Microwave loss

Garnets have played an important role in the development of highly sophisticated microwave devices since the development of yttrium—iron garnet, yttrium iron oxide [12063-56-8]. The iron is strongly constrained to be trivalent in order to maintain electrical neutraUty in the crystal, which is essential for low microwave losses. Garnets have lower values of saturation magneti2ation than spinels, but provide superior performance in microwave devices because they have a narrower resonance line width. 

For the microwave-assisted experiments, both solvents were replaced by 1,2-dichlorobenzene, as it couples very effectively with microwaves (loss-tangent (tan 5) at 20 °C 1,2-dichlorobenzene 0.280 as compared to 0.101 for chlorobenzene). Diels-Alder reactions of 3-methoxy or 3-phenyl pyrazinones with DMAD were performed at a pre-selected maximum temperature of 200 °C, whereas the intramolecular reaction of alkyne tethered pyrazinone required a higher temperature (220 °C). The yields obtained under microwave irradiation are comparable with those obtained under conventional conditions, while for the dihydrofuropyridinone the yield was improved from... 

Functional oxide materials play an important role for applications in microwave communication and sensor systems. Whereas silicon and GaAs represent the basic materials for the digital part of communication and sensor systems, the analogue parts require high quality factors and low losses, which cannot be fulfilled by semiconductors. Oxide insulators provide extremely low microwave losses expressed by the value of its loss tangent tan 5 = Im(er)/Re(er). The functionality of oxides in microwave devices or circuits can be classified as follows ... 

Non resonant techniques are only of limited use to determine microwave losses with high precision, in particular when the losses are very small. Flowever, for the investigation of nonlinear absorption phenomena (i.e. rf power dependent on surface impedance or loss tangent) by intermodulation distortion measurements broad-band test devices are more common. Typically, a planar transmission line with an impedance of 50 Ohms can be employed for intermodulation... 

Rupprecht G, Bell RO, Silverman BD (1961) Nonlinearity and microwave losses in cubic strontium-titanate. Phys Rev 123 97-98... 

Small amounts of the individual fullerenes ( 0.5mg) were placed in quartz tubes with alkali metals and sealed under vacuum. These samples were subjected to a series of heat treatments and tests for superconductivity by 9-GHz microwave-loss experiments. Preliminary tests indicated that only the K-doped Cfto showed a response consistent with a superconducting transition (Fig. 1). For this reason, together with the fact that K Cgo showed the highest film conductivity, we focused our studies on the K-doped compound. 

FIG. 2. Microwave loss vs temperature for a sample of nominal composition RbBCso in a static field of 100 Oe. 

Note that td determines the frequency Vd = cdd/(2tic) = (271ctd)- of the microwave loss maximum, where cod is the angular frequency of this maximum. 

The loss and absorption peaks at v 700 cm-1, located near the border of the IR region, arise due to mechanism a—that is, due to reorientation of a rigid (permanent) dipole in the hat well. This mechanism is also responsible for the microwave loss peak located between the frequencies 0.1 and 1cm-1. The complex permittivity s of the corresponding relaxation band is actually governed by Debye theory, which is involved formally in our calculation scheme. 

The semiconductor wafer is mounted at the end of an X-band microwave waveguide so that microwave radiation probes the reflectivity of the sample. The ohmic contact is applied as a grid of thin lines in order to minimise microwave losses. The front of the wafer is in contact with an electrolyte solution, and a modulated light source (for example a light emitting diode) illuminates the sample. The changes in... 

R. C. Kell, A. C. Greenham, and G. C. E. Olds, High-Permittivity Temperature-Stable Ceramic Dielectrics with Low Microwave Loss, J. Am. Ceram. Soc., 56, 352-4 (1973). 

Semiconductive materials having conductivity between 10 and 10 S/cm have been studied more extensively. They are obtained by dispersion of conductive fillers—metallic and carbon based—in an insulating matrix. High filler contents are required, from 10 to 15% for fibres and 30% for spheres, to reach a percolation level leading to microwave losses. 

Likewise our assignment of the major loss peak in PDMS may be subject to some uncertainty. In supporting our original assignment 16) we made use of microwave loss data (45). It may be that the microwave loss is collision induced (46) rather than caused by backbone motion, so that the loss peak at 240 K may correspond to the p relaxation, while the rather weaker loss peak at about 320 K may correspond to the a relaxation (47). 

Electron spin resonance has been reported on Yb and Nd impurities in EusGasOn (Hodges, 1975). The particular interest in this host is that the Eu ion has a singlet ground state and the lines are not broadened by the host. Microwave losses of GdsGasOn have been measured at 30 GHz, X-band, and 2 GHz by Adam et al. (1975). These measurements in this host are important in technological applications, as it is used as a nonreciprocal thin film substrate. 

S. Nomura, K. Toyama and K. Kaneta, Ba(Mgiz3Ta2flP3 Ceramics with Temperature-Stable High Dielecbic Constant and Low Microwave Loss , Jpn. J. Appl. Phsy. 21(10), L624-626 (1982). 

VARIATION OF DIELECTRIC MICROWAVE LOSSES IN POLYETHYLENE AS THE RESULT OF DIFFERENT SAMPLE TREATMENTS... 

Variation of Dielectric Microwave Losses in Polyethylene as the Result of Different Sample Treatments... 

The dielectric loss spectrum at 293 K consists of the main microwave peak at 10 Hz (3.3 cm ) with a barely resolved shoulder at v ca 1.5x10 Hz corresponding to the maximum of the power absorption coefficient at v ca SO cm in the far infrared spectrum. If the measurement temperature is towered, the microwave loss peak, ascribed by the authors to p relaxation process, moves out ccmsidetably to lower frequendes and becomes resolved from the far infrared peak which is shifted with decreasing temperature in the opposite direction. 

Within the last five years or so, several studies have been performed on dynamic (switchable) microwave absorbers. Studies at the US Navy s China Lake, CA facility, although highly secretive, are thought to involve metallic grids deposited on high microwave loss substrates such as Teflon on which a CP layer is deposited... 

Nomura S., Toyama K. and Tanaka K. Ba(Mgi/3Ta2/3)03 ceramics with temperature-stable high dielectric constant and low microwave loss, Jpn. J. Appl. Phys., (1982), 21, L624-626. 

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