Dynamics of water bearing silicate melts as seen by quasielastic neutron scattering at high temperature and pressure [Elektronische Ressource] / Fan Yang

Fan Yang

Description

PHYSIK - DEPARTMENTDynamicsofWaterBearingSilicateMeltsasseenbyQuasielasticNeutronScatteringatHighTemperatureandPressureDissertationvonFanYang¨TECHNISCHE UNIVERSITAT¨MUNCHENPhysik-DepartmentderTechnischenUniversita¨tMu¨nchenLehrstuhlfu¨rExperimentalphysikIVDynamics of Water Bearing Silicate Melts asseen by Quasielastic Neutron Scatteringat High Temperature and PressureFanYangVollsta¨ndigerAbdruckdervonderFakulta¨tfu¨rPhysikderTechnischenUniversita¨tMu¨nchenzurErlangungdesakademischenGradeseinesDoktorsderNaturwissenschaften(Dr.rer.nat.)genehmigtenDissertationVorsitzender: Univ.-Prof.Dr.R.MetzlerPru¨ferderDissertation:1. Univ.-Prof.Dr.A.Meyer2. Univ.-Prof.Chr.Pfleiderer,Ph.D.Die Dissertation wurde am 29.01.2009 bei der Technischen Universita¨tMu¨nchen eingereicht und durch die Fakulta¨t fu¨r Physik am 03.04.2009angenommen.QitoContentsSummary vZusammenfassung vii1 Introduction and motivation 11.1 Geologicalimportance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Theglasstransition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Interplayofstructure anddynamicsinsilicatemelts . . . . . . . . . . . . 41.4 Thehydroussilicatesystem . . . . . . . . . . . . . . . . . . . . . . . . . . 71.5 Presentwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Sample synthesis 112.1 Preparationofthedrysilicates . . . . . . . . . . . . . . . . . . . . . . . . 112.2 Dissolutionofwater . . . . . .

Informations

• Publié par: technische_universitat_munchen
• Publié le: 01 janvier 2009
• Nombre de lectures: 26
• Langue: English
• Poids de l'ouvrage: 2 Mo

Extrait

PHYSIK - DEPARTMENT
DynamicsofWaterBearingSilicateMeltsas
seenbyQuasielasticNeutronScattering
atHighTemperatureandPressure
Dissertation
von
FanYang
¨TECHNISCHE UNIVERSITAT
¨MUNCHENPhysik-Departmentder
TechnischenUniversita¨tMu¨nchen
Lehrstuhlfu¨rExperimentalphysikIV
Dynamics of Water Bearing Silicate Melts as
seen by Quasielastic Neutron Scattering
at High Temperature and Pressure
FanYang
Vollsta¨ndigerAbdruckdervonderFakulta¨tfu¨rPhysikderTechnischen
Universita¨tMu¨nchenzurErlangungdesakademischenGradeseines
DoktorsderNaturwissenschaften(Dr.rer.nat.)
genehmigtenDissertation
Vorsitzender: Univ.-Prof.Dr.R.Metzler
Pru¨ferderDissertation:
1. Univ.-Prof.Dr.A.Meyer
2. Univ.-Prof.Chr.Pfleiderer,Ph.D.
Die Dissertation wurde am 29.01.2009 bei der Technischen Universita¨t
Mu¨nchen eingereicht und durch die Fakulta¨t fu¨r Physik am 03.04.2009
angenommen.
to
QiContents
Summary v
Zusammenfassung vii
1 Introduction and motivation 1
1.1 Geologicalimportance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Theglasstransition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 Interplayofstructure anddynamicsinsilicatemelts . . . . . . . . . . . . 4
1.4 Thehydroussilicatesystem . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Presentwork . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2 Sample synthesis 11
2.1 Preparationofthedrysilicates . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2 Dissolutionofwater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.1 Samplecapsules . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.2.2 Hightemperatureandpressureprocess . . . . . . . . . . . . . . . 15
2.3 Samplecharacterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.1 Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.2 Dilatometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3 Theoretical background 24
3.1 Neutronscattering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1.1 Basictheory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
3.1.2 Coherent/incoherentscatteringandcorrelationfunctions . . . . 27
3.1.3 Neutronscatteringonliquidsandglasses . . . . . . . . . . . . . . 29
3.2 Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.1 Diffusioninsimpleliquid . . . . . . . . . . . . . . . . . . . . . . . 33
3.2.2 Hoppingmodelandanomalousdiffusion . . . . . . . . . . . . . . 34
3.3 Modecouplingtheoryandtheglasstransition . . . . . . . . . . . . . . . 37
3.3.1 Basicapproach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.3.2 Structural relaxationprocessesandscalinglaws . . . . . . . . . . 38
3.3.3 SchematicmodelsofglasstransitioninMCT . . . . . . . . . . . . 40
iii4 Sample environment 42
4.1 Basicconcepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.1.1 Highpressurehightemperaturevessel . . . . . . . . . . . . . . . 42
4.1.2 Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.2 Construction andoperationoftheautoclave . . . . . . . . . . . . . . . . 46
4.2.1 Highpressurecomponents . . . . . . . . . . . . . . . . . . . . . . 47
4.2.2 Hightemperaturefurnace . . . . . . . . . . . . . . . . . . . . . . . 49
4.2.3 OthercomponentsandadaptiontoTOFTOF . . . . . . . . . . . . 51
4.3 Neutronscatteringcharacteristics . . . . . . . . . . . . . . . . . . . . . . . 55
5 Neutron scattering experiment 59
5.1 Time-of-flightspectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5.2 Experimentalsetup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
5.3 Reductionofneutrondata . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
5.4 Neutronbackscatteringspectroscopy . . . . . . . . . . . . . . . . . . . . . 70
6 Dynamics in hydrous silicate melts 73
6.1 Drysodiumtrisilicates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
6.2 Hydroussodiumtrisilicatemelt . . . . . . . . . . . . . . . . . . . . . . . . 74
6.2.1 Energydomainanalysis . . . . . . . . . . . . . . . . . . . . . . . . 75
6.2.2 Timedomainanalysis . . . . . . . . . . . . . . . . . . . . . . . . . 79
6.3 Deuteratedsodiumtrisilicatemelt . . . . . . . . . . . . . . . . . . . . . . 83
6.4 Backscatteringmeasurement. . . . . . . . . . . . . . . . . . . . . . . . . . 84
6.5 Othersilicatesystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
7 Outlook 89
7.1 Neutronscatteringexperiments . . . . . . . . . . . . . . . . . . . . . . . . 89
7.2 Diffusioncouplemeasurements . . . . . . . . . . . . . . . . . . . . . . . . 90
7.2.1 Ex-situexperiments . . . . . . . . . . . . . . . . . . . . . . . . . . 90
7.2.2 Neutronradiography . . . . . . . . . . . . . . . . . . . . . . . . . 91
Appendix 93
A Neutron scattering cross sections 93
B List of neutron scattering experiments 94
List of figures 97
List of tables 99
Acknowledgement 101
Bibliography 103
Declaration of authorship 109
ivSummary
The aims of current thesis are to construct a high temperature high pressure sam-
ple environment for neutron time-of-flight experiments and to study water dynamics
in hydrous silicate melts under high temperature and high pressure conditions with
quasielasticneutronscatteringtechniques.
The understanding of the relaxation behaviour of silicate melts is very important
for many geological processes, especially active volcanism. The addition of water to
silicatescausesadrastic,non-lineardropofthemeltviscosityby5-10ordersofmagni-
tude. Waterisknowntopartiallyreactwithsilicatesuponitsdissolution,whichresults
intwodifferentspeciestobepresentinthesilicates: OH-groups andmolecularwater.
The knowledge of the water dynamics represents an essential key to understand the
meltproperties. However,the dissolution andtransport mechanismsofwaterspecies
insilicatemeltsisstillnotunderstood.
Neutronscatteringtechniquesgiveaccesstoinvestigatedynamicsonmicroscopic
timescales in the order of picoseconds to nanoseconds and on interatomic distances
˚in the A range. The intrinsicq resolution of the quasielastic neutron scattering allows
to study diffusion mechanisms in great detail by analyzing the q dependence of the
scatteringamplitudeandlineshapeofthequasielasticsignal.
Hydroussilicatesamplesarepreparedbyfusionofdrysilicateswithwaterathigh
temperature under high pressure with a total water content of 10 mol%. The charac-
terization of sample glass transition temperatures using calorimetric anddilatometric
methodsshowthatwaterishomogeneouslydissolvedinthesilicateglasses.
To study the hydrous silicates at temperatures higher than their glass transition
temperatures, high pressure in the order of 1-2 kbars is simultaneously required to
suppress water evaporation. Therefore, a high temperature high pressure sample en-
vironment was built which is optimized for the neutron time-of-flight spectrometer
TOFTOF at FRMII.Nb1Zr alloy ischosen asthe cellmaterial. Nbisarefractory class
metalwhichprovidessufficientmechanicalstrengthatelevatedtemperatures. Nbhas
alsoanextremelysmallincoherentneutron scattering crosssection. Hence,anaccept-
able signal-to-background ratio of about 10:1 can be achieved within the elastic and
quasielasticregionevenwithsuchamassive pressure cellwitha35mmouterdiame-
terand12mmwallthickness. Aninternalheatingsetupisusedtoheatupthesamples.
Thisisfavorableforhighpressureapparatusoperatedathightemperatures. Withsuch
setup the sample environment provides a temperature range from ambient tempera-
ture up to 1500 K at pressures up to 2 kbar at the sample position. Samples with a
3volumeofaround1cm canbemeasured,whichmeetstheintensityrequirementsfor
theneutrontime-of-flightexperiments.
vThe realization of the high temperature high pressure sample environment opens
anewpossibilityofusingquasielasticneutronscatteringtechniquestostudyhydrous
silicates. This enables direct observations of water dynamics in silicate melts under
magmachamberconditionsonanabsolutescale. Neutronscatteringalsoprovidesthe
possibilitytoperformacontrastvariationviaH O/D Osubstitution. Asaresultpure2 2
protonsignalscanbeextracted.
In the hydrous NaAlSi O and SiO system, The proton dynamics is not so fast3 8 2
asexpected,althoughthemacroscopicglasstransitiontemperatureofthesampleshas
already dropped almost by a factor of 2 compared to that of the dry silicates. No
resolvablequasielasticbroadeningordecayoftheintermediatescatteringfunctionhas
been observed with the instrumental energy resolutions available on TOFTOF at the
highestmeasuredtemperature. Thelowerboundaryvalueofthediffusioncoefficients
−10 2 −1isontheorderof10 m s intheinvestigatedtemperaturerange.
Anunusual relaxation behaviour of the proton in hydrous sodium trisilicate melt
has been observed. In the energy domain analysis, the scattering function S(q,ω) of
the pure proton signal shows a clear elastic contribution which cannot be described
by a simple Lorentzian or Kohlrausch-Williams-Watts (KWW, Fourier transformed
stretchedexponential)function. Theanalysisofthehighfrequencywingofthespectra