Seasonal changes in growth, biochemical constituents and phycocolloid of some marine algae of Mandapam coast

Seasonal variation in growth and biochemical constituents such as protein, carbohydrate and lipid in Hypnea valentiae, Acanthophora spicifera, Laurencia papillosa, Enteromorpha compressa, Ulva lactuca and Caulerpa recemosa were observed for one year from April, 1995 to March 1996. Carrageenan content was estimated from H. valentiae, A. spicifera and L. papillosa. In general, peak growth and biomass of these algae occurred during the period June -August and January - March. The maximum values in these algae varied from 12.5 to 13.2% for protein, 13.0 to 13.3% for carbohydrate and 10.3 to 12.0% for lipid. The yield of phycocolloid recorded 11.3%, 6.0% and 8.1% in H. valentiae, A. spicifera and L. papillosa respectively

The influence of Bi2O3Bi_2O_3 non-stoichiometry on the non-linear property of ZnO varistors

Proper choice of the starting compounds for the additive oxides enhances the non-linearity coefficient $(\alpha)$ to 40-60. High or-values can be achieved by using $Bi_2O_3$ containing $Bi^{5+}$ and consequently higher oxygen content. $Bi_20_3$ prepared by various low temperature chemical routes are more effective in increasing the a values. Ceramics formulated from ZnO + oxygen - excess $Bi_20_3$ have $\alpha$ = 6 to 14, whereas in presence of the transition metal oxides, the same fomulations exhibit $\alpha$-values up to 60, with the sharp turn-on point on the current-voltage curves. The oxygen-excess behaviour of $Bi_20_3$ is related to its defect-fluorite structure, with the excess oxygen located at the vacant tetrahedral voids and are charge com- pensated by $Bi^{5+}$ ions. This is supported by X-ray diffraction, opticaf reflectance spectra and electron paramagnetic resonance data. The depletion layer is formed at the pre-sintering stage as a result of electron trapping by oxygen evolved from the additives being chemisorbed on ZnO particles. The depletion layer is stabilized on either sides of the grain boundary by the higher talent transition metal ions that are preserved through charge compensation by cation vacancies, after the sintering

Low‐voltage varistors based on zinc antimony spinel Zn7Sb2O12

It is possible to prepare low‐voltage varistors from the zinc antimony spinel Zn7Sb2O12 with breakdown voltages in the range of 3–20 V and nonlinearity coefficient α=7–15. The varistor property is due to the formation of high ohmic potential barriers at the grain boundary regions on low‐ohmic n‐type grain interiors of the polycrystalline samples. The method of preparation of the spinel, synthesized by coprecipitation followed by annealing under restricted partial pressures of oxygen, controls the mixed valence states for antimony, namely, Sb3+ and Sb5+. This is critical in attaining high nonlinearity and lower breakdown voltages

Dependence of non-linearity coefficients on transition metal oxide concentration in simplified compositions of ZnO+Bi203+MOZnO+Bi_20_3+MO varistor ceramics (M=Co or Mn)

Ceramics with simplified compositions of $ZnO + Bi_20_3 + CoO \hspace{2mm} or \hspace{2mm} MnO$ show non-linearity coefficients $(\alpha)$ of 40-65 provided the concentration of transition metal ions is > 1.5mo1%. Samples doped with Co have higher non-linearity coefficients than those with Mn. This is attributed to the stability of multiple oxidation states of Co(Ill) + Co(ll) in contrast to Mn(ll) as the stable species in sintered ZnO ceramics. Electron paramagnetic resonance and diffuse reflectance spectral studies establish this fact. Low-signal capacitance-voltage measurements show that donor density $(N_d)$ in these ceramics ranges from 0.3 to $1.8 \times 10^{19} cm^{-3}$, which is comparatively larger than those of commercial varistors. The barrier height $(\phi_b)$ reaches up to 0.66 eV and the breakdown voltage is around 3.4-3.7 eV. Admittance spectroscopy and isothermal capacitance transient spectroscopy (ICTS) are used for characterizing the bulk traps originating from the transition metal dopants. Capacitance-voltage analyses above the breakdown voltages show negative capacitance, indicative of oscillatory charge redistribution involving multivalent states of Co and the shallower interface states. Multiple trapping relaxations are evident from the complex-plane capacitance studies

Low-voltage varistors based on zinc antimony spinel Zn7Sb2O12Zn_7Sb_2O_{12}

It is possible to prepare low-voltage varistors from the zinc antimony spinel $Zn_7Sb_2O_{12}$ with breakdown voltages in the range of 3–20 V and nonlinearity coefficient \alpha = 7–15. The varistor property is due to the formation of high ohmic potential barriers at the grain boundary regions on low-ohmic n-type grain interiors of the polycrystalline samples. The method of preparation of the spinel, synthesized by coprecipitation followed by annealing under restricted partial pressures of oxygen, controls the mixed valence states for antimony, namely, $Sb^{3+}$ and $Sb^{5+}$. This is critical in attaining high nonlinearity and lower breakdown voltages

High-Frequency Capacitance Resonance of ZnO-based Varistor Ceramics

Capacitance resonance of ZnO varistor ceramics can be shifted from gegahertz to a few megahertz frequency region with increasing amounts of specific secondary phases. The observed resonance is due to the total inductance of the varistor ceramics wherein the grain boundaries are shorted at high frequencies, which arises from the dynamic exchange of charge carriers at the grain interiors and the trap states that are formed at the depletion regions around the grain boundaries. This is applicable to ZnO/ZnO grain boundaries as also to ZnO/secondary phase boundaries. The capacitance-voltage studies show direct correlation between the magnitude of capacitance resonance and the nonlinearity coefficient in current-voltage relations

Zinc oxide ceramic varistors formulated with barium orthosilicate for operation in the 3- 15 V battery range

Vatistors working in the 3-15 V battery voltage range can be obtained from ZnO ceramics formulated with $Ba_2SiO_4$ as the extra additive. The presence of a large surface state density and a changing pattern of trap states at the grain boundary interfaces are more significant than large grain size in attaining lower breakdown voltages, yet preserving higher nonlinear coefficients (\alpha) up to 25

Dielectric resonance of zinc oxide varistor ceramics

Polycrystalline ZnO ceramics formulated with higher than normally known concentrations of grain boundary layer modifying additives, show resonance in capacitance around 10 to 30 MHz, which is in a much lower frequency range than so far reported. The observed resonance is due to the total inductance of the varistor ceramics wherein the grain boundaries are shorted at higher frequencies following a distinct relaxational maximum, arising from the dynamic exchange of charge carriers at the grain interiors and the trap states that are formed at the depletion regions around the grain boundaries. Methods of formulation, total content of the additives and processing of ceramics seem to be crucial in attaining capacitance resonance at megahertz frequencies. Capacitance above the breakdown voltages show negative values, indicative of oscillatory charge redistribution involving multivalent states of Co and the shallower interface states. Multiple trapping relaxations are evident from the complex-plane capacitance studies. The admittance spectroscopy data show that the type of trap states changes with the addition of grain boundary layer modifiers accompanied by considerable increase in the density of traps. The capacitance-voltage studies show direct correlation between the magnitude of capacitance resonance and the nonlinearity coefficient in current voltage relations