7 research outputs found
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Nano-indentation and avalanches in compressed porous SiO2
The ânano-indentation continuous stiffness measurement techniqueâ has been employed to analyze the failure dynamics of mesoporous SiO2 based material (Vycor). The depth dependence of the indentation hardness (H), elastic modulus (E), and elastoplastic parameter (S2/P) shows crackling noise, which has been analyzed to monitor the jerky strain release. The noise is power law distributed with exponents near âŒ1.5 over several decades, confirming avalanche criticality. This value is in good agreement with literature results obtained by other techniques and with earthquake statistics.EPSR
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Avalanche dynamics of ferroelectric phase transitions in BaTiO3 and 0.7Pb(Mg2â3Nb1â3)O3-0.3PbTiO3 single crystals
The motion of phase fronts during a ferroelectric phase transition is intermittent and follows avalanche dynamics. In the present study, we show that an intermittent propagation mode generates spikes of depolarization currents at an extremely slow heating rate of 0.05âK/min in BaTiO3 (BTO) and 0.7Pb(Mg2â3Nb1â3)O3-0.3PbTiO3 (PMN-PT) single crystals. Such âjerksâ are indicative of avalanche dynamics, and their energy exhibits a power law distribution with exponents of Δâ=â1.3â±â0.10 and Δâ=â1.5â±â0.10 for BTO and PMN-PT, respectively. The rate of aftershocks after big events decays as an Omori-like power-law and interevent times are characterized by a universal double power-law distribution, indicating the critical temporal correlations between the avalanche events.EPSR
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Annealing of metamict gadolinite-(Y): X-ray diffraction, Raman, IR, and Mössbauer spectroscopy
Radiation induced disorder in gadolinite that led to metamictization with an upper degree of amorphization of 18% was thermally annealed between room temperature and 1273 K. The degree of annealing was calibrated using the anti-symmetric SiâOâSi Raman-active stretching mode near 902 cmâ1. Annealing increased with increasing temperature with a rapid critical recrystallization at ca. 943 K. This annealing on a short length scale was then complemented by investigations of long-range ordering seen by X-ray diffraction. The same critical temperature was found, and in addition further increase of long-range order extended to 1073 K. Metamict gadolinite contains only Fe2+ within experimental uncertainty.EPSR
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Domain Dynamics in Quantum-Paraelectric SrTiO3
Twin dynamics forced by acoustic waves shows several linear and non-linear response modes below Tc=106K. In the quantum paraelectric state a âquantum domain glassâ at 25K<T<40K shows intense relaxation and temperature hysteresis. Domains float collectively in a complex, smooth landscape with long relaxations times. In the âquantum domain solidâ state below 25K new phenomena occur. A temperaturedependent memory effect of the elastic response after anneal at 36K depends on the lowest temperature reached in the âquantum domain solidâ state below 25K. The glassiness of twin boundary dynamics vanishes for temperatures approaching absolute zero.EPSR
The noise of many needles: Jerky domain wall propagation in PbZrO3 and LaAlO3
Measurements of the sample length of PbZrO3 and LaAlO3 under slowly increasing force (3-30 mN/min) yield a superposition of a continuous decrease interrupted by discontinuous drops. This strain intermittency is induced by the jerky movement of ferroelastic domain walls through avalanches near the depinning threshold. At temperatures close to the domain freezing regime, the distributions of the calculated squared drop velocity maxima N(Ï
m2) follow a power law behaviour with exponents Δ=1.6±0.2. This is in good agreement with the energy exponent Δ=1.8±0.2 recently found for the movement of a single needle tip in LaAlO3 [R. J. Harrison and E. K. H. Salje, Appl. Phys. Lett. 97, 021907 (2010)]. With increasing temperature, N(Ï
m2) changes from a power law at low temperatures to an exponential law at elevated temperatures, indicating that thermal fluctuations increasingly enable domain wall segments to unpin even when the driving force is smaller than the corresponding barrier
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The interaction between vacancies and twin walls, junctions, and kinks, and their mechanical properties in ferroelastic materials
Vacancies strongly interact with twin boundaries and often change dramatically the properties of the ferroelastic material. However, the understanding of this behavior at an atomic-level is still deficient. Here we study vacancy diffusion processes across very large length- and time-scales using a combination of molecular dynamics and Monte Carlo simulations. We find that vacancies reduce their energy by residing at twin boundaries, kinks inside domain boundaries, and junctions between domain boundaries. Vacancies have the largest binding energy inside junctions and co-migrate with the motion of the junctions. For the weaker trapping inside twin boundaries, a âghost lineâ may be generated because vacancies do not necessarily diffuse with moving boundaries and are left behind, leaving a trace of a previous position of the domain boundary. Needle twins act as channels for fast diffusion with almost one order of magnitude higher vacancy diffusivity than in bulk. The relative concentration of vacancies at twin boundaries (ÏVa) is a function of the average vacancy concentration (CVa) with ÏVaâŻâŒâŻCVaα and αâŻ=âŻ0.61, in contrast to that of immobile vacancies case with αâŻ=âŻ0.4. The concentration of vacancies at twin boundaries is enriched ca. 5 times at low temperatures. With increasing temperature, the enrichment drops as the trapping potential at the twin boundaries decreases (thermal release). The distribution of energy-drop upon twin pattern evolution follows a power law. The exponent Δ increases from âŒ1.44 to 2.0 when the vacancy concentration increases. The power law exponent is the same in the athermal region, while Vogel-Fulcher behavior is found at high temperatures.EPSR
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Change of crackling noise in granite by thermal damage: Monitoring nuclear waste deposits
High-sensitivity detection of acoustic emission from granite under uniaxial stress, together with advanced statistical analysis, shows changing collapse mechanisms when a sample is pre-heated. Massive microstructural changes occur at temperatures >500 °C while low-temperature (<<500 °C) treatment leads to scale invariant crackling noise with a mixed fix-point behavior. After treatment at higher temperatures, the collapse occurs via acoustic signals that show energy distributions with systematic deviations from the Gutenberg-Richter law while the Omoriâs and BĂ„thâs laws are not influenced by the thermal treatment. The granite samples stem from the site in the Beishan mountains where a new burial site for nuclear waste will be constructed. According to the 13th Five-Year Plan of the P.R. China, Chinese nuclear power installed capacity will reach 58 million kilowatts in 2020 and produce about 3200 tons of high-level nuclear waste every year. Monitoring the stability of the host rock at high temperatures becomes hence a key issue. Our analysis can serve as a blueprint for a protocol for continuous monitoring of the burial site.EPSR