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.

Dual Surface Modifications of Silicon Surfaces for Tribological Application in MEMS

Pham, Duc-Cuong,Singh, R. Arvind,Yoon, Eui-Sung Korean Tribology Society 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).

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.

Nanotribological Properties of Nano-channels Overcoated by DLC Films

Duc Cuong Pham,Kyounghwan Na,Sungwook Yang,Jinseok Kim,Van Hung Pham,Eui-Sung Yoon 한국트라이볼로지학회 2009 한국트라이볼로지학회 학술대회 Vol.2009 No.11

Hydrophobicity and Nanotribological Properties of Silicon Channels coated by Diamond-like Carbon Films

Pham, Duc Cuong,Na, Kyung-Hwan,Pham, Van Hung,Yoon, Eui-Sung Korean Tribology Society 2009 KSTLE International Journal Vol.10 No.1

This paper reports an investigation on nanotribological properties of silicon nanochannels coated by a diamond-like carbon (DLC) film. The nanochannels were fabricated on Si (100) wafers by using photolithography and reactive ion etching (RIE) techniques. The channeled surfaces (Si channels) were then further modified by coating thin DLC film. Water contact angle of the modified and unmodified Si surfaces was examined by an anglemeter using the sessile-drop method. Nanotribological properties, namely friction and adhesion forces, of the Si channels coated with DLC (DLC-coated Si channels) were investigated in comparison with those of the flat Si, DLC-coated flat Si (flat DLC), and Si channels, using an atomic force microscope (AFM). Results showed that the DLC-coated Si channels greatly increased hydrophobicity of silicon surfaces. The DLC coating and Si channels themselves individually reduced adhesion and friction forces of the flat Si. Further, the DLC-coated Si channels exhibited the lowest values of these forces, owing to the combined effect of reduced contact area through the channeling and low surface energy of the DLC. This combined modification could prove a promising method for tribological applications at small scales.

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.

Nanotribological properties of silicon nano-pillars coated by a Z-DOL lubricating film

Pham, Duc Cuong,Na, Kyung-Hwan,Yang, Sung-Wook,Kim, Jin-Seok,Yoon, Eui-Sung 대한기계학회 2010 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.24 No.1

This paper reports a novel approach for improving the nanotribological properties of silicon (Si) surfaces by topographically and chemically modifying the surfaces. In the first step, Si (100) wafers were topographically modified into nano-pillars by using the photolithography and reactive ion etching (RIE) techniques. Various patterns, including nano-pillars of varying diameters and pitches (distance between pillars), were fabricated. Then, the patterns were coated with a Z-DOL (perfluoropolyether (PFPE)) lubricating film using a dipcoating technique, and this process was followed by thermal treatment. These modified surfaces were tested for their nanotribological properties, namely adhesion and friction forces, using an atomic force microscope (AFM). The results showed that the topographical modification and Z-DOL coating each independently reduced the adhesion and friction forces on the Si surfaces. However, the combination of the two surface treatments was most effective in reducing these forces. This is attributed to the combined effects of the reduction in the real area of contact due to patterning and the low surface energy of the Z-DOL lubricant. Further, it was found that adhesion and friction forces of the surfaces with combined modification varied significantly depending on the diameter of the pillars and the pitch. It is proposed that such a combination of surface modifications promises to be an effective method to improve the nanotribological performance of miniaturized devices, such as MEMS, in which Si is a typical material.

Tribochemical Interactions of Si-doped DLC Film Against Steel in Sliding Contact

Pham, Duc-Cuong,Ahn, Hyo-Sok,Oh, Jae-Eung,Yoon, Eui-Sung 대한기계학회 2007 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.21 No.7

This study concerns the effects of tribochemical interactions at the interface of 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 over-coated on pure DLC coating by radio frequency plasma-assisted chemical vapor deposition (r.f. PACVD) with different Si concentration. Friction tests against steel ball using a reciprocating type tribotester 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 films and mating balls after tests. Results showed a dark-gray film consisting of carbon, oxygen and silicon on the worn steel ball surface with different thickness. On the contrary, such film was not observed on the surface of the ball slid against pure DLC coating. The oxidation of Si-DLC surface and steel ball was also found at particular regions of contact area. This demonstrates that tribochemical interactions occurred at the contact area of Si-DLC and steel ball during sliding to form a tribofilm (so called transfer film) on the ball specimen. While the pure DLC coating exhibited high coefficient of friction $(\sim0.06)$, the Si-DLC film showed a significant lower coefficient of friction $(\sim0.022)$ with the presence of tribofilm on mating ball surface. However, the Si-DLC film possesses a very high wear rate in comparison with the pure DLC. It was found that the tribochemical interactions strongly affected tribological properties of the Si-DLC film in sliding against steel.