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J L Vicente

Publications and source records attributed to J L Vicente.

13 recordsLinked to original sources
PubMed2004

Dielectric breakdown model for conductor-loaded and insulator-loaded composite materials.

In the present work we generalize the dielectric breakdown model to describe dielectric breakdown patterns in both conductor-loaded and insulator-loaded composites. The present model is an extension of a previous one [F. Peruani et al., Phys. Rev. E 67, 066121 (2003)] presented by the authors to describe dielectric breakdown patterns in conductor-loaded composites. Particles are distributed at random in a matrix with a variable concentration p. The generalized model assigns different probabilities P(i,k-->i('),k(')) to breakdown channel formation according to particle characteristics. Dielectric breakdown patterns are characterized by their fractal dimension D and the parameters of the Weibull distribution. Studies are carried out as a function of the fraction of inhomogeneities, p.

Journal Article↗
PubMed2003

Dielectric breakdown model for composite materials.

This paper addresses the problem of dielectric breakdown in composite materials. The dielectric breakdown model was generalized to describe dielectric breakdown patterns in conductor-loaded composites. Conducting particles are distributed at random in the insulating matrix, and the dielectric breakdown propagates according to new rules to take into account electrical properties and particle size. Dielectric breakdown patterns are characterized by their fractal dimension D and the parameters of the Weibull distribution. Studies are carried out as a function of the fraction of conducting inhomogeneities, p. The fractal dimension D of electrical trees approaches the fractal dimension of a percolation cluster when the fraction of conducting particles approximates the percolation limit.

Journal Article↗
PubMed2000

Fractal analysis of electrical trees in a cross-linked synthetic resin.

A statistical picture of dielectric breakdown in cross-linked polyester resins for a two-dimensional geometry is presented and discussed in this paper. A connection is established between the dielectric breakdown model (DBM) and the physical properties of the resin. Distribution propagation times of simulated trees obey a Weibull statistics, as was experimentally found. This adjustment is achieved by a redefinition of the unit of time, which is different from the one employed up to date. The experimental dependence of characteristic propagation times on the fractal dimension D can be reproduced in the range 1.2<D<1.5. A relationship is established between the glass transition temperature T(g) and the DBM parameter eta, which is in agreement with thermodynamical considerations. It is suggested that fractal characteristics of electrical trees should be related to a basic material property, such as the cross-linking density, which implies a notion of universality that deserves to be explored.

Journal Article↗
PubMed2000

[Frozen section biopsy in the assessment of organs for transplantation].

BACKGROUND: The success of transplantation and the increase in its indications has caused organ donations to be unable to meet the global demand. By broadening the donor selection criteria and accepting so-called suboptimal donors in an attempt to solve this problem, a careful individualized assessment is required of the validity of each donor. This often implies the need for a pretransplantation biopsy study. MATERIAL AND METHODS: Evaluation of frozen-section biopsy studies in the assessment of graft and donor validness in the the transplantation program of the La Fe University Hospital (Valencia, Spain) during 1998, when a total of 301 solid organ transplantations were performed. RESULTS: Sixty pretransplantation frozen-section biopsies were performed (29 of the liver, 21 renal, 3 lymph nodes, 3 brain, 2 prostatic, 1 myometrial and 1 pericardial). As a result of the bioptic study, 18 transplantations were invalidated (30% of all biopsies). The most frequent causes of invalidation were liver macroesteatosis (7 cases), malignancies (5 cases) and renal parenchymal lesions (5 cases). Among the tumor diagnosis there were one false-positive and one false-negative results of malignancy. CONCLUSION: Frozen-section biopsies evaluation in suboptimal organ donors or donors suspected of suffering disease integrates morphological analysis in the transplantation team decision process, and affords a quality factor in the selection of donors.

Adolescent↗
PubMed1999

Theoretical model of diatomic molecules interacting on a two-dimensional lattice.

The coverage-dependent binding energy of interacting dimers is investigated using the lattice gas model. Dimers with distinguishable ends are assumed to interact if they occupy only nearest-neighbor lattice sites. The interaction energy between a pair of dimers will be repulsive if identical ends are in nearest-neighbor positions and attractive if they are not. The grand partition function is derived by using the Bethe approximation. The energy of the system of interacting dimers is evaluated as a function of the lattice coverage at different temperatures and pair interaction potentials. Under the same conditions the average number of all types of nearest-neighbors is evaluated.

Journal Article↗
PubMed1997

[Predictive variables of early mortality after orthotopic heart transplant in adults].

OBJECTIVE: The purpose of the study was to analyze some variables of donors, recipients and surgical procedures in order to discover factors that could predict mortality during the early stage (< 30 days) of orthotopic heart transplants. MATERIAL AND METHOD: 125 consecutive orthotopic heart transplants in adults were analyzed. The average age was 51 +/- 11 (range: 12-67), 109 (87%) were men, 16 were women (13%). Two groups were compared: 15 patients who died within 30 days after heart transplant and 110 who survived during that period. Immunosuppressive protocol: preoperative: Cyclosporin + Azathioprine. Intraoperative: Methylprednisolone Postoperative: Methylprednisolone (first 24 h), antilymphocyte monoclonal antibodies (7-10 days after heart transplant) + Cyclosporin + Azathioprine + Corticoids. The following parameters of the recipient were analyzed: sex, age, weight, size, thoracic perimeter, pretransplant cardiopathy, previous thoracic operations, functional stage or need for catecholamines during the days prior to the transplant, pulmonary artery pressure and resistance, history of systemic arterial hypertension, elevation of creatinine, blood type, urgent transplant indication, receptor/donor weight relationship. The following parameters of donors and operation were analyzed: sex, age, weight, thoracic perimeter, period in intensive care unit, dose of dopamine and dobutamine, blood type, origin of the organ, cause of death, ischaemia time, cardiopulmonary by-pass time and cardioplegia type. RESULTS: The rate of early mortality was 12%. The univariate analysis showed differences in: prior cardiovascular surgery, receptor blood type, need for urgent transplantation, pulmonary artery resistance > 2.5 Wood Units, cardiopulmonary by-pass time, weight relationship between receptor and donor. The death cause of the donor proved significant. On multivariate analysis, the following parameters independently predicted early mortality: history of operation with extracorporeal circulation, high pulmonary artery resistance, urgent transplant, receptor/donor weight relation and time of extracorporeal circulation. CONCLUSIONS: We believe that the results of our experience can help to stratify the risk in the orthotopic heart transplant recipient and even to contraindicate the procedure in some cases showing an accumulation of poor prognostic factors in borderline recipients.

Adolescent↗
PubMed1996

[Analysis of the block design test in a geriatric sample group].

We have carried out a study to determine the normal values of the 'block design test' in a sample group of 58 healthy elderly persons, a test which is usually used in the clinical evaluation of constructive apraxia. We therefore describe a grading scale which has not been previously published, in order to check the possible usefulness of this drawing as a rapid test of cognitive evaluation. A study was made of the possible interaction of the test with sample parameters, age, sex, educational level and also the inter-examiner reliability. We have observed a significant interaction between the variables of sex and educational level. Also the inter-examiner reliability is not very good. In view of our results, we consider that the 'block design test' is not very useful in the evaluation of constructive praxis in healthy elderly persons, and therefore it does not seem to us to be a good test for the rapid evaluation of cognitive function in this group.

Aged↗
PubMed1996

[The 'clock drawing test' in healthy elderly people].

INTRODUCTION AND OBJECTIVE: In this paper we aim to determine the normal values of the clock drawing test in a sample group of 58 healthy elderly persons as a neuropsychological parameter for the evaluation of certain cognitive functions. MATERIAL AND METHODS: For this we have used our own adaptation of the quantitative parameters of the clock drawing test in the evaluation 'on command' and 'on copying'. We have also evaluated the possible interaction of the test with the following sample parameters: sex, age, and educational level and we have calculated the inter-examiner reliability of the test. RESULTS: The average scores obtained were: 8.04 'on-command' (SD 1.60) and 9.20 (SD 0.93) 'on-copying'. CONCLUSIONS: On analysis of our results we have not observed interaction of the parameters used (age, sex and educational level) with the scores obtained in the test, while there was a high level of inter-examiner reliability.

Aged↗
PubMedDate not supplied

[A proposal for application and scoring of the Clock Drawing Test in Alzheimer's disease].

INTRODUCTION: The Clock Drawing Test (CDT) has been used in recent years as a simple neuropsychological instrument to assess cognitive deterioration associated with dementia, even though uniform operative criteria with respect to its application and scoring have not been established. OBJECTIVE: To present application normatives and establish the most relevant psychometric criteria of the CDT in a sample of healthy subjects (HS) and patients with Alzheimer's disease (AD). PATIENTS AND METHODS: 56 patients were selected of which 35 were female and 21 were male. The patients' mean age was 72.7 with a standard deviation of 7.64. All of whom where probable AD patients according to the NINCDS-ADRDA criteria at stage 1 CDR. The group of HS was made up of 56 control subjects (34 female, 22 male) with a mean age of 72.14 and a standard deviation of 7.2. The CDT was applied in both its command (COM) and copy (COP) experimental conditions. RESULTS: The main psychometric parameters analysed in the studied series showed the following values: internal consistency (Cronbach's alpha coefficient 0.9029); cut off point CT COM 6 with 92.80, sensitivity; false negatives (FN) 7.2 with a specificity rating 93.48; false positive (FP) 6.52 with 93.16 efficacy; cut off point CT COP 8 with 73.11 sensitivity; FN 26.89 with 90.58 specificity; FP 9.42 with 82.49 efficacy. CONCLUSION: The CDT can be used to discriminate between HS and those in the initial stages of AD in the given sample using the established application and scoring criteria.

Aged↗