# Piezoelectric Transformers: An Historical Review
## 1 Historical Introduction
### 1.1.1920s-1930s: Early Studies on Piezoelectric Transformers Using Salt Rochelle Single Crystal
By that time, [PERSON] entered in the doctoral program at Syracuse University and developed an extended work that established the foundation on piezoelectric ceramic transformer technology. His research work on this area is gathered in several U.S. Patents [13; 14; 15] and in his doctoral work published in 1956 [16; 17]. Unlike the earlier attempts by [PERSON] to make piezoelectric transformers using two pieces of piezoelectric solid crystals bonded together and clamped with an external structure, [PERSON]'s approach involved the use of a single rectangular block of BaTiO\({}_{3}\) polycrystalline ceramic comprising two distinct polarization zones (Figure 3). [PERSON] created the two zones by applying separated electrodes and different polarizing voltages to the halves of the block. Half of the block was polarized in the longitudinal direction and the other half in the thickness direction. The total length of the ceramic block determines the resonant frequency of the transformer, while the ratio of the input and output sections determines the voltage conversion. The concept was great but the results were poor, because it was very difficult to make a bipolarized block that would not break into pieces when the alternating current was applied. [PERSON] needed just one unit to prove his concept, but manufacturers had to be able to produce them reliably. The rectangular piezoelectric transformer of Figure 3 is traditionally known as the \"Rosen-type\" transformer. [PERSON] presented several other PT configurations including a cylindrically-segmented PT, a radially polarized disc PT, and a ring-type PT with double polarization. This is summarized in Figure 4. However, none of these other embodiments have gained the attention of the rectangular-type PT. In 1957, [PERSON] moved to California and joined the Stanford Research Institute where he switched his research into artificial intelligence [10] and became a relevant figure in that field. [PERSON] passed away in California in 2002, at the age of 85 [10; 11]. Figure 4: Other piezoelectric transformer embodiments presented by [PERSON] in 1954 [13]. Figure 3: Rectangular piezoelectric transformer presented by [PERSON] in 1954 [13]. The work of [PERSON] was continued by [PERSON] at the University of Illinois (Urbana, IL, USA). [PERSON], also a member of the Electronic Laboratory at General Electric Co. (Syracuse, NY, USA), extended his work to new ferroelectric materials and magnetostrictive materials. His doctoral thesis was published in 1958 [18]. From 1960 to 1962, the team of General Electric at Syracuse continued the work of [PERSON] and [PERSON] on ceramic power transformers through a research contract granted by the U.S. Navy [19]. In 1959, [PERSON], also a member of the Electronic Laboratory at General Electric Co., edited the book \"Solid State Magnetic and Dielectric Devices\" [20], with the collaboration of [PERSON] (by then at Stanford Research Institute, Menlo Park, CA, USA) and [PERSON], among others. This is the first published book discussing piezoelectric transformer technology in detail and is still a key reference to any researcher in this field.