I. Hadzaman

Controlled thermistor effect in the system CuxNi1–x–yCo2yMn2–yO4

Abstract The problem of selection guidelines for oxide semiconducting ceramics with controlled thermistor effect, prepared on the basis of complex CuxNi1–x–yCo2yMn2–yO4 (0.1⩽x⩽0.8; 0.1⩽y⩽0.9–x) chemical system and restricted by CuMn2O4, MnCo2O4 and NiMn2O4 spinel-structured compounds, is discussed. It is shown that the developed ceramic thermistors with a wide range of predetermined exploitation parameters (rated resistance from 2 to 21,500 Ω, material-specific constant B of NTC thermistor from 2435 to 3365 K), as well as a high level of reliability can be obtained using the proper methods of technological, compositional and design modifications. The attempt to establish a general relationships between chemical composition, technological features of sintering and electrical properties for single-phase ceramics is made, using the method of simplex matrices of the 4-fold D-optimum plan.

Thermal modification of ceramic composites based on manganese-containing cube spinels

The nature of thermostimulated diffusion of silver in ceramic composites based on manganese-containing cube spinels has been studied using the methods of electron-sonde microanalysis, X-ray diffraction, optical and Auger spectrometry.

Analytical Description of Degradation-Relaxation Transformations in Nanoinhomogeneous Spinel Ceramics

Mathematical models of degradation-relaxation kinetics are considered for jammed thick-film systems composed of screen-printed spinel Cu0.1Ni0.1Co1.6Mn1.2O4 and conductive Ag or Ag-Pd alloys. Structurally intrinsic nanoinhomogeneous ceramics due to Ag and Ag-Pd diffusing agents embedded in a spinel phase environment are shown to define governing kinetics of thermally induced degradation under 170 °C obeying an obvious non-exponential behavior in a negative relative resistance drift. The characteristic stretched-to-compressed exponential crossover is detected for degradation-relaxation kinetics in thick-film systems with conductive contacts made of Ag-Pd and Ag alloys. Under essential migration of a conductive phase, Ag penetrates thick-film spinel ceramics via a considerable two-step diffusing process.

Extended defects in insulating MgAl2O4 ceramic materials studied by PALS methods

Extended positron-trapping defects in technological modified insulating nanoporous MgAl2O4 ceramics are characterized by positron annihilation lifetime spectroscopy. The results are achieved using three-component fitting procedure with arbitrary lifetimes applied to treatment of measured spectra. Within this approach, the first component in the lifetime spectra reflects microstructure specificity of the spinel structure, the second component responsible to extended defects near intergranual boundaries and the third component correspond to ortho-positronium pick-off decaying in nanopores of ceramics. It is shown that in ceramics of different technological modifications the same type of positron traps prevails.

Grain Porous Structure and Exploitation Properties of Humidity-Sensitive Magnesium Aluminate Spinel-Type Ceramics

Grain porous structure in functional humidity-sensitive MgAl2O4 and modified MgO-Al2O3 ceramics prepared at 1200, 1300, and 1400 °C at 2 h is studied using combined X-ray diffraction, scanning electron microscopy, Hg-porosimetry, and positron annihilation lifetime spectroscopy. It is shown that the increase in sintering temperature from 1200 to 1400 °C results in the transformation of the pore size distribution in ceramics. Microstructure of these ceramics is improved with the increase of sintering temperature, which results in decreased amount of additional phases located near grain boundaries. These phase extractions serve as specific trapping centers for positrons penetrating the ceramics. The positron trapping and ortho-positronium decaying components are considered in the mathematical treatment of the measured spectra. Classic Tao-Eldrup model is used to draw the correlation between the ortho-positronium lifetime and the size of nanopores, which is complementary to porosimetry data. The studied ceramics with optimal nanoporous structure are highly sensitive to humidity changes with minimal hysteresis in adsorption-desorption cycles.

O-Ps-related modes for study of free-volume entities in nanostructured MgAl 2 O 4 ceramics

The o-Ps-related modes are used for study of free-volume entities in nanostructured spinel MgAl2O4 ceramics after drying and water-immersion treatment. It is established that that water-sorption processes in magnesium aluminate ceramics leads to corresponding increase in positron trapping rates of extended defects located as addition phases near grain boundaries. These phase extractions and free-volume entities in ceramic structure serve as specific trapping centers for positrons penetrating ceramics. It is demonstrated that Tao-Eldrup model can be adequately used for the calculation free volume (or nanopores size) in MgAl2O4 ceramics using o-Ps lifetimes of the third and the fourth components.

Structural and magnetic properties of sintered materials on the basis of zinc oxide

Structural and magnetic properties of ceramics Zn1-xCoxO and Zn1-xCrxO are studied. Average sizes of grains are determined by scanning electron microscopy. The magnetic field dependences of magnetic susceptibility are investigated by Faraday method. The relevant theoretical models are chosen.

Sintering-modified oxymanganospinel ceramics for NTC thermistor application

Mixed Ni-Co-Cu oxymanganospinels of Cu<inf>0.1</inf>Ni<inf>0.8</inf>Co<inf>0.2</inf>Mn<inf>1.9</inf>O<inf>4</inf> chemical composition are first developed for possible application as high-precise NTC thermistors using nanophase segregation effects controlled by sintering technological route. It is shown that rack-salt NiO phase in these ceramics occurs a decisive role on parasitic degradation caused by thermal storage of the ceramics at the elevated temperatures.

The influence of γ-irradiation on electrophysical properties of spinel-based oxide ceramics

Abstract The influence of 60Co γ-irradiation with 1.25 MeV average energy and 1 MGy absorbed dose on electrophysical properties of Cu-, Ni-, Co- and Mn-based spinel ceramic materials in the CuxNi1−x−yCo2yMn2−yO4 (0,1⩽x⩽0,8;0,1⩽y⩽0,9−x) system is investigated. The γ-induced increasing of the electrical resistance is observed for the investigated samples of various compositions. It is supposed that these changes are explained by cationic redistribution in the spinel sublattices of the ceramics.