Papers by Author: Akhmad Herman Yuwono

Abstract: Bi-Pb-Sr-Ca-Cu-O (BPSCCO) superconductors are recognized as a projectable high-temperature superconductor for high-efficiency electrical applications. The addition of Ti enhances the formation of the Bi-2223 phase from the BPSCCO superconductor. The process of producing BPSCCO superconducting materials with TiO₂ dopants is performed by the solid-state process and the production of wire rolling, consisting of bismuth (III) oxide powder (Bi₂O₃ = 99%), Strontium Carbonate powder (SrCO₃ = 99%), Calcium Carbonate powder (CaCO₃ = 99%), Copper Oxide powder (CuO₂ = 99%), Lead Oxide powder (PbO₂ = 98%) Bi: Pb: Sr: Ca: Cu ratio: 1.6: 0.4:2:2:3 doped by 1 %wt Titanium Oxide powder (TiO₂ = 98.5%). The variables used in this study were the comparison of the sintering method at 860°C for 24 hours and 820 °C calcination for 20 hours, and 850°C sintering for 20 hours. The superconductor characterization was tested through the X-Ray Diffraction (XRD) test, Scanning Electron Microscopy (SEM), and Resistivity test. XRD test results showed the formation of Bi₂Sr₂CuO₆ and Bi₂Sr₅Cu₃O₁₆ phase. SEM results showed an increase in grain size. The resistivity test results showed that all samples formed critical temperatures, 9.6 and 9.5K respectively.

Abstract: In this work, the development of solution-processed bulk heterojunction hybrid solar cells based on CdSe quantum dot (QD) and conjugated polymer poly [2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta [2,1-b;3,4-b] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)], PCPDTBT was performed. The photoactive layer was formed by integrating CdSe QDs onto multiwalled carbon nanotubes (CNTs). A simple method of thiol functionalization in the interface CNTs and CdSe QDs has been investigated. Integration of CNTs enhances long-term performance of solar cells devices. Initial PCE values of about 1.9 % under AM1.5G illumination have been achieved for this hybrid CNT-CdSe photovoltaic device. In addition, the long-term stability of the photovoltaic performance of the devices was investigated and found superior to CdSe QD only based devices. About 84 % of the initial PCE remained after storage in a glove box for one year without any further encapsulation. It is concluded that the improvement is mainly due to a strong binding between thiol functionalized CNTs and CdSe QDs, resulting preservation of the nanomorphology of the hybrid film over time.

Abstract: MgB₂ superconductor with relatively high critical temperature (Tc=40 K) has been developed for possibilities utilization in various practical applications such as Magnetic Resonance Imaging. In order to enhance the process, the material was prepared by Powder-In-Tube (PIT) method, while the superconducting properties was improved by incorporating 0 wt.%, 10 wt.% and 20 wt.% SiC nanoparticles into MgB₂ structure. This study aimed at analyzing the effect of sintering temperature on the microstructure, resistivity and phase of Fe-sheathed MgB₂ superconducting wires. Three different compositions of MgB₂ powders were inserted into Fe tube with inner ø of 4 mm and outer ø of 6 mm. This tube was then rolled and drawn into a ø 2.5 mm wire. Wire samples with three different compositions of SiC were heat treated at 600°C and 800°C respectively. All samples were characterized to analyze the morphology, resistivity and crystal structure. X-ray diffraction (XRD) analysis showed that some Mg may react with SiC to form MgSi and with oxygen to form MgO. Scanning electron microscope (SEM) images revealed that with no addition of SiC, MgB₂ was formed and dispersed uniformly in wire, but with 10 wt.% and 20 wt.% SiC nanoparticles, the whisker morphology was observed resulting in degradation of the superconducting properties.

Abstract: TiO₂ nanoparticles have been successfully synthesized using the sol-gel method with main materials of titanium tetraisopropoxide (TTIP) and HClO₄ solutions. Mass ratios (Rw) of aquadest and TTIP were 0.85, 2.00, and 3.50 which were going to be investigated in crystallization of TiO₂ phases. Pre-heating was performed on TiO₂ at 60°C for one day then it was annealed at 150°C for 3 hours. The DSSC structure was formed by using the synthesized-TiO₂ as semiconductor material and beta-carotene as dye sensitizer. The x-ray diffraction (XRD) spectrum indicated that TiO₂ peaks had anatase phases on crystal orientation of (101), (004), and (200) while TiO₂ of rutile phase only appeared on orientation of (211). The highest intensity for all Rw was dominated by (101) anatase phase. From XRD spectrum data of (101) peak, the Scherrer’s method predicted that crystal size of TiO₂ was 3.48 nm, 4.36 nm, and 4.47 nm for Rw of 0.85, 2.00, and 3.50, respectively. The Tauc’s method was applied on the UV-Vis data that predicted the bandgap energy (Eg) of TiO₂ for Rw of 2.00 (Eg=3.14 eV) was higher than Rw of 0.85 (Eg=3.02 eV) and 3.50 (Eg=3.04 eV). The I-V characteristic calculation of DSSC structures were obtained that the efficiency optimum is 0.01% for Rw of 2.00. It is considered that bandgap energy value correlated to stability of Ti-OH bonds that caused the exited-electrons are easily injected to conduction band of TiO₂. The performance of DSSC using the synthesized-TiO₂ which consists of anatase and rutile can be improved about ten times compared to that using the pure-TiO₂ rutile.

Abstract: ZnO nanoparticles have been used for many applications, including in cell labeling application. Its light emission can be used to determine and identify biology cells. Wet chemical precipitation method has been successfully done to synthesize the nanoparticle and it was subsequently continued by encapsulating with silica to keep ZnO stabilized in water to be properly used in cell labeling application. Varying precipitation temperatures has been performed to control the nanoparticle size and the addition of F127 surface active agent was carried out to prevent the agglomeration. The results showed the smallest nanoparticle (3.49 nm) was obtained from the process with temperature of 25oC, with the highest band gap energy, 3.12 eV. On the other hand, the largest nanoparticle (13.16 nm) was obtained from synthesis at temperature of 65oC, with the lowest band gap energy, 3.08 eV. These levels of band gap energy are potentially suitable for cell labeling application.

Abstract: The synthesis and characterization of TiO₂ nanostructure has become intensive nowadays because of its superior properties among other semiconductor materials. In this work, TiO₂ nanostructures have been derived from ilmenite mineral by using precipitation technique with various pH and calcination temperature. The resulting nanostructures were characterized to investigate the effects of those variables on the phase, crystallite size, and band gap energy. The characterization was performed by using XRD, FT-IR, UV-Vis DRS, SEM, EDS, and TEM. The results showed that TiO₂ sample prepared under low pH value of 0.3 demonstrated porous structures although they are not well-ordered yet, while the sample with a pH adjustment up to 7.0 provided nanotube structure. The biggest crystallite size of 3.43 nm and low band gap energy of 3.07 eV was obtained in the TiO₂ samples synthesized without pH adjustment and calcined at a a temperature of 300°C. This characteristics shows that TiO₂ nanostructure in this study is potential for the applications of dye sensitized solar cell (DSSC) and photocatalysist.

Abstract: The potential use of ZnO nanoparticles for cell labeling application has been improved over past several years. Focusing to overcome the tendency of the nanoparticles to aggregation, in this work ZnO nanoparticles have been synthesized by using surfactant-assisted precipitation method. The samples were then characterized by using XRD and UV-Vis Spectroscope. The results showed that the presence of surfactant could help controlling the crystallite size to become smaller (4.02 nm) as compared to the conventional precipitation method (9.45 nm). ZnO nanoparticles that had been coated by the surfactant was then re-coated again by silica shell to form ZnO@SiO2 core-shell. The presence of F-127 coating on the surface of the nanoparticles made the dispersion and the stability of crystallite size better in various encapsulation pH value (4.04 4.32 nm). The band gap energy of the ZnO nanoparticles (3.145 3.085 eV) also showed a good correlation with the crystallite size (4.02 10.38 nm). Therefore, the resulting ZnO@SiO2 core-shell in the present work are potential to be used in cell labeling application.

Abstract: Porous titanium dioxide (TiO₂) nanostructure has been successfully synthesized by a modified solgel method using non-ionic triblock copolymer pluronic F-127 as surfactant template and titanylsulfate (TiOSO₄) solution as the inorganic precursor derived from the sulfuric process of Bangka-Indonesia ilmenite ore. The resulting nanostructure samples were characterized by XRD, SEM, TEM and UV-Vis spectroscopy. The results showed that porous titania particles have sphere-like shape and can be indexed as the anatase phase with average crystallite size of about 5-7 nm, narrow pore size distribution with an averange pore diameter of about 3-5 nm and band gap energy in the range of 3.10 3.16 eV.

Abstract: lmenite (FeO.TiO₂) ore from Bangka island-Indonesia is a potential raw material for synthesizing titanium dioxide (TiO₂), which can be used further as pigmen and photocatalyst. The fabrication of TiO₂ particles from ilmenite can be carried out through the solvent extraction using sulfuric acid route. Therefore, the solubility of the ilmenite ore in sulfuric acid environment is one of the key factors to obtain the desired TiO₂ particles. The current research is aimed at comparing the solubility of pristine Bangka ilmenite ore with that of precedingly decomposed by sodium hidroxide (NaOH) in pressurized and atmospheric reflux reactors. The dissolution of both precursors was carried out in those reactors under various temperatures of 75, 100, 125, 150 and 175°C. The results showed that the optimum dilution was achieved at 150°C. The obtained recovery of ilmenite was 88.8 % for the pressurized reactor and 75.5% for the atmospheric reflux reactor. The solubility of titanium (Ti) element increased steadily to reach a recovery of 68% at 150°C and decreased significantly afterwards. It was also found that the increase of iron (Fe) element solubility was proportional to the increase of processing temperatures.

Abstract: Dye-sensitized solar cell (DSSC) is one of the very promising alternative renewable energy sources to anticipate the diminishing in the fossil fuel reserves in the next few decades and to make use of the abundance of intensive sunlight energy in tropical countries like Indonesia. TiO₂ nanoparticles have been used as the photo electrode in DSSC because of its high surface area and allow the adsorption of a large number of dye molecules. In the present study, TiO₂ aerogel have been synthesized via sol-gel process with water to inorganic precursor ratio (R_w) of 2.00, followed with subsequent drying by CO₂ supercritical extraction (SCE). As comparison, the TiO₂ xerogel was also prepared by conventional drying and annealing. Both types of gels were subjected to conventional and multi-step annealing. The resulting nanoparticles in aerogel and xerogel have a band-gap energy of 3.10 and 3.04 eV, respectively. The open circuit voltage (V_oc) measurement reveals that the DSSC fabricated with aerogel provided a higher voltage (21,40 mV) than xerogel (1,10 mV).