电容器介电和压电陶瓷

电容器电介质和压电陶瓷是先进的工业材料，由于其导电性差，可用于生产蓄电或发电设备。

电容器是存储电能的装置，该电能以在两个分离的，带相反电荷的电极之间的空间中产生的电场的形式存储。它们存储能量的能力使其成为许多电路中必不可少的组成部分，并且可以通过将固态介电材料插入分隔电极的空间中来大大提高该能力。介电材料是不良的电导体。陶瓷的非导电特性是众所周知的，并且某些陶瓷被制成极为有效的电介质。实际上，所有电容器的90％以上都是用陶瓷材料作为电介质来生产的。

压电是在受到机械压力时会产生电压的材料。相反，当受到电磁场影响时，它们的尺寸会发生变化。许多压电器件由与电容器电介质相同的陶瓷材料制成。

本文介绍了最杰出的介电和压电陶瓷的性能，并概述了它们的实际应用。

钛酸钡的铁电性能

静电现象在电学中有详细描述：静电：电容。在那篇文章中，解释了低电导率是形成材料的化学键的一个因素。在电介质中，与诸如金属之类的导电材料不同，将原子保持在一起的强离子键和共价键不会使电子自由地在电场的影响下穿过材料。取而代之的是，该材料变成电极化的，其内部的正负电荷有些分离，并平行于电场轴排列。当用在电容器中时，这种极化作用会降低保持在电极之间的电场强度，这反过来又增加了可以存储的电荷量。

Most ceramic capacitor dielectrics are made of barium titanate (BaTiO₃) and related perovskite compounds. As is pointed out in the article ceramic composition and properties, perovskite ceramics have a face-centred cubic (fcc) crystal structure. In the case of BaTiO₃, at high temperatures (above approximately 120° C, or 250° F) the crystal structure consists of a tetravalent titanium ion (Ti⁴⁺) sitting at the centre of a cube with the oxygen ions (O²⁻) on the faces and the divalent barium ions (Ba²⁺) at the corners. Below 120° C, however, a transition occurs. As is shown in Figure 1, the Ba²⁺ and O²⁻ ions shift from their cubic positions, and the Ti⁴⁺ ion shifts away from the cube centre. A permanent dipole results, and the symmetry of the atomic structure is no longer cubic (all axes identical) but rather tetragonal (the vertical axis different from the two horizontal axes). There is a permanent concentration of positive and negative charges toward opposite poles of the vertical axis. This spontaneous polarization is known as ferroelectricity; the temperature below which the polarity is exhibited is called the Curie point. Ferroelectricity is the key to the utility of BaTiO₃ as a dielectric material.

Within local regions of a crystal or grain that is made up of these polarized structures, all the dipoles line up in what is referred to as a domain, but, with the crystalline material consisting of a multitude of randomly oriented domains, there is overall cancellation of the polarization. However, with the application of an electric field, as in a capacitor, the boundaries between adjacent domains can move, so that domains aligned with the field grow at the expense of out-of-alignment domains, thus producing large net polarizations. The susceptibility of these materials to electric polarization is directly related to their capacitance, or capacity to store electric charge. The capacitance of a specific dielectric material is given a measure known as the dielectric constant, which is essentially the ratio between the capacitance of that material and the capacitance of a vacuum. In the case of the perovskite ceramics, dielectric constants can be enormous—in the range of 1,000–5,000 for pure BaTiO₃ and up to 50,000 if the Ti⁴⁺ ion is replaced by zirconium (Zr⁴⁺).

Chemical substitutions in the BaTiO₃ structure can alter a number of ferroelectric properties. For example, BaTiO₃ exhibits a large peak in dielectric constant near the Curie point—a property that is undesirable for stable capacitor applications. This problem may be addressed by the substitution of lead (Pb²⁺) for Ba²⁺, which increases the Curie point; by the substitution of strontium (Sr²⁺), which lowers the Curie point; or by substituting Ba²⁺ with calcium (Ca²⁺), which broadens the temperature range at which the peak occurs.

Disk, multilayer, and tubular capacitors

Barium titanate can be produced by mixing and firing barium carbonate and titanium dioxide, but liquid-mix techniques are increasingly used in order to achieve better mixing, precise control of the barium-titanium ratio, high purity, and submicrometre particle size. Processing of the resulting powder varies according to whether the capacitor is to be of the disk or multilayer type. Disks are dry-pressed or punched from tape and then fired at temperatures between 1,250° and 1,350° C (2,280° and 2,460° F). Silver-paste screen-printed electrodes are bonded to the surfaces at 750° C (1,380° F). Leads are soldered to the electrodes, and the disks are epoxy-coated or wax-impregnated for encapsulation.

The capacitance of ceramic disk capacitors can be increased by using thinner capacitors; unfortunately, fragility results. Multilayer capacitors (MLCs) overcome this problem by interleaving dielectric and electrode layers (see Figure 2). The electrode layers are usually palladium or a palladium-silver alloy. These metals have a melting point that is higher than the sintering temperature of the ceramic, allowing the two materials to be cofired. By connecting alternate layers in parallel, large capacitances can be realized with the MLC. The dielectric layers are processed by tape casting or doctor blading and then drying. Layer thicknesses as small as 5 micrometres (0.00022 inch) have been achieved. Finished “builds” of dielectric and electrode layers are then diced into cubes and cofired. MLCs have the advantages of small size, low cost, and good performance at high frequencies, and they are suitable for surface mounting on circuit boards. They are increasingly used in place of disk capacitors in most electronic circuitry. Where monolithic units are still employed, tubular capacitors are often used in place of disks, because the axial wire lead configuration of tubular capacitors is preferred over the radial configuration of disk capacitors for automatic circuit-board insertion machines.

As is noted above, barium titanate-based MLCs usually require firing temperatures in excess of 1,250° C. To facilitate cofiring with electrode alloys of lower melting temperatures, the sintering temperature of the ceramic can be reduced to the neighbourhood of 1,100° C (2,000° F) by adding low-melting glasses or fluxing agents. In order to reduce the costs associated with precious-metal electrodes such as palladium and silver, ceramic compositions have been developed that can be cofired with less expensive nickel or copper at lower temperatures.