全文获取类型
收费全文 | 241篇 |
免费 | 25篇 |
国内免费 | 11篇 |
专业分类
化学 | 64篇 |
力学 | 9篇 |
综合类 | 18篇 |
数学 | 102篇 |
物理学 | 84篇 |
出版年
2024年 | 1篇 |
2023年 | 8篇 |
2022年 | 9篇 |
2021年 | 22篇 |
2020年 | 11篇 |
2019年 | 12篇 |
2018年 | 8篇 |
2017年 | 9篇 |
2016年 | 11篇 |
2015年 | 11篇 |
2014年 | 9篇 |
2013年 | 25篇 |
2012年 | 6篇 |
2011年 | 14篇 |
2010年 | 13篇 |
2009年 | 7篇 |
2008年 | 9篇 |
2007年 | 11篇 |
2006年 | 8篇 |
2005年 | 12篇 |
2004年 | 12篇 |
2003年 | 4篇 |
2002年 | 6篇 |
2001年 | 3篇 |
2000年 | 5篇 |
1999年 | 10篇 |
1998年 | 1篇 |
1997年 | 3篇 |
1996年 | 3篇 |
1995年 | 2篇 |
1994年 | 1篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 1篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1982年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有277条查询结果,搜索用时 15 毫秒
121.
The Xidatan rock series consist of mylonite, gneiss and granite. The U-Pb age showsthat the granite was formed at 206 Ma, and the ~(40)Ar/~(39)Ar ages of the biotite display that the my-lonite was formed at 145 Ma when the Xidatan ductile shear zone occurred. At about 110-100 Mathe Xidatan Fault reactivated again. As a result, the biotite and muscovite of Xidatan rock seriesopened their argon isotope system. 相似文献
122.
周斌峰 《广东微量元素科学》2016,(5):23-26
目的对应用血清CEA、CA19-9联合检测方式对患有胃癌疾病的老年患者的病情实施诊断的临床价值进行研究。方法选择广丰县中医院收治的患有胃癌疾病和胃部良性疾病的老年患者各60例,再抽取同期接受健康体检的健康老年人资料60例,分别将其定义为研究1组、研究2组、对照组。采用化学发光法对三组研究对象的血清CEA、CA19-9水平进行测定,对比分析检测结果和两项指标的阳性率。结果研究1组的血清CEA、CA19-9水平明显高于对照组和研究2组,组间数据比较差异显著(P0.05);研究2组的血清CEA、CA19-9水平明显高于对照组,组间数据比较差异显著(P0.05)。研究1组的CEA、CA19-9水平检测阳性人数明显多于对照组和研究2组,组间数据比较差异显著(P0.05);研究2组的CEA、CA19-9水平检测阳性人数明显多于对照组,组间数据比较差异显著(P0.05)。结论患有胃癌疾病的老年患者的血清CEA、CA19-9水平会异常升高,临床上可以将其作为老年胃癌疾病诊断的重要依据。 相似文献
123.
A generic type age‐structured fishery population model consisting of two harvestable age classes is formulated. Optimal harvest rates are determined with uniform fishing mortality and perfectly selective fishing, respectively. Selectivity allows for differentiating the fishing mortality among different age classes. Sustainable yield–biomass functions are developed, and the maximum sustainable yield (MSY) solutions are found under both exploitation schemes. The gain of perfectly selective fishing over uniform (or biomass) fishing is examined under various assumptions, and it is proved that the benefit of selective harvesting increases when the harvestable fish population becomes more heterogeneous in terms of weights, or values. In contrast to the surplus production model, or Clark model, the analysis also demonstrates that MSY with different age classes is not purely a biological concept. 相似文献
124.
Fenglong Yang Jialin Qian Hongzhu Qu Zhimin Ji Junli Li Wenjing Hu Feng Cheng Xiangdong Fang Jiangwei Yan 《Electrophoresis》2023,44(9-10):835-844
The use of DNA methylation to predict chronological age has shown promising potential for obtaining additional information in forensic investigations. To date, several studies have reported age prediction models based on DNA methylation in body fluids with high DNA content. However, it is often difficult to apply these existing methods in practice due to the low amount of DNA present in stains of body fluids that are part of a trace material. In this study, we present a sensitive and rapid test for age prediction with bloodstains based on pyrosequencing and random forest regression. This assay requires only 0.1 ng of genomic DNA and the entire procedure can be completed within 10 h, making it practical for forensic investigations that require a short turnaround time. We examined the methylation levels of 46 CpG sites from six genes using bloodstain samples from 128 males and 113 females aged 10–79 years. A random forest regression model was then used to construct an age prediction model for males and females separately. The final age prediction models were developed with seven CpG sites (three for males and four for females) based on the performance of the random forest regression. The mean absolute deviation was less than 3 years for each model. Our results demonstrate that DNA methylation-based age prediction using pyrosequencing and random forest regression has potential applications in forensics to accurately predict the biological age of a bloodstain donor. 相似文献
125.
2002~2003年通过对精河甘家湖荒漠梭梭林国家级自然保护区大沙鼠的实地调查,共进行野外作业9次,设置100m×100m样方40个,采用样方面积夹子法,按5×20进行布夹,共布夹3964次,捕获大沙鼠(Rhomboms opimus)173只,其中雄94只,雌79只.通过对大沙鼠体重、体长、胴体重、胚胎及产仔情况、性腺发育状况的分析与比较,我们利用差异显著的胴体重为标准,将其划分为5个年龄组.其中雌鼠:1龄组幼体30~67g,2龄组亚成体62-85g,3龄组成体1组85~113g,4龄组成体2组113~147g,5龄组老体139~173g.雄鼠:1龄组幼体33~84g,2龄组亚成体61~102g,3龄组成体1组102~158g,4龄组成体2组113~153g,5龄组老体131~193g. 相似文献
126.
以启发式科学写作策略框架指导下的教学活动为基础,结合化学实验教学的特点,构建了“五阶段”化学实验教学模式,该模式下的教学过程分为5个阶段,分别为任务准备、实验参与、合作协商、探究反思、完成书面写作,将学生的论证活动与实验活动相联系,促进学生的实验论证,结合写作促进对论证过程的反思,进一步达成对科学本身和实验原理的理解。并且结合高中化学“碱性锌锰电池”实验案例,分析启发式科学写作策略指导下的教学实施,启迪中学教师展开该策略的教学。 相似文献
127.
Gwi-Taek Jeong Don-Hee Park Hwa-Won Ryu Baik Hwang Je-Chang Woo 《Applied biochemistry and biotechnology》2004,116(1-3):1193-1203
Plants have a potential to produce a large number of important metabolites such as pharmaceuticals, food additives, pigments,
flavors, fragrances, and fine chemicals. Large-scale plant cell and tissue cultures for producing useful products has been
considered an attractive alternative to whole plant extraction for obtaining valuable chemicals. In plant cell and tissue
cultures, cell growth and metabolite production are influenced by nutritional and environmental conditions as well as physical
properties of the culture system. To obtain a high growth rate of plant cell and tissue cultures, the culture tem. To obtain
a high growth rate of plant cell and tissue cultures, the culture conditions should be maintained at an optimum level. We
studied the relationship between inoculum conditions and the growth of Panax ginseng hairy root culture, and found that the growth rate varied with the inoculum conditions such as the number of root tips, the
length of root tips, the part of root tips, and the inoculum size and age of hairy roots. 相似文献
128.
K. Fröhlich 《Isotopes in environmental and health studies》2013,49(12):557-560
Groundwater dating is known to be determination of the residence time of groundwater. This residence time is commonly defined as the length of time the water has been isolated from the atmosphere. The paper reviews isotope methods available for dating of old groundwater. Their residence time is supposed to be far outside the range of C-14 dating which is about 50000 years. These dating methods are based on the radioactive decay of long-lived radionuclides produced by interaction of cosmic radiation with gases in the atmosphere (Kr-81, Cl-36, I-129), on the accumulation of noble gases suo-surface produced by nuclear reactions including radioactive decay of U/Th series radionuclides (He-4, He-3), and on the disequilibrium between uranium isotopes due to water-rock interaction and radioactive decay (U-234, U-238). The paper presents the principles of these dating methods, refers to recent case studies and summarizes problem areas for further development and application of these methods. 相似文献
129.
高效液相色谱法鉴定蓝色圆珠笔油墨字迹的书写时间 总被引:6,自引:2,他引:4
由圆珠笔油墨形成的契约、合同、收据和借条等可疑文件的真伪及形成时间的鉴定是目前法庭科学实验室经常遇到的问题, 因此建立一种简便、灵敏和准确的检验圆珠笔油墨字迹色痕的异同及形成时间的方法是十分必要的。文章介绍了一种可以检验圆珠笔油墨的种类及形成时间的方法。这种方法是依据高效液相色谱法分析圆珠笔油墨中染料的种类及染料随时间的变化关系来确定圆珠笔油墨字迹的异同及书写时间,染料随时间的变化可以通过相应的色谱峰的峰面积比计算而得。 相似文献
130.
黄怡 《新疆大学学报(理工版)》2009,26(2):159-163
研究具有年龄和病程等结构的传染病动力学模型具有重要的实际和理论意义,它们在研究与年龄区间和病程区间的疾病的传播起到了重要的作用,本文据此建立一个考虑上述因素的具年龄和病程结构的传染病偏微分模型. 相似文献