Low Curie Temperature Materials, The Next Generation of High Energy Density Ceramic Dielectrics?

SUMMARY

• Methodology of using E-P hysteresis loop for dielectric material characterisation have been applied as the best suitable method for the first efficient evaluation at the existing lab scale environment.
• Ferroelectric BT-11BS known from its use in piezo devices was proved as the best candidate for high energy density ceramic dielectrics.
• BT-11BS material particle size and sintering temperatures have been subjected to optimization resulting in improvement of the material prepared by conventional solid-state synthesis method exceeding energy density of currently mass use pure BaTiO3 material.
• An interesting finding is that lowering Curie temperature of ferroelectric materials may increase electric density even further as the permittivity does not drop with voltage so rapidly when operating above the Curie temperature. This has been demonstrated on low Curie Temperature BT-11BS, BCST but also on pure BT material (at temperature of 160°C above CT). This behaviour is also not dependant to the fabrication process – observed in the same materials prepared by both Process 1., Process 2. and Process 3. methods, thus it can be considered as a general feature of ferroelectric materials.
• Advanced method of material preparation using Process 3. method yielded into a sample with fine, dense particle structure and further improved ED values in range of 10x versus reference pure BT samples. BT-11BS samples prepared by the Process 3. method were too thick that was difficult to analyse, and it had to be grinded that may impact on the final performance. Thus, further room present for the process optimisation to achieve even better results.
• Other preparation technologies such as Tape Casting or Cold Sintering may be more suitable to prepare thinner layer of the BT-11BS material to fully evaluate its potential for MLCC technology.

CONCLUSIONS

The research presented in this report identified BT-11BS ceramic ferroelectric material as a potential candidate for high energy density storage dielectric material with advantages of use especially as a high voltage, high energy capacitor with a stable capacitance vs BIAS and temperature performance.

Particle size, sintering temperature and use of different manufacturing methods to prepare BT-11BS layer were subjected to our lab optimization to maximize its energy density and capacitance stability. The maximum achieved energy density in our lab was ~ 4-10x higher energy density compared to the reference pure BT material depending to the manufacturing process.

The highest energy density on BT-11BS (10x higher ED than BT reference) was achieved using Process 3. method that produced very fine, dense ceramic dielectric structure with lower permittivity ~3K at room temperature but increased electrical strength that allowed application of very high electrical field up to 0.09 MV/cm (vs 0.03 MV/cm in the case of conventional solid-state method). Permittivity/capacitance drop with DC voltage has been also much lower (~ 30% at high electric field) compare to conventional methods (~ 90%). Permittivity/capacitance is also very stable with temperature – within 10% change between 25°C and 100°C. Process 3. method may be especially suitable for high voltage SLCC single layer ceramic capacitors to further increase its energy density at high operating voltages over kV range.   

Interesting finding of the research is that permittivity/capacitance of general ferroelectric ceramic materials operated above Curie temperature decrease less with BIAS voltage and thus its overall energy density is higher as demonstrated on all tested materials pure BT, BCST and BT-11BS. This may be especially beneficial at high voltage field i.e., high energy density and high voltage capacitors.

These findings present a potential focus for development of the next generation capacitor ceramic materials with low Curie temperature featuring higher energy densities compare to the current state of the art.

BT-11BS is readily available material used and known for its piezoelectric features in piezo devices. It can be prepared and processed into a standard paste form used in mass ceramic capacitor manufacturing today such as screen printing and tape casting, thus it can be considered as compatible with the existing ceramic capacitor high manufacturing volume process and equipment.

The presented results of our research may pose a potential for further domination and growth of ceramic technology over the other capacitor technologies in wide range of higher energy density capacitor applications from ultraminiature smartphones through automotive to renewable energies and medical.  

ACKNOWLEDGEMENT

Authors would like to express thanks to Bernd Pretzlaff, Janosch Lichtenberger, and Volker Lang for their support and advise for this scientific result.

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