Search Results

You are looking at 1 - 10 of 64 items for

  • Author or Editor: M. Martin x
  • Refine by Access: All Content x
Clear All Modify Search

Polymer crystallization

Isothermal and non-isothermal spherulite growth parameters from optical microscopy and differential scanning calorimetry

Journal of Thermal Analysis and Calorimetry
Authors:
J. Martins
,
J. Cruz-Pinto
, and
M. Oliveira

Abstract  

Differential scanning calorimetry (dual furnace, null-balance, DSC) and optical microscopy (OM) have been used to study the isothermal crystallization kinetics of poly(oxymethylene)-POM. The non-isothermal crystallization of the same material has also been studied by optical microscopy. A very controversial problem is whether the isothermal kinetic parameters may be applied to describe the non-isothermal crystallization. The results show that the kinetic spherulite growth parameters obtained by non-isothermal optical microscopy are, within the experimental errors involved, the same as those obtained by isothermal optical microscopy or isothermal DSC. The importance of this finding is highlighted.

Restricted access

Abstract  

Three hundred ancient Turkish potsherds were analyzed by instrumental neutron activation analysis, and the resulting data analyzed by several techniques of multivariate statistical analysis, some only recently developed. The programs AGCLUS, MASLOC, and SIMCA were sequentially employed to characterize and group the samples by type of pottery and by site of excavation or collection. Comparison of the statistical analyses by each method provided archaeological insight into the site/type relationships of the samples and ultimately evidence relevant to commercial relations between the ancient communities and to specialization of pottery production over time. The techniques used for statistical analysis were found to be of significant potential utility in the future analysis of other archaeometric data sets.

Restricted access

Abstract  

This paper analyzes the scientific relationships between Spain and the Eastern European countries (EEc), including the former USSR, from 1982 to 1992. The study considers the number of co-authored papers as well as the number of stays which reflect to the exchange of scientists among them. The total number of co-authored papers of Spanish scientists with their colleagues of those countries is 664 as recorded in theScience Citation Index. During the last three years, a significative increase in the number of co-signed papers has been observed. From the sample considered, the percentage of bilateral and multilateral co-authored papers is 43.8 and 56.2 respectively. In absolute figures, USSR and Poland are the countries with highest number of collaborative papers with Spain. However, in relation to their scientific output, Poland and Hungary show a higher percentage of co-authored papers than the rest of EEc. On the other hand, previous data, provided by the Spanish Secretary of State for Research and Universities (SEUI), related to the flow of scientists on sabbatical year to and from Spain,1 showed that from 1984 to 1994, a total of 213 researches come from the EEc to Spain, while the number of Spanish scientists going to those countries was extremely scarce.

Restricted access

Abstract  

The role of domestic journals in disseminating research results in the filed of Earth Sciences in Spain is discussed. The analysis is based on bibliometric indicators of Spanish scientific production, as well as on the opinion of Spanish researchers in this field, obtained through a specially designed survey. A reasonable correspondence has been found between the results of bibliometric analysis and scientists' judgements. Results show that 69% of Spanish articles in Earth Sciences were published in Spanish journals during the period 1990–1994. Scientists use both national and international journals to communicate, their research results, although due to the nature of the discipline, geographically oriented and therefore mostly devoted to local problems, they use basically domestic journals. In terms of international visibility, although none of the Spanish journals in this field is covered by the SCI, most of them are covered by some of the most representative international databases in the field concerned. The study points out the importance of domestic journals in the field of Earth Sciences in Spain.

Restricted access

We study the pairs (S,L)of positive random variables which arise in the a .ne de- compositions T (law) =S +L 2 T of the stable

Restricted access

Among other objectives, forest inventories are aimed to identify ecological communities and to correlate community composition with environmental variables. The identification of different communities would require several forest inventories, each covering small sampling areas with relatively homogeneous environmental conditions. The multiple plot sampling method, traditionally used in local inventories of tropical forests, cannot assure such homogeneity, since even small sampling areas would show environmental heterogeneity influencing vegetation. In this paper we assessed the consequences of this heterogeneity for sampling by quantifying the variability of species abundance ranks for species sampled with 10 or more individuals in a set of plots covering a small sampling area. The species reference abundance ranks were obtained from a sample of 100 plots of 10 m × 10 m each randomly set in a sampling area of 6.5 ha in a tropical forest fragment (Southeastern Brazil). For each species we used resamplings (30 trials) to obtain the species abundance ranks in sub-samples, considering different sampling intensities (n = 25, 50 and 75 plots), and compared these ranks with the species reference rank (n = 100 plots). Then, we compared the species ranks in sub-samples of 50 plots (10.000 trials) with the reference rank and assessed the frequency and extent of rank displacements. Species rank was highly variable across resampling trials for the sampling intensities of n = 25 and n = 50, but decreased considerable with a sampling intensity of n = 75 plots. The mean rank variability and especially the maximum displacement raised significantly from the seventh most abundant species on, and some species occupied quite discrepant abundance ranks in up to 10% of the 10.000 resampling trials. This high internal variability of forest samples may impair the search for floristic patterns as scale lessens, say, to the meso-scale (1–100 km2). We discussed some possible ways to increase internal homogeneity of tropical forest samples with the multiple plot sampling method. Among these, objective entitation, based on an ancient phytosociological procedure, is suggested as the most appropriate for use on the hilly relieves of the Atlantic forest biome.

Restricted access

Abstract  

Calorimetric measurements were made of the heat of immersion in water of cassiterite that was either untreated or treated with 60% HNO3. The heats of immersion of cassiterite and fluorite were also calculated theoretically from the surface Gibbs energy components, and compared with the heat of immersion measured for cassiterite and that taken from the literature for fluorite. The results of the measurements and calculation revealed that the heat of immersion depends on the degree of hydration of the surface of cassiterite and fluorite. It was also found that it is possible to predict the heats of immersion in water of cassiterite and fluorite from the Lifshitz-van der Waals and acid-base components of the surface Gibbs energy.

Restricted access

Abstract  

Thermally Stimulated Current (TSC) spectroscopy and Differential Scanning calorimetry (DSC) have been applied to the characterization of the microstructure of Poly (Ether Ether Ketone)/PEEK. the dielectric relaxation spectra show two modes, dependent upon crystallinity: –  * for the mode stuated in the vicinity of the glass transition temperature, two components have been distinguished and attributed to the molecular mobility in the ‘true amorphous phase’ and in the ‘rigid amorphous region’. –  * below 0°C, two sub-modes appear, situated around-110°C and-75°C, due to the two different crystal entities, beads and laths.

Restricted access