Papers by Author: Andanastuti Muchtar

Abstract: In this study, samarium strontium cobaltite, Sm_0.5Sr_0.5CoO_3−δ (SSC) and samarium doped ceria, Sm_0.2 Ce_0.8O_1.9 (SDC) carbonate or (SDCC) was used as the new composite cathode powder materials. This composite cathode powder was prepared via high energy ball milling (HEBM) technique for LTSOFC application. Various weight percentages of SDCC ranging from 50 wt.% to 70 wt.% was chosen to be added with SSC powder. The prepared samples of SSC–CE55, SSC-CE64 and SSC-CE73 composite cathode powders were characterized by using TGA, XRD , FTIR in order to investigate their physical structural, morphological and chemical compatibility. Result shows that this new composite cathode powders obtained good thermal stability, chemical compatibility and exhibit no trace of dissimilar phase from both SSC and SDCC. The existence of carbonates layer in the SSC-SDCC composite cathodes powder has been verified by analysis of the FTIR spectra.

Abstract: The microstructure, specific surface area, and pore size distribution of nickel oxide–samarium doped ceria carbonate (NiO–SDCC) composite anodes were investigated with respect to wt.% NiO. Four types of composite anode powders were prepared by mechanically mixing NiO with SDCC electrolytes at different wt.% (50:50, 60:40, 70:30 and 80:20). XRD results demonstrated that the crystalline phase structure of NiO and SDCC does not show any phase change, but only exhibits the NiO and SDCC interface within the NiO–SDCC composite anode powder. SEM and BET results show that wt.% NiO considerably influenced the particle connectivity between NiO and SDCC, particle size, specific surface area, and pore size distribution. Specific surface area, and pore size increased whereas particle size decreased with an increase in wt.% NiO. The highest surface area (11.9 m²/g) and the largest pore volume (0.14 cm³/g) were obtained for NiO–SDCC (60:40) composite anode powder. Therefore, the composite anode with large specific surface area displays the highest catalytic activity and the lowest interfacial polarization resistance at the anode/electrolyte interface.

Abstract: The demand for tetragonal zirconia as a dental restorative material has been increasing because of its excellent mechanical properties and resemblance to natural tooth color, as well as its excellent biological compatibility. Cerium oxide (CeO₂) has been added to yttria-stabilized zirconia (Y-TZP), and studies have demonstrated that the stability of the tetragonal phase can be significantly improved. Y-TZP with 5wt% CeO₂ as a second stabilizer was developed via colloidal process, followed by a suitable sintering process. According to the literature, the sintering process is the most crucial stage in ceramic processing to obtain the most homogeneous structure with high density and hardness. This study aims to investigate the effect of sintering temperature on the mechanical properties of nanostructured ceria–zirconia fabricated via colloidal processing and slip casting process with cold isostatic pressing (CIP). Twenty-five pellet specimens were prepared from ceria–zirconia with 20 nm particle size. CeO₂ nanopowder was mixed with Y-TZP nanopowder via colloidal processing. The consolidation of the powder was done via slip casting followed by CIP. The samples were divided into five different sintering temperatures with. Results from FESEM, density and hardness analyses demonstrated statistically significant increase in density and hardness as the sintering temperature increased. The hardness increased from 4.65 GPa to 14.14 GPa, and the density increased from 4.70 to 5.97 (g/cm3) as the sintering temperature increased without changing the holding time. Sintering Ce-Y-TZP at 1600 °C produced samples with homogenous structures, high hardness (14.14 GPa), and full densification with 98% of the theoretical density.

Abstract: In addition to the good electrochemical performance criteria in solid oxide fuel cell (SOFC) applications, cathode material must match thermal expansion with other SOFC components. Thus, effects of Ag on thermal mismatch, chemical reactions, and microstructure are investigated. Ag (1 wt% to 5 wt. %) was mixed with La₀.₆Sr₀.₄Co₀.₂Fe₀.₈O_3-δ (LSCF6428) and Sm-doped ceria carbonate (SDCC) composite cathode powder. LSCF6428-SDCC-Ag samples were sintered at 600 °C for 2 h. The thermal expansion coefficients (TECs), which were determined using a dilatometer, indicated relatively less TEC mismatch between LSCF-SDCC-Ag cathodes composite and SDCC electrolyte. The average TEC value obtained from 20 °C to 600 °C implied that LSCF-SDCC-A5 (5 wt. % Ag) showed better thermal matching (13.18×10⁻⁶ K⁻¹) with SDCC electrolyte (12.84×10⁻⁶ K⁻¹) and achieved better compatibility. The X-ray diffraction patterns indicated that the LSCF6428-SDCC-Ag peak increased with the increase in the amount of Ag. Scanning electron microscopy analysis showed that Ag was capable of maintaining the porosity that is required for cathodes (20%–40%). Results showed that Ag exhibited desirable thermal and chemical compatibility with LSCF-SDCC. Thus, LSCF6428-SDCC-Ag can be used as a composite cathode for low-temperature SOFCs.

Abstract: Development of tin/multi-walled carbon nanotube (SnO₂/MWCNTs) thin films were prepared by sol-gel method. The synthesis of tin oxide (SnO₂) was carried out by dissolving tin (II) chloride (SnCl₃) in a solvent of 2-methoxyethanol. Different annealing temperatures of 400 °C, 450 °C, 500 °C, 550 °C and 600 °C were proposed in this study. The changes in the structural properties were analyzed by means of transmission electron microscopy (TEM) and atomic force microscopy (AFM) analysis. AFM results indicated very rough surface area of SnO₂/MWCNTs thin films where roughness values increased linearly from 1.8 nm to 11 nm by increasing the annealing temperatures from 400 °C to 600 °C. The SnO₂/MWCNTs-based DSSC exhibited good photovoltaic performance with power conversion efficiency (η), photocurrent density (J_sc), open circuit voltage (V_oc) and fill factor (FF) of 0.62 %, 5.6 mA cm^-2, 0.55 V and 0.65 respectively. The obtained structural and photovoltaic performance analysis was proposed as a suitable benchmark for Sn/MWCNTs based dye-sensitized solar cell (DSSC) application.

Abstract: Ceramics are increasingly popular in dental restoration after metal restoration has been found to be less esthetic. One such example is yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP). However, one of the challenges of this application is its insufficient translucency to ensure high esthetic restoration. This study reviews the effect of sintering parameters, primary particle size, microstructure homogeneity, and thickness of zirconia on its translucency. Several studies remarked that the thickness of the framework had the greatest effect on zirconia translucency. Thus, a rigorous thickness control is necessary. The consideration for optimal sintering parameters (sintering temperature and holding time) and the use of smaller particle sizes help in the densification and elimination of porosity in zirconia, which, consequently, improve its translucency. Finally, a homogeneous microstructure can reduce the light scattering effect in zirconia and increase its translucency. Identifying the factors that influence zirconia translucency can contribute to future research in improving the esthetic dental restoration.

Abstract: Cathode-carbonate is a prospective material for low-temperature solid oxide fuel cells (LT-SOFCs). The influence of carbonate on the properties of cathode LT-SOFCs was studied. Different amounts of binary carbonate (Li₂CO₃:Na₂CO₃) were prepared at molar ratios of 67:33 and 62:38. The cathode composite powders were calcined at 750 °C for 2 h before uniaxial press. X-ray diffraction results confirmed that the carbonate in cathode composite existed as amorphous phase. Therefore, carbonate has a good compatibility with cathode composite powders. High-energy ball milling maintained the composite cathode powders at the nanoscale. The developed cathode composite also gave acceptable porosity values between 26% and 27%. The findings show that that the amount of binary (Li/Na) carbonate in cathode composite influences its properties, indicating potential for LT-SOFCs applications.

Abstract: The use of tetragonal zirconia as a dental restorative material has recently increased because of its unique mechanical and optical properties, as well as high biological compatibility with the oral cavity environment. However, the mechanical properties of zirconia can be severely degraded, which leads to the failure of dental restorations. This review focuses on the low-temperature degradation of dental zirconia and its effects on the properties of zirconia and on the oral environment. The purpose is to show the importance of this negative phenomenon and suggest guidelines for minimizing the aging of zirconia that is used as a dental restoration material.

Abstract: Zirconia-based ceramics exhibit excellent mechanical properties and biocompatibility in dental applications. However, the production of translucent zirconia that offers resemblance to real teeth remains a challenge. This study aims to fabricate zirconia compacts by cold isostatic pressing (CIP) and investigate the influence of sintering temperature on translucency, microstructure, hardness, and density of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP). Zirconia stabilized with 3 mol% yttria (3Y-TZP) was pressed by uniaxial pressing and later by CIP to produce green bodies in pellet form. Subsequently, the green bodies were sintered at different temperatures (1100 °C to 1300 °C). The specimens were then investigated in terms of translucency, density, and hardness. X-ray diffraction was also performed and the microstructure of the specimens was observed under a scanning electron microscope (SEM). Density and light transmittance tests results showed that zirconia sintered at 1200 °C exhibits the highest density (5.957 g/cm³) and light transmittance intensity. Vickers hardness test showed that higher sintering temperatures result in higher hardness of the sintered zirconia. SEM micrographs illustrate the effect of microstructural changes on the translucency of zirconia. A temperature of 1200 °C is found to be the recommended sintering temperature at which zirconia exhibiting optimum translucency and mechanical properties is produced. CIP is found to be a suitable consolidation method to produce high-density translucent zirconia.

Abstract: Consolidation of ceramic parts may be achieved by several techniques, including the slip casting and cold isostatic pressing (CIP) methods. In the present work, the performances of the two methods are compared in the fabrication of nanostructured zirconia compacts for dental crown applications. First, a zirconia suspension suitable for slip casting was prepared. The rheological properties of the zirconia suspension were optimized by adding a dispersant agent and controlling the pH. Zirconia slurries were then slip-cast into a pellet. Second, another group of zirconia pellets were fabricated using uniaxial pressing and were then cold-isostatically pressed. Both slip-cast and CIP samples were sintered at 1300 °C with a soaking time of 2 hrs. The mechanical properties of both samples were compared. The samples prepared by slip casting were denser compared with those prepared via CIP. Slip casting technique produced samples with 98.8% of the theoretical density, which resulted in the high Vickers hardness (11.4 GPa) of the slip-cast samples. Morphological studies revealed that the microstructures of the slip cast-sample were more homogeneous and contain no porosity. The formation of such a structure is due to the enhancement of the particle packing efficiency by slip casting as well as to the removal of larger agglomerates by colloidal processing prior to casting. As a consolidation stage, slip casting appears to be more suitable than the CIP technique in preparing reliable nanostructural ceramic parts.