全文获取类型
收费全文 | 5804篇 |
免费 | 598篇 |
国内免费 | 452篇 |
专业分类
化学 | 1857篇 |
晶体学 | 37篇 |
力学 | 1427篇 |
综合类 | 138篇 |
数学 | 1257篇 |
物理学 | 2138篇 |
出版年
2023年 | 38篇 |
2022年 | 101篇 |
2021年 | 179篇 |
2020年 | 177篇 |
2019年 | 142篇 |
2018年 | 134篇 |
2017年 | 186篇 |
2016年 | 246篇 |
2015年 | 185篇 |
2014年 | 254篇 |
2013年 | 380篇 |
2012年 | 354篇 |
2011年 | 365篇 |
2010年 | 302篇 |
2009年 | 370篇 |
2008年 | 385篇 |
2007年 | 336篇 |
2006年 | 355篇 |
2005年 | 261篇 |
2004年 | 274篇 |
2003年 | 219篇 |
2002年 | 203篇 |
2001年 | 201篇 |
2000年 | 159篇 |
1999年 | 149篇 |
1998年 | 110篇 |
1997年 | 117篇 |
1996年 | 98篇 |
1995年 | 88篇 |
1994年 | 74篇 |
1993年 | 61篇 |
1992年 | 48篇 |
1991年 | 48篇 |
1990年 | 48篇 |
1989年 | 43篇 |
1988年 | 21篇 |
1987年 | 29篇 |
1986年 | 24篇 |
1985年 | 16篇 |
1984年 | 13篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 17篇 |
1980年 | 4篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1975年 | 2篇 |
1959年 | 1篇 |
1957年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有6854条查询结果,搜索用时 46 毫秒
51.
将交通需求级别进行划分后,通过数据包络分析法计算交通需求匹配度和交通运行效率,近乎绝对细化交通运行表现,依据具有时间轴的四维交通状况模拟建立了全新多维度的交通评价体系。此外,利用模糊层次分析法确定道路交通安全评价指标体系中各个指标的权重,同时作出基于差异驱动原理的综合评价。最后,将交通的污染指标提炼为机动车的排放指标和电动车的耗电指标来衡量交通工具对环境的影响。 相似文献
52.
《Comptes Rendus Chimie》2017,20(7):704-709
Aluminophosphate and silicoaluminophosphate molecular sieves with both five (AFI) and eleven (AEL) type structures are synthesized by hydrothermal crystallization at 473 K, using tripropylamine and dipropylamine as a structure-directing template. The as-prepared AFI and AEL sieves are characterized and then assessed as sorbents for uranium (VI) from radioactive effluents. The sorption process is used to reduce the volumes of effluents and convert them into a stable solid waste. The batch experimental studies are carried out to evaluate the AEL and AFI structure effect on the removal of uranium. The AlPO4-5, SAPO-5, AlPO4-11 and SAPO-11 are applied to radioactive effluents with different activities obtained from Nuclear Research Center of Draria, Algeria. Important decontamination factor values are obtained for AFI sorbents. Thermodynamic parameters, namely, the enthalpy (ΔH), entropy (ΔS) and free energy (ΔG) for each sorption process are calculated. The collected results indicated that sorbents are effective materials for the removal of uranium (VI) ions, the sorbent with AFI structure being a highly effective material for the removal of uranium (VI) ions from nuclear effluents. 相似文献
53.
《Macromolecular bioscience》2017,17(2)
An in situ‐forming gel system comprised of diblock copolymer formed from polyethylene glycol (PEG) and polycaprolactone (PCL) {MPEG‐b‐(PCL‐ran‐PLLA)} could be used in controlled drug delivery for tissue remodeling. The purpose of this study is to demonstrate favorable vocal folds (VF) regeneration by using MPEG‐b‐(PCL‐ran‐PLLA) diblock copolymers (C97L3; CL/LA ratio 97:3) incorporating hepatocyte growth factor (HGF). Gradual release of HGF from C97L3 is detected and biochemical properties of released HGF are maintained. A scar is made with microscissors on both VFs in 32 rabbits, followed by injection of HGF‐only, C97L3‐only, or HGF‐C97L3 composite gel in the left side VF, while the right side VF is left untreated. In vivo fluorescence live imaging system demonstrates that C97L3 enables the sustained release of injected HGF in the scarred VF for 12 weeks. The histological analysis shows increased glycosaminoglycan including hyaluronic acid accumulation and decreased collagen deposition. Videokymographic analysis shows more favorable vibrations of HGF‐C97L3 treated VF mucosa, compared to other treatment groups. In conclusion, the controlled HGF release helps to regulate extracellular matrix synthesis, and leads to the eventual functional improvement of the scarred VF.
54.
《Macromolecular bioscience》2017,17(3)
Producing meiosis‐competent germ cells (GCs) from embryonic stem cells (ESCs) is essential for developing advanced therapies for infertility. Here, a novel approach is presented for generation of GCs from ESCs. In this regard, microparticles (MPs) have been developed from alginate sulfate loaded with bone morphogenetic protein 4 (BMP4). The results here show that BMP4 release from alginate sulfate MPs is significantly retarded by the sulfated groups compared to neat alginate. Then, BMP4‐laden MPs are incorporated within the aggregates during differentiation of GCs from ESCs. It is observed that BMP4‐laden MPs increase GC differentiation from ESCs at least twofold compared to the conventional soluble delivery method. Interestingly, following meiosis induction, Dazl , an intrinsic factor that enables GCs to enter meiosis, and two essential meiosis genes (Stra8 and Smc1b ) are upregulated significantly in MP‐induced aggregates compared to aggregates, which are formed by the conventional method. Together, these data show that controlled delivery of BMP4 during ESC differentiation into GC establish meiosis‐competent GCs which can serve as an attractive GC source for reproductive medicine.
55.
David A. Towers 《代数通讯》2017,45(3):1284-1293
The purpose of this paper is to continue the study of chief factors of a Lie algebra and to prove a further strengthening of the Jordan–Hölder theorem for chief series. 相似文献
56.
Juping Tang 《代数通讯》2017,45(7):3017-3021
A subgroup A of a finite group G is called {1≤G}-embedded in G if for each two subgroups K≤H of G, where K is a maximal subgroup of H, A either covers the pair (K,H) or avoids it. Moreover, a subgroup H of G is called nearly m-embedded in G if G has a subgroup T and a {1≤G}-embedded subgroup C such that G?=?HT and H∩T≤C≤H. In this paper, we mainly prove that G is solvable if and only if its Sylow 3-subgroups, Sylow 5-subgroups and Sylow 7-subgroups are nearly m-embedded in G. 相似文献
57.
For any integer n ≥ 2, let P(n) be the largest prime factor of n. In this paper, we prove that the number of primes p ≤ x with P(p-1) ≥ p~c is more than(1-c + o(1))π(x) for 0 c 1/2. This extends a recent result of Luca, Menares and Madariaga for1/4≤ c ≤1/2. We also pose two conjectures for further research. 相似文献
58.
Mihaela Tertis Oana Hosu Bogdan Feier Andreea Cernat Anca Florea Cecilia Cristea 《Molecules (Basel, Switzerland)》2021,26(11)
Food safety and quality control pose serious issues to food industry and public health domains, in general, with direct effects on consumers. Any physical, chemical, or biological unexpected or unidentified food constituent may exhibit harmful effects on people and animals from mild to severe reactions. According to the World Health Organization (WHO), unsafe foodstuffs are especially dangerous for infants, young children, elderly, and chronic patients. It is imperative to continuously develop new technologies to detect foodborne pathogens and contaminants in order to aid the strengthening of healthcare and economic systems. In recent years, peptide-based sensors gained much attention in the field of food research as an alternative to immuno-, apta-, or DNA-based sensors. This review presents an overview of the electrochemical biosensors using peptides as molecular bio-recognition elements published mainly in the last decade, highlighting their possible application for rapid, non-destructive, and in situ analysis of food samples. Comparison with peptide-based optical and piezoelectrical sensors in terms of analytical performance is presented. Methods of foodstuffs pretreatment are also discussed. 相似文献
59.
Bao-Hong Lee Wei-Hsuan Hsu Chih-Yao Hou Hao-Yuan Chien She-Ching Wu 《Molecules (Basel, Switzerland)》2021,26(12)
Mango peels are usually discarded as waste; however, they contain phytochemicals and could provide functional properties to food and promote human health. This study aimed to determine the optimal lactic acid bacteria for fermentation of mango peel and evaluate the effect of mango peel on neuronal protection in Neuron-2A cells against amyloid beta (Aβ) treatment (50 μM). Mango peel can be fermented by different lactic acid bacteria species. Lactobacillus acidophilus (BCRC14079)-fermented mango peel produced the highest concentration of lactic acid bacteria (exceeding 108 CFU/mL). Mango peel and fermented mango peel extracts upregulated brain-derived neurotrophic factor (BDNF) expression for 1.74-fold in Neuron-2A cells. Furthermore, mango peel fermented products attenuated oxidative stress in Aβ-treated neural cells by 27%. Extracts of L. acidophilus (BCRC14079)-fermented mango peel treatment decreased Aβ accumulation and attenuated the increase of subG1 caused by Aβ induction in Neuron-2A cells. In conclusion, L. acidophilus (BCRC14079)-fermented mango peel acts as a novel neuronal protective product by inhibiting oxidative stress and increasing BDNF expression in neural cells. 相似文献
60.
Bernardita Cayupe Carlos Morgan Gustavo Puentes Luis Valladares Hctor Burgos Amparo Castillo Alejandro Hernndez Luis Constandil Miguel Ríos Patricio Sez-Briones Rafael Barra 《Molecules (Basel, Switzerland)》2021,26(12)
Prenatally malnourished rats develop hypertension in adulthood, in part through increased α1-adrenoceptor-mediated outflow from the paraventricular nucleus (PVN) to the sympathetic system. We studied whether both α1-adrenoceptor-mediated noradrenergic excitatory pathways from the locus coeruleus (LC) to the PVN and their reciprocal excitatory CRFergic connections contribute to prenatal undernutrition-induced hypertension. For that purpose, we microinjected either α1-adrenoceptor or CRH receptor agonists and/or antagonists in the PVN or the LC, respectively. We also determined the α1-adrenoceptor density in whole hypothalamus and the expression levels of α1A-adrenoceptor mRNA in the PVN. The results showed that: (i) agonists microinjection increased systolic blood pressure and heart rate in normotensive eutrophic rats, but not in prenatally malnourished subjects; (ii) antagonists microinjection reduced hypertension and tachycardia in undernourished rats, but not in eutrophic controls; (iii) in undernourished animals, antagonist administration to one nuclei allowed the agonists recover full efficacy in the complementary nucleus, inducing hypertension and tachycardia; (iv) early undernutrition did not modify the number of α1-adrenoceptor binding sites in hypothalamus, but reduced the number of cells expressing α1A-adrenoceptor mRNA in the PVN. These results support the hypothesis that systolic pressure and heart rate are increased by tonic reciprocal paraventricular–coerulear excitatory interactions in prenatally undernourished young-adult rats. 相似文献