Biomedical subjects

H F Poulsen

Publications and source records attributed to H F Poulsen.

8 recordsLinked to original sources

PubMed2002

Grain nucleation and growth during phase transformations.

The mechanical properties of polycrystalline materials are largely determined by the kinetics of the phase transformations during the production process. Progress in x-ray diffraction instrumentation at synchrotron sources has created an opportunity to study the transformation kinetics at the level of individual grains. Our measurements show that the activation energy for grain nucleation is at least two orders of magnitude smaller than that predicted by thermodynamic models. The observed growth curves of the newly formed grains confirm the parabolic growth model but also show three fundamentally different types of growth. Insight into the grain nucleation and growth mechanisms during phase transformations contributes to the development of materials with optimal mechanical properties.

Journal Article↗

PubMed2001

In situ measurement of grain rotation during deformation of polycrystals.

Texture evolution governs many of the physical, chemical, and mechanical properties of polycrystalline materials, but texture models have only been tested on the macroscopic level, which makes it hard to distinguish between approaches that are conceptually very different. Here, we present a universal method for providing data on the underlying structural dynamics at the grain and subgrain level. The method is based on diffraction with focused hard x-rays. First results relate to the tensile deformation of pure aluminum. Experimental grain rotations are inconsistent with the classical Taylor and Sachs models.

Journal Article↗

PubMed2000

A conical slit for three-dimensional XRD mapping.

Traditionally, depth resolution in diffraction experiments is obtained by inserting pinholes in both the incoming and diffracted beam. For materials science investigations of local strain and texture properties this leads to very slow data-acquisition rates, especially when characterization is performed on the level of the individual grains. To circumvent this problem a conical slit has been manufactured by wire-electrodischarge machining. The conical slit has six 25 microm-thick conically shaped openings matching six of the Debye-Scherrer cones from a face-centred-cubic powder. By combining the slit with a microfocused incoming beam of hard X-rays, an embedded gauge volume is defined. Using a two-dimensional detector, fast and complete information can be obtained regarding the texture and strain properties of the material within this particular gauge volume. The average machining and assemblage errors of the conical slit are found both to be of the order of 5 microm. An algorithm for alignment of the slit is established, and the potential of the technique is illustrated with an example of grain mapping in a 4.5 mm-thick Cu sample.

Journal Article↗

PubMed1999

Results from screening for gestational diabetes mellitus in a Danish county.

OBJECTIVE: To examine the complications in gestational diabetes mellitus with special reference to the significance of the threshold for the oral glucose tolerance test (OGTT, plasma glucose > or = 7.8 mmol/l (equivalent to blood glucose 6.9 mmol/l) (WHO criteria) or blood glucose > 6.7 mmol/l (local criteria) after 2 h 75 g oral glucose load). DESIGN: Prospective descriptive study. SETTING: From April 1, 1995 to April 1, 1997 a screening for gestational diabetes mellitus (GDM) according to the recommendations from the Danish board of health was performed in Ribe county. SUBJECTS: Pregnant women in Ribe county (n = 6158). MAIN OUTCOME MEASURES: Complications during pregnancy and the outcome. RESULTS: Two hundred and twenty (3.6%) women had GDM by the local selection criteria and 2.8% according to the WHO criteria. The study revealed a significantly increased frequency of malformations (7.3% vs. 1.2%, p < 0.0001) and preeclampsia (15.5% vs. 1%, p < 0.0001) in the group with GDM independent of selection criteria (obesity vs. family history and glucosuria). The frequency of preeclampsia appeared higher in the group with obesity (p = 0.040). Malformations appeared with identical frequency in the groups with OGGT threshold higher than blood glucose 6.9 mmol/l (plasma glucose 7.8 mmol/l) (WHO criteria) and with blood glucose between 6.7 and 6.9 mmol/l, whereas preeclampsia did not occur with higher frequency in the latter group. CONCLUSION: Complications appear frequently in women with an abnormal OGGT, and obesity predisposes to preeclampsia. The use of the WHO criteria for screening for GDM would miss a number of women with complications.

Adult↗

PubMed1998

Focusing Optics for High-Energy X-ray Diffraction.

Novel focusing optical devices have been developed for synchrotron radiation in the energy range 40-100 keV. Firstly, a narrow-band-pass focusing energy-tuneable fixed-exit monochromator was constructed by combining meridionally bent Laue and Bragg crystals. Dispersion compensation was applied to retain the high momentum resolution despite the beam divergence caused by the focusing. Next, microfocusing was achieved by a bent multilayer arranged behind the crystal monochromator and alternatively by a bent Laue crystal. A 1.2 micro m-high line focus was obtained at 90 keV. The properties of the different set-ups are described and potential applications are discussed. First experiments were performed, investigating with high spatial resolution the residual strain gradients in layered polycrystalline materials. The results underline that focused high-energy synchrotron radiation can provide unique information on the mesoscopic scale to the materials scientist, complementary to existing techniques based on conventional X-ray sources, neutron scattering or electron microscopy.

Journal Article↗

PubMed1998

A triple-crystal diffractometer for high-energy synchrotron radiation at the HASYLAB high-field wiggler beamline BW5.

The triple-crystal diffractometer installed at HASYLAB beamline BW5 with a high-field wiggler of critical energy 27 keV for DORIS III, operated at 4.5 GeV electron energy, is described. Samples can be mounted in large cryostats or furnaces normally used in neutron scattering experiments. The instrument has been successfully applied to measure structure factors S(Q) in liquids and amorphous materials, to collect full data sets of highly accurate structure factors for charge-density work, to measure the spin component of the ground-state magnetization in transition-metal and rare-earth compounds, to study the diffuse scattering from stacking faults and dislocation loops in Si single crystals, and for the investigation of various aspects of structural phase transitions: critical scattering in SrTiO3, oxygen order and stripe order in high-To materials. A crossed-beam technique allows for local studies of texture, internal strain and phase changes in the bulk of materials.

Journal Article↗

PubMed1997

Applications of high-energy synchrotron radiation for structural studies of polycrystalline materials.

The large penetration power of high-energy X-rays (>60 keV) raises interesting prospects for new types of structural characterizations of polycrystalline materials. It becomes possible in a non-destructive manner to perform local studies, within the bulk of the material, of the fundamental materials physics properties: grain orientations, strain, dislocation densities etc. In favourable cases these properties may be mapped in three dimensions with a spatial resolution that matches the dimensions of the individual grains. Imbedded volumes and interfaces become accessible. Moreover, the high energies allow better in-situ studies of samples in complicated environments (industrial process optimization). General techniques for research in this energy range have been developed using broad-band angle-dispersive methods, on-line two-dimensional detectors and conical slits. Characterizations have been made at the level of the individual grains and grain boundaries as well as on ensembles of grains. The spatial resolution is presently of the order of 10-100 micom. Four examples of applications are presented along with an outlook.

Journal Article↗

PubMed1990

[Hemoglobin concentration prior to the 20th week of pregnancy correlated with complications in the third trimester].

The haemodynamic changes in pregnancy result in decrease in haemoglobin and haematocrit. It has previously been demonstrated that absence of this decrease or even increase in the haemoglobin concentration (Hb) and haematocrit (Hct) values measured in the third trimester or late in the second trimester can be correlated to increased risk of development of hypertension, pre-eclampsia and intrauterine growth retardation. In the present investigation, the authors have investigated whether the abovementioned complications of pregnancy can also be correlated to high Hb and high Hct measured early in the third trimester. The investigation included 373 healthy pregnant women, 46 of whom had Hb greater than or equal to 8.3 mmol/l in the 13th-19th weeks. Complications of pregnancy occurred in 17.4% of the 46 women. Among the remaining 327 pregnant women with Hb less than 8.3 mmol/l, complications of pregnancy were observed in only 5.8%. This difference is significant. The blood volume increases by approximately 1.5 litre during pregnancy. The plasma volume increases by 50% from the seventh to the 32nd week and thereafter remains constant until delivery. The erythrocyte volume increases first from approximately the 16th week and increases by a total of 25%. The early increase in plasma volume results in a decrease in the haemoglobin concentration (Hb) and in the haematocrit (Hct) value. During the third trimester, Hb and Hct increase again as a result of the increased erythrocyte volume. A positive correlation has been demonstrated between the magnitude of the plasma volume in the third trimester and the birth weight and a negative correlation between Hb in the third trimester and the birth weight.(ABSTRACT TRUNCATED AT 250 WORDS)

Erythrocyte Volume↗