Increasing the power of piezoelectric energy harvesters by magnetic stiffening
W. Al-Ashtari, M. Hunstig, T. Hemsel, W. Sextro, Journal of Intelligent Material Systems and Structures 24 (2013) 1332–1342.

Enhanced energy harvesting using multiple piezoelectric elements: Theory and experiments
W. Al-Ashtari, M. Hunstig, T. Hemsel, W. Sextro, Sensors and Actuators A: Physical 200 (2013) 138–146.

Characteristics of Piezoelectric Energy Harvesters in Autonomous Systems
W. Al-Ashtari, M. Hunstig, T. Hemsel, W. Sextro, in: Proceedings of 10th International Workshop on Piezoelectric Materials and Applications and 8th Energy Harvesting Workshop, Hannover, Germany, 14.-17.7.2013, Hannover, Germany, 2013, pp. 159–161.

Leistungs- und Bandbreitensteigerung von Energy-Harvesting-Generatoren für Energieautarke Systeme
M. Hunstig, W. Al-Ashtari, T. Hemsel, W. Sextro, in: J. Gausemeier, R. Dumitrescu, F.  Rammig, W. Schäfer, A. Trächtler (Eds.), 9. Paderborner Workshop Entwurf Mechatronischer Systeme, Heinz Nixdorf Institut, Universität Paderborn, Paderborn, 2013, pp. 359–372.

Model based enhancement of an autonomous system with a piezoelectric harvester
W. Al-Ashtari, Model Based Enhancement of an Autonomous System with a Piezoelectric Harvester, Shaker, 2013.

Frequency tuning of piezoelectric energy harvesters by magnetic force
W. Al-Ashtari, M. Hunstig, T. Hemsel, W. Sextro, Smart Materials and Structures 21 (2012) 035019.

Analytical determination of characteristic frequencies and equivalent circuit parameters of a piezoelectric bimorph
W. Al-Ashtari, M. Hunstig, T. Hemsel, W. Sextro, Journal of Intelligent Material Systems and Structures 23 (2012) 15–23.