Publications by authors named "Bo Jakobsen"

13 Publications

  • Page 1 of 1

High-pressure cell for simultaneous dielectric and neutron spectroscopy.

Rev Sci Instrum 2018 Feb;89(2):023904

Glass and Time, IMFUFA, Department of Science and Environment, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark.

In this article, we report on the design, manufacture, and testing of a high-pressure cell for simultaneous dielectric and neutron spectroscopy. This cell is a unique tool for studying dynamics on different time scales, from kilo- to picoseconds, covering universal features such as the α relaxation and fast vibrations at the same time. The cell, constructed in cylindrical geometry, is made of a high-strength aluminum alloy and operates up to 500 MPa in a temperature range between roughly 2 and 320 K. In order to measure the scattered neutron intensity and the sample capacitance simultaneously, a cylindrical capacitor is positioned within the bore of the high-pressure container. The capacitor consists of two concentric electrodes separated by insulating spacers. The performance of this setup has been successfully verified by collecting simultaneous dielectric and neutron spectroscopy data on dipropylene glycol, using both backscattering and time-of-flight instruments. We have carried out the experiments at different combinations of temperature and pressure in both the supercooled liquid and glassy state.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.5007021DOI Listing
February 2018

Communication: High pressure specific heat spectroscopy reveals simple relaxation behavior of glass forming molecular liquid.

J Chem Phys 2015 Dec;143(22):221101

DNRF Centre "Glass and Time," IMFUFA, Department of Sciences, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark.

The frequency dependent specific heat has been measured under pressure for the molecular glass forming liquid 5-polyphenyl-4-ether in the viscous regime close to the glass transition. The temperature and pressure dependences of the characteristic time scale associated with the specific heat is compared to the equivalent time scale from dielectric spectroscopy performed under identical conditions. It is shown that the ratio between the two time scales is independent of both temperature and pressure. This observation is non-trivial and demonstrates the existence of specially simple molecular liquids in which different physical relaxation processes are both as function of temperature and pressure/density governed by the same underlying "inner clock." Furthermore, the results are discussed in terms of the recent conjecture that van der Waals liquids, like the measured liquid, comply to the isomorph theory.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.4936867DOI Listing
December 2015

Communication: Supramolecular structures in monohydroxy alcohols: insights from shear-mechanical studies of a systematic series of octanol structural isomers.

J Chem Phys 2014 Sep;141(10):101104

DNRF Centre "Glass and Time", IMFUFA, Department of Sciences, Roskilde University, Postbox 260, DK-4000 Roskilde, Denmark.

A recent study [C. Gainaru, R. Figuli, T. Hecksher, B. Jakobsen, J. C. Dyre, M. Wilhelm, and R. Böhmer, Phys. Rev. Lett. 112, 098301 (2014)] of two supercooled monohydroxy alcohols close to the glass-transition temperature showed that the Debye peak, thus far mainly observed in the electrical response, also has a mechanical signature. In this work, we apply broadband shear-mechanical spectroscopy to a systematic series of octanol structural isomers, x-methyl-3-heptanol (with x ranging from 2 to 6). We find that the characteristics of the mechanical signature overall follow the systematic behavior observed in dielectric spectroscopy. However, the influence from the molecular structure is strikingly small in mechanics (compared to roughly a factor 100 increase in dielectric strength) and one isomer clearly does not conform to the general ordering. Finally, the mechanical data surprisingly indicate that the size of the supramolecular structures responsible for the Debye process is nearly unchanged in the series.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.4895095DOI Listing
September 2014

Communication: Identical temperature dependence of the time scales of several linear-response functions of two glass-forming liquids.

J Chem Phys 2012 Feb;136(8):081102

DNRF centre Glass and Time, IMFUFA, Department of Sciences, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark.

The frequency-dependent dielectric constant, shear and adiabatic bulk moduli, longitudinal thermal expansion coefficient, and longitudinal specific heat have been measured for two van der Waals glass-forming liquids, tetramethyl-tetraphenyl-trisiloxane (DC704) and 5-polyphenyl-4-ether. Within the experimental uncertainties the loss-peak frequencies of the measured response functions have identical temperature dependence over a range of temperatures, for which the Maxwell relaxation time varies more than nine orders of magnitude. The time scales are ordered from fastest to slowest as follows: Shear modulus, adiabatic bulk modulus, dielectric constant, longitudinal thermal expansion coefficient, and longitudinal specific heat. The ordering is discussed in light of the recent conjecture that van der Waals liquids are strongly correlating, i.e., approximate single-parameter liquids.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.3690083DOI Listing
February 2012

Frequency-dependent specific heat from thermal effusion in spherical geometry.

Phys Rev E Stat Nonlin Soft Matter Phys 2010 Jun 25;81(6 Pt 1):061505. Epub 2010 Jun 25.

DNRF Centre Glass and Time, IMFUFA, Department of Sciences, Roskilde University, Postbox 260, DK-4000 Roskilde, Denmark.

We present a method of measuring the frequency-dependent specific heat at the glass transition applied to 5-polyphenyl-4-ether. The method employs thermal waves effusing radially out from the surface of a spherical thermistor that acts as both a heat generator and a thermometer. It is a merit of the method compared to planar effusion methods that the influence of the mechanical boundary conditions is analytically known. This implies that it is the longitudinal rather than the isobaric specific heat that is measured. As another merit the thermal conductivity and specific heat can be found independently. The method has highest sensitivity at a frequency where the thermal diffusion length is comparable to the radius of the heat generator. This limits in practice the frequency range to 2-3 decades. An account of the 3ω technique used including higher-order terms in the temperature dependence of the thermistor and in the power generated is furthermore given.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1103/PhysRevE.81.061505DOI Listing
June 2010

Prevalence of approximate square root(t) relaxation for the dielectric alpha process in viscous organic liquids.

J Chem Phys 2009 Apr;130(15):154508

Department of Sciences, DNRF Centre Glass and Time, IMFUFA, Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark.

This paper presents dielectric relaxation data for organic glass-forming liquids compiled from different groups and supplemented by new measurements. The main quantity of interest is the "minimum slope" of the alpha dielectric loss plotted as a function of frequency in a log-log plot, i.e., the numerically largest slope above the loss peak frequency. The data consisting of 347 spectra for 53 liquids show prevalence of minimum slopes close to -1/2, corresponding to approximate square root(t) dependence of the dielectric relaxation function at short times. The paper studies possible correlations between minimum slopes and (1) temperature (quantified via the loss peak frequency); (2) how well an inverse power-law fits data above the loss peak; (3) degree of time-temperature superposition; (4) loss peak half width; (5) deviation from non-Arrhenius behavior; (6) loss strength. For the first three points we find correlations that show a special status of liquids with minimum slopes close to -1/2. For the last three points only fairly insignificant correlations are found, with the exception of large-loss liquids that have minimum slopes that are numerically significantly larger than 1/2. We conclude that--excluding large-loss liquids--approximate square root(t) relaxation appears to be a generic property of the alpha relaxation of organic glass formers.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.3098911DOI Listing
April 2009

Supercooled liquid dynamics studied via shear-mechanical spectroscopy.

J Phys Chem B 2008 Dec;112(51):16320-5

DNRF Centre "Glass and Time", IMFUFA, Department of Sciences, Roskilde University, Postbox 260, DK-4000 Roskilde, Denmark.

We report dynamical shear-modulus measurements for five glass-forming liquids (pentaphenyltrimethyltrisiloxane, diethyl phthalate, dibutyl phthalate, 1,2-propanediol, and m-touluidine). The shear-mechanical spectra are obtained by the piezoelectric shear-modulus gauge (PSG) method. This technique allows one to measure the shear modulus (10(5)-10(10) Pa) of the liquid within a frequency range from 1 mHz to 10 kHz. We analyze the frequency-dependent response functions to investigate whether time-temperature superposition (TTS) is obeyed. We also study the shear-modulus loss-peak position and its high-frequency part. It has been suggested that when TTS applies, the high-frequency side of the imaginary part of the dielectric response decreases like a power law of the frequency with an exponent -1/2. This conjecture is analyzed on the basis of the shear mechanical data. We find that TTS is obeyed for pentaphenyltrimethyltrisiloxane and in 1,2-propanediol while in the remaining liquids evidence of a mechanical beta process is found. Although the high-frequency power law behavior w(-alpha) of the shear loss may approach a limiting value of alpha = 0.5 when lowering the temperature, we find that the exponent lies systematically above this value (around 0.4). For the two liquids without beta relaxation (pentaphenyltrimethyltrisiloxane and 1,2-propanediol) we also test the shoving model prediction, according to which the relaxation time activation energy is proportional to the instantaneous shear modulus. We find that the data are well described by this model.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1021/jp805097rDOI Listing
December 2008

Investigation of the shear-mechanical and dielectric relaxation processes in two monoalcohols close to the glass transition.

J Chem Phys 2008 Nov;129(18):184502

DNRF Centre Glass and Time, IMFUFA, Department of Sciences, Roskilde University, Postbox 260, DK-4000 Roskilde, Denmark.

Shear-mechanical and dielectric measurements on the two monohydroxy (monoalcohol) molecular glass formers 2-ethyl-1-hexanol and 2-butanol close to the glass-transition temperature are presented. The shear-mechanical data are obtained using the piezoelectric shear-modulus gauge method covering frequencies from 1 mHz to 10 kHz. The shear-mechanical relaxation spectra show two processes, which follow the typical scenario of a structural (alpha) relaxation and an additional (Johari-Goldstein) beta relaxation. The dielectric relaxation spectra are dominated by a Debye-type peak with an additional non-Debye peak visible. This Debye-type relaxation is a common feature peculiar to monoalcohols. The time scale of the non-Debye dielectric relaxation process is shown to correspond to the mechanical structural (alpha) relaxation. Glass-transition temperatures and fragilities are reported based on the mechanical alpha relaxation and the dielectric Debye-type process, showing that the two glass-transition temperatures differ by approximately 10 K and that the fragility based on the Debye-type process is a factor of 2 smaller than the structural fragility. If a mechanical signature of the Debye-type relaxation exists in these liquids, its relaxation strength is at most 1% and 3% of the full relaxation strength of 2-butanol and 2-ethyl-1-hexanol, respectively. These findings support the notion that it is the non-Debye dielectric relaxation process that corresponds to the structural alpha relaxation in the liquid.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.3007988DOI Listing
November 2008

[Laparoscopic repair of giant incisional hernia after abdominal wall reconstruction].

Ugeskr Laeger 2007 Sep;169(38):3209-10

Gentofte Hospital, Kirurgisk Afdeling D.

Laparoscopic repair of giant incisional hernias, traditionally treated using the open technique with abdominal wall reconstruction, represents a development in the operative method with fewer peri- and post-operative complications. The authors present a patient with a giant incisional hernia after primary right hemipelvic chondrosarcoma and pelvic resection. The patient was treated with laparoscopic repair, in which a large prolene mesh was implanted, and the patient had an uncomplicated post-operative course.
View Article and Find Full Text PDF

Download full-text PDF

Source
September 2007

Formation and subdivision of deformation structures during plastic deformation.

Science 2006 May;312(5775):889-92

Center for Fundamental Research: Metal Structures in Four Dimensions, Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark.

During plastic deformation of metals and alloys, dislocations arrange in ordered patterns. How and when these self-organization processes take place have remained elusive, because in situ observations have not been feasible. We present an x-ray diffraction method that provided data on the dynamics of individual, deeply embedded dislocation structures. During tensile deformation of pure copper, dislocation-free regions were identified. They showed an unexpected intermittent dynamics, for example, appearing and disappearing with proceeding deformation and even displaying transient splitting behavior. Insight into these processes is relevant for an understanding of the strength and work-hardening of deformed materials.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1126/science.1124141DOI Listing
May 2006

Dielectric and shear mechanical alpha and beta relaxations in seven glass-forming liquids.

J Chem Phys 2005 Dec;123(23):234511

Department of Mathematics and Physics (IMFUFA), Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark.

We present shear mechanical and dielectric measurements taken on seven liquids: triphenylethylene, tetramethyltetra-phenyltrisiloxane (Dow Corning 704 diffusion pump fluid), polyphenyl ether (Santovac 5 vacuum pump fluid), perhydrosqualene, polybutadiene, decahydroisoquinoline (DHIQ), and tripropylene glycol. The shear mechanical and dielectric measurements are for each liquid performed under identical thermal conditions close to the glass transition temperature. The liquids span four orders of magnitude in dielectric relaxation strength and include liquids with and without Johari-Goldstein beta relaxation. The shear mechanical data are obtained by the piezoelectric shear modulus gauge method giving a large frequency span (10(-3)-10(4.5) Hz). This allows us to resolve the shear mechanical Johari-Goldstein beta peak in the equilibrium DHIQ liquid. We moreover report a signature (a pronounced rise in the shear mechanical loss at frequencies above the alpha relaxation) of a Johari-Goldstein beta relaxation in the shear mechanical spectra for all the liquids which show a beta relaxation in the dielectric spectrum. It is found that both the alpha and beta loss peaks are shifted to higher frequencies in the shear mechanical spectrum compared to the dielectric spectrum. It is in both the shear and dielectric responses found that liquids obeying time-temperature superposition also have a high-frequency power law with exponent close to -12. It is moreover seen that the less temperature dependent the spectral shape is, the closer it is to the universal -12 power-law behavior. The deviation from this universal power-law behavior and the temperature dependencies of the spectral shape are rationalized as coming from interactions between the alpha and beta relaxations.
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.2136887DOI Listing
December 2005

Dielectric and shear mechanical relaxations in glass-forming liquids: a test of the Gemant-DiMarzio-Bishop model.

J Chem Phys 2005 Dec;123(23):234510

Department of Mathematics and Physics (IMFUFA), Roskilde University, P.O. Box 260, DK-4000 Roskilde, Denmark.

The Gemant-DiMarzio-Bishop model, which connects the frequency-dependent shear modulus to the frequency-dependent dielectric constant, is reviewed and a new consistent macroscopic formulation is derived. It is moreover shown that this version of the model can be tested without fitting parameters. The reformulated version of the model is analyzed and experimentally tested. It is demonstrated that the model has several nontrivial qualitative predictions: the existence of an elastic contribution to the high-frequency limit of the dielectric constant, a shift of the shear modulus loss peak frequency to higher frequencies compared with the loss peak frequency of the dielectric constant, a broader alpha peak, and a more pronounced beta peak in the shear modulus when compared with the dielectric constant. It is shown that these predictions generally agree with experimental findings and it is therefore suggested that the Gemant-DiMarzio-Bishop model is correct on a qualitative level. The quantitative agreement between the model and the data is on the other hand moderate to poor. It is discussed if a model-free comparison between the dielectric and shear mechanical relaxations is relevant, and it is concluded that the shear modulus should be compared with the rotational dielectric modulus, 1(epsilon(omega)-n2), which is extracted from the Gemant-DiMarzio-Bishop model, rather than to the dielectric susceptibility or the conventional dielectric modulus M=1epsilon(omega).
View Article and Find Full Text PDF

Download full-text PDF

Source
http://dx.doi.org/10.1063/1.2136886DOI Listing
December 2005

[Are the Danish Society of General Practitioners' clinical guidelines concerning "Identification and investigation of dementia and dementia-like conditions" useful as the basis of dementia investigation in general practice?].

Ugeskr Laeger 2003 May;165(20):2105-7

Lunds Universitet, Forskningsmetodisk grundkurs VMF 521, og Sundhedsforvaltningen i Nykøbing Falster, Kvalitetsenheden FUAP.

Introduction: During the last five years better possibilities have appeared for investigation and medical treatment of dementia. Society and the national health authorities have a growing demand for systematical identification of dementia. In 1999, DSAM published a clinical guideline for identifying dementia. The aim of the study was to test the usefulness of that guideline.

Material And Methods: A total of 22 out of 29 general practitioners (GPs) agreed to use the guidelines of the Danish Society of General Practitioners (DSAM) and the questionnaires from three consultations in order to systematically identify dementia. At the same time, the GPs gave their opinion about the usefulness of the guidelines.

Results: A total of 22 GPs sent results from 69 patients. A group of 49 patients had been CT-scanned, 13 patients had been examined by neuropsychologists, and nine patients had started acetylcholinesterase inhibitor treatment.

Conclusion: Some GPs found it embarrassing to offer dementia identifying to their patients. Identifying dementia was complicated and time-consuming. Identifying dementia offered opportunities to look further into polypharmacy and cooperation with other health sectors. Necessary but not sufficient preconditions for starting identifying dementia in general practice will be fees, direct admission to CT-scanning and neuropsychologist, and possibly right to prescribe acetylcholinesterase inhibitors. The conclusion was that all GPs found the guidelines of DSAM useful for identifying dementia in general practice.
View Article and Find Full Text PDF

Download full-text PDF

Source
May 2003
-->