Radio frequency microwave detector is an important electronic device in microwave system. It has wide application in communication, radar, navigation, remote sensing, electronics industry, medical treatment and scientific research. In recent years, with the rapid development of communication technology, higher requirements for future microwave detectors are put forward, such as high sensitivity detection for weak signals (below μW), low power consumption and easy miniaturization and integration. The use of electron spin properties rather than electronic charge properties to build microwave detectors is expected to solve the above challenges. Recently, Zeng Zhongming team of Suzhou Institute of Nanotechnology and Nanostructure Biology, Chinese Academy of Sciences, has made new progresses in the research on microwave detection devices based on electron spin characteristics in cooperation with domestic and foreign scientists. They use thin film preparation technology to precisely control the interface properties of nano-magnetic thin films. The magnetic moments of the free layer are subtly perpendicular to the film plane in the "magnetic free layer / isolation / magnetic fixed layer" sandwich nanostructures. The magnetic moment of the fixed layer Parallel to the film plane (Figure a). Since the magnetic moments of the two magnetic layers are arranged nearly 90 degrees, the spin injection efficiency is greatly improved. The structure offers excellent microwave detection performance: up to 75,400 mVmW-1 at 1 nW of weak signal, 20 times the detection limit of a semiconductor Schottky diode detector. At the same time, the device is the volume of semiconductor microwave detectors 1/50, easy to integrate. In addition, the device can work under zero magnetic field, eliminating the dependence on the external magnetic field, simplifying the device structure, reducing power consumption. The results of this study provide important guidance for the design of new and highly sensitive nano-devices. Relevant research results published in the recent Nature Communications (Nature Communications, 2016, 7: 11259). The work of this research has been funded by the Ministry of Science and Technology major instruments and the National Natural Science Foundation. (A) Schematic diagram of device structure and test principle; (b) Microwave response curve under different applied bias currents
Percussion drilling is employed when auger or wash boring is not possible in very stiff soil or rock. It also can be used in most soil types.
Here the advancement of a hole is achieved by alternatively lifting and dropping a heavy cutting or hammering bit that is attached to a rope or cable that is lowered into an open hole or inside a temporary casing (casings are hollow cylindrical pipes used for borehole stability and to prevent the loss of drilling fluid through the boreholes).
Usually a tripod is used to support the cable. The stroke of bit varies according to the ground condition. The major disadvantage of this method is that it is not possible to get good-quality undisturbed samples. In very hard rock (and especially fractured hard rock), down-the-hole (DTH) drilling can be employed. In this case the hammer, applying repeated percussive pressure, is located just behind the drill bit inside the hole, unlike the open percussion drilling, where the hammer is on top of the drilling string.
The drilling string provides the necessary force and rotation to the hammer and bit, as well as compressed air or fluids to the hammer and for the flushing of cuttings.
This arrangement also allows for much deeper percussion drilling. However, the DTH drills are typically more expensive.
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