Barium titanate transitions

Historically, materials based on doped barium titanate were used to achieve dielectric constants as high as 2,000 to 10,000. The high dielectric constants result from ionic polarization and the stress enhancement of k associated with the fine-grain size of the material. The specific dielectric properties are obtained through compositional modifications, ie, the inclusion of various additives at different doping levels. For example, additions of strontium titanate to barium titanate shift the Curie point, the temperature at which the ferroelectric to paraelectric phase transition occurs and the maximum dielectric constant is typically observed, to lower temperature as shown in Figure 1 (2). 

This kind of microstructure also influences other kinds of conductors, especially those with positive (PTC) or negative (NTC) temperature coefficients of resistivity. For instance, PTC materials (Kulwicki 1981) have to be impurity-doped polycrystalline ferroelectrics, usually barium titanate (single crystals do not work) and depend on a ferroelectric-to-paraelectric transition in the dopant-rich grain boundaries, which lead to enormous increases in resistivity. Such a ceramic can be used to prevent temperature excursions (surges) in electronic devices. 

A ferroelectric model material is barium titanate BaTi03. On cooling from high temperatures, the permittivity increases up to values well above 10,000 at the phase transition temperature Tc. The inverse susceptibility as well as the dielectric permittivity follows a Curie-Weiss law x1 f 1 oc (T — O). The appearance of the spontaneous polarization is accompanied with a spontaneous (tetragonal) lattice distortion. 

The phase transition in barium titanate is of first order, and as a result, there is a discontinuity in the polarization, lattice constant, and many other properties, as becomes clear in Figure 1.7. It is also clear in the figure that there are three phase transitions in barium titanate having the following sequence upon cooling rhombohedral, orthorhombic, tetragonal and cubic. 

Figure 1.9 Reciprocal dielectric susceptibility at the phase transition of lithium tantalate (second order phase transition) and of barium titanate (first order phase transition).

The dielectric behavior closed to a phase transistion is displayed in Figure 1.9 for barium titanate (first-order transition with Tc = 135°C, C = 1.8 105oC) and for lithium tantalate (second-order transition with Tc = 618°C and C = 1.6 105 °C). 

The compositions of most dielectric materials used for ceramic capacitors are based on ferroelectric barium titanate. As discussed in detail in Pragraph 1.3 the permittivity of ferroelectric perovskites shows marked changes with temperature, particularly close to the phase transition. From the device point of view a high dielectric permittivity with stable properties over a wide temperature range is required. There are various specifications which have to be fulfilled (e.g. X7R AC/C(T = 25°C) < 0.15 in a range between -55°C and 125°C). 

Ferroelectric behaviour is limited to certain materials and to particular temperature ranges for a given material. As shown for barium titanate in Section 2.7.3, Fig. 2.40(c), they have a Curie point Tc, i.e. a temperature at which the spontaneous polarization falls to zero and above which the properties change to those of a paraelectric (i.e. a normal dielectric). A few ferroelectrics, notably Rochelle Salt (sodium potassium tartrate tetrahydrate (NaKC406.4H20)) which was the material in which ferroelectric behaviour was first recognized by J. Yalasek in 1920, also have lower transitions below which ferroelectric properties disappear. 

Lattice dynamics in bulk perovskite oxide ferroelectrics has been investigated for several decades using neutron scattering [71-77], far infrared spectroscopy [78-83], and Raman scattering. Raman spectroscopy is one of the most powerful analytical techniques for studying the lattice vibrations and other elementary excitations in solids providing important information about the stmcture, composition, strain, defects, and phase transitions. This technique was successfully applied to many ferroelectric materials, such as bulk perovskite oxides barium titanate (BaTiOs), strontium titanate (SrTiOs), lead titanate (PbTiOs) [84-88], and others. 

Darlington CNW, Cemik RJ (1993) The effects of isovalent and non-isovalent impurities on the ferroelectric phase transition in barium titanate. J Phys Condens Matter 5 5963-5970... 

Barium titanate BaTi03 is usually considered as the prototype of compounds having a purely displacive ferroelectric phase transition, which exhibits a soft mode describable by an anharmonic phonon. The nonferro-electric compound NH4C1 is another example of compound with pure order-disorder phase transition, with two phases that differ from the ordering of the ammonium (NH/) cation in the unit cell. 

Barium titanate goes through the following phase transitions upon heating Rhombohedral —> orthorhombic —> tetragonal —cubic... 

ON THE INFLUENCE OF A DC FIELD ON THE THERMAL CONDUCTIVITY OF BARIUM TITANATE BASE CERAMIC SPECIMENS AND TRIGLYCINE SULFATE SINGLE CRYSTALS IN THE PHASE TRANSITION REGION. //ENGLISH TRANSLATION OF IZV. AKAD. NAUK SSSR, SER. FIZ. 31 /11/1842-4,... 

EFFECT OF A CONSTANT ELECTRIC FIELD ON THE THERMAL CONDUCTIVITY OF CERAMIC SAMPLES OF SOLID SOLUTIONS BASED ON BARIUM TITANATE AND OF A TRIGLYCINE SULFATE SINGLE CRYSTAL IN THE REGION OF PHASE TRANSITIONS. 

Choi, S.-Y. and Kang, S.-J. L., Sintering kinetics by structural transition at grain boundaries in barium titanate, Acta Mater., 52, 2937 3, 2004. 

Attempts are still being made to produce improved bolometers operating at or near room temperature. These include vanadium oxide metal-semiconductor transition devices [8.7], bismuth-lead layers [8.8], metallic nickel [8.8a], and aluminium [8.8b], silicon carbide [8.8c], and doped barium titanate ceramic [8.8d] elements. New designs of radiometers and power meters using thermistor bolometers have been described [8.9-11]. Microelectronic techniques have been used to produce fast but sensitive (NEP 10 WHz at 25 MHz and 100 pm wavelengths) uncooled for improved bolometers [8.11a]. 

NaKC4H40e 4H2O), monopotassium dihydrophosphate (KH2PO4), or barium titanate (BaTiOs). At sufficiently high temperatures ferroelectrics show normal dielectric behavior. However, below a certain critical temperamre (so called. Curie temperature), even a small electric field causes a large polarization, which is preserved even if the external field is switched off. This means that below the Curie point ferroelectric materials show spontaneous polarization. The phase transition at the Curie temperature is related to the change of the lattice symmetry of the sample. 

Cowley RA (1977) Structural phase transitions. In Balkanski M (ed) Proceedings of the international conference on lattice dynamics. Flammarion Sciences, Paris, p 625 Devonshire AF (1949) Theory of barium titanate - part 1. Phil Mag 40 Serie 7(309) 1040-1063 Devonshire AF (1951) Theory of barium titanate - part 11. Phil Mag 42 Serie 7(333) 1065-1079 Ehrenfest P (1933) Phase changes in the ordinary and extended sense classified according to the eorresponding singularities of the thermodynamic potential. Proc Acad Sci Amsterdam 36 153-157... 

Barium titanate has been studied extensively since the end of World War II when it was identified independently in Russia, Japan and USA as a promising material with high permittivity for ceramic capacitors. Its ferroelectric activity is known since 1946 when it was discovered on ceramic samples. Since 1950 s also the single-crystals of desired size and quality are available. BaTiOs undergoes several phase transitions... 

Barium titanate is also the first material with the ferroelectricity studied theoretically by Devonshire (1949). Theoretical description is based on the idea of the thermodynamic potentials known from the Landau-Ginzburg theory of phase transitions (Devonshire 1949, 1951). Barium titanate is mainly used in multilayer ceramic capacitors and in positive temperature coefficient (PTC) elements (Bhalla et al. 2000). 

These are primarily concerned with the degree of disorder in the solid. Factors which increase this disorder enhance the reactivity of the material. The existence of a brief period of enhanced reactivity during a solid i solid2 phase transition has been controversial and is referred to as the Hedvall effect (see References 17, 18, and references therein). An example in ceramic processing is the use of metastable anatase instead of the stable rutile form of titanium dioxide for the preparation of barium titanate. ... 

The second class of sol-gels contains redox-active metal oxides, such as tungsten oxide, vanadium pentoxide, manganese oxide, and other transition metal oxides. Moreover, many n-type semiconductors such as zinc oxide, barium titanate, and titanium dioxide can be used in this class (113). The structures of these gels are sensitive to the pH and oxidation state of the precursors. Many redox-active sol-gels exhibit electrochromism (different oxidation states exhibit different colors, allowing spectroscopic determination of redox states). These gels can also accommodate the reductive insertion of lithium and other moieties. 

Among the multiple oxide particles, barium titanate has been successfully prepared from reverse microemulsions (Herrig and Hempelmann, 1996 Beck et al., 1998) by using isopropanolic solution of Ba- and Ti-alkoxides in 1 1 molar ratio, cyclohexane as the oil phase, and various non-ionic surfactants. Such particles were nanometric (less than 20 nm) in size. A series of nanoparticulate aluminates of transition metals Co, Ni and Cu have been synthesized from microemulsions by Meyer et al. (1999). The noteworthy point in this synthesis is the application of heterobimetallic alkoxides as the single source materials of the cations in each case. 

Measurement of the temperature dependence of the dielectric constant is an important characterization for a ferroelectric material. As mentioned in the introduction section, when a phase transition occurs in a ferroelectric material, the dielectric constant always behaves anomalously. In general, the real part ofthe dielectric constant, e, shows a maximum at the phase transition temperature, where a change from a ferroelectric phase to the paraelectric one (or vice versa) occurs the value of e being higher than that at room temperature usually by 3 orders of magnitude. Figure 22-6 shows how the dielectric constant of barium titanate changes with temperature. At temperatures above the transition temperature, the variation ofthe dielectric constant (at low frequency) as a function of temperature obeys the Curie-Weiss law (see equation (22-2)). Usually, the peak ofthe dielectric constant versus temperature curve determines the Curie point F, and the Curie-Weiss constant C is determined by the slope ofthe curve of(e ) versus T. 

Interpret the DSC curves given in Fig. 2, below. Figure a curve 1, evaporation of n-decane, curve 2, empty sample cup (base line). Figure b tempering of an Al/Zn alloy. Figure c barium titanate solid state transitions. 

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