Ruben Menchavez - Academia.edu (original) (raw)

Papers by Ruben Menchavez

Mindanao Journal of Science and Technology

This study fabricated a wall-panel board made of rice husk and red clay-based geopolymer. The geo... more This study fabricated a wall-panel board made of rice husk and red clay-based geopolymer. The geopolymer paste was studied at varying molarities of alkali-activator solution with a fixed ratio of Na2SiO3/NaOH at 2.5. A calcined red clay was used as the aluminosilicate precursor at a mass ratio of 1:1 to the activator solution. The geopolymer paste was heated at 80 °C for 12 h in a conventional oven and then aged for seven days at room condition. It was found out that 12 molars of NaOH in geopolymer paste provided the maximum flexural strength of 5.48 MPa. The rice husk was then introduced to the geopolymer paste at varying amounts from 10 to 20 wt% based on the mass of the paste. It was observed that the flexural strength and bulk density of the solidified composites decreased with an increasing amount of rice husks. Correspondingly, the measured flexural strengths varied from 4.21 to 3.13 MPa, whereas the bulk densities measured from 1.84 to 1.51 g/cm3. It was further observed that...

Materials Science Forum, 2007

This study investigates the phase compositions in gelcast dense alumina and porous alumina that w... more This study investigates the phase compositions in gelcast dense alumina and porous alumina that were sintered in an inert atmosphere. Both materials were gelcasted using identical slurry composition except for adding surfactant to slurry intended for porous shaping. The resulting gelcasted samples were dried and then sintered in argon gas at 1700oC. The X-ray photoelectron analysis was used for identifying the phase compositions in sintered samples, which was supported with TG/DTA and electrical measurements. Results showed that, in addition to alumina and carbon phases, the dense sample yielded Al4C3 and Al, while porous sample yielded aluminum oxycarbides. The formation of these phases reduced the carbon content through different chemical reactions to a limited extent because they formed a protective layer on alumina grains, which prevented the direct contact between alumina and carbon. Subsequent measured electrical resistivities of dense and porous alumina were, respectively, 0....

Journal of the European Ceramic Society, 2009

... Ruben L. Menchavez a , Masayoshi Fuji a and Minoru Takahashi Corresponding Author Contact Inf... more ... Ruben L. Menchavez a , Masayoshi Fuji a and Minoru Takahashi Corresponding Author Contact Information , a , E-mail The Corresponding Author. ... 3 As the part dries, the gel matrix ruptures and coats on individual ceramic particles. ...

Journal of Colloid and Interface Science, 2005

The colloidal stability of suspensions of alumina particles has been investigated by measuring pa... more The colloidal stability of suspensions of alumina particles has been investigated by measuring particle size distribution, sedimentation, viscosity, and zeta potential. Alumina particles were found to be optimally dispersed at pH around 3 to 7.8 without dispersant and at pH 8.5 and beyond with dispersant. The above results corroborate zeta potential and viscosity measurement data well. The surface charge of alumina powder changed significantly with anionic polyelectrolyte (ammonium polycarboxylate, APC) and the iep shifted toward more acidic range under different dispersant conditions. It was found that the essential role played by pH and dispersant (APC) on the charge generation and shift in the isoelectric point of alumina manifests two features: (i) the stability decreases on approaching the isoelectric point from either side of pH, and (ii) the maximum instability was found at pH 9.1 for alumina only and at pH 6.8 for alumina/APC, which is close to the isoelectric points for both the system, respectively. Using the model based on the electrical double-layer theory of surfactant adsorption through shift in isoelectric points, the authors could estimate the specific free energy of interaction (7.501 kcal/mol) between particles and dispersant. The interaction energy, zeta potential, sedimentation, and viscosity results, were used to explain the colloidal stability of the suspension.

Journal of Colloid and Interface Science, 2006

Electrochimica Acta, 2008

... The gelcasted part is usually dried, debindered, and subsequently densified by pressureless s... more ... The gelcasted part is usually dried, debindered, and subsequently densified by pressureless sintering, oxidation sintering or reduction-sintering [2]. In recent years, much attention has been focused on the development of ceramic nano-composites with nano-sized conductive ...

This study explores a method of synthesizing a clay-based ceramic/carbon composite by starch cons... more This study explores a method of synthesizing a clay-based ceramic/carbon composite by starch consolidation casting and reductive sintering. Starch is harnessed not only as a binder but also as precursor for carbon material, which presents a simple, less expensive and environmental-friendly approach to synthesize ceramic/carbon composite. Ceramic slurry from ternary powder mixture of red-clay, quartz, and feldspar in water was consolidated with potato starch in a mould and heated inside a pressure cooker to obtain a cylindrical green body. The moulded samples were then sintered at 1000-1250 oC in a vertical tube furnace under argon to synthesized bulk ceramic composites. The sintered samples turned black that indicates the formation of pyrolyzed carbon in the ceramic matrix and in graphite form with decreasing carbon yield (3.957-1.273%) as sintering temperature increases. Phase composition and microstructure of the clay-based ceramic/carbon were examined by fourier transform infrare...

Abstract This paper reports a facile and environment-friendly process to synthesize electrically ... more Abstract This paper reports a facile and environment-friendly process to synthesize electrically conductive porous alumina/graphite composites by starch consolidation technique followed by reductive sintering. Green ceramic composites were consolidated with different starches and sintered at different temperatures in an argon atmosphere. Electrical measurements, carbon contents and Raman analyses of carbon structures determined an optimal sintering temperature of 1700 °C, which lead to a uniform formation of conductive graphitic networks at an optimal concentration of about 3.8 vol% in the porous composites. These carbon networks resulted into porous composites having high electrical conductivities measured in the range from 3 to 7 S/cm, which depended on the starch types and their porous properties. Correspondingly, the bulk porosities of the sintered composites were measured from 42 to 46%, with rounded micropores having diameters ranging from 14 to 39 μm. These porous properties of the sintered composites offer promising applications for conductive membrane and porous electrode.

Nihon Reoroji Gakkaishi

We have observed a progression of the shape and velocity of a bubble rising in aqueous gelatin so... more We have observed a progression of the shape and velocity of a bubble rising in aqueous gelatin solution under gelation process. Two experimental systems were designed to produce two different bubble sizes that were allowed to rise in test sections filled with aqueous gelatin solution. The physical gelling was allowed to proceed slowly in a temperature-controlled cooling system. The viscoelasticity of the aqueous gelatin solution was measured using a stress controlled rheometer under the same temperature history with the bubble rising observations. It was found that the change in bubble shape corresponds to the elastic behavior of solution due to the progress in gelation process. The gel point of the gelatin solution was conveniently determined by detecting the change in the shape of the rising bubble from a sphere shape to prolate and cuspidal shape. It was further observed that a small bubble is preferable during measurement because of its less impact on the gelation structure around the bubble. Subsequently, the measuring system designed in the present study is simple and convenient to install for offline process monitoring.

Transactions of the Indian Ceramic Society, 2006

Journal of The Electrochemical Society, 2008

ABSTRACT A novel, highly conductive porous alumina/nanocarbon network (alumina/NCN) composites we... more ABSTRACT A novel, highly conductive porous alumina/nanocarbon network (alumina/NCN) composites were fabricated. The electrochemical behaviors of the composite materials were studied by electrochemical measurements, X-ray diffraction patterns, scanning electron microscopy observations, and so on. The results showed that the reproducibility and stability of the fabricated nanocarbon composites are good enough for use as electrodes in strong acidic and alkaline environments. The fabricated electrode in electrochemical performances was to some extent closer to graphite electrode rather than glassy carbon in these aqueous solutions. The porous nanocarbon electrodes seemed to have certain electrocatalysis capabilities for hydrogen evolution or oxygen reduction in aqueous solutions. These performances are attributed to the existence of pyrolyzed NCN derived from polymer and its high porosity in the composite matrix. The possible cathodic and anodic electrode reactions on this nanocarbon electrode surface were proposed and discussed according to their standard potentials reported in the literature. In addition, alumina particles or grain boundary phase dissolution in the electrode was a simple hydrolysis reaction with very low dissolution rate at ambient conditions, which did not involve the electrode reactions.

Ceramic Transactions, 2006

Key Engineering Materials, 2006

In this study, we propose a new process to fabricate electrically conductive alumina by gelcastin... more In this study, we propose a new process to fabricate electrically conductive alumina by gelcasting and reduction sintering. The process used the conventional gelcasting method except for varying amounts of monomer at 2.8, 5.5, and 8.0 wt.% relative to the weight of the slurry. In the plastic mould, the slurry was under in situ solidification for 3 hrs at 25oC to achieve gelation. The freshly gelled bodies were demolded, carefully dried, and then sintered at 1100oC, 1300oC, and 1550oC in nitrogen atmosphere. The holding times at 1100oC and 1300oC was 2 hours, while at 1550oC were 2, 4, and 6 hrs. The sintered alumina body was characterized by electrical property, X-ray diffraction, and scanning electron microscopy. Results showed that monomer additions and sintering schedule significantly affect in lowering electrical resistance. The low value was 3.6×106 +cm at 8.0wt.% monomer addition and sintering at 1550oC for 2 hrs. The effect of physical properties on electrical conductivity an...

Advanced Materials Research, 2006

This work demonstrated an in-situ pyrolysis of gelcast alumina under reduction sintering to make ... more This work demonstrated an in-situ pyrolysis of gelcast alumina under reduction sintering to make alumina and carbon composite in providing semi-electrical conductivity. To increase the carbon content, the monomer was varied in the premix solution with reduction sintering in nitrogen gas. Two-probe method was used to measure electrical resistance of the sintered samples. The results revealed that the increase of monomer addition and sintering treatment were effective in reducing electrical resistance. The lowest value was 3.6×106-cm, which is a potential candidate for electrostatic shielding application. The reduction-sintered sample was re-sintered in an air in order to gain insight on the conductive path due to carbon network. Further tests such as XRD, TGA/DTA, and scanning electron microscopywere used to explain the semi-conductive property of the material.

Ceramics International, 2014