## 5 Werkstofftechnik

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#### Erscheinungsjahr

- 2019 (13) (entfernen)

#### Dokumenttyp

- Vortrag (8)
- Posterpräsentation (3)
- Zeitschriftenartikel (2)

#### Schlagworte

- Ceramic spring (3)
- Ceramics (2)
- Slurry (2)
- VSSA (2)
- 5G (1)
- Alumina (1)
- Analytische Zentrifuge (1)
- Atomization (1)
- Breakdown strength (1)
- Calcination (1)
- Calcium cobaltite (1)
- Ceramic (1)
- Deformation behavior (1)
- Destabilization (1)
- Dielectric breakdown (1)
- Dielectric strength (1)
- Failure test (1)
- Fine Powder (1)
- Force-distance diagram (1)
- Freeze Drying (1)
- Hard machining (1)
- High-voltage testing (1)
- LTCC multilayer (1)
- Machine Learning (1)
- Microstructure (1)
- Nano particle (1)
- Nano screening (1)
- Nanomaterial (1)
- Nanoparticles (1)
- Particle size (1)
- Partikelgrößenverteilung (1)
- Phase transformation (1)
- Pressure-assisted sintering (1)
- Sensor (1)
- Spring constant (1)
- Spring constant (1)
- Thermoelectric properties (1)
- Ultrasound (1)
- alumina (1)
- ceramics (1)
- volume resistivity (1)

#### Organisationseinheit der BAM

- 5.5 Technische Keramik (13) (entfernen)

Dielectric breakdown of ceramics is widely believed to originate from microstructural defects. Still, there is no commonly accepted model for the origin and process of dielectric failure that covers all observed phenomena and dependencies. In analogy to mechanical strength, the Weibull distribution is commonly used to evaluate dielectric strength data. This works well for a given group of specimens with constant geometry. But unlike mechanical strength, dielectric strength scales with the inverse square root of sample thickness. This cannot be explained by the classic Weibull concept. The Griffith type energy release rate model of dielectric breakdown proposed by Schneider is based on space charge injection and conducting filaments from the sample surface. This model incorporates the distinct thickness dependence and the pronounced influence of surface defects. Based on this model and the classic Weibull probability of failure, Schneider’s group theoretically derived a probability of breakdown that predicts an increase of failure probability with increasing electrode area. In our study we tested this model with dielectric strength data measured on dense alumina samples using different electrode areas. Weibull modulus and characteristic dielectric strength (scale parameter) were determined for a set of measurements using small electrodes. These values were used to calculate the failure probability under large electrodes according to the model. The calculated data excellently fits the measured values. Thus, our experiments substantiate the assumptions made in the breakdown model and the significance of surface defects for dielectric failure.

If the particle size decreases, the ratio of surface area to volume increases considerably. This provides benefits for all surface-driven processes that run faster or at lower temperatures than larger particles. However, handling and characterization of the nanopowders are much more difficult. Particularly polydisperse powders with irregular shape, as grinding products, represent a challenge. Granulometry in the submicron and nanoscale often leads to incorrect results without knowledge of particle morphology.
This presentation demonstrates potentials of using the volume-specific surface area (SV or VSSA) in the granulometric characterization of nanopowders, for instance, correlations between the volume-specific surface area and the median particle size are discussed considering the particle morphology and the model of the logarithmic normal distribution.
Moreover, the presentation deals with the optimal dispersion of nanopowders during sample preparation. Indirect ultrasound device with defined cooling was developed to prevent both contamination by sonotrode abrasion and sample changes by heat.
Successful granulometric characterization of nanopowders demands both improved dispersion technology and very often an effective combination of two or more measurement methods.

Considerations for nanomaterial identification of powders using volume-specific surface area method
(2019)

The EC’s recommendation for a definition of nanomaterial (2011/696/EU) should allow the identification of a particulate nanomaterial based on the number-based metric criterion according to which at least 50% of the constituent particles have the smallest dimension between 1 and 100 nm. However, it has been recently demonstrated that the implementation of this definition for regulatory purposes is conditioned by the large deviations between the results obtained by different sizing methods or due to practical reasons such as high costs and time-consuming.
For most measurement methods for particle size determination it is necessary to initially disperse the particles in a suitable liquid. However, as the particle size decreases, the adhesion forces increase strongly, making it more difficult to deagglomerate the particles and to assess accurately the result of this process. Therefore, the success of the deagglomeration process substantially determines the measurement uncertainty and hence, the comparability between different methods.
Many common methods such as dynamic light scattering (DLS), centrifugal liquid sedimentation (CLS) or ultrasound attenuation spectroscopy (US) can give good comparable results for the size of nanoparticles, if they are properly separated and stabilized (e.g. in reference suspensions).
In order to avoid the use of hardly available and expensive methods such as SEM / TEM for all powders, an agglomeration-tolerant screening method is useful.
One of the measurement methods well suited to probe the size of particulate powder is the determination of the volume-specific surface area (VSSA) by means of gas adsorption as well as skeletal density. The value of 60 m2/cm3 corresponding to spherical, monodisperse particles with a diameter of 100 nm constitutes the threshold for decisioning if the material is a nano- or non-nanomaterial. The identification of a nanomaterial by VSSA method is accepted by the EU recommendation.
However, the application of the VSSA method was associated also with some limitations. The threshold of 60 m2/cm3 is dependent on the particle shape, so that it changes considerably with the number of nano-dimensions, but also with the degree of sphericity of the particles. For particles containing micro-pores or having a microporous coating, false positive results are induced. Furthermore, broad particle size distributions made necessary to additionally correct the threshold. Based on examples of commercially available ceramic powders, the applicability of the VSSA approach was tested in relation with SEM and TEM measurements. The introduction of a correction term for deviations from sphericity and further additions improved the applicability of VSSA as a screening method.