全文获取类型
收费全文 | 2412篇 |
免费 | 78篇 |
国内免费 | 107篇 |
专业分类
系统科学 | 90篇 |
丛书文集 | 81篇 |
教育与普及 | 42篇 |
理论与方法论 | 4篇 |
现状及发展 | 14篇 |
研究方法 | 9篇 |
综合类 | 2356篇 |
自然研究 | 1篇 |
出版年
2024年 | 21篇 |
2023年 | 23篇 |
2022年 | 35篇 |
2021年 | 54篇 |
2020年 | 34篇 |
2019年 | 19篇 |
2018年 | 25篇 |
2017年 | 37篇 |
2016年 | 38篇 |
2015年 | 48篇 |
2014年 | 81篇 |
2013年 | 86篇 |
2012年 | 93篇 |
2011年 | 111篇 |
2010年 | 93篇 |
2009年 | 143篇 |
2008年 | 131篇 |
2007年 | 149篇 |
2006年 | 116篇 |
2005年 | 117篇 |
2004年 | 111篇 |
2003年 | 81篇 |
2002年 | 88篇 |
2001年 | 76篇 |
2000年 | 72篇 |
1999年 | 91篇 |
1998年 | 92篇 |
1997年 | 81篇 |
1996年 | 78篇 |
1995年 | 66篇 |
1994年 | 49篇 |
1993年 | 42篇 |
1992年 | 46篇 |
1991年 | 45篇 |
1990年 | 39篇 |
1989年 | 23篇 |
1988年 | 21篇 |
1987年 | 13篇 |
1986年 | 23篇 |
1985年 | 3篇 |
1982年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有2597条查询结果,搜索用时 9 毫秒
171.
OBJECTIVE: To determine the range of body temperature in a group of healthy Chinese term neonates over the first 72 hours of life and to assess the influence of body weight, gestational age and route of delivery.METHOD: All 200 consecutive cases of neonates delivered at our hospital from March to August 2001 were included in this retrospective study. Temperatures were measured immediately after delivery, after 30 minutes, 1 hour, 2 hours, 8 hours and 15 hours and on the 2nd and 3rd day. Axillary temperatures ranging from 36.5 degrees C to 37 degrees C were regarded as normal. No cases of maternal fever or systemic infection of the newborns were discovered. All infants were discharged in good general condition. RESULTS: The mean rectal temperature at birth was 37.19 degrees C. The lowest average temperature was reached at 1 hour after delivery (36.54 degrees C) with a significant difference between natural delivery (36.48 degrees C) and section (36.59 degrees C) (P<0.05). Temperature subsequently rose to 36.70 degrees C at 8 hours and 36.78 degrees C at 15 hours (P<0.05). Hypothermia was seen in 51.8% and hypothermia in 42.5% of the patients. On the 3rd day after delivery, 96% of all temperatures were in the normal range. A significant relation was found between hypothermia and both low birth weight (P<0.001) and low gestational age (P<0.05). CONCLUSION: The reference range presently used did not include all physiological temperatures in the first 72 hours of life. Considering other factors, such as birth weight, route of delivery, gestational age and body temperature on the 2nd and 3rd day of life, may help to correctly assess the significance of temperatures beyond the reference range. 相似文献
172.
加强实践教学提高人才培养质量 总被引:1,自引:0,他引:1
职业教育应重视培养学生的能力,必须加强实践教学,明确教学指导思想,强化实践教学环节,鼓励学生社会实践活动,抓好毕业实训工作。 相似文献
173.
Involvement of chemokine receptors in breast cancer metastasis 总被引:344,自引:0,他引:344
Müller A Homey B Soto H Ge N Catron D Buchanan ME McClanahan T Murphy E Yuan W Wagner SN Barrera JL Mohar A Verástegui E Zlotnik A 《Nature》2001,410(6824):50-56
Breast cancer is characterized by a distinct metastatic pattern involving the regional lymph nodes, bone marrow, lung and liver. Tumour cell migration and metastasis share many similarities with leukocyte trafficking, which is critically regulated by chemokines and their receptors. Here we report that the chemokine receptors CXCR4 and CCR7 are highly expressed in human breast cancer cells, malignant breast tumours and metastases. Their respective ligands CXCL12/SDF-1alpha and CCL21/6Ckine exhibit peak levels of expression in organs representing the first destinations of breast cancer metastasis. In breast cancer cells, signalling through CXCR4 or CCR7 mediates actin polymerization and pseudopodia formation, and subsequently induces chemotactic and invasive responses. In vivo, neutralizing the interactions of CXCL12/CXCR4 significantly impairs metastasis of breast cancer cells to regional lymph nodes and lung. Malignant melanoma, which has a similar metastatic pattern as breast cancer but also a high incidence of skin metastases, shows high expression levels of CCR10 in addition to CXCR4 and CCR7. Our findings indicate that chemokines and their receptors have a critical role in determining the metastatic destination of tumour cells. 相似文献
174.
In this paper, the Life Cycle of Urban Development was firstly analyzed, and the phases of Life Cycle Assessment applied to Urban Development (ULCA) were described. As a case study, ULCA was applied in the environmental impact assessment of the land readjustment project of Hyogo District of Saga, Japan. In addition, mitigation proposals for reducing CO2 were also presented and the relevant environmental ef-fects were simulated. 相似文献
175.
LiQiang WANGGe CHENNailu LIAOBo WANGTongzhen 《科学通报(英文版)》2003,48(8):739-742
Using finite element method (FEM), the complex flow fields of quenching medium in a quenching tank are calculated for the cases with and without a work-piece in it. The influence of the flow homogenization device on the flow fields is studied. The computer simulation results are in consistency with the actual measurement results. According to the simulated results, the structure of the quenching tank and the flow homogenization device are optimized and more uniform flow fields are obtained. 相似文献
176.
以六自由度地空导弹拦截仿真为背景,阐述联合建模与仿真环境(JMASE)的基本概念和功能描述,介绍基于JMASE-SIMULINK的工程级与交战级一体化联合建模与仿真方法,给出某地空导弹全系统模型及交战仿真结果。仿真实践表明:通过将SIMULINK模型自动转换为JMASE兼容的C 模型组件,并在JMASE环境下组装、配置和运行,则获得SIMULINK工程级武器系统建模能力的同时,也获得JMASE提供的具有一致时空及环境效应描述的交战级仿真能力,既提高了建模与仿真的效率,又促进了模型重用和互操作,是实现工程级与交战级联合建模与仿真的有效方法。 相似文献
177.
DANG Ge MA ShuYing ZHOU YunLiang 《科学通报(英文版)》2007,52(24):3403-3415
With energetic ion measurements on FAST satellite, the morphologic features of quiet-time ionospheric O ion upflowing at altitudes of 2000 to 4000 km are drawn out for the first time. The pre-noon cusp/cleft is the predominant region of upflowing occurrence for O with lower energy. Meanwhile the pre-dawn sector near the equatorward edge of the plasma convection dominates the occurrence for the higher energy O ions. No matter whether the energy is lower or higher, the upflows occur often over a wide MLT range of lower latitudes outside the auroral oval. The upflowing within the pre-midnight (21:00― 22:00 MLT) auroral oval carrys larger energy fluxes, with extremely large fluxes for higher energy O appearing near the polar cap boundary. For altitudes of 2000―4200 km under observation, the ion conics occur much more frequently than ion beams. Ion beams are rarely found below 3000 km, while the conics occur uniformly over the observed altitudes. 相似文献
178.
基于DDS技术的FM信号发生器的设计及其FPGA实现 总被引:1,自引:0,他引:1
以FPGA为主要硬件,采用直接数字频率合成技术结合嵌入式锁相环,开发出了一种具有数字调制功能的FM信号发生器,并在自行研制的ALTERA Cyclone实验板上得到实现.经调试,该信号发生器的频率分辨率为0.596 HZ,最高输出载波频率达到10 MHz,同时具有输出相位连续,抗干扰能力强等优点. 相似文献
179.
180.
WANG LiMin GE Wei CHEN FeiGuo 《科学通报(英文版)》2007,52(4):450-455
The velocity profiles and temperature distributions of gas flow in microchannels, for Knudsen numbers ranging from 0.01 to 0.20, are investigated with pseudo-particle modeling (PPM). It has been found that the velocity profiles are mainly affected by Knudsen number and the external force fields applied. When Knudsen number was increased, the slip velocities on the walls increased at the beginning, and then decreased. The temperature distributions were also significantly affected by the external force. The Darcy friction factor increased with increasing Knudsen number, and its variation with Mach number under increased Knudsen number was similar to the so-called premature laminar-turbulent transition observed in experiments. 相似文献