首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   109852篇
  免费   12783篇
  国内免费   14376篇
化学   84695篇
晶体学   2428篇
力学   4218篇
综合类   587篇
数学   9538篇
物理学   35545篇
  2023年   1166篇
  2022年   1913篇
  2021年   3008篇
  2020年   3964篇
  2019年   3550篇
  2018年   2940篇
  2017年   3487篇
  2016年   4343篇
  2015年   4238篇
  2014年   5346篇
  2013年   8983篇
  2012年   7828篇
  2011年   7408篇
  2010年   5969篇
  2009年   7442篇
  2008年   7406篇
  2007年   7740篇
  2006年   6717篇
  2005年   5436篇
  2004年   5022篇
  2003年   4457篇
  2002年   3706篇
  2001年   2855篇
  2000年   2474篇
  1999年   2130篇
  1998年   1882篇
  1997年   1635篇
  1996年   1585篇
  1995年   1494篇
  1994年   1424篇
  1993年   1124篇
  1992年   1120篇
  1991年   697篇
  1990年   542篇
  1989年   418篇
  1988年   410篇
  1987年   371篇
  1986年   351篇
  1985年   472篇
  1984年   346篇
  1983年   196篇
  1982年   370篇
  1981年   535篇
  1980年   484篇
  1979年   510篇
  1978年   399篇
  1977年   298篇
  1976年   256篇
  1975年   93篇
  1973年   176篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
1.
2.
Preparation of pea protein isolate-xylan (PPI-X) conjugate-stabilized nanoemulsions using ultrasonic homogenization and the corresponding structure and environmental stability were investigated in this study. Conditions used to prepare nanoemulsions were optimized using a response surface methodology as follows: protein concentration 8.86 mg/mL, ultrasound amplitudes 57 % (370.5 W), and ultrasound time 16 min. PPI-X conjugate-stabilized nanoemulsions formed under these conditions exhibited less mean droplet size (189.4 ± 0.45 nm), more uniform droplet distribution, greater absolute value of zeta-potential (44.8 ± 0.22 mV), and higher protein adsorption content compared with PPI-stabilized nanoemulsions. PPI-X conjugate-stabilized nanoemulsions also exhibited even particle distribution and dense network structure, which might be reasons for the observed high interfacial protein adsorption content of conjugate-stabilized nanoemulsions. Moreover, better stability against environmental stresses, such as thermal treatment, freeze–thaw treatment, ionic strength and type, and storage time was also observed for the conjugate-stabilized nanoemulsions, indicating that this type of nanoemulsions possess a potential to endure harsh food processing conditions. Therefore, results provide a novel approach for the preparation of protein-polysaccharide conjugate-stabilized nanoemulsions to be applied as novel ingredients to meet special requirements of processed foods.  相似文献   
3.
The process of the formation of silver nanoparticles (AgNPs) via the method of galvanic replacement (GR) of Ag+ with aluminum powder in sodium polyacrylate (NaPA) solutions in the ultrasonic (US) field has been studied. It was observed, that the yellow colloidal solutions of stabilized AgNPs with the absorption maximum at ∼ 410 nm were obtained under the application of US power by 20 W and frequency by 20 kHz in the wide range of AgNO3 and NaPA concentrations (0.1 – 0.5 mM and 0.5 – 5.0 g/L respectively) at 25 0C. It was shown, that the GR process under US field occurs without of the significant induction period. Using the UV–vis spectroscopy the kinetics of AgNPs formation has been studied and it was observed the first order kinetics with respect to Ag+ ions both for the nucleation and growth processes. It was found that observable rate constants of nucleation are close for the all experimental conditions but the observable rate constants of growth decreased with increasing of initial concentration of AgNO3. Based on the obtained kinetic data it was proposed a mechanism of the formation of AgNPs consisted of the following two main stages: 1) the nucleation with the formation of primary nanoclusters (AgNCs) on aluminum surface followed by their ablation from the surface of the sacrificial metal by ultrasound into bulk of solution; 2) the transformation of AgNCs in AgNPs via growth from the Al surface and / or agglomeration of AgNCs. Using TEM it was found that the size of obtained AgNPs does not exceed of 25 nm and slightly depends on the initial concentrations of precursors. High antimicrobial activity of obtained colloidal solutions against gram-negative and gram-positive bacteria as well as against fungi was observed.  相似文献   
4.
This study aimed to investigate the impact of multi-mode S-type ultrasound treatment on the protein extraction level of walnut dregs. The structural properties of the walnut protein (WP) were characterized, and the correlation between protein structure and extraction level was analyzed. The in situ real-time monitoring model for the ultrasound-assisted WP extraction process was established by a miniature fiber near-infrared (NIR) spectrometer. Results showed that the protein yield, purity, and comprehensive extraction index (CEI) of extracted WP were 71.07 %, 72.69 %, and 71.72, respectively, under optimal conditions (dual-frequency 20/28 kHz, ultrasonic treatment duration 30 min, and ultrasound power density 120 W/L). The secondary structure of extracted WP displayed that the proportion of α-helix and β-sheet reduced, while the contents of β-turn and random coil increased after ultrasonic treatment. Besides, sonication decreased the disulfide bond content and increased free sulfhydryl (-SH) and surface hydrophobicity compared to the control. The microstructures of WP confirmed that appropriate sonication could unfold the protein aggregates and reduce the particle size. The extraction level of WP is positively correlated with the -SH content (p < 0.01). The quantitative prediction model of Si-PLS for -SH content in the ultrasound-assisted WP extraction process was established and performed a good correction and prediction performance (Rc = 0.9736; RMSECV = 0.446 μmol/L; Rp = 0.9342; RMSEP = 0.807 μmol/L). This study exploited a high-efficiency way for the WP extraction industry, and provided theoretical support for the development of the intelligent system in industrial protein extraction process.  相似文献   
5.
采用水热-煅烧法制备Cd2SnO4,之后通过超声混合法得到一系列MoS2/Cd2SnO4复合材料。采用X射线衍射、扫描电子显微镜、X射线光电子能谱对Cd2SnO4和一系列MoS2/Cd2SnO4复合材料进行结构和形貌的表征。研究了MoS2掺杂量对于MoS2/Cd2SnO4复合材料的气敏性能影响。实验结果表明,当MoS2与Cd2SnO4的质量比为2.5%,MoS2/Cd2SnO4复合材料制备的气敏元件在170 ℃时对浓度为100 μL·L-1的甲醛气体的灵敏度为40.0,最低检测限为0.1 μL·L-1。  相似文献   
6.
ortho-Alkynylarylketone derivatives were employed as key precursors for a one-pot synthesis of arylnaphthalenelactone and furanonaphthol libraries. In this work, we discovered a cost-effective protocol to prepare arylnaphthalenelactones in one-pot using inexpensive starting material, malonate ester, which was conveniently functionalized leading to a variety of structures. Moreover, we also found an unexpected oxy-dearylation reaction which could be used to synthesize furanonaphthol analogs. These novel methods could be applied to a broad range of substrates to give the corresponding products in up to 83% yield. Notably, these classes of compounds exhibited more significant inhibition against protein-tyrosine phosphatase 1B (PTP1B) enzyme than a standard compound, ursolic acid.  相似文献   
7.
A magnetically separable palladium nanocatalyst has been synthesized through the immobilization of palladium onto 3-aminopropylphenanthroline Schiff based functionalized silica coated superparamagnetic Fe3O4 nanoparticles. The nanocatalyst (Fe3O4@SiNSB-Pd) was fully characterized using several spectroscopic techniques, such as FT-IR, HR-SEM, TEM, XRD, ICP, and XPS. The microscopic image of Fe3O4 showed spherical shape morphology and had an average size of 150 nm. The Pd-nanoparticles exhibited an average size 3.5 ± 0.6 nm. The successful functionalization of Fe3O4@SiNSB-Pd was identified by FT-IR spectroscopy and the appearance of palladium species in Fe3O4@SiNSB-Pd was confirmed by XRD analysis. While XPS has been utilized for the determination of the chemical oxidation state of palladium species in Fe3O4@SiNSB-Pd. Several activated and deactivated arene halides and olefines were employed for Mizoroki-Heck cross-coupling reactions in the presence of Fe3O4@SiNSB-Pd, each of which produced the respective cross-coupling products with excellent yields. The Fe3O4@SiNSB-Pd shows good reactivity and reusability for up to seven consecutive cycles.  相似文献   
8.
Ti2O3 thin films have been prepared through atomic layer deposition and subjected to electrical resistivity measurements as a function of temperature. The as-prepared films were stable for up to three weeks. In Ti2O3 thin films, the insulator-metal transition is observed at ∼80 K, with nearly 3–4 orders of magnitude change in resistivity. The anomalous increase in electrical resistivity in the films is in accordance with the two-band model. However, the energy interval between the bands depending on the crystallographic c/a ratio leads to a change in electrical resistivity as a function of temperature.  相似文献   
9.
One of the most common problems in wounds is delayed healing and complications such as infection. Therefore, the need for novel materials accelerates the healing of wounds especially abdominal wounds after surgery besides high efficiency and safety is mandatory. The rate of wound healing, anti-inflammatory and biocompatibility of Zn-Al LDH (Zn-Al layer double hydroxide) alone and loaded with Curcumin (Zn-Al LDH/Curcumin) was screened via in-vivo assays through intramuscular implantation in rat abdominal wall with intact peritoneum cavity. The implanted drugs were formed through Curcumin loaded into LDH of Zn-Al with drug release of 56.78 ± 1.51% within 24 h. The synthesized nanocomposite was characterized by (TGA/DTA) thermal analysis, (XRD) X-ray diffraction, (FESEM) Field emission scanning electron microscopy, (HRTEM) high resolution transmission electron microscope, energy dispersive X-ray (EDX) and low-temperature N2 adsorption, pore volume and average pore size distribution. The integrity of blood circulation, inflammatory signs, wound healing rate, capacity of tissue integration, antigenicity and composite biocompatibility, auto fluorescence ability of collagen bundles and the tensile strength of the muscle were assessed histopathologically after 7 and 30 days’ post-implantation. Excellent wound healing ability was achieved with shortest length between the wound gap edges and higher tensile strength of the muscle. Besides emit florescence very well followed by good healing and tensile muscles strength in Curcumin while very low strength with scar formation in Zn-Al LDH/Curcumin in both acute and chronic wound. No signs of inflammation in Curcumin & Zn-Al LDH. No vessels obstruction or bleeding observed in both Zn-Al LDH and Curcumin more than Zn-Al LDH/Curcumin and control which examined through candling. Good healing & infiltrated immune cells in same groups through histopathological examination. This work supports the anti-inflammatory, wound healing and biocompatibility of both LDH and Curcumin with living matter, increasing their biomedical applications in this era with safety and increasing efficacy with prolonged drug release.  相似文献   
10.
Datura metel L. is an important medicinal plant of Solanaceae family which has extensive pharmacological properties. The present investigation was aimed to identify the presence of phytoconstituents and assess in vitro antibacterial, anti-biofilm, anti-diabetic, anti-inflammatory, antioxidant, cytotoxicity, and wound healing efficacy of D. metel leaves extract. Among different solvent extracts, methanolic extract showed higher amount of phenolic (124.61 ± 0.68 mg GAE/g), alkaloid (88.77 ± 1.01 mg AE/g), flavonoids (42.24 ± 0.18 mg QE/g), and tannins contents (38.72 ± 0.51 mg GAE/g). The extract exhibited not only significantly (P < 0.05) different antibacterial activities against pathogens tested but also showed maximum biofilm inhibition of 94, 88, and 92% against B. subtilis, MRSA, and E. coli, respectively. Anti-diabetic assay depicted 22.55 ± 0.62–79.41 ± 1.13% and 24.31 ± 1.47–72.59 ± 0.22% of α-amylase and α-glucosidase inhibition abilities of methanolic extract, respectively at varied concentrations. The methanolic extract showed potential anti-inflammatory effect (P < 0.05) by showing 28.11 ± 0.13, 34.94 ± 1.11, 55.73 ± 0.42, 73.28 ± 0.72, and 92.62 ± 1.33% of inhibition of protein denaturation at different concentrations with an IC50 value of 52.45 µg/mL. The extract revealed significant (P < 0.05) rate of ABTS scavenging, DPPH degradation, and reducing power assay in a concentration dependent manner. The cytotoxicity assay was demonstrated on L929 mouse fibroblast cell line and found > 90% of cell viability in the presence of methanolic extract, thereby indicating its non-toxicity effect. Wound healing assay indicated that methanolic extract at 50 µg/mL closed 100% of wound gap after 24 h with high rate of migration and proliferation. Furthermore, GC–MS chromatogram revealed the presence of several components in methanolic extract, including neophytadiene, hexadecanoic acid, and hentriacontane as principal phytoconstituents. In conclusion, methanolic extract of D. metel leaves could be used as potent therapeutic agent not only for treating metabolic diseases but also superficial chronic diabetic wounds.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号