| dc.description.abstract | The way similar to monomerisation or at higher temperatures. CD studies indicated no major change in the secondary structure of the proteins of lipovitellins during chloroform absorption.
The interaction of Ca²? and Na? ions with VLDL as monitored by ANS fluorescence indicated Ca²? ion can enhance ANS fluorescence considerably more than Na?. Effect of pH on ANS fluorescence indicated a decrease in ANS fluorescence with an increase in the pH. These two observations are in keeping with data obtained for phospholipids. This indicates that phospholipids of VLDL are the probable binding sites of ANS.
Proteins situated on the surface of lipovitellins and VLDL were determined on polyacrylamide gel using fluorescamine as a surface label. Some preliminary data on the spatial relationship of proteins in ?-lipovitellin has been obtained using 1,5-difluoro, 2,4-dinitrobenzene as a bifunctional reagent.
ApoC of hen's egg yolk VLDL has been recombined with egg yolk lecithin or a mixture of egg yolk lecithin and triolein. Lipid-bound ApoC was separated and its lipid and protein content was determined. The lipid-bound ApoC had increased amount of helical content as compared to VLDL.
Complex substitution of lanthanides, free electrons and oxygen vacancy are simultaneously generated. The reflectance spectra of Ln-doped BaTiO? ceramics and reduced BaTiO? show two weak bands around 440 and 640 nm in the visible region. Therefore, it is obvious that the color centres similar to that in reduced BaTiO? can be generated in doped samples as well. This may be due to the excitation of electrons from valence band to the donor centres.
EPR studies are used to probe into the molecular nature of these color centres in n-type semiconducting BaTiO? ceramics with innovations. The general nature of these electronic centres is represented as (Ti³?–V?e?) and O??. The O?? centre is stabilized by the cation vacancies, by hydrogen (as HO??), or through isomorphous or interstitial defects with positive charge deficit and including the formation of MO? radicals type. Ba vacancies in PTCR-specimens prevail at the Ti-rich grain-boundary phase and are compensated by the complex centres like (Ti³?–O??) or (Ti³?–O??)²?. The defect centres in reduced BaTiO? are as well present in the bulk of the grains of Ln-doped PTCR-samples i.e., (Ti³?–V?e?) centres remain identical in molecular structure as in the reduced sample. However, in the grain-boundary of the reduced BaTiO? there are (Ti³?–O?)²? centres. In the Ln-doped samples there exist (Ln³?–O?)²? centres at the grain boundary layers. EPR studies indicate that the mechanism of PTCR-effect enhancement in the case of Mn addition is different from that when Cu is added. In these systems, donors can be defined as electron traps and acceptors as hole traps in such complex semiconductors. Mn acts as a donor while Cu is an acceptor.
DTA studies have been attributed to the crystallization of secondary phases at grain boundary (exotherm around 1150°C in AST added samples) under the controlled cooling rate. SEM photographs of TiO? added sample showed the presence of a thin layer of an additional phase all around the grain. The important feature is the continuation of the domains into the grain-boundary phase. This indicates the epitaxial growth relation between the grain phase and grain-boundary phase.
Different models currently sustaining can be grouped into any one of the following alternatives:
(I) There are electron acceptor surface states at the grain-boundary region of each crystallite which generate a charge accumulation and consequently a band bending or potential barrier of Schottky type prevails. In the ferroelectric region, the negative space-charge is compensated at least partly, by the spontaneous polarization. This approach is further modified as a 3-dimensional negatively charged zone at the grain-boundary due to Ba-vacancies. On the whole, there is only a grain-boundary layer of finite thickness and no separate phase is segregated.
(II) A layer of a second phase is present in between grains of BaTiO? which possesses (a) acceptor character or (b) traps whose activation energy has a dependence of permittivity on temperature and the field strength. Resistivity of the second phase increases above the Curie temperature. It has been shown from single-grain boundary measurements, that the barrier layer model does not explain the I-V characteristics of the grain-boundary. PTCR-effect is not related to the increased barrier height through decreased permittivity but to the activation of traps associated with the disappearance of spontaneous polarization (P?) at T?. Activation of traps decreases the electron mobility and hence the increased resistivity.
The new proposed model on the basis of experimentally observed facts invokes the idea that bulk of the BaTiO? grains in the PTCR-ceramics, which is designated as the grain phase (GP), should be distinguished from the surface layer (grain-boundary layer, GBL) of finite thickness with electrical properties different from that of grain phase. The GP is enveloped by a second phase designated as grain-boundary phase (GBP). One gradation of GBL and GBP can be continuous and not abrupt i.e., they have certain crystallographic relation deduced as epitaxial. The epitaxial relation is possible since the major GBP is also a titanate of the composition BaO·nTiO? with n > 1. These phases have reaction relations with BaTiO? as:
BaTi?O? + Ba²? + O²? ? Ba?TiO?
BaTi?O? + 2Ba²? + 2O²? ? Ba?TiO? + 2e?
where Ba²? and O²? are the lattice components abstracted from BaTiO? while vacancies are generated thereupon. In the PTCR-samples, GP is n-type semiconductor with (Ti³?–V?e?) centres located nearer to conduction band. (Ln³?–O?) centres are deeper activated traps compared to (Ti³?–O?) centres which exist at the GBL such that there is decrease in electron mobility. The (Ln³?–O?) in the GBL with the modified composition acts as the electron trap centre whereas Cu increases the density of (Ln³?–O??) centres at GBL and hence the enhanced PTCR-effect. GBP will have modified composition e.g., BaO·nTiO? in Ln-doped samples and epitaxial relation with GP.
The I-V characteristics in the GBP follow Ohm's law (I ? V) over the lower voltage range at all temperatures and Child's law (I ? V²) over the higher voltage range above T?, which is indicative of conduction by a space-charge-limited current (SCLC), over the higher voltage range. The I-V curves show concavity and such conditions are explained according to the trap filling limit. These observations are not in accordance with the barrier layer model (which essentially Schottky-type depletion layer) which should exhibit an exponential dependence exp(qV/kT). Therefore, PTCR-effect relates to activation of traps associated with the disappearance of spontaneous polarization around T?. This mechanistic proposed model explains the PTCR-effect most satisfactorily and answers a set of questions on PTCR-effect in a self-consistent manner in the following way. | |
