取类型
收费全文 | 36764篇 |
免费 | 6595篇 |
国内免费 | 4594篇 |
专业分类
化学 | 26674篇 |
晶体学 | 532篇 |
力学 | 2041篇 |
综合类 | 356篇 |
数学 | 4066篇 |
物理学 | 14284篇 |
出版年
2024年 | 29篇 |
2023年 | 669篇 |
2022年 | 762篇 |
2021年 | 1065篇 |
2020年 | 1409篇 |
2019年 | 1522篇 |
2018年 | 1286篇 |
2017年 | 1155篇 |
2016年 | 1695篇 |
2015年 | 1661篇 |
2014年 | 2060篇 |
2013年 | 2754篇 |
2012年 | 3521篇 |
2011年 | 3576篇 |
2010年 | 2608篇 |
2009年 | 2491篇 |
2008年 | 2784篇 |
2007年 | 2352篇 |
2006年 | 2347篇 |
2005年 | 1988篇 |
2004年 | 1578篇 |
2003年 | 1169篇 |
2002年 | 1137篇 |
2001年 | 913篇 |
2000年 | 722篇 |
1999年 | 731篇 |
1998年 | 489篇 |
1997年 | 442篇 |
1996年 | 434篇 |
1995年 | 384篇 |
1994年 | 355篇 |
1993年 | 321篇 |
1992年 | 210篇 |
1991年 | 236篇 |
1990年 | 179篇 |
1989年 | 135篇 |
1988年 | 119篇 |
1987年 | 65篇 |
1986年 | 75篇 |
1985年 | 72篇 |
1984年 | 62篇 |
1983年 | 39篇 |
1982年 | 41篇 |
1981年 | 38篇 |
1980年 | 31篇 |
1979年 | 22篇 |
1978年 | 20篇 |
1977年 | 19篇 |
1976年 | 18篇 |
1975年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Science China Mathematics - Linear factor models are familiar tools used in many fields. Several pioneering literatures established foundational theoretical results of the quasi-maximum likelihood... 相似文献
2.
当煤层上方为坚硬顶板时,在工作面回采期间容易出现大面积悬顶,导致巷旁充填体出现大变形破坏,极大威胁采煤安全.为解决这一问题,以某矿1205工作面沿空留巷为工程背景,分析了爆炸围岩裂隙范围和坚硬顶板破断特征,建立了切顶卸压围岩结构力学模型,研究表明,相邻炮孔爆炸后,炮孔间裂隙互相贯穿,降低了顶板强度,同时当切顶角为15°时,巷道稳定性较好.工程实践结果表明:采用爆破切顶技术后,顶板下沉量减小了62.3%,较好地保证了围岩稳定性. 相似文献
3.
4.
硒是动植物及人体生长必需的十五种微量元素之一,具有清除体内自由基、抗氧化、增强免疫力等功能,但其安全剂量的范围却很窄。利用扫描电子显微镜(SEM)和X射线衍射仪(XRD)对湿法球磨制备的硫铁矿形貌进行了表征。SEM观测发现加乙醇助磨后的硫铁矿为粒径大小较均匀的球形颗粒团聚体,粒径范围在17~200 nm之间,平均粒径138 nm。XRD衍射图谱中的特征峰与FeS2衍射图谱中各峰位置基本一致,因此判定硫铁矿中主要化学组分为FeS2,且图谱中基本没有杂峰,表明制备过程中并未混入杂质,样品纯度较高。实验结果表明,该法制备的硫铁矿具有颗粒粒径小、比表面积大、反应活性高等优点。研究中利用X射线光电子能谱仪(XPS)对硫铁矿去除水体中SeO2-3的机理进行了研究。研究结果表明, (1)在较为广泛的实验pH范围(pH 2.2~11.5),硫铁矿均能有效去除水体中SeO2-3,去除效率(除pH值7.8以外)均达到90%以上;(2)硫铁矿与SeO2-3发生反应后,其主要组成元素的XPS特征峰结合能有所减小,表明硫铁矿表面发生了一定化学变化;(3)酸碱环境下硫铁矿去除SeO2-3的机理不完全相同,酸性环境下,硫铁矿对SeO2-3的去除是单纯的氧化还原过程,即硫铁矿中被酸活化的S2-2将SeO2-3还原为单质Se(0),并且酸性越强,SeO2-3去除效果越好;碱性环境下,SeO2-3的去除过程中氧化还原与络合反应并存,硫铁矿表面有络合态Fe(OH)SeO3和单质Se(0)两种存在形态,且碱性越强,络合态Fe(OH)SeO3含量越高。以上研究结果为硫铁矿去除固定水体和土壤中以SeO2-3为代表的可变价金属阴离子提供重要理论依据和应用基础。 相似文献
5.
Haicun Yang Shuipi Cai Yu Jiang Zheng Cao Wenzhong Ma Fanghong Gong Guoliang Tao Chunlin Liu 《Journal of polymer science. Part A, Polymer chemistry》2022,60(10):1571-1587
The commonly used multi-center initiation methods always lead to the formation of quantities of homopolymer in the surface tailoring based on reverse atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain-transfer (RAFT) polymerization. In this study, a monocenter redox pair constructed of silica bearing tert-butyl hydroperoxide groups and ascorbic acid (SiO2-TBHP/AsAc) was applied to substitute the commonly used initiation method of R-supported RAFT grafting polymerization. All the propagating radicals were restricted on the surface of solid particles during the whole procedure theoretically, resulting in a higher grafting efficiency of 95.1% combined with the “controllable” feature at 10 h. This redox pair was also used to initiate the reverse ATRP in miniemulsion successfully with a grafting efficiency of 86.3% at 10 h. The grafting efficiency obtained under this monocenter initiation method was significantly higher than that of the frequently reported surface modification by reverse ATRP and RAFT polymerization. In addition, the high-efficient surface tailoring was traced and confirmed by nuclear magnetic resonance, Fourier transform infrared, X-ray photoelectron spectroscopy, thermogravimetric analysis, transmission electron microscopy, and other analysis tests. The advantage of this monocenter redox pair will open a new avenue for the potential “high-efficient” surface tailoring of various materials. 相似文献
6.
Nonlinear Dynamics - This paper proposes an optimal parameter design of control scheme for mechanical systems by adopting the Stackelberg game theory. The goal of the control is to drive the... 相似文献
7.
Hanghang Zhou Dr. Ying Luo Dr. Jun Xu Dr. Lidan Deng Prof. Dr. Zheng Wang Prof. Dr. Hong He 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(68):e202202044
Cubic-like CaTaO2N photocatalysts with high crystallinity and uniform particle size were successfully prepared by the flux-assisted nitridation method. The growth of CaTaO2N single crystals under different synthesis conditions was systematically investigated to understand the effects of the crystallinity and optical property on photocatalytic performance of CO2 reduction. Moreover, the modification of CaTaO2N single crystals with core-shell Ni−Ag bicomponent cocatalyst by two-step decoration process gave a 2.4 times higher amount of CO evolution than the deposition of sole Ag cocatalyst, because of the synergistic effects of bicomponent cocatalyst on the interfacial electron transfer and surface catalytic process. This study provides a valuable way to construct high-crystalline photocatalysts with effective bicomponent cocatalyst for visible-light-driven CO2 reduction with H2O. 相似文献
8.
Guo Jiayun Ma Dongge Sun Fulin Zhuang Guilin Wang Qi Al-Enizi Abdullah M. Nafady Ayman Ma Shengqian 《中国科学:化学(英文版)》2022,65(9):1704-1709
Science China Chemistry - The heterojunction constructed of covalent organic frameworks (COFs) with adjustable structure and other photocatalysts has great potential in the field of photocatalysis.... 相似文献
9.
A capillary electrophoresis-acid barrage stacking online enrichment method has been established to detect the four isoflavones which are Daidzein, Genistein, Formononetin, and Biochanin A. The proposed method was optimized using a single factor alternative method, and the optimal conditions obtained from the optimization were: the BGE was 25 mM borax and 2 mM β-cyclodextrin, the applied separation voltage was 20 kV, and the detection wavelength was 260 nm. The time ratio of the injection of sample and the injection of acid was 150 s:20 s, and the acid used was 250 mM acetic acid. The sample solvent used was 60% v/v acetonitrile. The established method had the enrichment factor of these four isoflavones at 24.5, 32.0, 29.2, and 33.7, respectively, LOD and LOQ are as low as nanograms per milliliter. Finally, the CE-acid barrage stacking method was successfully applied to the determination of four isoflavones in rat plasma and red clover extract, verifying the applicability and feasibility of the method. 相似文献
10.
Dr. Zaifa Shi Dr. Yihuang Jiang Dr. Jingxiong Yu Prof. Shanjun Chen Prof. Jun Chen Prof. Zichao Tang Prof. Lansun Zheng 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(8):e202103546
At present, the reactivity of cyclic alkanes is estimated by comparison with acyclic hydrocarbons. Due to the difference in the structure of cycloalkanes and acycloalkanes, the thermodynamic data obtained by analogy are not applicable. In this study, a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer (MB-VUV-PI-TOFMS) was applied to study the low-temperature oxidation of cyclopentane (CPT) at a total pressure range from 1–3 atm and low-temperature range between 500 and 800 K. Low-temperature reaction products including cyclic olefins, cyclic ethers, and highly oxygenated intermediates (e. g., ketohydroperoxide KHP, keto-dihydroperoxide KDHP, olefinic hydroperoxides OHP and ketone structure products) were observed. Further investigation of the oxidation of CPT – electronic structure calculations – were carried out at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+ G(d,p) level to explore the reactivity of O2 molecules adding sequentially to cyclopentyl radicals. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPT radicals with O2 is HO2 elimination yielding cyclopentene. The pathways of second and third O2 addition – the dissociation of hydroperoxide – were further confirmed. The results of this study will develop the low-temperature oxidation mechanism of CPT, which can be used for future research on accurately simulating the combustion process of CPT. 相似文献