Semra Kurama

The effect of processing conditions, amount of additives and composition on the microstructures and mechanical properties of α-SiAlON ceramics

Abstract A series of samples with yttrium α-SiAlON compositions and different amounts of additive has been fabricated from α-Si 3 N 4 , AlN, Al 2 O 3 and Y 2 O 3 starting powders, using gas pressure sintering and three different sintering procedures. One series of samples was heated up to 1825°C and then held for 3 h, another group of samples was held at a lower temperature (1500 or 1600°C) for 1 h and then heated up to 1825°C and held for 3 h. The results of investigations using scanning electron microscopy showed the effect of composition and sintering procedure on the morphology of α-SiAlON grains. It was found that the amount of elongated grains increased with increasing amount of liquid phase. The mechanical tests showed that all of the samples exhibited HV10 values in the range of 1800–1976 kg/mm and K IC values in the range of 3.9–6.3 MPam 1/2 .

Synthesis and Sintering of Cordierite at Low Temperature from Kaolin and Magnesium Hydroxide

Thermal reactions of ultrafine Mg(OH)2 mixtures of ultrafine particles and kaolin mixtures were investigated to obtain cordierite at low temperatures. Magnesium hydroxide was precipitated from magnesium chloride (MgCl2) solution. Kaolin with three different chemical composition (KI, KS and KK) was mixed with Mg(OH)2 to form cordierite at different temperatures (900-1200°C). Mole ratio of mixtures were MgO:Al2O3:2SiO2. To obtain dense cordierite, boron oxide in different quantities (in the form of magnesium borate 2MgO.B2O3) were added into mixtures of kaolin and Mg(OH)2. After sintering, it was observed that, by increasing the amount of B2O3 in the mixture, the density was not changed significantly. Introduction Rankin and Merwin found cordierite, Mg2Al3Si5AlO18, to be the only ternary phase in the MgO-Al2O3-SiO2 system [1]. Cordierite as a ceramic material has a wide range of uses and applications stemming from its important properties of low thermal expansion and dielectric constant coupled with high chemical and thermal stability. In addition, cordierite is expected to have great potential as a substrate material for integrated circuit boards [2] replacing alumina, which has a relatively lower dielectric constant in the high frequency region. There are no known large deposits of natural cordierite, and it probably is never available for introduction to ceramic bodies. A body having the theoretical composition of cordierite may consist of 39.6 % talc, 47.0 % clay and 13.4 % alumina. Bodies of this composition, however, have so short vitrification range that they cannot be fired in industrial kilns. Preparing dense cordierite glassceramics also has long been a problem because of this problem [3-5]. In this study, ultrafine particles of Mg(OH)2 were precipitated from aqueous solution of magnesium chloride. Cordierite ceramics were obtained by mixing Mg(OH)2 particles with different type of kaolin and firing between 900-1200°C. To obtain dense cordierite ceramic different amounts of B2O3 were added into mixtures. Experimental Procedure Sample preparation. Three types of kaolin, designated as KI, KS and KK, mined around Turkey were used as raw material to synthesise the cordierite ceramics. The particle size distribution and chemical composition of kaolins are given in Fig. 1 and Table 1, respectively. The classified kaolin consisted of tabular particles of kaolin with mean particle sizes of 1.5, 1.3 and 0.8 μm. As a source of MgO, ultrafine particles of magnesium hydroxide were prepared as in our previous work [6]. The kaolin and Mg(OH)2 were weighed according to the molar ratio of MgO:Al2O3:2SiO2 and were mixed using an agate mortar in ethanol media to prevent aggregation. Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 925-928 doi:10.4028/www.scientific.net/KEM.264-268.925

Utilization of Coal Combustion Fly Ash in Terracotta Bodies

In this present work, coal combustion fly ash from a power plant in Turkey was used in combination with a traditional raw material in terracotta production with the aim of having a product with improved physico-mechanical properties and lower production cost. Several compositions were prepared by adding different amounts of fly ash (ranging from 0 to 20 %) in a yellow firing terracotta formulation and shaped by wet pressing. Following firing at a suitable temperature, some of the physical properties of the resultant tiles were determined as a function of the fly ash content. A combination of XRD, SEM and EDX techniques were also employed to correlate the properties with the phase composition. The results indicated that fly ash could be utilized easily in certain amounts in such an application.

Investigation properties of BaO/RO-Al2O3-R2O3-B2O3-SiO2 glass-ceramic sealants for solid oxide fuel cell

Glass-ceramics are commonly used as sealing materials for planar solid oxide fuel cells (SOFCs). The major requirements of stack and module builders for these materials are the stability of the coefficient of thermal expansion (CTE), the excellent bonding (sticking) behaviour and the absence of volatile ingredients, which can lead to changes of the material properties and the sealing ability. In this study, the thermal stability of glasses in the BaO-Al2O3-La2O3-B2O3-SiO2 system was investigated, to develop a suitable sealing glass for planar solid oxide fuel cell operating at 800–850°C. The developed glasses were characterised through measurement of different properties, coefficient of thermal expansion (CTE), glass transition temperature (Tg), dilatometric softening temperature (Td), crystallisation behaviour during prolonged heat-treatment, density etc. At a target operating temperature of 750°C, the long-term coefficient of thermal expansion (CTE) of one particular composition (G4) was found to be particularly stable, due to devitrification to a mixture of glasses and ceramic phases.

Defect Detection in Porcelain Tiles Using Ultrasound

The main point of this work is the optimization of ultrasonic methods for the non-destructive testing of sintered porcelain tiles containing defects. Zirconia and steel balls of different diameter and rubber with different thicknesses were imbedded in porcelain tile granules before pressing. After being sintered at 1473 K, the tiles were inspected using ultrasound. This method may allow defect detection using an A-scan. It can be observed that the amplitude of the back wall echo for a defective part is smaller than for a part without defects. Depending of the size, shape and position of the defect, a defect peak can be observed. An immersion pulse-echo C-scan was used to differentiate defects. The shape and place of the defects can be defined.

Mg+2 and Ce+3 Doped α-SiAlON Ceramics

 In the present work, stabilization of MgO and CeO2 in α-SiAlON structure is studied. Mg and CeO2 were used in 50:50 molar ratios as additives. The mixture was sintered at 1830°C for 1 hour in gas pressure sintering furnace (GPS). The results show that sintered sample was composed of α-SiAlON and β-SiAlON phases. According to X-ray diffraction (XRD) and energy dispersive X-ray measurements (EDX) analyses, Mg and Ce cation could be incorporated into α-SiAlON structure higher than single R cation (R=Mg or Ce) doped compositions.

Densification of Sr-Mg Doped SiAlONs with GPS and SPS

At temperature above 1200°C, the thermal stability of α-SiAlON phases has been debated since 1992; however, it has been discussed if any α-SiAlON phase can be formed in Ce, La, Eu and Sr-doped SiAlON systems. In our previous studies it was shown that the use of Mg-Ce and Mg-Sr elements as dopants SiAlON compositions, in which all elements just have very low or no stability in the α-SiAlON structure, would promote the stability of Mg-Ce elements in the α- SiAlON phase [1, 2]. However, in Mg-Sr systems, it was obtained that Mg2+ is predominantly incorporated in α-SiAlON structure whereas Sr2+ mainly remains in the grain boundaries [2]. In this study, by applying spark plasma sintering (SPS) (at 1400-1700°C) and post-sintering thermal heat treatment (at 1500°C for 5 hrs and 1700°C for 2hrs) Mg or Mg-Sr doped SiAlON (50:50 mole ratios) ceramics were prepared. The results were compared with GPS sintered samples data. The effect of sintering temperature on densification process, phase transformation, microstructure and mechanical properties of samples were investigated. The results showed that by using SPS, Sr-Mg doped samples can be sintered at lower temperature (at 1600°C) than at GPS (at 1800°C) and it has no Sr-doped grain boundary phases.

Microstructural Evolution R-α-SiAlON Ceramics (R=Y, Y+Sm and Sm)

Yttrium or samarium doped α-SiAlON composition was sintered using gas pressure sintering through two different sintering procedures. One group of samples was heated up to 1800°C and then held for 1h while another group of samples was held at a 1600°C for 1hr before sintered at 1800°C for 1h. Microstructural characterization of the two stage sintered materials revealed the presence of elongated α-SiAlON form related with the type of additive cations.

The Design of Compositions for Multi-Cation Doped α-β SiAlON Ceramics by the Rietveld Method

Although densification of α-β-SiAlON ceramics is easier by using sintering additives, it is difficult to achieve the designed composition and therefore the me chanical properties after sintering. This is mainly due to the stability regions of α-SiAlONs which are related to cation size of the rare earth. In the present work, α-β-SiAlON starting composition was densified by gas pressure sintering using oxides of Nd 2O3, Y2O3 and equimolar mixture of Nd 2O3-Y2O3. The Rietveld technique was used for characterisation of the diffracti on pattern and calculation of lattice parameters of α-SiAlON. On the basis of obtained data, the stability regions of Y, Nd and Y-Nd doped α-SiAlONs were established. The Rietveld results were further confirmed the previous results obtained by classical XRD technique that the stability region for Nd-Y multi-cation doped α-SiAlON is larger than Nddoped system.

SERAMİK SAĞLIK GEREÇLERİNİN SİNTERLEME SICAKLIĞININ FİZİKSEL ÖZELLİKLERE VE TAHRİBATSIZ MUAYANE YÖNTEMİNE ETKİSİ

Bu calisma, insaat malzemeleri icinde siniflandirilan seramik saglik gereclerinin, sicakliga bagli olarak fiziksel ozelliklerinin ve ultrasonik hizlarinin degisimini incelemek amaciyla yapilmistir. Laboratuvar sartlarinda hazirlanan seramik saglik gereclerinin bunyeleri, gorunur porozite olusturmak ve gorunur poroziteye bagli olarak degisen miktarlarda su emme degerlerine sahip olmalarini saglamak amaciyla farkli sicakliklarda sinterlenmistir. Farkli sicakliklarda sinterlenen numuneler, tahribatsiz muayene yontemlerinden ultrasonik (darbe-yanki) test yontemiyle incelenmistir. Elde edilen ultrasonik hiz degerleri, kalibrasyon egrilerini hazirlamak ve sicakliga bagli su emme ile gorunur porozite gibi fiziksel ozelliklerin tahmini icin kullanilabilir.