ZnO-%6 Bi2O3 ikili sisteminde CoO ilavesinin mikroyapı ve tane büyüme kinetiğine etkileri

Abstract

Sunulan bu tez çalışması, varistör seramikleri üzerine yapılan bir dizi çalışmanın bir halkası olup, çalışmada ZnO - %6 Bİ203 ikili seramik sisteminde CoO ilavesinin sistemin tane boyutu kinetiğine ve mikroyapısma olan etkisi incelenmiştir. Bu amaçla 3 ayrı bileşimde hazırlanan toz karışımları; presleme, farklı sıcaklık ve sürelerde sinterleme süreçlerinden geçmiş, daha sonra numunelerin fiziksel ve mikroyapısal karakterizasyomı yapılmıştır. Fiziksel karakterizasyon çalışmasında yoğunluk ölçümleri ve ağırlık kayıplarının sinterleme sıcaklığı ile değişimi saptanmıştır. Mikroyapısal karakterizasyon çalışmalarında ise bileşim, sıcaklık ve süre ile birlikte tane boyutlarının ne şekilde değiştiği saptanmış ve her bileşime ait aktivasyon enerjisi hesaplamaları yapılarak CoO 'in sisteme etkisi saptanmıştır.

%6 Bh03 BINARY SYSTEM During 1950's, first investigations related with usage of ZnO ceramics as a resistor to provide protection against over voltage in electronic devices was began. Besides successive results at Rusia on ZnO / Bi203 ceramics in 1961, industrial vise usage of these materials was not practical. In 1967, the first project on ZnO devices development started in Japan with following discovery of bulk nonlinearity in ZnO ceramics with Ag electrodes in 1967 and fast development of industrial prototype and commercial production realised in 1968 (1). The reasons of preference of ZnO material besides the other semi-conductive materials are the simplisity of sintering of it and ability of keeping under control of its electrical resistance. Realisation of some specifications such as non-ohmic exponent, threshold voltage region, surge withstand capability and long term reliability, made ZnO ceramics more popular and ensured a lot of investigations on it (2). ZnO varistors are produced by classical ceramic production methods. A typical commercial varistor has a combination of; % 98 ZnO, % 0.5 Bi203, CoO, MnO, Sb203. The Production of the material with this combination is as follows;. Mixing and wet grinding. Drying and granulating. Shaping by press (0 = 4- 10 mm, h = 1 - 40 mm). Sintering at 1100 - 1300 °C in an electrical resistance furnace.. Electroding with Ag or Al (Ag for low voltage applications and Al for high voltage applications).. Characterisation (electrical properties). There are three important parameters effecting on electrical properties of varistors during production (3); Thicness and grain size effects on breakdown voltage. Electrode surface area effects on surge current withstanding capability. Ceramic bulc volume effects on transient energy capability. The breakdown voltage which has to be achieved is controlled by changing grain size or the thickness of the varistor. Depending on the thickness, the bulk volume of the varistor will also change, and this will effect the transient energy capability. The surge current withstand capability is directly controlled by electrode surface area. Increasing of surface area increases withstanding capability against to surge current i.e. while the withstanding capability of 1cm2 surface area is 5000 A, it rises up to 10000A for 2 cm2. The primery function of the varistors is to protect electronic device against high voltage by limiting surge voltage which has been produced by electronic curcuits. Varistors ensure this by their non-ohmic current - voltage characteristics. Varistors can be used in AC or DC fields and over a wide range of voltages, from a few volts to tens or kilovolts, and the wide range of currents from microamperes to kiloamperes. Varistors have additional property of high energy absorption capability, ranging from a few joules to thousand of joules (5). The non-ohmic current - voltage relationship which is the most important characteristics of these ceramics can be formulated as follows; I = KV" I - Current (A) V = Voltage (V) K = Constent a = Non-ohmic exponent a, which is used in the equation is a measure of the device nonlinearity and varies with voltage. The increase in a increases the protection property. This value is about 30 for commercial varistors (3). The advantages and properties of varistors can be summerised as follows; Static voltage - current relationship Capacitance and dielectric losses Response time Surge withstand capability Surge energy capability Reliability VI ZnO varistors are polycrystalline ceramics with more then one phase. Microfractures of them consists of ZnO grains, spinel phases and intergranular Bizmuth-rich phases. İntergranular regions inhibit elecron transitions and act as barriers against to current. This phenomenon gives varistor characteristics to these materials. At the beginning these barriers which occured at the grain boundries was realized by physicists and generally, first investigations was made by them. According to these investigations too much models related with conductivity mechanisms of ZnO ceramics was reported. Because of the electrical properties of the varistors are the subjects of another study, in this thesis conductivity mechanisms are not investigated. Because of the direct effect of the microstructure on varistör properties, and composition and production paremeters effect on microstructure, material scientists began to interest with this material. Since 1970's the effect of some oxides on microstructure and electrical properties have been investigated. As a result of these investigations, addition of Bİ2O3 has been found to be essential for forming non-ohmic behaviour. However, addition of transition oxides such as Co304, and Mn02 also enhance nonlinearity. Similarly multiple dopants such as a combination of Bi203, Sb203, Mn02, Si02, Co304 etc. have produced greater nonlinearity then that by a single dopant. While, Bi203 and MnO contribute to grain coarsening, Sb203 inhibit grain growth (4). The major findings of the microstructural analyses are summarized in below figure (4); Bi203 MnO yS Co304 Sb203 To outline the effect of individual oxides on microstructure and electrical properties, last few years investigations oriented to binary and ternary systems instead of multisystems. Among these investigations ZnO-Sb203, ZnO-Bi203 binary siystems, and effect of some oxides on these systems can be told. R.C. Bradt and his work group have vu been researching ZnO- Bi203 and ZnO- Sb203 systems and effect of some oxides such as Ti02, A1203, Nb203 on kinetics of grain growth. In these thesis it is aim at outlining the effect of CoO on microstructure and grain growth on ZnO- %6 Bİ203 binary siystem. The reason of chosing ZnO-%6 Bİ203 binary siystem is the result of the previous investigations of Bradt and well known charecteristics of it. Because there is'nt any research about CoO effects on grain growth of ZnO-Bi203 system it was selected. Three type of composition was prepeared. This blends was wet mixed in a conventionel attritör with 600 rpm for 1 hour. The mixture dried in etuv at 100-150oC and granulated A uniaxial press and stainless steel moud was used for shaping. !44 samples with about 123 mm diameter, 3 mm height and 2.5 gr weight was prepeared. The combination of samples as fallows; % 94 ZnO - % 6 Bİ2O3 % 92 ZnO - % 6 Bi203 - % 2 CoO % 90 ZnO - % 6 Bi20 3- % 4 CoO For sintering oxide athmosphere electrical furnace was used. Sintering curcle included; 5°C heating up rate, 1000-1 100- 1200-1 3 00°C for sintering temperatures; 1, 3, 5, 10 hours for sintering times and atmosphere cooling. Sintered samples was characterised by means of microstructure (grain size measurements), density (weight loss) and XRD analysis ( phase composition). Microstructurel analysis was made by using optical microscopy and SEM. 48 photos from optical microsopy and 12 photes from SEM was obtained and used for grain size measurmentsto outline kinetic of grain growth. Activation energies and grain growth rate exponencial was determined by using fallowing equation; G°-G0n = kotexp(-Q/RT) In this equation n is the kinetic grain growth exponent, Go is the initial grain size, G is the avarage grain size at the time, ko is aconstant, Q is the activation energy. Results of density measurements show decrease on densities with increasing temperature and time. In ceramic systems usually densty of sintering materials increase with temperature and time and sometimes % 100 of theoretical densities can be obtained. In these system volatization of Bi203- can be coused a decrease on densty value. Weight measurements which was made on four different temperature similarly show weight lossin the system espacially after 1200°C. Similar results was obtained by Bradt and Wong in ZnO- Bi203 system (14,16). VUl Grain measurements by using linear intercept method show there is no important effect of CoO on ZnO grain size. Activation analysis by using kinetic grain growth equation gave following results; 204 Kj/mol for %0 CoO, 187 Kj/mol for %2 CoO, 203 Kj/mol for % 4 CoO additions. Kinetic grain growth exponents were 5.3 for %0 CoO, 5.6 %2CoO, 5.2 for %4 CoO. From XRD analsis of six samples whih have three different composition sintered at lOOOoC and 1300oC during 1 hour only ZnO and p Bi203 phases were obtained. There is'nt any compound of CoO with Bi203 or ZnO and Bi203 with ZnO. CoO may exist in system as soid solition. SEM and optical microphotos show effects of sintering temperature and time on grain size. Coarsening at grain sizes are occcuring with increasing temperature and time.