全文获取类型
收费全文 | 45470篇 |
免费 | 4461篇 |
国内免费 | 6447篇 |
专业分类
化学 | 30532篇 |
晶体学 | 1367篇 |
力学 | 1144篇 |
综合类 | 290篇 |
数学 | 8931篇 |
物理学 | 14114篇 |
出版年
2024年 | 33篇 |
2023年 | 450篇 |
2022年 | 662篇 |
2021年 | 987篇 |
2020年 | 1388篇 |
2019年 | 1439篇 |
2018年 | 1227篇 |
2017年 | 1501篇 |
2016年 | 1545篇 |
2015年 | 1456篇 |
2014年 | 2124篇 |
2013年 | 3526篇 |
2012年 | 2343篇 |
2011年 | 3588篇 |
2010年 | 2826篇 |
2009年 | 3262篇 |
2008年 | 3409篇 |
2007年 | 3265篇 |
2006年 | 2920篇 |
2005年 | 2682篇 |
2004年 | 2324篇 |
2003年 | 1881篇 |
2002年 | 1653篇 |
2001年 | 1242篇 |
2000年 | 1115篇 |
1999年 | 954篇 |
1998年 | 781篇 |
1997年 | 717篇 |
1996年 | 670篇 |
1995年 | 701篇 |
1994年 | 640篇 |
1993年 | 557篇 |
1992年 | 448篇 |
1991年 | 330篇 |
1990年 | 227篇 |
1989年 | 218篇 |
1988年 | 176篇 |
1987年 | 129篇 |
1986年 | 128篇 |
1985年 | 135篇 |
1984年 | 111篇 |
1983年 | 72篇 |
1982年 | 99篇 |
1981年 | 85篇 |
1980年 | 90篇 |
1979年 | 64篇 |
1978年 | 63篇 |
1977年 | 46篇 |
1976年 | 31篇 |
1973年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
101.
本文探究了多个影响因素对大豆脲酶诱导碳酸钙沉淀(SICP)的影响,以优选出主要影响因素并提供其最佳范围。首先分析了脲酶浓度和温度对脲酶活性的影响;之后通过正交实验设计,进行25种工况的SICP水溶液实验,研究不同因素组合下Ca2+利用率的变化规律;最后借助扫描电子显微镜观测不同工况下生成碳酸钙的形态。结果表明:低温有利于脲酶的保存及活性发挥,5 ℃时脲酶活性能保持21 d以上;同一温度下,脲酶浓度越大,脲酶初始活性越高,脲酶完全失活所需时间越短。pH值、脲酶与胶结液体积比是影响Ca2+利用率的主要因素。为达到较高的Ca2+利用率,脲酶和胶结液最佳体积比为1,氯化钙与尿素最佳浓度比为1.5,Ca2+最佳浓度为1 mol/L。当脲酶浓度较低时生成的六面体状碳酸钙较多;随着脲酶浓度的增大,所沉淀的碳酸钙向球形转变。大豆中富含的天冬氨酸是控制碳酸钙形态的重要因素。 相似文献
102.
采用湿化学法合成了Eu原子掺量5%的Lu2O3陶瓷前驱体,通过SEM、XRD研究了煅烧前后前驱体和1 100 ℃煅烧4 h后粉体的形貌、结构以及物相。结果表明煅烧后的粉体为纳米类球形、高分散且结晶性良好的颗粒。颗粒尺寸为68.5 nm。使用煅烧后的粉体为原料,在1 650 ℃真空烧结30 h制备了高透过率的Eu:Lu2O3陶瓷,晶粒尺寸为46 μm,在611 nm处的直线透过率可以达到66.3%。此外对陶瓷的吸收曲线、光致激发和发射光谱特性以及X射线激发发射光谱进行研究。可观察到,Eu:Lu2O3陶瓷存在基质和激活离子两类吸收,光致发光光谱和X射线激发发射光谱均可以看出Eu:Lu2O3陶瓷存在极强的5D0→7F2跃迁发光,位于611 nm处。对比商业的BGO单晶的X射线发射光谱,可得本实验中制备的陶瓷的光输出为85 000 ph/MeV。Eu:Lu2O3陶瓷本身有着高X射线以及高能粒子的阻止能力,结合高光输出特性,表明Eu:Lu2O3陶瓷在X射线成像等领域具有巨大的潜在应用价值。 相似文献
103.
通过对角化364×364完全能量矩阵的理论方法,对掺杂在Bi4Ge3O12晶体中的Er3+的Stark能级和EPR参数进行了研究,同时,定量分析了高阶晶体场混合效应和J-J混合效应对EPR g因子的影响。研究结果表明:对Er3+来说,最主要的J-J混合效应来源于多重态谱项2K15/2,其对EPR g因子的贡献约占2.5%,而最主要的高阶晶体场混合效应来源于第一激发多重态4I13/2和基态多重态4I15/2之间的晶体场混合,其对各向异性g因子中g⊥的贡献大致是g//的两倍(即g⊥约占 0.21%,g//约占0.092%),其他更高阶的晶体场混合和J-J混合效应可以忽略不计。因此,对于Er3+掺杂的络合物系统来说,只考虑基态多重态4I15/2对EPR g因子的贡献应该是一个很好的近似。 相似文献
104.
Many efforts are currently devoted to improving the stability and crystallinity of imine-based two-dimensional (2D) covalent organic frameworks (COFs) given their wide range of potential applications. The variation in the relative orientations of the imine bonds has been found to be a critical factor that impacts the stacking of the 2D COF layers, leads to the formation of isomer structures, and influences the crystallinity of the final product. Most investigations to date have focused only on the structural properties, while the role of the imine orientations on the electronic properties has not been studied systematically. Here, we explore this effect by examining how the electronic band structures, electronic couplings, and effective masses evolve when considering four isomeric structures of an imine-linked tetraphenyl-pyrene naphthalene-diimide COF. Our results provide an understanding of the impact of the imine orientations and how they need to be controlled to realize COF inter-layer stackings that can lead to efficient cross-plane electron transport. They can be used to guide the design and synthesis of imine-based COFs for applications where charge transport needs to be optimized. 相似文献
105.
《Arabian Journal of Chemistry》2021,14(11):103421
Processing of Carapa guianensis seeds to obtain oil on an industrial scale generates a significant amount of by-product, approximately 66% w/w, which is called cake and is a potential source of biomolecules, including simple phenolic structures. For this reason, studies were carried out on the chemical profiles of hydrolyzed extract from this agro-industrial by-product through High Performance Thin-Layer Chromatography (HPTLC) and Gas Chromatography coupled to Mass Spectrometry (GC–MS). These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product. The sample antioxidant capacity was determined by methods of 2,2-diphenyl-1-picrylhydrazyl (DPPH)and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS+) radicals direct sequestration. The hydrolyzed fraction showed a total of 63.47% in the relative abundance of the total of compounds, standing out: p-hydroxybenzoic acid (39.19%) and protocatechuic acid (3,4-dihydroxybenzoic acid) (5.62%), both from hydroxybenzoic acids and 3-(3,4-dihydroxyphenyl)lactic acid, (7.76%) hydroxycinnamic acids derivatives. In these results, the fraction rich in simple phenolic acids was obtained, attributing the prominent behavior of this matrix antioxidant activity, expressed by (IC50: of 16.42 µg/mL and 6.52 µg/mL for DPPH and ABTS+ radicals, respectively). The research demonstrated an alternative to applicability that involves sustainability from agro-industrial. These techniques were used to detect metabolic classes and/or groups, and to identify, for the first time, thirteen simple phenolic acids in this by-product, generating a process capable of converting biomass into a bioproduct, consisting of bioactive compounds, in addition to adding value to the industrial chain. 相似文献
106.
107.
108.
Six new carbonate-bridged Zn2Ln2 cluster complexes derived from salen-type Schiff base ligands [H2La = N, N′-bis(3-methoxysalicylidene)-1,3-diaminopropane and H2Lb = N, N′-bis(3-methoxysalicylidene)- 1,2-diaminoethane] have been synthesized. The bis-imine chain in Schiff base ligands have an obvious influence on the cluster complexes' structures, magnetic and luminescence properties. The carbonate bridging ligand exactly comes from autoimmobilization of carbon dioxide, which may mediate ferromagnetic coupling between Ln3+ ions, favoring magnetocaloric effects and single molecule magnet (SMM) properties. Complexes Zn2Dy2(μ3-CO3)2(La)2(NO3)2(MeOH)2 ( 1 ) and [Zn2Dy2(μ3-CO3)2(Lb)2(NO3)2]·2MeOH ( 2) show field-induced SMM properties; complexes Zn2Tb2(μ3-CO3)2(La)2(NO3)2(MeOH)2 ( 3 ) and [Zn2Tb2(μ3-CO3)2(Lb)2(NO3)2]·2MeOH ( 4 ) display both luminescence and field-induced SMM behaviors; while complexes [Zn2Gd2(μ3-CO3)2(La)2(NO3)2]·2MeOH ( 5 ) and [Zn2Gd2(μ3-CO3)2(Lb)2(NO3)2]·2MeOH ( 6 ) exhibit medium magnetic entropy changes, which are candidates for cryogenic molecular magnetic refrigerants. 相似文献
109.
In this study, a new, economical and green method was reported for synthesizing Fe3O4@CuO nanoparticles without adding any surfactants using Euphorbia polygonifolia extract as a renewable, mild and safe reducing agent and effective stabilizer. The green synthesized NPs were analyzed by various methods such as XRD, FESEM, FT-IR, EDS, VSM, UV–visible, DRS, BET and TGA-DTA. Based on the BET analysis, the Fe3O4@CuO NP had a surface area of 69.20 m2/g. The FTIR analysis verified the existence of different functional groups of phytochemicals from Euphorbia polygonifolia extract which were accountable for the NPs formation. The catalytic performance of the catalyst for the degradation of metronidazole, ciprofloxacin and cephalexin antibiotics was examined in aqueous mediums at room temperature. The results showed an extraordinary catalytic performance, easy reusability and long-term stability of the composite for reducing antibiotic pollution. In this process, the effects of environmental conditions such as initial pH of the environment, initial concentration of antibiotics, the concentration of modified photocatalyst and reaction time were studied. According to the results, at the optimal conditions, the highest removal efficiency for metronidazole, ciprofloxacin and cephalexin antibiotics using Fe3O4@CuO nanoparticles, were 89%, 94%, and 96%, respectively. Also, it was observed that even after recycling, the NPs presents good nanocatalytic stability for the degradation of antibiotics. Using the NPs for five cycles did not significantly alter the photocatalyst efficiency, showing that the photocatalytic stability of the NPs was excellent. 相似文献
110.
Mahsa Kiani Mojtaba Bagherzadeh Soraia Meghdadi Farzaneh Fadaei-Tirani Maryam Babaie Kurt Schenk-Joß 《应用有机金属化学》2020,34(11):e5911
The new Co(II) - carboxamide complex ( 1 ) and Co3O4 nanoparticles ( 2 ), by way of thermal decomposition of ( 1 ) have been efficiently synthesised in the environment-friendly. X-ray diffraction reveals a slightly distorted octahedral coordination of cobalt (four nitrogens and two oxygens) in ( 1 ) and regular octahedral or tetrahedral ones (oxygens only) in ( 2 ). The investigation of ( 1 ) and ( 2 ) in the Mizoroki-Heck and epoxidation of alkens reactions showed them both to be powerful, green and inexpensive catalysts. 相似文献