Dielectric properties composite

Historically, materials based on doped barium titanate were used to achieve dielectric constants as high as 2,000 to 10,000. The high dielectric constants result from ionic polarization and the stress enhancement of k associated with the fine-grain size of the material. The specific dielectric properties are obtained through compositional modifications, ie, the inclusion of various additives at different doping levels. For example, additions of strontium titanate to barium titanate shift the Curie point, the temperature at which the ferroelectric to paraelectric phase transition occurs and the maximum dielectric constant is typically observed, to lower temperature as shown in Figure 1 (2). 

Fig. 1. Effect of compositional variations on the dielectric properties of strontium titanate-barium titanate solid solutions. A, BaQ SrQ QTiO B,...

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

The introduction of organotin residues into an epoxide resin is known to improve the dielectric properties and thermal stability of hardened compositions 71). 

In dilute solutions it is possible to relate the activity coefficients of ionic species to the composition of the solution, its dielectric properties, the temperature, and certain fundamental constants. Theoretical approaches to the development of such relations trace their origins to the classic papers by Debye and Hiickel (6-8). For detailpd treatments of this subject, refer to standard physical chemistry texts or to treatises on electrolyte solutions [e.g., that by Harned... 

As indicated above, the dielectric properties are the chief consideration for applications of these fluoromethylene cyanate ester resins. As such, processing as coatings for microelectronic applications and as nonstructural castings and composites for radome-related applications is being investigated. In this respect the n = 6 system has been identified as the best compromise for synthesis, properties, and processing. Results in this section pertain only to this member of the series, which is currently under evahation for the above applications.13... 

This research was an attempt to develop new polymers with the mechanical properties of polyarylene ethers and the dielectric properties of fluoropolymers. After initially testing the viability of the [2n+ 2n] cyclodimerization reaction for preparing high-molecular-weight polymers and testing the dielectric properties of these polymers, two polymers (one thermoplastic and one thermoset) were prepared in larger quantities to evaluate the thermal and mechanical performance of these novel compositions. The high Te thermoset was also quantitatively tested for thermal/oxidative stability. 

One interesting application of the dielectric properties of the resins in these composite systems is that it provides an opportunity to monitor the cure characteristics of... 

Some of the more important dielectric properties are dielectric loss, loss factor, dielectric constant (or specific inductive capacity), dc conductivity, ac conductivity, and electric breakdown strength. The term dielectric behavior usually refers to the variation of these properties within materials as a function of frequency, composition, voltage, pressure, and temperature. 

Several workers have employed monomodal cavities for microwave chemistry on the sub-gram scale. In some cases in which monomodal cavities have been used7, special benefits of so-called focussed microwaves have been claimed. As mentioned earlier, the dielectric properties of a sample can alter substantially with temperature and/or with changing chemical composition. Hence, regardless of whether multi-modal or unimodal cavities are employed, frequent tuning may be necessary if heating efficiency is to be retained. This aspect has often been overlooked by proponents of focussed microwaves. The nett result is that transfer of microwave conditions between monomodal to multi-modal cavities is usually facile. With the MBR (which had a tunable multimodal cavity), Cablewski et al. performed five reactions that had been conducted earlier on the gram scale or below with focussed microwaves (T. Cablewski, B. Heilman, P. Pilotti, J. Thorn, and C.R. Strauss, personal communication see also Ref. 117 for conference poster). These were scaled-up between 40- and 60-fold and reaction conditions... 

Solid food materials have dielectric properties dependent upon their composition. In many instances, particularly when developing microwavable food products, it is necessary to know the effective bulk microwave properties of the product, crushed, as is, or when agglomerated together. Typical examples are peas, beans, com, pasta, flour... 

Early work using microwaves as a diagnostic tool relied upon measuring a secondary effect of the dielectric properties of the material under interrogation, i.e., reflection, absorption and transmission. The two fundamental microwave parameters, e and e" are related to the food or material composition. These two fundamental parameters also determine the reflection, absorption and transmission of the materials exposed to a microwave signal. Thus by measuring the amplitude and phase of the reflected or transmitted wave, or the characteristics of absorption of a wave through the material, one is able to empirically establish a relationship to the constituency of the product. 

Ohlsson, T., Enriques, M. and Bengtsson, N. 1974. Dielectric properties of model meat emulsions at 900 and 28 MHz in relation to their composition. Journal of Food Science. 39 1153. 

Steady-State Fluorescence. The fluorescence characteristics of PRODAN are extremely sensitive to the physicochemical properties of the solvent (38). As benchmarks, the steady-state emission spectra for PRODAN in several liquid solvents are presented in Figure 1. It is evident that the PRODAN emission spectrum red shifts with increasing solvent polarity. This red shift is a result of the dielectric properties of the surrounding solvent and the large excited-state dipole moment (ca. 20 Debye units) of PRODAN (38). It is the sensitivity of the PRODAN fluorescence that will be used here to investigate the local solvent composition in binary supercritical fluids. 

Table VI summarizes the effect of heating medium on the loss of acids after 3 minutes of microwave heating. Loss of volatile acids varied widely dependent on the microwave medium. Acetic and caproic acids had losses ranging from 20-80% and 0-73%, respectively, depending on medium composition. The dielectric property, specific heat, or other physical/chemical properties of individual flavor compounds can provide valuable insight into the potential behavior of these compounds during the microwave process. The dielectric property of the total food system and the affinity of the flavor compound for the microwave medium, however, were primarily responsible for the behavior of these flavor compounds during microwave heating.

Interest turned to compositions of high permittivity titanates and zirconates in which one member exhibited a negative and the other a positive TC , the objective being to achieve xf = 0. The effect of composition on TC, for two such systems is shown in Fig. 5.35. Another system which attracted interest is MgTi03-CaTi03 in which the end members have the dielectric properties shown in Table 5.7. The composition Mg0 95Ca0 05TiO3 yields xf 0, er = 21 and Q = 8000 at 7 GHz. 

Fig. 5.44 The effect of compositional heterogeneity on dielectric properties curve A, 43cat.% Ba-7cat.%Bi 49cat.%Ti(BaTiO, and Bi4Ti3012 calcined separately) curve B, 46cat.%Ba-5.5cat.%Bi-48cat.%Ti (starting materials calcined together). Both are sintered at 1300 °C for lh.

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