Search results

This paper aims to fabricate and characterize ZnO-based multilayer varistors.

Tape casting technique was utilized for preparation of multilayer varistors based on ZnO doped with Pr, Bi, Sb, Co, Cr, Mn and Si oxides. Scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) methods were used to study the microstructure, elemental and phase compositions, respectively, of the varistors. Dielectric properties were investigated by impedance spectroscopy. Current–voltage (I–U) dependences were measured to characterize nonlinear behavior of the fabricated varistors.

XRD, SEM and EDS studies revealed dense microstructure of ceramic layers with ZnO grains sized 1-4 μm surrounded by nanometric Bi-rich films, submicrometer Zn₇Sb₂O₁₂ spinel grains and needle-shaped Pr₃SbO₇ crystallites. Praseodymium oxide was found to be very effective as an additive restricting the ZnO grain growth. I–U characteristics of the fabricated multilayer varistors were nonlinear, with the nonlinearity coefficients of 23-27 and 19-51 for the lower and higher Pr₂O₃ content, respectively. The breakdown voltages were 60-150 V, decreasing with increasing sintering temperature.

Low-temperature cofired ceramics technology enables attaining a significant progress in miniaturization of electronic passive components. Literature concerning application of this technology for multilayer varistors fabrication is limited. In the present work, the results of XRD, SEM and EDS studies along with the I–U and complex impedance dependences are analyzed to elucidate the origin of the observed varistor effect. The influence of sintering temperature and Pr₂O₃-doping level was investigated.

The purpose of this study was to develop fabrication procedure of multilayer varistors based on doped ZnO and to investigate their microstructure and electrical properties.

Two ceramic compositions based on ZnO doped with Bi₂O₃, Sb₂O₃, CoO, MnO, Cr₂O₃, B₂O₃, SiO₂ and Pr₂O₃ were used for tape casting of varistor tapes. Multilayer varistors were prepared by stacking of several green sheets with screen printed Pt electrodes, isostatic lamination and firing at 1,050-1,100°C. Scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) studies were carried out to examine the microstructure and elemental composition of the varistors. Current-voltage characteristics were measured in the temperature range from −20 to 100°C.

The desired compact and fine-grained microstructure of multilayer varistors and nonlinear current-voltage characteristics were attained as a result of the applied fabrication procedure. The breakdown voltage of the varistors is 33-35 V and decreases slightly in the temperature range from −20 to 100°C. The nonlinearity coefficient changes from 14 to 23 with rising measurement temperature.

New improved formulations of varistor ceramic foils based on doped ZnO were developed using tape casting method and applied for fabrication of multilayer varistors with good electrical characteristics. The influence of temperature in the range from −20 to 100°C on the varistor parameters was studied.

Various thick film varistor constructions were made and characterised: ‘sandwich’, ‘interdigitated’ and ‘segmented’ varistors. The varistor active layer thickness, the electrode surface value and shape were varied. The Ul characteristics of these varistors were compared mutually, and with the Ul characteristics of the smallest chip varistors. In accordance with the results obtained, it has been shown that thick film printed varistors composed of ZnO and with additives could be applied as discrete components or integrated into a hybrid circuit.

This paper aims to present a method for the reduction of dielectric constant of low-temperature co-fired ceramics (LTCC) substrates with the use of controlled internal porosity.

A glass-ceramic green tape with addition of graphite as a pore former was developed. The green tapes were laminated and then sintered into multilayer structures with porous interior and thin external dense layers. Microstructure of green and fired structures was studied using optical and scanning microscopy. The behavior of the samples during heating was examined in a heating microscope. Impedance spectroscopy was applied for investigation of dielectric properties of the fabricated substrates.

Microstructure and dielectric properties of the fabricated LTCC structures were compared with the characteristics for non-porous samples with the similar composition. Introduction of 50 Wt.% admixture of graphite in the internal layers of the LTCC substrate was found to result in decrease in dielectric constant value down to about 3. Application of non-porous outer layers improved mechanical strength of the structure and smoothness of its surface, allowing screen printing of conductive pastes on both sides of the substrate.

The rapid growth of the wireless communication industry has created a great demand for the development of new and improved materials and devices operating properly at high frequencies. The fabricated materials can be useful for substrates of microwave devices.

The paper presents an innovative method of dielectric constant decrease of substrate materials. Getting insight into the phenomena responsible for formation of pores is crucial for designing materials for microwave electronics.

The purpose of the paper was to present a comparative study on the microstructure and magnetoelectric effect of new magnetoelectric composites based on TbFe₂ compound and Ni_0.3Zn_0.62Cu_0.08Fe₂O₄, CoFe₂O₄ ferrites as a magnetostrictive phase, Pb(Fe_1/2Ta_1/2)O₃ (PFT), Pb(Fe_1/2Nb_1/2)O₃ relaxors as a ferroelectric phase and polyvinylidene fluoride (PVDF) as piezoelectric phase.

The ceramic components of composites were prepared by the standard solid-state reaction method. The intermetallic compound TbFe₂ was prepared with an arc melting system with a contact-less ignition in a high purity argon atmosphere. The metal – ceramic – polymer composites were prepared in a container in which powder of PVDF were dissolved in N,N-dimethylformamide with continuous mixing and at the controlled temperature. Ceramic composites were prepared as bulk samples and multilayer tape cast and co-sintered laminates. The microstructure of the composites was investigated using scanning electron microscopy (SEM). The magnetoelectric effect of the composites was evaluated at room temperature by means of the dynamic lock-in method.

SEM analysis revealed a dense, fine-grained microstructure and uniform distribution of the metallic, ferrite and relaxor grains in the bulk composites. The SEM image for multilayer composite illustrates the lack of cracks or delaminations at the phase boundaries between the well-sintered ferrite and relaxor layers. For all studied composites, the magnetoelectric coefficients at a lower magnetic field increase, reaches a maximum and then decreases.

The progress in electronic technology is directly linked to advances made in materials science. Exploring and characterizing new materials with interesting magnetoelectric properties, in the rapidly growing field of functional materials, is an important task. The paper reports on processing, microstructure and magnetoelectric properties of novel magnetoelectric composites.

The purpose of this paper is to report on fabrication procedure and present microstructure and dielectric behavior of willemite ceramic material with addition of 5% Li₂CO₃ as a sintering aid.

The samples were fabricated by ball milling of the ceramic powders, preparation of granulate and pressing and co-firing using temperature profile based on heating microscope observation. The dielectric properties of the material were measured by impedance spectroscopy (Hz-MHz), transmission method (GHz) and time domain spectroscopy (THz). The composition and microstructure of the material were investigated using X-ray diffraction, scanning electron microscopy and energy-dispersive spectroscopy analysis. Ceramic powder was used to fabricate a green tape and low temperature co-fired ceramics (LTCC) multilayer structures, which in the next steps of the research were examined at the angle of cooperation with conductive pastes, strength and geometric repeatability.

The fabricated material showed low sintering temperature (920°C–960°C), low dielectric constant 6.2–6.34 and low dissipation factor at the level of 0.004–0.007. As LTCC material, willemite with 5% Li₂CO₃ addition showed good compatibility with AgPd conductive paste.

Search for new materials with low dielectric constant, applicable in LTCC technology, and development of their fabrication procedure are important tasks for the progress in modern microwave circuits.

ISHM invites papers for the above Conference, to be held on 29–31 May 1991 in Rotterdam, The Netherlands. Papers should cover areas such as: design, manufacturing, packaging and interconnection, materials and processing, applications, reliability, components, new technologies, marketing and economics, optoelectronics. Summaries should be in English, length 200–300 words. The deadline for receipt of summaries is 30 September 1990. (For full details, see announcement on pp. 54–55.)