Optical Spectra of the Colloidal Fe-doped Manganate CaMn1−xFexO3 (x = 0, 0.01, 0.03, 0.05)

Duc Huyen Yen Pham,Duc Tho Nguyen,Duc Thang Pham,Nam Nhat Hoang,The Tan Pham 한국물리학회 2013 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.62 No.12

We report the optical behaviors of the Fe-doped CaMnO3 family of compounds at low dopingconcentrations x 5%. The study aims at assisting the evaluation of the competition between ferroandantiferromagnetic orderings, which is believed to be a cause of many interesting properties of thisclass of compounds, including the magnetization reversal effect recently discovered. The structuralcharacterization showed a predominant orthorhombic phase with slightly increased cell constantsdue to doping. The Raman spectra revealed changes associated with the Mn sites, and the IRabsorption spectrum showed a characteristic Fe band at 1.2 eV, which should be accompanied bya change of spin. The analysis of the magnetization data allowed us to predict that while thedoping reduced the ferromagnetic coupling strength, and therefore the TC, the maximal dopingconcentration for the effective exchange to be zero was around 14%.

Multiferroic CoFe2O4-Pb(Zr,Ti)O3 Nanostructures

Pham Duc Thang,Mai T. N. Pham,G. Rijnders,D. H. A. Blank,Nguyen Huu Duc,J. C. P. Klaasse,E. Bruck 한국물리학회 2008 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.52 No.5

Multiferroic CoFe₂O₄-Pb(Zr,Ti)O₃ films were prepared on TiO₂-terminated (001) Nb-doped SrTiO3 substrates by using pulsed laser deposition (PLD). The lms were epitaxial and exhibited a large in-plane magnetic anisotropy and good ferroelectric properties. A decrease in the magneti- zation around the ferroelectric Curie temperature indicated magnetoelectric coupling between the magnetostrictive and the piezoelectric phases, which allows interconversion of energy stored in the electric and the magnetic elds and provides great potential for applications as next-generation multi-functional devices. Multiferroic CoFe₂O₄-Pb(Zr,Ti)O₃ films were prepared on TiO₂-terminated (001) Nb-doped SrTiO3 substrates by using pulsed laser deposition (PLD). The lms were epitaxial and exhibited a large in-plane magnetic anisotropy and good ferroelectric properties. A decrease in the magneti- zation around the ferroelectric Curie temperature indicated magnetoelectric coupling between the magnetostrictive and the piezoelectric phases, which allows interconversion of energy stored in the electric and the magnetic elds and provides great potential for applications as next-generation multi-functional devices.

Adaptive hierarchical sliding mode control using an artificial neural network for a ballbot system with uncertainties

Hai Le Xuan,Quoc-Dong Hoang,Soon Geul Lee,Dat Pham Xuan,Hoang Tran Viet,Minh Pham Van,Hung Pham Van,Hung Pham Viet,PHAM DUC TUAN,Duc Anh Nguyen 대한기계학회 2022 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.36 No.2

Ballbots, which have been studied for over ten years, are under-actuated mobile robots that operate using the inverted pendulum paradigm. Controlling a ballbot poses a number of challenges, including maintaining the stable upright posture from the ground in all directions and making sure it follows the desired trajectory. External factors such as a minor change in contact surface properties or fabrication errors can affect the system's stabilization and transfer capabilities. In this study, an adaptive hierarchical sliding mode control algorithm based on an artificial neural network is developed to make the ballbot robust to external factors. The use of the proposed controller ensures system stability despite uncertainties including friction, accidental centrifugal forces and gravity that occur when the ballbot follows the reference trajectory. The system stability is guaranteed on the basis of Lyapunov theory. Control efficiency and robot stability under system uncertainties are demonstrated by numerical simulation.

Effect of tip size on adhesion and friction characteristics of patterned polymeric surface

Duc-Cuong Pham,R. Arvind Singh,Nguyen Xuan Toan,Euu-Sung Yoon 한국트라이볼로지학회 2007 한국트라이볼로지학회 학술대회 Vol.2007 No.11

Nano-pillars made of poly (methyl-methacrylate) (PMMA) were fabricated on silicon wafer by capillary force lithography technique. The pattern was investigated for its tribological properties at micro/nano-scales in comparison with flat polymeric surface. An atomic force microscope (AFM) was used to investigate adhesion and friction properties of the patterns and flat surface at nano-scale. Two AFM tips with diameter of 2.5 and 10㎛ were used to study the effect of the tip size. A reciprocating micro-tribotester using glass balls (1㎜ in diameter) as counter-body was used to study friction characteristic of the samples at micro-scale. Results indicated that the patterns exhibited significant lower nano-adhesion and micro/nano-friction when compared to those of the flat film. The reasons for the patterns to exhibit such low adhesion and friction were assigned to its reduced real contact area and increased hydrophobicity. Moreover, the adhesion and friction forces of the patterned and non-patterned surfaces increased with the AFM tip size. It was due to the increase in real contact area when the larger tip contacts with the sample surfaces.

Enhancing some characteristic properties of magnetic micro-/nano- materials

Pham Duc Thang 한국자기학회 2016 한국자기학회 학술연구발표회 논문개요집 Vol.2016 No.05

Friction and Wear Properties of Boron Carbide Coating under Various Relative Humidity

Duc-Cuong Pham,Hyo-Sok Ahn,Eui-Sung Yoon 한국트라이볼로지학회 2005 KSTLE International Journal Vol.6 No.2

Friction and wear properties of the Boron carbide (B₄C) coating 100 ㎚ thickness were studied under various relative humidity (RH). The boron carbide film was deposited on silicon substrate by DC magnetron sputtering method using B₄C target with a mixture of Ar and methane (CH₄) as precursor gas. Friction tests were performed using a reciprocation type friction tester at ambient environment. Steel balls of 3 ㎜ in diameter were used as counter-specimen. The results indicated that relative humidity strongly affected the tribological properties of boron carbide coating. Friction coefficient decreased from 0.42 to 0.09 as the relative humidity increased from 5% to 85%. Confocal microscopy was used to observe worn surfaces of the coating and wear scars on steel balls after the tests. It showed that both the coating surface and the ball were significantly wornout even though boron carbide is much harder than the steel. Moreover, at low humidity (5%) the boron carbide showed poor wear resistance which resulted in the complete removal of coating layer, whereas at the medium and high humidity conditions, it was not. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses were performed to characterize the chemical composition of the worn surfaces. We suggest that tribochemical reactions occurred during sliding in moisture air to form boric acid on the worn surface of the coating. The boric acid and the tribochemcal layer that formed on steel ball resulted in low friction and wear of boron carbide coating.

Social Solidarity as The Foundation for Realization of Democracy in Vietnam

Pham Van Duc 전남대학교 5.18연구소 2014 민주주의와 인권 Vol.14 No.3

Dual Surface Modifications of Silicon Surfaces for Tribological Application in MEMS

Duc-Cuong Pham,R. Arvind Singh,Eui-Sung Yoon 한국트라이볼로지학회 2007 KSTLE International Journal Vol.8 No.2

Si(100) surfaces were topographically modified i.e. the surfaces were patterned at micro-scale using photolithography and DRIE (Deep Reactive Ion Etching) fabrication techniques. The patterned shapes included micro-pillars and microchannels. After the fabrication of the patterns, the patterned surfaces were chemically modified by coating a thin DLC film. The surfaces were then evaluated for their friction behavior at micro-scale in comparison with those of bare Si(100) flat, DLC coated Si(100) flat and uncoated patterned surfaces. Experimental results showed that the chemically treated (DLC coated) patterned surfaces exhibited the lowest values of coefficient of friction when compared to the rest of the surfaces. This indicates that a combination of both the topographical and chemical modification is very effective in reducing the friction property. Combined surface treatments such as these could be useful for tribological applications in miniaturized devices such as Micro-Electro-Mechanical-Systems (MEMS).

Superior micro/nano-tribological properties of polymeric channels

Duc-Cuong Pham,R. Arvind Singh,Pham Van Hung,Eui-Sung Yoon 한국트라이볼로지학회 2007 한국트라이볼로지학회 학술대회 Vol.2007 No.11

This work investigates micro/nano-scale tribological properties of sub-micrometer channels fabricated on thin polymeric surfaces by a soft lithographic technique. This technique involves placement of an e1astomeric mold onto a spin-coated polymeric film on the silicon substrate, and then the negative replica of the mold is formed by raising the temperature above the glass transition of the polymer. Water contact angle of the channels and the flat film was measured by an anglemeter using the sessile-drop method. Nano-scale adhesion and friction characteristics of the samples were measured by a commercial atomic force microscope (AFM) whereas micro-scale friction was measured by a micro-tribotester. Results show that water contact angle of the polymeric surface increased significantly by the channeling. The nano-scale adhesion and friction of channeled surface as well as its micro-scale friction reduced markedly when compared to those of PMMA thin film. Friction behavior of the channeled surface significantly depended on sliding direction for both micro/nano-scales.

Tribochemical interactions of Si-doped DLC film against steel in sliding contact

Duc-Cuong Pham,Eui-Sung Yoon(윤의성),Ho-Sung Kong(공호성),Hyo-Sok Ahn(안효석) 한국트라이볼로지학회 2006 한국트라이볼로지학회 학술대회 Vol.2006 No.6

This study concerns the effects of tribochemical interactions at the interface between Si-DLC (silicon-doped diamond-like carbon) film and steel ball in sliding contact on tribological properties of the film. The Si-DLC film was deposited on DLC/Si substrate using radio frequency plasma-assisted chemical vapor deposition (r.f. PACVD) with different Si concentration. Friction tests against steel ball using a reciprocating type wear tester were performed in ambient environment. X-Ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) were used to study the chemical characteristics and elemental composition of the film and mating balls after tests. As results, a dark-gray transfer film consists of carbon, oxygen and silicon elements were observed on worn surface of steel ball with different thickness. The oxidation of Si-DLC surface and steel ball was also identified at particular regions of contact area. On the contrary, the transfer film could not be found on surface of the ball slid against pure DLC coating. This demonstrates that chemical reactions were occurred at contact area of Si-DLC and steel ball during sliding. While friction test of DLC film versus steel ball exhibited high friction coefficient (~0.06), the Si-DLC in its turn showed very low friction (~0.022) with the presence of tribofilm on mating ball surface. However, Si-DLC film possesses poor wear resistance in comparison with the pure DLC. We suggest that the tribochemical process strongly affected to tribological properties of the Si-DLC film in sliding against steel.