Some science behind the scenes

Ceramics

A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous (e.g., a glass). For our purposes and because most common ceramics are crystalline, this section will be restricted to inorganic crystalline materials, as opposed to the noncrystalline glasses.

Although all of the following types of man made ceramics exhibit piezoelectric properties they are of little use to us spiritually, I have provided details to ‘complete the picture’. 

Crystals are naturally piezoelectric. They have a nearly perfect internal structure that results in a very stiff material, offering the potential for excellent material repeatability. Man made Ceramics, on the other hand, aren’t naturally piezoelectric and must be imparted with the piezoelectric characteristics through a polarization process in manufacturing.  All of the man made products we see here were invented to get over the problems of natural crystals such as a low current and difficulty of supply of the crystals themselves.  The family of ceramics with perovskite or tungsten-bronze structures, for example,  exhibits piezo-electricity:

  • Barium titanate (BaTiO3)—Barium titanate was the first piezoelectric ceramic discovered.
  • Lead titanate (PbTiO3)
  • Lead zirconate titanate (Pb[ZrxTi1-x]O3 0≤x≤1)—more commonly known as PZT, lead zirconate titanate is the most common piezoelectric ceramic in use today.
  • Potassium niobate (KNbO3)
  • Lithium niobate (LiNbO3)
  • Lithium tantalate (LiTaO3)
  • Sodium tungstate (Na2WO3)
  • Ba2NaNb5O5
  • Pb2KNb5O15
  • Sodium potassium niobate (NaKNb). In 2004, a group of Japanese researchers led by Yasuyoshi Saito discovered a sodium potassium niobate composition with properties close to those of PZT, including a high TC.
  • Bismuth ferrite (BiFeO3) is a promising candidate for the replacement of lead-based ceramics.
  • Sodium niobate NaNbO3

Some of the lead based ceramics specifically developed for their piezoelectric effects are also considered to be ‘hazardous’.  Lead zirconate titanate (Pb[ZrxTi1-x]O3 0≤x≤1) , also called PZT, is a ceramic perovskite material that shows a marked piezoelectric effect. PZT-based compounds are composed of the chemical elements lead and zirconium and the chemical compound titanate which are combined under extremely high temperatures. A mechanical filter is then used to filter out the particulates. PZT-based compounds are actually used in the manufacture of ultrasound transducers.

Electromechanical properties of lead zirconate titanate piezoceramics under the influence of mechanical stresses - Zhang QM, Zhao J; Dept. of Electr. Eng., Pennsylvania State Univ., University Park, PA.

In lead zirconate titanate piezoceramics, external stresses can cause substantial changes in the piezoelectric coefficients, dielectric constant, and elastic compliance due to nonlinear effects and stress depoling effects. In both soft and hard PZT piezoceramics, the aging can produce a memory effect that will facilitate the recovery of the poled state in the ceramics from momentary electric or stress depoling. In hard PZT ceramics, the local defect fields built up during the aging process can stabilize the ceramic against external stress depoling that results in a marked increase in the piezoelectric coefficient and electromechanical coupling factor in the ceramic under the stress. Although soft PZT ceramics can be easily stress depoled (losing piezo-electricity), a DC bias electric field, parallel to the original poling direction, can be employed to maintain the ceramic poling state so that the ceramic can be used at high stresses without depoling.

In theory it would be possible to get lead free ceramic beads to make into ear rings and necklaces but most of the ceramic materials on the market are designed to be used industrially in the manufacture of transducers used in industrial equipment, vehicles and so on.  They are not intended for personal use……

Large piezoelectric effect in Pb-free ceramics - Liu W, Ren X; Multi-disciplinary Materials Research Center and State Key Lab of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, 710049, China.

We report a non-Pb piezoelectric ceramic system Ba(Ti(0.8)Zr(0.2))O(3)-(Ba(0.7)Ca(0.3))TiO(3) which shows a surprisingly high piezoelectric coefficient of d(33) approximately 620 pC/N at optimal composition. Its phase diagram shows a morphotropic phase boundary (MPB) starting from a tri-critical triple point of a cubic paraelectric phase (C), ferroelectric rhombohedral (R), and tetragonal (T) phases. The high piezo-electricity of the MPB compositions stems from the composition proximity of the MPB to the tricritical triple point, which leads to a nearly vanishing polarization anisotropy and thus facilitates polarization rotation between 001T and 111R states. We predict that the single-crystal form of the MPB composition of the present system may reach a giant d(33) = 1500-2000 pC/N. Our work may provide a new recipe for designing highly piezoelectric materials (both Pb-free and Pb-containing) by searching MPBs starting from a TCP.

Some types are used in medical applications and because of their piezoelectric properties can cause problems………………

Piezoelectricity in dental materials, a conceivable cause of postrestorative sensitivity.

Sjögren G, Bergman M, Johansson K; Department of Dental Materials and Technology, University of Umeå, Sweden.

With the increased use of tooth-colored posterior inlays reports of postrestorative sensitivity have also increased. One of the symptoms the patients complain of is a sharp pain when the inlays are loaded through chewing and biting. Many explanations for the causes of dissimilar types of postrestorative sensitivity have been offered, but one conceivable explanation that has not hitherto been studied is the direct piezoelectric effect in dental materials. Direct piezoelectric effect means that when certain anisotropic crystals are mechanically loaded, a charge is generated on the surface. The aim of the present study was to examine whether this physical phenomenon occurs in certain materials intended for dental use. Specimens of four different dental ceramics and one indirect composite resin for inlays were mechanically loaded with various forces, and the current was directly recorded. Currents of up to 0.9 microA with a pulse duration of 0.4 msec were extracted, and it cannot be excluded that the piezoelectric phenomenon and related properties may cause postrestorative sensitivity. This has to be taken into consideration when posterior inlays of the types concerned are used.