Dielectric to pyroelectric phase transition induced by defect migration
This is a modal window.
The media could not be loaded, either because the server or network failed or because the format is not supported.
Formal Metadata
| Title |
| |
| Title of Series | ||
| Number of Parts | 62 | |
| Author | ||
| License | CC Attribution 3.0 Unported: You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal purpose as long as the work is attributed to the author in the manner specified by the author or licensor. | |
| Identifiers | 10.5446/38748 (DOI) | |
| Publisher | ||
| Release Date | ||
| Language |
Content Metadata
| Subject Area | ||
| Genre | ||
| Abstract |
|
26
00:00
Electric power distributionDielectricPyroelectricityCrystallographic defectPhase (matter)ElectromigrationParticle physicsPlain bearingVideoPyroelectricityCrystallographic defectElectromigrationElectricityPower (physics)MaterialComputer animation
00:14
Single crystalElectricityKeramikCircuit diagramTemperatureVolumetric flow rateCrystallizationHeatDielectricInversion <Meteorologie>MaterialColor chargeAM-Herculis-SternCooling towerLadungstrennungElectric currentElectric power distributionPyroelectricityRelative articulationEffects unitComputer animation
01:18
ElectricitySensorThermometerCrystal structureAnodeDiffractionShort circuitPower (physics)MaterialPyroelectricityPyrometerReflexionskoeffizientDistortionOrder and disorder (physics)Angle of attackFormation flyingGenerationAM-Herculis-SternX-rayYearThermographic cameraInfraredCrystallizationElectromigrationLattice constantSpannungsmessung <Elektrizität>Computer animation
02:37
AmplitudeSteelTurningElectricityMeasurementTemperatureElectric currentCrystallizationStress (mechanics)Angeregter ZustandChannelingSpare partPower (physics)MaterialAudio frequencyPyroelectricitySauerstoff-16ThermalSignal (electrical engineering)Current densityFormation flyingAM-Herculis-SternYearShotgunSingle crystalAmmeterElectromigrationProzessleittechnikDiagram
04:37
Model buildingProzessleittechnikCathodeSauerstoff-16Initiator <Steuerungstechnik>SteelCrystal structureAngeregter ZustandComputer animation
04:53
MeasurementTemperatureHot workingCrystal structureGroup delay and phase delayCathodePyroelectricitySauerstoff-16Crystallographic defectContinental driftElectromigrationElektrisches DipolmomentEngine displacementElectricitySemiconductorHauptsatz der Thermodynamik 2SpectroscopyForgingLunar nodePower (physics)KelvinWeekCompound engine
06:21
SteckverbinderComputer animation
Transcript: English(auto-generated)
00:03
Welcome and thank you for your interest in dielectric to pyroelectric phase transition induced by defect migration. What is the pyroelectric effect? Pyroelectric materials are a subgroup of dielectric matter marked by the presence of a
00:21
non-zero electric polarization in absence of an electric field, the so-called spontaneous polarization. At a constant temperature, surrounding charges compensate the arising charges of opposite sign on the crystal surfaces. Connecting an external electric circuit and heating the sample will cause a change
00:40
of the spontaneous polarization, which is accompanied by a redistribution of the surface charges. Compensating charges enable an electrical current flow in the circuit. Cooling of the sample will cause an increase in the spontaneous polarization, which causes a redistribution of surface charges and a reversed current flow.
01:02
This is a dynamic effect, only present when the temperature is changing and only possible within crystals having a symmetry without inversion center. Typical pyroelectric materials are single crystals or ceramics of barium titanate or polyvinylidene fluoride polymers.
01:22
Pyroelectric materials have become increasingly important for sensor applications like infrared cameras, approximation and motion sensors and for pyrometers as contactless temperature sensors. But is it possible to generate pyroelectricity in a central symmetric crystal structure?
01:45
Strontium titanate, a cubic transition metal oxide, responds to the application of an external electric field, which is in the order of 10 to the 6th volts per meter, with a symmetry change of the surface region near the anode. In-situ X-ray diffraction studies prove this structural change.
02:05
During electroformation the 002 reflection broadens towards smaller diffraction angles, which results in larger lattice constants. The developing new phase is referred to as the migration induced field stabilized polar
02:21
phase, in short MFP phase, which remains stable when an electric field is applied. It shows a polar distortion of the original cubic lattice and hence may be pyroelectric. But how can we measure the assumed pyroelectricity?
02:40
The important parameter is the pyroelectric coefficient, calculated by the thermal stimulation of the sample and simultaneously measuring the polarization change. The sample is placed on a programmable heater and connected to an ampere meter. Typical measurement techniques for pyroelectric properties, like the Bioroundary method, are unsuitable for characterizing the MFP phase,
03:06
because they cannot distinguish between pyroelectric and thermally stimulated signals originating from thermal detrapping of charge carriers. We therefore chose the Sharp-Garn method, which is based on a sinusoidal
03:20
temperature excitation, by simultaneously measuring the current response of the material and determining temperature amplitude, current amplitude, angular frequency and phase shift between temperature and current. A non-pyroelectric material shows only thermally stimulated currents which are in phase with the temperature excitation.
03:43
If the material becomes pyroelectric, a pyroelectric current out of phase arises. This principle is now adopted to strontium titanate, involving the sinusoidal temperature stimulation and simultaneously applying an electric field. The time-dependent forming current of a strontium titanate single crystal is composed of ionic and electronic parts, which
04:08
are caused by oxygen vacancy migration but are now superimposed by an additional current because of the sinusoidal temperature excitation. At the beginning of the electroformation, the oscillatory thermal current shows an in-phase temperature behavior.
04:24
When the overall current signal is declining, an out-of-phase current contribution can be detected, revealing the pyroelectricity of the formed MFP phase. What is the underlying process for this behavior? The following model can be derived.
04:41
In the initial state, strontium titanate shows statistically distributed oxygen vacancies, which move to the cathode when an electric field is applied. A closer look at the unit cells shows the oxygen vacancy drift towards the cathode along the oxygen octahedron edges by switching its position with an oxygen atom.
05:04
The MFP phase is established at the anode, where oxygen vacancies are depleted. Stretching of the unit cells now causes an inherent dipole moment due to the slight displacement of the titanium atom in the center of the octahedron, which is stabilized by the external electric field and remains stable even when the oxygen vacancy has left the unit cell.
05:28
The temperature dependence of the dipole moments is responsible for the observed pyroelectricity. In summary, a dielectric to pyroelectric phase transition induced by defect migration with
05:42
a pyroelectric coefficient of 30 microcoulomb per Kelvin and square meter was found. Symmetry considerations allow a structure prediction of the MFP phase. Furthermore, qualitative thermal current analysis reveals details of defect migration. This work has been undertaken at the TU Bergacademy Freiberg Institute
06:06
for Experimental Physics by the group Compound Semiconductors and Solid State Spectroscopy. We would like to thank all our partners and funding agencies for their continued interest in our work. Thanks for watching our video!