Dielectrics lithium niobate

Fig. 4.3. Typical normalized piezoelectric current-versus-time responses are compared for x-cut quartz, z-cut lithium niobate, and y-cut lithium niobate. The y-cut response is distorted in time due to propagation of both longitudinal and shear components. In the other crystals, the increases of current in time can be described with finite strain, dielectric constant change, and electromechanical coupling as predicted by theory (after Davison and Graham [79D01]).

It appears that the observed breakdown must be explained in terms of the transient behavior of stress-induced defects even though the stresses are well within the nominal elastic range. In lithium niobate [77G06] and aluminum oxide [68G05] the extent of the breakdown appears to be strongly influenced by residual strains. In the vicinity of the threshold stress, dielectric relaxation associated with defects may have a significant effect on current observed in the short interval preceding breakdown. 

Two approaches are proposed to resolve this difficulty. The first is based on generation of a high-purity acoustic field in a spherical, unharmonic resonator, and the second is based on comparison of a set of transducers with limited but overlapping dynamic ranges. In the second approach, simultaneous measurement of a broadband signal produces an accurate characterization over the entire range. Candidates for sensor fabrication include solid dielectric capacitive sensors, piezoelectric transducers, and high-temperature lithium niobate or quartz transducers. 

Pockels coefficients measured at the technologically important wavelengths 1.3 and 1.55 pm are higher than in the case of lithium niobate. Moreover, the difference in the dielectric constants is important s=2S (LiNb03) and =2.5-4 (EO polymer). The lower value corresponds to a decreased device power consumption and an enhanced speed of operation. 

Smith RT, Welsh FS (1971) Temperature dependence of the elastic, piezoelectric, and dielectric constants of lithium tantalate and lithium niobate. J Appl Phys 42 2219... 

For lithium niobate, dielectric constants are of the order of 28 while index of refraction is of the order of 2.2. The bandwidth-length product is 10 GHz-cm and the voltage-length product is 5 V-cm. By clever engineering (minimizing the... 

Since the electro-optic tensor has the same symmetry as the tensor of the inverse piezoelectric effect, the linear electro-optic (Pockels) effect is confined to the symmetry groups in which piezoelectricity occurs (see Table 8.3). The electro-optic coefficients of most dielectric materials are small (of the order of 10 m V ), with the notable exception of ferroelectrics such as potassium dihydrogen phosphate (KDP KH2PO4), lithium niobate (liNbOs), lithium tantalate (LiTaOs), barium sodium niobate (Ba2NaNb50i5), or strontium barium niobate (Sro.75Bao.25Nb206) (Zheludev, 1990). For example, the tensorial matrix of KDP with symmetry group 42m has the form... 

Ferroelectric and dielectric materials comprise Volume 4. This volume contains reviews on pyroelectricity the fundamentals and applications, crystal growth, characterization and domain studies in lithium niobate and lithium tantalate ferroelectrics, bismuth vanadate, the electric field influence on acoustic waves, dielectric ceramics, and low dielectric constant materials for microelectronics interconnects. 

For room temperature operation, a most attractive thermal detector is the pyroelectric element. It is a small capacitor with a dielectric material that possesses a temperature sensitive dipole moment. So far, the most successful dielectric is triglycine phosphate (TGS), particularly if doped with L-alanine. Its Curie point is at 49 °C and, consequently, it must be operated below that temperature. (Above the Curie point, these dielectrics lose their pyroelectric properties.) Other suitable materials include lithium tantalate and strontium barium niobate. The voltage across a capacitor of charge Q is... 

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