Tunable Dielectric Materials | Advanced Composite Materials Facility

Tunable dielectric materials are the key components of electronically beam-steerable antennas which play a fundamenal role in wireless communication systems at microwave and millimetre wave frequencies. Wireless communication technology including 5G satellite communications, entails for example mobile communications, ‘Internet of Things’ and deep space communication.

The basic challenge in the improvement of tunable dielectrics is the reduction of the dielectric losses at high frequencies. These materials are characterized by a tunable dielectric constant and loss upon the application of an electric field. The front-runner tunable dielectric is the perovskite ferroelectric Ba_0.6Sr_0.4TiO₃ (BST). Although it offers an excellent tunability at room temperature, the large values of dielectric losses hinder its potential for high frequency applications.

Tailoring the materials’ synthesis towards enhanced dielectric properties

Experimental results of co planar waveguides at high frequencies — Fig.1: Experimental results of waveguides at high frequencies.

The ACMF is applying the high throughput physical vapour deposition and characterization in order to identify the best compositions in dielectric thin films which result in lower dielectric losses while maintaining high tunability. Structural measurements are being carried out in collaboration with Professor Ian Reaney at the University of Sheffield.

We have investigated the doping with small amounts of Mn in BST and proved that the dielectric loss is reduced while the tunability remains above 50 %. In collaboration with Professor Kees de Groot in Electronic and Computer Science in Southampton University, we have deposited tunable dielectric materials in waveguide structures in order to fabricate co planar waveguides with excellent transmission characteristics at 10 GHZ (Figure 1:Experimental results of co planar waveguides phase shifters with varying DC bias voltage from 0 to 10 V: (a) Return loss; (b) Insertion loss; (c) Continual phase angle shifting; (d) Retention curve of phase angle shifting with forward and return DC biasing). The waveguide structures have been designed for incorporation in arrays for the high through put screening of tunable dielectrics at high frequency.(Figure 2)

Arrays of waveguides (left) and a thin film (right) — Fig.2: Arrays: of waveguides (left) and 196 BSTO capacitors (right).

Lead Niobate Pyrochlore Oxides (PbnNb2On+5) have exhibited the lowest dielectric loss known in any thin film system, as we have proved in one of our earlier studies. Currently we are investigating the improvement of the dielectric behaviour of the cubic PbNbO pyrochlore oxides by adding various dopants, checking the effect of different substrates and deposition temperatures. The optimised materials will be included in new phase shifters for incorporation in the phased array antenna in collaboration with Professor Steven Gao at the Kent University.

This multidisciplinary research is funded by the EPSRC (EP/N032470/1) grant.

References:

1) Moasted, A.; Bakaimi, I.;Hayden, B.; Sinclair, C.D.; Reaney, I.M. ‘Origin of improved tunability and loss in N2 annealed barium strontium titanate films’ Physics Review Materials 2020, 4, 094410.

2)Luo, Q.; Gao, S.; Li, W.; Sobhy, M.; Bakaimi, I.; De Groot, C. H. K.; Hayden, B.E.; Reaney, I.; Yang, X. 'Multibeam dual-circularly polarized reflectarray for connected and autonomous vehicles. IEEE Transactions on Vehicular Technology', 2019, 68, 3574-3585.

3) Bakaimi, I.; He, X., Guerin, S., Hashim,N., Luo, Q., Reaney, I. M., Gao, S.; Hayden. B.E.; De groot, C.H.K. 'Combinatorial synthesis and screening of (Ba,Sr)(Ti,Mn)O₃ thin films for optimization of tunable co-planar waveguides.' Journal of Materials Chemistry C, 2018, 6, 6222-6228.

4)Luo, Q.; Gao, S.; Wang, Y.; Bakaimi, I.; Moasted, A.;Kees De Groot, C. H.; Hayden, B.E.; Reaney, I. ' Electronically Beam-steerable Dual-band Reflectarray for Satellite Communications' 2019 IEEE-APS Topical Conference on Antennas and Propagation in Wireless Communications (APWC).Granada, Spain, 2019, pp. 050-055, doi: 10.1109/APWC.2019.8870431.

5) He, X.; Bakaimi, I.; Hashim, N.; Wang, Y.; Mostaed, A.; Reaney, I. M.; Kees De Groot, C. H. '(Ba, Sr)(Ti, Mn)O₃ perovskite films for co-planar waveguide tunable microwave phase shifters.'2018 13th European Microwave Integrated Circuits Conference (EuMIC), Madrid, 2018, pp. 166-169, doi: 10.23919/EuMIC.2018.8539889.