ORCID Profile
0000-0003-2337-3566
Current Organisation
Institut Teknologi Sepuluh Nopember
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Publisher: IOP Publishing
Date: 05-2018
Publisher: Elsevier BV
Date: 06-2017
Publisher: IOP Publishing
Date: 21-06-2019
Publisher: IOP Publishing
Date: 05-2018
Publisher: AIP
Date: 2013
DOI: 10.1063/1.4820291
Publisher: IOP Publishing
Date: 03-2019
Publisher: Informa UK Limited
Date: 02-10-2015
Publisher: Springer Science and Business Media LLC
Date: 09-11-2022
DOI: 10.1186/S12874-022-01773-9
Abstract: There are situations when we need to model multiple time-scales in survival analysis. A usual approach in this setting would involve fitting Cox or Poisson models to a time-split dataset. However, this leads to large datasets and can be computationally intensive when model fitting, especially if interest lies in displaying how the estimated hazard rate or survival change along multiple time-scales continuously. We propose to use flexible parametric survival models on the log hazard scale as an alternative method when modelling data with multiple time-scales. By choosing one of the time-scales as reference, and rewriting other time-scales as a function of this reference time-scale, users can avoid time-splitting of the data. Through case-studies we demonstrate the usefulness of this method and provide ex les of graphical representations of estimated hazard rates and survival proportions. The model gives nearly identical results to using a Poisson model, without requiring time-splitting. Flexible parametric survival models are a powerful tool for modelling multiple time-scales. This method does not require splitting the data into small time-intervals, and therefore saves time, helps avoid technological limitations and reduces room for error.
Publisher: Springer Science and Business Media LLC
Date: 04-07-2020
Publisher: Springer Science and Business Media LLC
Date: 05-04-2013
Publisher: Springer Science and Business Media LLC
Date: 11-03-2016
Publisher: Springer Science and Business Media LLC
Date: 06-1998
Publisher: Wiley
Date: 25-08-2021
DOI: 10.1002/APP.51565
Abstract: The thermomechanical properties of polyethylene glycol (PEG) composites filled with various zircon sizes were studied. The zircon powders were derived from natural (well‐known as puya) sand collected from Kereng Pangi, Central Kalimantan, Indonesia. The effects of the zircon size and content were examined to understand the thermomechanical properties of the composites using dynamic mechanical analysis in shear mode. Pure zircon powders with micron to nanometer sizes were prepared. The microzircon powders were prepared by heating zircon at 500, 1000, and 1200°C. Moreover, the nanozircon powders were prepared by a wet milling method with milling times of 5, 10, and 15 hours. Furthermore, the composites were prepared by a wet mixing method. According to elemental analysis of scanning electron microscopy/energy dispersive X‐Ray spectroscopy (SEM/EDX) data, it was found that the various zircon sizes caused different distribution effects, that is, in general, the smaller the size was, the better the distribution. Filler size variation also affected the thermomechanical properties of the composites. The addition of microzircon heated at 1200°C had the lowest storage moduli ( G' ), that is, 154.90 MPa and 155.55 MPa for 5 wt.% and 10 wt.%, respectively. Moreover, the maximum value of G' was obtained for the composite with the addition of nanozircon milled for 10 h (Z10h), that is, 679.27 MPa and 706.37 MPa for 5 and 10 wt.%, respectively. The addition of nanozircon slightly reduced room‐temperature G' , presumably due to the agglomerated filler, as confirmed by the SEM/EDX data. Moreover, a decrease in zircon size caused an increase in the melting temperature ( T m ) of the matrix. In contrast, 15 h of milling had a minor effect on T m and G' , whereas the loss modulus ( G" ) decreased with the addition of nanozircon. The effects of filler size on the thermomechanical properties of PEG/zircon composites are discussed in detail.
Publisher: IOP Publishing
Date: 17-04-2019
Publisher: IOP Publishing
Date: 09-2018
Publisher: Wiley
Date: 29-07-2022
DOI: 10.1002/APP.52923
Abstract: The thermomechanical and optical properties of poly(methyl methacrylate) (PMMA)/quartz and PMMA/zircon composites at filler content variations of 0, 1, 2.5, and 5 wt% have been studied. The silica quartz and zircon powders were derived from local sands collected from Tanah Laut, South Kalimantan, and Kereng Pangi, Central Kalimantan, Indonesia, respectively. The nanoquartz and nanozircon fillers (herewith were designated as quartz and zircon respectively) were obtained by milling these powders for 15 h. Dynamic mechanical analysis was used to acquire the storage modulus and glass transition temperature as the thermomechanical properties of the PMMA/quartz and PMMA/zircon composites. Meanwhile, the optical properties were determine using ultraviolet–visible spectroscopy. Results indicate that zircon fillers can improve the storage modulus of the composites up to 1.85 times that of PMMA alone. Zircon is more effective than silica in improving the storage modulus of PMMA. Implementing the degradation rate model shows us that all the composites have nearly the same degradation behavior in the glassy region. Moreover, the addition of fillers decreases the band gap energies.
Publisher: Elsevier BV
Date: 07-2023
Publisher: Cambridge University Press (CUP)
Date: 09-1998
DOI: 10.1017/S0885715600010046
Abstract: Mass attenuation coefficient corrections, for Rietveld phase analysis with an external compositional calibration standard, may be made using Compton scattering intensities measured by X-ray fluorescence spectrometry. The method is mainly useful for Rietveld phase analysis when mixing an internal standard is impossible or undesirable. The validity of the method has been demonstrated using a suite of alumina-zirconia powders of known composition. Also presented are results for a typical application—determination of phase composition depth profiles defining the graded compositional character of an aluminium titanate/zirconia-alumina ceramic composite.
Publisher: IOP Publishing
Date: 2017
Publisher: Trans Tech Publications, Ltd.
Date: 07-2015
DOI: 10.4028/WWW.SCIENTIFIC.NET/AMR.1112.47
Abstract: A study of liquid mixing route to synthesize high purity Mg 0.8 Zn 0.2 TiO 3 nanopowder, a candidate dielectric ceramics, has been successfully performed. Formation of the phases on the dried powder was studied using TG/DTA, XRD and FT-IR data. Rietveld analysis on the collected XRD patterns confirmed the formation of solid solution in the system. Such solid solution can be obtained from the powder calcined at 500 °C, but calcination at 550 °C gave rise to the most optimum molar purity up to 98.5% without intermediate phases. The role of Zn ions on the formation of solid solution was also discussed. Homogeneity of particle size distribution and nano-crystallinity of the system was verified from the particle size analyzer data, TEM image and the Rietveld analysis output.
Publisher: Springer Science and Business Media LLC
Date: 31-01-2018
Publisher: Author(s)
Date: 2016
DOI: 10.1063/1.4945517
Publisher: AIP
Date: 2010
DOI: 10.1063/1.3295577
Publisher: IOP Publishing
Date: 19-11-2018
Publisher: IOP Publishing
Date: 10-04-2017
Publisher: IOP Publishing
Date: 05-2023
Abstract: Ultra-high-density zircon (ZrSiO 4 ) ceramics were prepared using the spark plasma sintering (SPS) technique of zircon nanopowder with the addition of three different sintering agents, i.e., Bi 2 O 3 , V 2 O 5 and B 2 O 3 . The effect of each agent and the SPS parameters (temperature and pressure) on phase composition, microstructure, thermal and mechanical properties of the ceramics were evaluated. The identified crystalline phases of the sintered ceramics were zircon and monoclinic zirconia. The addition of a sintering agent affects the structure of zircon ceramics, i.e. the lattice parameter and the crystallite size. The sintered ceramics reached relative densities up to 99.9% of the theoretical one when V 2 O 5 or B 2 O 3 was added. SEM observations confirmed the densification of the zircon ceramics. We found the ceramics exhibited thermal conductivity ranging from 0.39 to 0.61 Wm −1 K −1 at 373 K while the coefficient of thermal expansion was 2.3–4.0 × 10 −6 /°C and the Vickers hardness was obtained to be 9.52–12.66 GPa. The Young’s ( E ), bulk ( B ), and shear ( G ) moduli, Poisson’s ratio ν, Pugh’s ratio B/G , and the ratio of H V 3 / E * 2 of the ceramics are in a range of 240 − 288 GPa, 207 − 267 GPa, 91 − 109 Pa, 1.95 − 2.45, and 0.011 − 0.019 respectively. We found that high-density, quasi-ductile zircon ceramics can be synthesized at a low sintering temperature and short holding time.
Publisher: AIP
Date: 2008
DOI: 10.1063/1.2906059
Publisher: Wiley
Date: 16-12-2020
DOI: 10.1002/APP.50384
Publisher: IOP Publishing
Date: 09-2018
Publisher: Springer Science and Business Media LLC
Date: 06-1998
Publisher: Trans Tech Publications, Ltd.
Date: 27-07-2023
DOI: 10.4028/P-CGAJ4B
Abstract: A study was carried out on the effect of hydrochloric acid (HCl) temperature and molarity during the dissolution process on the crystal cell of ZrSiO 4 of zircon powders derived from zircon sand mined in Kereng Pangi, Central Kalimantan. The study is a continuation of the previous work and is aimed at associating the process and structural parameters of the zircon phase in the product. The synthesis was started with a well-established route in our laboratory, i.e., magnetic separation and milling for 2 hours, but the subsequent leaching using the HCl step was carried out at varying temperatures and concentrations. The temperature variations used were 80°C, 90°C, and 100°C, while the molar variations of HCl were 0.5 and 2 M. Then, the various products were further leached using 7 M sodium hydroxide (NaOH) to yield zircon powders subjected to X-ray diffraction (XRD) investigation. The XRD data analysis using Rietica software found that at the concentration of 0.5 M, the increasing temperature decreased in zircon cell volume but increased the tetragonality parameter c/a . However, opposite results occurred at the concentration of 2 M HCl. In general, we found that the impurity levels in the zircon powders may be associated with its cell volume and tetragonality parameters, i.e., the more the quartz content in the zircon powder, the smaller the zircon cell volume and tetragonality.
Publisher: AIP
Date: 2013
DOI: 10.1063/1.4821002
Publisher: Elsevier BV
Date: 04-2019
Publisher: International Union of Crystallography (IUCr)
Date: 22-03-2002
DOI: 10.1107/S0021889801020507
Abstract: Strain–size evaluations from diffraction line broadening for MgO ceramic materials have been compared using single-line integral-breadth and Rietveld procedures with the Voigt function. Diffraction data were measured by Bragg–Brentano X-ray diffractometry (XRD), without incident beam monochromatization, and neutron diffractometry (ND) to encompass near-surface and bulk effects, respectively. The specimens consisted of sets of MgO ceramics and MgO–Y 2 O 3 ceramic composites sintered over a range of temperatures. An MgO ceramic sintered at 1723 K for 2 h exhibited slightly less XRD broadening than the standard LaB 6 NIST 660 SRM, and was therefore selected to make instrument profile corrections for both XRD and ND data. It was found for both data types that: ( a ) sintering initially relieves residual strain present in the MgO powder used to sinter the ceramics and also promotes grain growth ( b ) residual strain of the MgO ceramic minimizes as the sintering temperature increases, and then increases with further rise in the sintering temperature, presumably as a result of intragranular interactions associated with grain growth and ( c ) introduction of the second phase (Y 2 O 3 ) increases strain and inhibits crystal growth. The single-line and Rietveld methods gave similar strain values from both the XRD and ND data within the limits of experimental error, but there were substantial differences between the single-line and Rietveld size estimates determined with the XRD and ND data.
Publisher: Elsevier BV
Date: 08-2019
Publisher: Springer Science and Business Media LLC
Date: 1996
DOI: 10.1007/BF00274609
Publisher: IOP Publishing
Date: 05-2018
Publisher: Elsevier BV
Date: 03-2011
Publisher: Elsevier BV
Date: 09-2019
Publisher: Trans Tech Publications, Ltd.
Date: 08-2015
DOI: 10.4028/WWW.SCIENTIFIC.NET/MSF.827.213
Abstract: Ferrofluid (magnetite/Fe 3 O 4 magnetic fluid) is colloidal suspension containing Fe 3 O 4 nanoparticles dispersed in a liquid carrier. In this work, Fe 3 O 4 particles in the fluid have been prepared by a simple co-precipitation route. The nano-structural behaviors such as phase purity and crystal structure of magnetite particles in ferrofluid were studied by means of X-ray diffractometry (XRD). Meanwhile, the form and structure factors were investigated by small-angle neutron scattering (SANS) spectrometer. The XRD pattern confirmed a single phase of spinel cubic Fe 3 O 4 structure. Further XRD data analysis revealed that the magnetite has a lattice parameter of 8.38 Å. The SANS data was fitted by applying a lognormal spherical calculation as a form factor and a mass fractal model as a structure factor. It showed that the magnetite ferrofluid has primary particles of 7.6 nm in diameter with fractal dimension of 1.2, which can be associated with chain-like structure. The chain-like structured Fe 3 O 4 ferrofluid based on local natural iron sand in this work opens new opportunities to be applied for novel prospective applications.
Publisher: Wiley
Date: 28-12-2018
DOI: 10.1002/APP.47372
Publisher: Author(s)
Date: 2017
DOI: 10.1063/1.4968375
Publisher: IOP Publishing
Date: 07-05-2019
Publisher: World Scientific Pub Co Pte Lt
Date: 09-2017
DOI: 10.1142/S1793292017501107
Abstract: Manganese (Mn)-doped black iron oxide (Fe 3 O 4 ) magnetic fluids in the system of Mn x Fe[Formula: see text]O 4 were successfully synthesized from natural magnetite (iron sand) by using co-precipitation method at room temperature. The analyses of the small angle neutron scattering (SANS) data by applying a log-normal sphere with a mass fractal models for [Formula: see text] and [Formula: see text] and two log-normal spheres with a single mass fractal models for [Formula: see text], 0.75 and 1 revealed that the primary particles of the Mn x Fe[Formula: see text]O 4 fluids tended to decrease from 3.8[Formula: see text]nm to 1.5[Formula: see text]nm along with the increasing fraction of Mn contents. The fractal dimension ([Formula: see text]) increased from about 1.2 to 2.7 as the Mn contents were increasing which physically represents an aggregation of the Mn x Fe[Formula: see text]O 4 particles in the fluids growing up from 1 to 3 dimensions to consolidate a more compact structure. The magnetization curves of the fluids exhibited an increasing saturation magnetization from [Formula: see text] to [Formula: see text], and a decreasing on [Formula: see text] and 0.75, with the maximum achievement of [Formula: see text]. These phenomena may probably be due to the combined effects, arising from cationic and dopant distributions, aggregation and its size, and also fractal dimension. Furthermore, the decrease of blocking temperature of the Mn x Fe[Formula: see text]O 4 magnetic fluids could be associated with the reduced particle sizes, while the freezing temperature had its highest peak intensity when it collectively occurred with the blocking temperature at a similar point of about 270[Formula: see text]K.
Publisher: IOP Publishing
Date: 03-2018
Publisher: Wiley
Date: 15-08-2023
DOI: 10.1002/APP.54536
Abstract: Zircon‐filled poly(methyl methacrylate) (PMMA) composites were synthesized, and their physical properties after ambient heat treatments were investigated. The sub‐micron zircon filler was obtained by purifying local zircon sand. The heat treatments were at 25°C (untreated), 55°C, and 70°C, around the glass transition temperature of PMMA. The crystal and molecular structures of the s les were investigated using X‐ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy. The thermomechanical, optical, and thermal (between RT and 550°C) properties of all composites were examined using dynamic mechanical analysis (DMA), UV–Vis spectroscopy, and differential thermal/thermogravimetric analysis (DT/TGA) instruments. XRD patterns showed that there were no crystal structure changes. FTIR peaks were reduced due to the heat treatment indicating the presence of PMMA molecular degradation in the heat‐treated s les. Meanwhile, DMA data showed that the heat‐treated s les exhibit a much lower room temperature storage modulus, that is, up to half as compared to the untreated ones. Furthermore, the heat treatment also affects the optical properties, including a slight drop of transmittance in UV‐A and visible regions but a slight increase of transmittance in UV‐B and UV‐C regions. Finally, the differential scanning calorimetry/thermogravimetry (DSC/TG) data show that the heat‐treated pure PMMAs become more challenging to undergo thermal degradation (i.e., mass drop).
Publisher: AIP
Date: 2013
DOI: 10.1063/1.4820994
Publisher: Trans Tech Publications, Ltd.
Date: 07-2019
DOI: 10.4028/WWW.SCIENTIFIC.NET/MSF.964.40
Abstract: Investigation of Fe K-edge X-Ray Absorption Near Edge Spectroscopy (XANES) spectra of Fe 3 O 4 (FeO.Fe 2 O 3 ) from natural source compared with the Fe 3 O 4 standard is presented. The natural Fe 3 O 4 powder was prepared from ironstone of Tanah Laut, Kalimantan Selatan by co-precipitation method. XANES measurements in transmission mode were performed at the Synchrotron Light Research Institute (SLRI), Nakhon Ratchasima, Thailand. XRD phase analysis confirms that the synthesized Fe 3 O 4 powder is a single phase, but it cannot determine the proportion of Fe 2 O 3 and FeO in the structure. TEM measurement confirms that the particle size of natural Fe 3 O 4 about 10 nm. Qualitative analysis of the pre-edge XANES data revealed that the absorbing atom in the XAS measurement is Fe 3 + . Meanwhile, the absorption edge ( E 0 ) values of natural and standard Fe 3 O 4 powders were 7126.44 eV and 7125.02 eV, respectively. The proportion was then acquired using XANES data analysis through Linear Combination Fitting (LCF). It was found that the natural Fe 3 O 4 s le consisted of 98 wt. % Fe 2 O 3 and 2 wt.% FeO, while the standard Fe 3 O 4 powder consisted of 96 wt. % Fe 2 O 3 and 4 wt. % FeO. The mechanism of the absorption in both s les is also described and compared.
Publisher: Trans Tech Publications, Ltd.
Date: 08-2015
DOI: 10.4028/WWW.SCIENTIFIC.NET/AMR.1123.383
Abstract: A study to enhance the value of natural silica sand from Tanah Laut, South Kalimantan, Indonesia has been initiated. A number of local sands were selected as the candidates for the study. The selected sand contained more than 90% quartz and was further processed to obtain a high purity initial powder using magnetic separation and immersion with HCl. The sealing materials were prepared by mixing the natural-sand-based silica (SiO2) powder with magnesia (MgO) and boria (B2O3) with composition of 70:10:20 by weight followed by uniaxial pressing and finally sintering at 1150°C for 1h and 4 h to produce ceramic composites. XRD measurement revealed that the ceramic contained quartz, protoenstatite, and clinoenstatite. The 1h and 4h ceramics exhibited 1.89% and 0.43% apparent porosity, 7.00 106 and 6.63106 cm electrical resistivity, 3.60 and 2.29 GPa Vickers microhardness, and 11.2010-6 and 11.5510-6 ppm/°C thermal expansion coefficient respectively. The 4h s le is more appropriate for sealing function in fuel cell than the 1h s le.
Publisher: IOP Publishing
Date: 03-2018
Publisher: AIP Publishing LLC
Date: 2014
DOI: 10.1063/1.4897124
Publisher: IOP Publishing
Date: 04-2018
Publisher: IOP Publishing
Date: 02-2019
Publisher: AIP Publishing LLC
Date: 2016
DOI: 10.1063/1.4941506
Publisher: Informa UK Limited
Date: 08-03-2022
Publisher: Elsevier BV
Date: 08-2022
Publisher: Springer Science and Business Media LLC
Date: 30-05-2015
Publisher: IOP Publishing
Date: 03-2018
Publisher: Elsevier BV
Date: 11-2021
Publisher: Springer Science and Business Media LLC
Date: 08-2017
Publisher: Springer Science and Business Media LLC
Date: 08-07-2022
Publisher: Elsevier BV
Date: 04-2019
Publisher: Wiley
Date: 12-06-2019
DOI: 10.1002/APP.48130
Publisher: IOP Publishing
Date: 02-2019
Publisher: IOP Publishing
Date: 07-05-2019
Publisher: Trans Tech Publications, Ltd.
Date: 07-2015
DOI: 10.4028/WWW.SCIENTIFIC.NET/AMR.1112.262
Abstract: A prospective fuel cell sealing material using silica sand from Bancar, Tuban, East Java has been investigated. The investigation was aimed to enhance the value of such natural sand and was started by a treatment using magnetic separation and immersion with HCl to produce pure silica powder. Ceramic composites were then synthesized by a solid state reaction method by mixing the silica powder, magnesia (MgO), and 5-20wt% of boria (B 2 O 3 ), pressing the mixture into pellets and finally sintering them at 1150 °C for 4 hours. By Archimedes method, it was found that the addition of B 2 O 3 is favorable to reduce the porosity of composites with a maximum value reaching 0.68(0)% for the composite with 20% B 2 O 3 . An X-ray diffraction technique revealed that a liquid phase sintering had occurred, indicated by the appearance of quartz (SiO 2 ), forsterite (Mg 2 SiO 4 ) and protoenstatite (MgSiO 3 ) in the s le without B 2 O 3 and quartz and protoenstatite in the composites with 5 and 10 wt% of B 2 O 3 . Meanwhile, the composite SMB15h, in addition to quartz and protoenstatit, also contains a new phase, namely suanite (Mg 2 B 2 O 5 ). In SMB204h, there are quartz and suanite.The phase compositions were determined using the Rietveld method from which thermal expansion coefficient values of 9.5-12 ppm °C -1 had been predicted. These results showed that natural-sand-based composites could fairly satisfy the required conditions of fuel-cell sealing materials.
Publisher: Trans Tech Publications, Ltd.
Date: 07-2015
DOI: 10.4028/WWW.SCIENTIFIC.NET/AMR.1112.385
Abstract: The complex shear modulus of polyethylene glycol (PEG) 4000 + quartz compositesas a variation of processing temperature and applied temperature has been investigated using a Dynamic Mechanical Analysis (DMA) instrument. The quartz, obtained by processing silica sand from Tanah Laut, South Kalimantan, of as much as 40% by weight was used as the filler of the composite. The processing temperature was room temperature, 50°C, and 70°C after considering the theoretical melting point of PEG 4000. Results showed that such temperaturesgave several phenomena related with the complex modulus values from the pure PEG 4000 and the composite, i.e.they dropped with time indicating the presence of relatively rapid strain relaxation and were in the range of 50-300MPa. Furthermore, theaddition of quartz improved the values wherethe most improvement occurred on the 70°C s le,i.e. almost twice from the pure PEG 4000. Increasing the heating process, however, significantly reduced the values. The reduction was confirmed by further investigation where the result showed that the complex shear modulus value both in the pure PEG 4000 and the composite dropped with applied temperature at around 42°C and 45°C, respectively, whichcan be related to the melting PEG. A severe drop was particulartly observed in the composite which was believed caused by the addition of quartz in a lose powder form.
Publisher: Trans Tech Publications, Ltd.
Date: 07-2019
DOI: 10.4028/WWW.SCIENTIFIC.NET/MSF.964.50
Abstract: The synthesis of Fe 2 TiO 5 powders has been successfully performed by making use of Fe 2 O 3 and TiO 2 powders. The Fe 2 O 3 powder was obtained from local ironstone in Tanah Laut, South Kalimantan, while the TiO 2 powder was a commercial product. The Fe 2 O 3 powder was obtained from the local ironstone through coprecipitation method on pH 5, followed by calcination at 800 °C for 1 hour. The synthesis of Fe 2 TiO 5 powder was done by mixing the raw powders using Planetary Ball Mill method for 5 hours. Thermogravimetric dan Differential Thermal Analysis (TG-DTA) was performed to estimate the calcination temperature. The milled mixtures were then calcined at temperatures of 700 – 1100 °C. X-Ray Diffraction (XRD) data showed that Fe 2 TiO 5 formation started at 800 °C with a weight fraction of 3.60 wt%. The XRD data also showed that at 1100 °C the Fe 2 TiO 5 formation has completed. The crystallite size of Fe 2 TiO 5 powders was 50 and 66 nm after calcination at 900 and 1100 °C, respectively.
Publisher: AIP Publishing LLC
Date: 2014
DOI: 10.1063/1.4866726
No related grants have been discovered for Suminar Pratapa.