# Piezoelectric Transformers: An Historical Review
## 1 Historical Introduction  
### 1.1.1920s-1930s: Early Studies on Piezoelectric Transformers Using Salt Rochelle Single Crystal  
The first studies on electrical to electrical conversion using piezoelectric materials took place in the late 1920s and early 1930s. [PERSON] has the honor of being the first researcher to investigate the idea of a piezoelectric transformer. [PERSON] considered two single crystal blocks preloaded against each other with an external frame (Figure 1). One of the crystal elements was driven with an input electric signal generating a vibration in the crystal. The second crystal element was used to convert the vibration back to an electric energy. Figure 1: Representative sketch of the work of [PERSON] on piezoelectric crystal transformers [1]. The ratio of input to output electric energy was measured to evaluate the energy conversion. The work of [PERSON] on piezoelectric transformers, disclosed in multiple patents [1; 2; 3], was limited to the use of Salt Rochelle single crystals, the only extensively available material at the time. The use of this material led to obvious limitations in performance, design and applicability as compared to the later developed piezoceramic materials. s to Early 1960s: [PERSON]'s Work at General Electric, Syracuse, NY, with BaTiO\({}_{3}\) Ferroelectric Ceramics  
Prior to about 1940, only two types of ferroelectrics were known, Salt Rochelle and some closely related tartrates, and potassium dihydrogen phosphate and its isomorphs [4]. That changed in 1942 with the announcement by [PERSON] and [PERSON] [5] of barium titanate (BaTiO\({}_{3}\)) as a new ferroelectric ceramic. This discovery came to be associated with: (i) the discovery of the electrical poling process in polycrystalline ceramics by [PERSON] and [PERSON] [6; 7]; (ii) the development of the systematics of the piezoelectric effect in polarized ceramics by [PERSON] [8]; and (iii) the extraction of the first values for the piezoelectric coefficients of BaTiO\({}_{3}\) by [PERSON] in 1948 [9]. Ferroelectric ceramics were quickly recognized to have significant advantages for many electromechanical applications, including ease of fabrication in desired shapes, ability to operate under high humidity, control of response through selection of polarization directions, a high dielectric constant, high electromechanical coupling, and potential low cost. Furthermore, the physical properties of the ceramics could be altered selectively by the use of additive materials and by variations in firing procedures so as to produce the characteristics desired for particular applications. During the late 1940s and through the 1950s, BaTiO\({}_{3}\) became commercially available, triggering the proliferation of piezoelectric applications. During this period, [PERSON] (Figure 2), a young researcher that had just joined the team of the Electronics Laboratory at General Electric Company in Syracuse, NY, USA, started working on his doctoral research on piezoelectric transformers using the newly available BaTiO\({}_{3}\) and applying the theoretical work developed by [PERSON]. The impact of [PERSON]'s work in piezoelectric transformer technology has been tremendous. Although his work has been extensively referenced, his figure has been rarely remembered. We want to use some lines of this paper as a tribute to his relevancy in this field. Born in Canada, [PERSON] [10; 11] received a B.S. degree in Electrical Engineering at Cooper Union, NY, USA, in 1940. He returned to Canada where he spent the period of World War II supervising a group testing Canadian fighter planes before they were sent to battle in Britain. After the war, in 1950, he received his M. Eng. in Communications at McGill University in Montreal (QC, Canada). Shortly after, he joined the General Electric Company (G.E.) team in Syracuse, NY, USA. While at G.E., [PERSON]  
Figure 2: [PERSON], 7 December 1917–8 December 2002 [10]. worked on solid-state devices and coauthored an influential textbook on transistors [12].