L. Gracia - Academia.edu (original) (raw)

Papers by L. Gracia

Journal of the American Ceramic Society, 2014

ABSTRACT A systematic first-principles investigation, by using the density functional formalism w... more ABSTRACT A systematic first-principles investigation, by using the density functional formalism with the nonlocal B3LYP approximation including a long-range dispersion correction, has been performed to calculate the structural and electronic properties and phase transitions under pressure of the three phases of ZnS (cubic zinc blende, ZB, hexagonal wurtzite, W, and cubic rock salt, RS). Numerical and analytical fittings have been carried out to determine the equilibrium unit cell geometry and equation of state parameters for the ZnS phases. The band structures, energy gap, density of states, and vibrational frequencies and their pressure dependences are investigated. The present results illustrate that both phases, W and ZB, present very similar enthalpy and the RS phase becomes thermodynamically more stable than ZB and W structures at 15.0 and 15.5 GPa, respectively. These phase transitions are accompanied by an increase of the first shell coordination number of Zn atom and by a cell volume collapse of 13.9% and 14.3% for ZB and W phases, respectively. The atomic contributions of the conduction and valence bands, as well the binding energy for the Zn 3d orbital have been obtained.

Theoretical investigations concerning the high-pressure polymorphs, the equations of state, and t... more Theoretical investigations concerning the high-pressure polymorphs, the equations of state, and the phase transitions of SnO2 have been performed using density functional theory at the B3LYP level. Total energy calculations and geometry optimizations have been carried out for all phases involved, and the following sequence of structural transitions from the rutile-type (P42/mnm) driven by pressure has been obtained (the transition pressure is in parentheses): --&amp;gt; CaCl2-type, Pnnm (12 GPa) --&amp;gt; alpha-PbO2-type, Pbcn (17 GPa) --&amp;gt; pyrite-type, Pa (17 GPa) --&amp;gt; ZrO2-type orthorhombic phase I, Pbca (18 GPa) --&amp;gt; fluorite-type, Fmm (24 GPa) --&amp;gt; cotunnite-type orthorhombic phase II, Pnam (33 GPa). The highest bulk modulus values, calculated by fitting pressure-volume data to the second-order Birch-Murnaghan equation of state, correspond to the cubic pyrite and the fluorite-type phases with values of 293 and 322 GPa, respectively.

The Journal of Physical Chemistry A, 2003

ABSTRACT The complex reaction between VO2+ (1A1/3A‘ ‘) and C2H4 (1Ag/3A1) to yield VO+ (1Δ/3Σ) an... more ABSTRACT The complex reaction between VO2+ (1A1/3A‘ ‘) and C2H4 (1Ag/3A1) to yield VO+ (1Δ/3Σ) and CH3CHO (1A‘/3A‘ ‘) has been studied by means of B3LYP/6-31G* and B3LYP/6-311G(2d,p) calculations. The structures of all reactants, products, intermediates, and transition structures of this reaction have been optimized and characterized at the fundamental singlet and first excited triplet electronic states. Crossing points are localized, and possible spin inversion processes are discussed by means of the intrinsic reaction coordinate approach. Relevant stationary points along the most favorable reaction pathways have been studied at the CCSD/6-311G(2d,p)//B3LYP/6-311G(2d,p) calculation level. The theoretical results allow the development of thermodynamic and kinetic arguments about the reaction pathways of the title process. In the singlet state, the first step is the barrierless obtention of a reactant complex associated with the formation of a V−C bond, while in the triplet state a three-membered ring addition complex with the V bonded to the two C atoms is obtained. Similar behavior is found in the exit channels: the product complexes can be formed from isolated products without barriers. The reactant and product complexes are the most stable stationary points in the singlet and triplet electronic states. From the singlet state reactant complex, two reaction pathways are posssible to reach the triplet state product complex. (i) A mechanism in which a hydrogen transfer process is the first and rate limiting step and the second step is an oxygen transfer between vanadium and carbon atoms with a concomitant change in the spin state. The crossing point between singlet and triplet spin states is not kinetically relevant because it takes place at a later stage occurring in the exit channel. (ii) A mechanism in which the first stage renders a four-membered ring between vanadyl cation and the ethylene fragment and an oxygen−carbon bond is formed; on going from this minimum to the second transition structure, associated with a carbon−vanadium bond breaking process, the crossing point between singlet and triplet spin states is reached. The final step is the hydrogen transfer between both carbon atoms to yield the product complex. In this case the spin change opens a lower barrier pathway. The transition structures with larger values of relative energies for both reactive channels of VO2+ (1A1) + C2H4 (1Ag) → VO+ (3Σ) + CH3CHO (1A‘) present similar energies, and the two reaction pathways can be considered as competitive.

Physical Review B, 2009

Theoretical investigations concerning the possible titanium silicate polymorphs have been perform... more Theoretical investigations concerning the possible titanium silicate polymorphs have been performed using density functional theory at B3LYP level. Total-energy calculations and geometry optimizations have been carried out for all phases involved. The following sequence of pressure-driven structural transitions has been found: CrVO 4-type, Cmcm ͑in parenthesis the transition pressure͒, → zircon-type, I4 1 / amd ͑0.8 GPa͒, → scheelite-type, I4 1 / a ͑3.8 GPa͒. At higher pressure the last phase is found to be stable at least up to 25 GPa. The equation of state of the different polymorphs is also reported. We found that the highest bulk modulus corresponds to the zircon and scheelite phases with values of 248 and 238 GPa, respectively. The orthorhombic Cmcm phase is the most compressible of all the studied structures with a bulk modulus of 124 GPa, being also the most stable phase at ambient pressure. Finally, calculations of the electronic structure, vibrational and dielectric properties of TiSiO 4 are also reported.

Physical Chemistry Chemical Physics, 2014

Journal of Physics: Condensed Matter, 2013

An experimental and theoretical study of the structural properties of monoclinic bismuth oxide (... more An experimental and theoretical study of the structural properties of monoclinic bismuth oxide (-Bi 2 O 3) under high pressures is here reported. Both synthetic and mineral bismite powder samples have been compressed up to 45 GPa and their equations of state have been determined with angle-dispersive x-ray diffraction measurements. Experimental results have been also compared to theoretical calculations which suggest the possibility of several phase transitions below 10 GPa. However, experiments reveal only a pressure-induced amorphisation between 15 and 25 GPa, depending on sample quality and deviatoric stresses. The amorphous phase has been followed up to 45 GPa and its nature discussed.

Journal of Computational Chemistry, 2000

$Research paper thumbnail of Quantum-mechanical simulation of MgAl_ {2} O_ {4} under high pressure$

... These result should be 224114-6 QUANTUM-MECHANICAL SIMULATION OF MgAl2 O4 . . . ... 22 C.-H. ... more ... These result should be 224114-6 QUANTUM-MECHANICAL SIMULATION OF MgAl2 O4 . . . ... 22 C.-H. Hu and DP Chong, in Encyclopedia of Computational Chemistry, edited by P. von Rau'e Schleyer Wiley, Chichester, 1998 . 23 J. Muscat, A. Wander, and NM Harrison, Chem. ...

Lecture Notes in Electrical Engineering, 2009

Tracking of manoeuvring visual targets or visual features is an important issue in robotic vision... more Tracking of manoeuvring visual targets or visual features is an important issue in robotic vision. For this purpose, predictive techniques are used. In this sense, this work presents a new predictor that decreases the tracking error compared with classic filters for abrupt motion changes and can be used for unknown object’s dynamics. The proposed predictor is based on a fuzzy mix of several Kalman filters, but it can be extended to other algorithms like Circular Predictors. This fuzzy mix depends on what filter is working closer to their optimal settings. The performance and robustness of the proposed algorithm is verified by simulation and experiment and it is compared with other robust methods.

Revista Española de Cirugía Ortopédica y Traumatología (English Edition), 2008

... Therefore, the final ABG-I prosthesis model had 60,401 elements (33,504 were of the cortical ... more ... Therefore, the final ABG-I prosthesis model had 60,401 elements (33,504 were of the cortical bone, 22,088 of the cancellous bone and 4,809 of the ABG-I prosthesis). The ABG-I prosthesis is made of a wrought titanium al-loy type Ti-6Al-4V. ...

The implementation of a postprocessor for the NX™ platform (Siemens Corp.) is described in this p... more The implementation of a postprocessor for the NX™ platform (Siemens Corp.) is described in this paper. It is focused on a milling redundant robotic milling workcell consisting of one KUKA KR 15/2 manipulator (6 rotary joints, KRC2 controller) mounted on a linear axis and synchronized with a rotary table (i.e., two additional joints). For carrying out a milling task, a

2009 IEEE International Conference on Mechatronics, 2009

Abstract Nowadays, the use of kinematically redundant robotic workcells for machining tasks is in... more Abstract Nowadays, the use of kinematically redundant robotic workcells for machining tasks is increasing. It is due to their capacity to avoid singular configurations, although a choice among a set of possible configurations is required. Experience and knowledge of the ...

Revista Española de Cirugía Ortopédica y Traumatología, 2008

Palabras clave: remodelado óseo, estudio con DEXA, elementos finitos, prótesis de cadera.

International Journal of Control, Automation and Systems, 2009

This work presents a trajectory control for non-redundant serial-link manipulators that is valid ... more This work presents a trajectory control for non-redundant serial-link manipulators that is valid for trajectories with ordinary singularities of codimension one and non-ordinary singularities of any codimension. For this purpose, several singularity classifications are considered and a procedure is developed in order to solve the indeterminate motion of non-ordinary singularities. The proposed trajectory control is validated by simulation and by experiments with the six-revolute (6R) industrial robot KUKA KR 15/2.

International Journal of Electrical Engineering Education, 2011

Visual servoing uses the visual information provided by cameras to control the movement of a robo... more Visual servoing uses the visual information provided by cameras to control the movement of a robotic system. This paper presents an international on-line course aimed at BSc, MSc and PhD students in this subject. The course lasted for eight weeks, and its objective was to provide a general vision of those aspects related to the course theme including a special incentive: the professors teaching the course were the most important experts in this field at a global level, signifying that the students were able to gain access to a potentially higher quality of teaching than that available at an individual traditional university.

18th Mediterranean Conference on Control and Automation, MED'10, 2010

In the great majority of commercial robots the tracking speed along the task-space path usually h... more In the great majority of commercial robots the tracking speed along the task-space path usually has to be computed a priori by the robot operator to avoid hitting some input constraints. In this paper, an approach based on sliding mode conditioning of a path parametrization is proposed to achieve the greatest tracking speed which is compatible with the robot input constraints (e.g. joint speeds). The proposal avoids on-line computations required by conventional analytical methodologies, and it can be easily added to pre-existing path tracking schemes, since its switching law is confined to the low-power side of the system. A necessary and sufficient condition for sliding mode establishment is derived, which assures for a given path that the robot fully exploits the actuator power once sliding regime is reached. Simulation results on a 2R manipulator are presented in order to illustrate the effectiveness of the approach.