J. Gracio

University of Aveiro, Aveiro, Aveiro, Portugal

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Publications (158)187.17 Total impact

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    ABSTRACT: Nanoindentation-induced polar properties in (001)-oriented SrTiO3 single crystals were analysed using piezoresponse force microscopy and local Raman spectroscopy measurements. The area directly beneath the indenter showed a strong piezoelectric response, together with an enhanced response in the area along the direction tangential to the residual indent. Stress-induced stable polarization states near the crack areas were also observed. Local poling performed on the strained areas suggests a possibility of polarization reversal and the stability of field-induced polar states.
    Scripta Materialia 11/2014; 74:76. · 2.82 Impact Factor
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    ABSTRACT: An insight into the interaction of collagen type I with apatite in bone tissue was performed by using differential scanning calorimetry, Fourier transform infrared spectroscopy, and molecular modeling. Scanning electron microscopy shows that bone organic content incinerate gradually through the different temperatures studied. We suggest that the amide regions of the type I collagen molecule (mainly C=O groups of the peptide bonds) will be important in the control of the interactions with the apatite from bone. The amide I infrared bands of the collagen type I change when interacting to apatite, what might confirm our assumption. Bone tissue results in a loss of thermal stability compared to the collagen studied apart, as a consequence of the degradation and further combustion of the collagen in contact with the apatite microcrystals in bone. The thermal behavior of bone is very distinctive. Its main typical combustion temperature is at 360 ºC with a shoulder at 550 ºC compared to the thermal behavior of collagen, with the mean combustion peak at ca. 500 ºC. Our studies with molecular mechanics (MM+ force field) showed different interaction energies of the collagen-like molecule and different models of the apatite crystal planes. We used models of the apatite (100) and (001) planes; additional two planes (001) were explored with phosphate-rich and calcium-rich faces; an energetic preference was found in the latter case. We preliminary conclude that the peptide bond of collagen type I is modified when the molecule interacts with the apatite, producing a decrease in the main peak from ca. 500oC in collagen, up to 350oC in bone. The combustion might be related to collagen type I, as the H energies present only small variations between mineralized and non-mineralized samples. The data obtained here give a molecular perspective into the structural properties of bone and the change in collagen properties caused by the interaction with the apatite. Our study can be useful to understand the biological synthesis of minerals as well as the organic-inorganic interaction and the synthesis of apatite implant materials.
    Advances in Biological Chemistry 05/2013; 3?(2?):x.
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    ABSTRACT: The electrical properties of polymer composites based on polycarbonate (PC) and panipol CXM (CX), filled with reduced graphene oxide (rGO), were investigated. The composite preparation conditions allowed good dispersion of rGO in the polymer matrix. We show here that when used as a nanofiller in polymers, rGO offers an appreciable improvement of the electrical current in 3 orders of magnitude (from 10−10 A to 10−7 A at 10 V), as observed in current-voltage (I-V) data for both PC and CX polymers with rGO. The suggested mechanism for the observed switching effects is the migration of oxygen groups aided by both the electrical field and Joule heating. Moreover, some reset- and set- like changes similar to resistive switching were observed in the I-V data of PC and CX-based films upon the addition of rGO. Clockwise (resembling a memristive system type II) and counter-clockwise (resembling a memristive system type I) directions were detected in the I-V data of the analyzed films. According to the obtained results, rGO can be a good filler for PC and CX polymer-based films for application in electronic and photonic areas, due to the significant improvement of the electrical conductivity of these polymers.
    Journal of Applied Physics 02/2013; 113(6). · 2.21 Impact Factor
  • Diamond and Related Materials 01/2013; 31:81. · 1.71 Impact Factor
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    ABSTRACT: Micro and nanosystems-based products will be an important contributor to the industrial and economic future, as a key value adding element for many sectors. The adoption of micro and nano manufacturing technologies making use of a variety of materials, components and knowledge based technologies will most probably provide functionality and intelligence to highly miniaturised systems. Chemical Vapour Deposited (CVD) diamond is a hard material of high mechanical strength and thermal stability and therefore an ideal candidate for microelectromechanical devices. The latter, is already being used to manufacture sensors, resonators, actuators, biological devices or even plastic moulding impressions, by depositing the film on a structured substrate, by selective growth of diamond on Si using SiO2 masks, or by pattern etching on oxygen-containing plasma. The diamond shaping may also be performed by a fine tuning of the nucleation and growth process. In this paper, it is presented the results of diamond coated structures and an evaluation of its morphology variation with the substrate configuration. The grown films were characterized for quality, surface roughness and microstructure using scanning electron microscopy, Raman and X-ray diffraction spectroscopy and surface profilometry.
    Journal of Nanoscience and Nanotechnology 08/2012; 12(8):6700-6. · 1.15 Impact Factor
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    ABSTRACT: Silver (Ag) nanoparticles were synthesized on the surface of graphene sheet by the simultaneous reduction of Ag+ and graphene oxide (GO) in the presence of simple reducing agent, hydrazine hydrate (N2H4 x H2O). Both the Ag+ and GO were reduced and Ag+ was nucleated onto graphene. GO flakes were prepared by conventional chemical exfoliation method and in the presence of strong acidic medium of potassium chlorate. Silver nanoparticles were prepared using 0.01 M AgNO3 solution. The reduced GO sheet decorated with Ag is referred as G-Ag sample. G-Ag was characterized by FTIR (Fourier transform infrared) spectroscopy using GO as standard. An explicit alkene peak appeared around 1625 cm(-1) was observed in G-Ag sample. Besides, the characteristic carbonyl and hydroxyl peaks shows well reduction of GO. The FTIR therefore confirms the direct interaction of Ag into Graphene. SEM (scanning electron microscopy) and TEM (transmission electron microscopy) analysis were performed for morphological probing. The average size of Ag nanoparticles was confirmed by around 5-10 nm by the high-resolution TEM (HRTEM). The Ag quantum dots incorporated nanocomposite material could become prominent candidate for diverse applications including photovoltaic, catalysis, and biosensors etc.
    Journal of Nanoscience and Nanotechnology 08/2012; 12(8):6731-6. · 1.15 Impact Factor
  • J A Santos, V F Neto, D Ruch, J Grácio
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    ABSTRACT: A mechanical seal is a type of seal used in rotating equipment, such as pumps and compressors. It consists of a mechanism that assists the connection of the rotating shaft to the housings of the equipments, preventing leakage or avoiding contamination. A common cause of failure of these devices is end face wear out, thus the use of a hard, smooth and wear resistant coating such as nanocrystalline diamond would be of great importance to improve their working performance and increase their lifetime. In this paper, different diamond coatings were deposited by the HFCVD process, using different deposition conditions. Additionally, the as-grown films were characterized for, quality, morphology and microstructure using scanning electron microscopy (SEM) and Raman spectroscopy. The topography and the roughness of the films were characterized by atomic force microscopy (AFM).
    Journal of Nanoscience and Nanotechnology 08/2012; 12(8):6835-9. · 1.15 Impact Factor
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    ABSTRACT: Although large focus has been placed into the deposition of nanocrystalline and ultra-nanocrystalline diamond films, most of this research uses microwave plasma assisted CVD systems. However, the growth conditions used in microwave systems cannot be directly used in hot-filament CVD systems. This paper, aims to enlarge the knowledge of the diamond film depositing process. H2/CH4/Ar gas mixtures have been used to deposit micro, nano and ultra-nanocrystalline diamond films by hot-filament CVD systems. Additionally, the distance between the filaments array and the substrate was varied, in order to observe its effect and consequently the effect of a lower substrate temperature in the nucleation density and deposition. All the samples were characterized for microstructure and quality, using scanning electron microscopy and Raman spectroscopy.
    Journal of Nanoscience and Nanotechnology 08/2012; 12(8):6822-7. · 1.15 Impact Factor
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    Frontiers in Electronic Materials; 06/2012
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    Diamond and Related Materials 02/2012; · 1.71 Impact Factor
  • Thermoplastic Elastomers, 02/2012: chapter Microinjection of enhanced thermoplastics; InTech.
  • PMI 2012 - Int. Conf. on Polymers & Moulds InnovationsPMI 2012 - Int. Conf. on Polymers & Moulds Innovations; 01/2012
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    ABSTRACT: The resistive switching (RS) mechanism in Ni-doped graphene oxide (GO) devices is studied. We found that RS depends strongly on the fabrication method of the GO sheet and on the electrode material. Resistive switching in GO-devices can be caused by the diffusion of ions from metallic electrode or by the migration of oxygen groups, depending on the fabrication process. We also show that GO-based structures possess activity-dependent modification capabilities, emphasized by the increase/decrease of device conductance after consecutive voltage sweeps of opposite polarity. Our results allow a better understanding of bipolar RS, towards future non-volatile memories and neuromorphic systems. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4742912]
    Applied Physics Letters 01/2012; 101(6). · 3.52 Impact Factor
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    ABSTRACT: We used the micro-Raman spectroscopy to evaluate residual stresses developed in deposited diamond films on Ti6Al4V substrates. A comparison is made between specimens with and without a diamond like carbon (DLC) interlayer. The effect of seeding the specimens prior to the deposition of the diamond coating is also investigated. In addition, AFM analysis is carried out to evaluate the quality of the surface toughness. The results show that the introduction of a DLC interlayer lowered the residual stress intensity compared to the specimens without DLC. The seeding process enabled the generation of smaller grains compared to the specimens without seeding. The measured averaged residual stresses on specimens with a DLC interlayer were about half of those measured on specimens without DLC. However the seeding process, which led to a better surface finish, had a partial opposite effect on the stress intensities. This is probably due to the grain growth depending on the nucleation sites available from the seeding process.
    Diamond and Related Materials 01/2012; 22:105-112. · 1.71 Impact Factor
  • 6º Encontro Nacional do Colégio de Engenharia Mecânica da Ordem dos Engenheiros; 01/2012
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    ABSTRACT: The percolation threshold problem in insulating polymers filled with exfoliated conductive graphite nanoplatelets (GNPs) is re-examined in this 3D Monte Carlo simulation study. GNPs are modelled as solid discs wrapped by electrically conductive layers of certain thickness which represent half of the electron tunnelling distance. Two scenarios of 'impenetrable' and 'penetrable' GNPs are implemented in the simulations. The percolation thresholds for both scenarios are plotted versus the electron tunnelling distance for various GNP thicknesses. The assumption of successful dispersion and exfoliation, and the incorporation of the electron tunnelling phenomenon in the impenetrable simulations suggest that the simulated percolation thresholds are lower bounds for any experimental study. Finally, the simulation results are discussed and compared with other experimental studies.
    Journal of Physics D Applied Physics 10/2011; 44(45):455306. · 2.53 Impact Factor
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    ABSTRACT: Addition of an adequate amount of carbon nanotubes (CNTs) to electrically insulating polymers can make them conductive. The conductivity behavior of such nanocomposites, also known as the percolation behavior, is mainly due to the formation of pathways of touching particles. In this Monte Carlo simulation study, CNTs are modeled as penetrable cylindrical sticks also known as the "soft-core" model which are randomly scattered inside a representative volume element (RVE) of the nanocomposite. As it brings about a new configuration of constituents, the mechanical loading effects on the percolation are investigated assuming simple linear elastic behavior. To evaluate the impact of the mechanical deformation on the percolation, we first propose a two-step homogenization technique aimed at evaluating the effective homogenized stiffness at any configuration is proposed. The displacement field of the RVE is related to the applied stress via this effective stiffness. As the spatial configuration of the composing constituents is altered during the course of stressing the RVE, the effective stiffness and the percolation state change as well. An incremental procedure is therefore proposed for updating the stiffness tensor and for the checking the percolation state. The simulation results indicate that a percolating nanocomposite becomes non-percolating by applying a unidirectional tensile stress. Finally a convincing comparison with several independent experimental results is provided which confirms the results of the proposed methodology.
    Journal of Computational and Theoretical Nanoscience 09/2011; 8(10):2087-2099. · 1.03 Impact Factor
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    ABSTRACT: We used piezoresponse force microscopy (PFM) to study the dc bias field-induced domain structure of the (001) oriented single crystals of 0.77PbMg1/3Nb2/3O3-0.23PbSc1/2Nb1/2O3 relaxor. In the as-grown state, no polarization contrast was observed due to the average cubic symmetry of the crystal. Upon application of a dc bias applied via a PFM tip, a stable complex domain pattern, corresponding to a hedgehog-type topological defect, appeared on the surface of the crystal. The shape of these domains depended on the magnitude of the field. If the field was below a threshold, the domains possessed a cylindrical form. Above the threshold, the border between domains became close to a straight line. At high biases and large separation between the pulses, the development of the domains obeyed an avalanche-type dynamics.
    Journal of Applied Physics 09/2011; 110(5):052002-052002-5. · 2.21 Impact Factor
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    ABSTRACT: Tungsten carbide alloys possess a large number of applications, due to its high hardness, high resistance to heat and to wear, which makes it ideal to be used in wear parts of machinery or on surfaces liable to corrosion, abrasion or high temperatures. For practical applications, it is alloyed with cobalt (Co) or nickel (Ni) in order to improve its properties. The increasing demand of broadening the operation limits of some components, impose the need to improve its life time. Coating these parts with hard and smooth diamond thin film may improve the wear performance and decrease their roughness and friction coefficient. In this work, a comparative study of nanodiamond films deposit onto WC-Co and WC-Ni, by means of a hot-filament Chemical Vapour Deposition (HFCVD) system, is presented. The study was accomplished by varying the CH4/H2/Ar gas ratio of the deposition process. The substrate temperature was kept low (< 700 degrees C) to minimize the thermal stress in the coating-substrate system. The microstructure of the deposited diamond film was characterized using Scanning Electron Microscopy (SEM), Raman spectroscopy and X-ray diffraction spectroscopy. Roughness of the working surfaces were also accessed prior and after the depositions.
    Journal of Nanoscience and Nanotechnology 06/2011; 11(6):5388-93. · 1.15 Impact Factor
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    ABSTRACT: The inscription of a Fibre Bragg Grating (FBG) in optical fibres allows them to be used as sensors, being capable of decoding small variations of strain; temperature; pressure; loading; bending; or even refractive index, by means of a shift in the reflected wavelength. Nevertheless, broadening their sensitivity and operation range would be desirable. This may be achieved by appropriated fibre coating. Diamond possesses a set of extreme properties, such as high thermal conductivity, hardness and resistance to hazard environments. Furthermore, it is known for its excellent biocompatible response, so it may be suitable to be used as a coating material for biological sensors. In this paper, the results of the optimization process of diamond coatings on optical fibre sensors is presented, considering their potential use for practical biological purposes.
    Journal of Nanoscience and Nanotechnology 06/2011; 11(6):5408-12. · 1.15 Impact Factor

Publication Stats

367 Citations
187.17 Total Impact Points

Institutions

  • 1997–2014
    • University of Aveiro
      • • Division of Mechanical Engineering
      • • Tecnologia Mecânica e Automação (TEMA)
      • • Department of Mechanical Engineering
      • • Department of Physics
      Aveiro, Aveiro, Portugal
  • 2011
    • Carnegie Mellon University
      • Department of Mechanical Engineering
      Pittsburgh, PA, United States
  • 2010
    • University of Strasbourg
      • Institut de Mécanique des Fluides et des Solides (IMFS)
      Strasburg, Alsace, France
    • University of Central Lancashire
      • School of Computing, Engineering and Physical Sciences
      Preston, ENG, United Kingdom
  • 2006
    • University of Rome Tor Vergata
      • Dipartimento di Scienze e Tecnologie Chimiche
      Roma, Latium, Italy
    • Purdue University
      • Birck Nanotechnology Center
      West Lafayette, IN, United States
  • 2003
    • Russian Academy of Sciences
      • Institute of Problems in Mechanics
      Moscow, Moscow, Russia