Publications by authors named "Holger Fritze"

8 Publications

  • Page 1 of 1

Linking the Electrical Conductivity and Non-Stoichiometry of Thin Film CeZrO by a Resonant Nanobalance Approach.

Materials (Basel) 2021 Feb 5;14(4). Epub 2021 Feb 5.

Institute of Energy Research and Physical Technologies, Clausthal University of Technology, 38640 Goslar, Germany.

Bulk ceria-zirconia solid solutions (CeZrO, CZO) are highly suited for application as oxygen storage materials in automotive three-way catalytic converters (TWC) due to the high levels of achievable oxygen non-stoichiometry δ. In thin film CZO, the oxygen storage properties are expected to be further enhanced. The present study addresses this aspect. CZO thin films with 0 ≤ x ≤ 1 were investigated. A unique nano-thermogravimetric method for thin films that is based on the resonant nanobalance approach for high-temperature characterization of oxygen non-stoichiometry in CZO was implemented. The high-temperature electrical conductivity and the non-stoichiometry δ of CZO were measured under oxygen partial pressures O in the range of 10-0.2 bar. Markedly enhanced reducibility and electronic conductivity of CeO-ZrO as compared to CeO and ZrO were observed. A comparison of temperature- and O-dependences of the non-stoichiometry of thin films with literature data for bulk CeZrO shows enhanced reducibility in the former. The maximum conductivity was found for CeZrO, whereas CeZrO showed the highest non-stoichiometry, yielding δ = 0.16 at 900 °C and O of 10 bar. The defect interactions in CeZrO are analyzed in the framework of defect models for ceria and zirconia.
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http://dx.doi.org/10.3390/ma14040748DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7915746PMC
February 2021

Single crystals of ferroelectric lithium niobate-tantalate LiNbTaO solid solutions for high-temperature sensor and actuator applications.

Acta Crystallogr B Struct Sci Cryst Eng Mater 2020 Dec 16;76(Pt 6):1071-1076. Epub 2020 Nov 16.

Universität Potsdam, Institut für Physik, Karl Liebknecht Str. 24-25, Potsdam, 14471, Germany.

Ferroelectric LiNbTaO solid solutions with various Nb/Ta ratio were grown from the melt by the Czochralski method. The exact composition of the grown crystals was determined by inductively coupled plasma atomic mass spectrometry. The dependence of the crystal composition on the composition of the initial melt was obtained and explained by a wide separation between the phase boundaries of the liquid and solid phases on the LiNbO-LiTaO phase diagram. Using high-resolution X-ray diffraction, the parameters a and c of a crystal unit cell were determined (LiNbTaO: a = 5.1574 Å and c = 13.8498 Å). Further, the Curie temperature T of the crystals was measured using the differential scanning calorimetry technique. T was found to depend on the composition of the crystals that allowed conditions for the monodomainization of the grown crystals to be defined (LiNbTaO: T = 1102°C; LiNbTaO: T = 794°C). Finally, the velocity of surface acoustic waves was determined by scanning electron microscopy and X-ray diffraction techniques (YZ-cut of a LiNbTaO crystal: V = 3440 m s).
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http://dx.doi.org/10.1107/S2052520620014390DOI Listing
December 2020

Determination of the Dielectric Properties of Storage Materials for Exhaust Gas Aftertreatment Using the Microwave Cavity Perturbation Method.

Sensors (Basel) 2020 Oct 23;20(21). Epub 2020 Oct 23.

Bayreuth Engine Research Center (BERC), Department of Functional Materials, University of Bayreuth, 95440 Bayreuth, Germany.

Recently, a laboratory setup for microwave-based characterization of powder samples at elevated temperatures and different gas atmospheres was presented. The setup is particularly interesting for investigations on typical materials for exhaust gas aftertreatment. By using the microwave cavity perturbation method, where the powder is placed inside a cavity resonator, the change of the resonant properties provides information about changes in the dielectric properties of the sample. However, determining the exact complex permittivity of the powder samples is not simple. Up to now, a simplified microwave cavity perturbation theory had been applied to estimate the bulk properties of the powders. In this study, an extended approach is presented which allows to determine the dielectric properties of the powder materials more correctly. It accounts for the electric field distribution in the resonator, the depolarization of the sample and the effect of the powder filling. The individual method combines findings from simulations and recognized analytical approaches and can be used for investigations on a wide range of materials and sample geometries. This work provides a more accurate evaluation of the dielectric powder properties and has the potential to enhance the understanding of the microwave behavior of storage materials for exhaust gas aftertreatment, especially with regard to the application of microwave-based catalyst state diagnosis.
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http://dx.doi.org/10.3390/s20216024DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7660336PMC
October 2020

Full set of piezo-optic and elasto-optic coefficients of CaTaGaSiO crystals at room temperature.

Appl Opt 2020 Oct;59(28):8951-8958

The photoelasticity of the (CTGS) crystal was studied by an interferometric method based on a single-pass Mach-Zehnder interferometer. The maximum number of sample orientations for the piezo-optic experiments was applied to prove accuracy in the determination of the piezo-optic coefficients. Based on the matrices of the piezo-optic coefficients and the elastic stiffness coefficients, all the coefficients of the elastic-optic matrix are calculated. For the highest coefficient, the acousto-optic efficiency is evaluated. The results obtained for CTGS are compared with the corresponding results for (langasite) crystals. The highest acousto-optic figure of merit of CTGS =1.66⋅10/ is two and three times higher, compared with langasite and strontium borate, respectively, which are often used for acousto-optic modulation of light in the ultraviolet spectral range.
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http://dx.doi.org/10.1364/AO.398428DOI Listing
October 2020

Compressional-Wave Effects in the Operation of a Quartz Crystal Microbalance in Liquids:Dependence on Overtone Order.

Sensors (Basel) 2020 Apr 29;20(9). Epub 2020 Apr 29.

Institute of Physical Chemistry, Clausthal University of Technology, Arnold-Sommerfeld-Str. 4, D-38678 Clausthal-Zellerfeld, Germany, (F.M.).

The operation of the quartz crystal microbalance (QCM) in liquids is plagued by small flexural admixtures to the thickness-shear deformation. The resonator surface moves not only in the transverse direction, but also along the surface normal, thereby emitting compressional waves into the liquid. Using a simple analytical model and laser Doppler vibrometry, we show that the flexural admixtures are stronger on the fundamental mode than on the overtones. The normal amplitude of motion amounts to about 1% of the transverse motion on the fundamental mode. This ratio drops by a factor of two on the overtones. A similar dependence on overtone order is observed in experiments, where the resonator is immersed in a liquid and faces an opposite planar wall, the distance of which varies. Standing compressional waves occur at certain distances. The amplitudes of these are smaller on the overtones than on the fundamental mode. The findings can be rationalized with the tensor form of the small-load approximation.
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http://dx.doi.org/10.3390/s20092535DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7249204PMC
April 2020

High-temperature electroacoustic characterization of Y-cut and singly-rotated Ca3TaGa3Si2O14 resonators.

IEEE Trans Ultrason Ferroelectr Freq Control 2014 Aug;61(8):1433-41

Synthetic piezoelectric crystals in the P321 crystal class have been a focus of substantial research that is largely driven by applications in high-temperature resonant BAW and SAW sensing. Fully ordered crystals in this class, such as Ca3TaGa3Si2O14 (CTGS), have been suggested as offering the potential of electroacoustic performance that is superior to more extensively studied langasite (LGS) and langatate (LGT), which are partially disordered. In this study, the resonant frequencies, acoustic damping, and electrical conductivity of CTGS bulk acoustic resonators with Y-cut and (YXl)-30° crystal orientations and fundamental frequencies near 5 MHz are investigated at temperatures between ambient and 1100°C. (YXl)-30° resonators are found to have turnover temperatures near 200°C for the third and fifth overtones, in contrast to a monotonic decrease in resonant frequencies of Y-cut crystals with increasing temperature. The maximum temperature derivative of fractional changes in fifth-overtone frequency of (YXl)-30° CTGS is 40 × 10-6K-1 (near 1100°C), and this value is not greatly different from the temperature derivative of Y-cut CTGS frequencies over a broader range of temperatures. At ambient temperatures, the acoustic loss Q-1 of CTGS with both crystal orientations is found to be greater than the lowest values previously reported for LGS and LGT. The electrical conductivity of the CTGS specimens between 500°C and 1100°C is substantially lower than that previously reported for LGS. Corresponding to this lower conductivity, the piezoelectric/conductive contribution to Q-1 at elevated temperatures is reduced. Additional anelastic relaxation peaks observed between 100°C and 700°C are similar to those previously reported for LGS and LGT.
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http://dx.doi.org/10.1109/tuffc.2014.3052DOI Listing
August 2014

Defect chemistry, redox kinetics, and chemical diffusion of lithium deficient lithium niobate.

Phys Chem Chem Phys 2011 Apr 23;13(15):6925-30. Epub 2011 Feb 23.

Institute of Physical and Theoretical Chemistry, Technische Universität Braunschweig, Hans-Sommer-Str. 10, 38106 Braunschweig, Germany.

High-temperature optical in situ spectroscopy was used to investigate the defect absorption, redox kinetics, and chemical diffusion of a lithium deficient (48.4 mol% Li(2)O) congruent melting lithium niobate single crystal (c-LN). Under reducing atmospheres of various oxygen activities, a(O(2)), UV-Vis-NIR spectra measured at 1000 °C are dominated by an absorption band due to free small polarons centered at about 0.93 eV. The polaron band intensity was found to follow a power law of the form a(O(2))(m) with m = -1/4. A chemical reduction model involving electrons localized on niobium ions on regular lattice sites can explain the observed defect absorption and its dependence on oxygen activity. The kinetics of reduction and oxidation processes upon oxygen activity jumps and the associated chemical diffusion coefficients are found in close agreement over a range from -0.70 to -14.70 in log a(O(2)), indicating a reversible redox process. Assuming coupled fluxes of lithium vacancies and free small polarons for the attainment of stoichiometry, the diffusion coefficients of lithium vacancies as well as of lithium ions in the lithium deficient c-LN have been determined at 1000 °C.
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http://dx.doi.org/10.1039/c0cp02703kDOI Listing
April 2011

Diffusion-related implications for langasite resonator operation.

IEEE Trans Ultrason Ferroelectr Freq Control 2004 Nov;51(11):1381-7

Department of Physics, Metallurgy and Materials Science, Technische Universität Clausthal, Clausthal-Zellerfeld, D-38678 Germany.

Oxygen and gallium diffusivities in langasite were experimentally determined by analysis of diffusion profiles of 18O and 71Ga tracers by SIMS analysis as functions of temperature and doping. Strontium-enhanced diffusivities and activation energies of approximately 1.2+/-0.2 eV confirm the predominant role of oxygen vacancies in controlling the electrical conductivity of langasite at elevated temperature and oxygen partial pressure. The potential impact of high levels of porosity and the use of an oxygen primary ion beam on the accuracy of some of the data is discussed. The gallium diffusivity, with activation energy of 3.13 eV, was found to be more than two orders of magnitude lower than that of oxygen. Surface exchange measurements enabled estimation of gallium loss at elevated temperatures and oxygen partial pressure; the level is not believed to be of major concern for resonator performance.
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http://dx.doi.org/10.1109/tuffc.2004.1367476DOI Listing
November 2004