Size effect on properties of varistors made from zinc oxide nanoparticles through low temperature spark plasma sintering

Conditions for the elaboration of nanostructured varistors by spark plasma sintering (SPS) are investigated, using 8-nm zinc oxide nanoparticles synthesized following an organometallic approach. A binary system constituted of zinc oxide and bismuth oxide nanoparticles is used for this purpose. It is synthesized at roomtemperature in an organic solution through the hydrolysis of dicyclohexylzinc and bismuth acetate precursors. Sintering of this material is performed by SPS at various temperatures and dwell times. The determination of the microstructure and the chemical composition of the as-prepared ceramics are based on scanning electron microscopy and X-ray diffraction analysis. The nonlinear electrical characteristics are evidenced by current–voltage measurements. The breakdown voltage of these nanostructured varistors strongly depends on grain sizes. The results show that nanostructured varistors are obtained by SPS at sintering temperatures ranging from 550 to 600 8C

Studies on salinity tolerance and acclimatization of some commercially important seaweeds

Studies were made on salinity tolerance and acclimatization of 13 economically important red, brown and green algae at different salinities ranging from 5 to 55%. Caulerpa rocemosa degenerated after 3 days in all these salinities. Hypnea valentiae tolerated a wide saliniiy range of 15 to 45% and Gracilaria crassa and Acanthophora spicifera from 15 to 55%. All other algae tolerated only 25 and 35%. Experiments conducted on salinity acclimatization showed that A.spicifera could be acclimatized at the salinities from 55 to 15% and all other algae from 55 to 25% except C. racemosa

Effect of growth promoters on the onshore culture of Gracilaria edulis (Gmelin) Silva

Culture of agar yielding red alga Gracilaria edulis was carried out in fibreglass tanks by providing running seawater and aeration under a shed with transparent roof. The seed material was pretreated for 12 hours at different concentrations of growth promoters IAA, IBA, GA, Ascorbic acid, EDTA and Inositol. In general, more increase in growth and biomass was obtained in the plants pretreated with lower concentrations of these growth promoters

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