This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

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Physics of thin-film ferroelectric oxides

An overview of the state of art in ferroelectric thin films is presented. First, we review applications: microsystems' applications, applications in high frequency electronics, and memories based on ferroelectric materials. The second section deals with materials, structure (domains, in particular), and size effects. Properties of thin films that are important for applications are then addressed: polarization reversal and properties related to the reliability of ferroelectric memories, piezoelectric nonlinearity of ferroelectric films which is relevant to microsystems' applications, and permittivity and loss in ferroelectric films-important in all applications and essential in high frequency devices. In the context of properties we also discuss nanoscale probing of ferroelectrics. Finally, we comment on two important emerging topics: multiferroic materials and ferroelectric one-dimensional nanostructures. (c) 2006 American Institute of Physics.

/pdf/ferroelectric-thin-films-review-of-materials-properties-and-rc7umdkxk5.pdf

Ferroelectric thin films: Review of materials, properties, and applications

Ultrathin crystalline films offer the possibility of exploring phase transitions in the crossover region between two and three dimensions. Second-order ferromagnetic phase transitions have been observed in monolayer magnetic films1,2, where surface anisotropy energy stabilizes the two-dimensional ferromagnetic state at finite temperature3. Similarly, a number of magnetic materials have magnetic surface layers that show a second-order ferromagnetic–paramagnetic phase transition with an increased Curie temperature4. Ferroelectricity is in many ways analogous to ferromagnetism, and bulk-like ferroelectricity and finite-size modifications of it have been seen in nanocrystals as small as 250 A in diameter5, in perovskite films 100 A thick6 and in crystalline ferroelectric polymers as thin as 25 A (7-10). But these results can be interpreted as bulk ferroelectricity suppressed by surface depolarization energies, and imply that the bulk transition has a minimum critical size11,12,13. Here we report measurements of the ferroelectric transition in crystalline films of a random copolymer of vinylidene fluoride and trifluoroethylene just 10 A (two monolayers) thick. We see a first-order ferroelectric phase transition with a transition temperature nearly equal to the bulk value, even in these almost two-dimensional films. In addition, we see a second first-order transition at a lower temperature, which seems to be associated with the surface layers only. The near-absence of finite-size effects on the bulk transition implies that these films must be considered as two-dimensional ferroelectrics.

/pdf/two-dimensional-ferroelectric-films-4asku9bxo3.pdf

Two-dimensional ferroelectric films

Ferroelectric materials, because of their robust spontaneous electrical polarization, are widely used in various applications. Recent advances in modelling, synthesis and characterization techniques are spurring unprecedented advances in the study of these materials. In this Review, we focus on thin-film ferroelectric materials and, in particular, on the possibility of controlling their properties through the application of strain engineering in conventional and unconventional ways. We explore how the study of ferroelectric materials has expanded our understanding of fundamental effects, enabled the discovery of novel phases and physics, and allowed unprecedented control of materials properties. We discuss several exciting possibilities for the development of new devices, including those in electronic, thermal and photovoltaic applications, and transduction sensors and actuators. We conclude with a brief survey of the different directions that the field may expand to over the coming years. Strain engineering can be used to control the properties of thin-film ferroelectric materials, which are promising for electronic, thermal, photovoltaic and transduction applications. This Review addresses fundamental aspects, novel ways to control materials properties and the development of new ferroelectric-based devices.

https://escholarship.org/content/qt51w3s3s1/qt51w3s3s1.pdf?t=p8f5p7

Thin-film ferroelectric materials and their applications

▪ Abstract This paper reviews the literature on size effects in ferroelectric materials, with an emphasis on thin film perovskite ferroelectrics. The roles of boundary conditions, defect chemistry, electrode interfaces, surface layers, and microstructure in controlling the measured properties of ferroelectric films, as well as the observed deviation from bulk properties are discussed. Examples of the manifestation of size effects in terms of the low and high field dielectric properties, the piezoelectric effect, and the leakage behavior of films are given.

The Properties of Ferroelectric Films at Small Dimensions

Ferroelectric thin films : synthesis and basic properties

For the purpose of converting the mechanical energy of surface waves on bodies of water, a power generating piezoelectric structure (10) is provided comprising piezoelectric material members, preferably in the form of one or a laminate of sheets (14) of such material, each sheet having an electrode (24) on opposite surfaces thereof Each pair of electrodes and the piezoelectric material therebetween define a power generating element, each of which is preferably dimensioned, relative to the wave lengths of selected waves on the body of water in which the generator is to be used, for increasing the efficiency of power conversion A support (12) is provided for maintaining the structure in a preselected position within and below the surface of the water Preferably, the pizoelectric generating elements are flexible and are supported in such manner to allow flexure thereof in response to movement of the surrounding water In certain embodiments the elements are designed to enter into mechanical resonance in response to the passage of waves thereover, such flexure and/or resonance increasing the mechanical coupling efficiency between the waves and the elements

Power generation from waves on the surface of bodies of water.

A wave energy converter (WEC) including an electric generator (42) for capturing energy contained in ocean waves and converting it to electrical energy at the output of the electric generator (42) may be characterized as an effective capacitive or inductive element; systems embodying the invention include an inductive or capacitive element inserted in the circuit between the output of the electric generator (42) and a load (60) in order to achieve resonance with the effective capacitance 'inductance' of the WEC and so as to increase the efficiency of the power transfer to the load (60); in certain embodiments the load (60) coupled to the system has a optimum value which is made a function of the frequency of the ocean waves and the effective capacitance or inductance; the effective capacitance 'inductance' of the converter (40a) varies as a function of the frequency of the ocean waves; accordingly, systems embodying the invention may include a controller (54) for varying the load (60) and/or the inductive 'capacitive' element coupled in the system resonate with the converter (54) for increasing the efficiency of the system.

Apparatus and method for optimizing the power transfer produced by a wave energy converter (wec)

Apparatus for converting energy contained in surface waves on a body of water to useful energy comprises a float from which is suspended a fully or effectively fully submerged inverted cup-shaped member having a closed top end and an open bottom end. Disposed within the top space is a compressible fluid separated by a flexible membrane from a column of water filling the bottom open end of the submerged member. Passing surface waves cause pressure variations at the bottom end of the member causing changes in the water column height and corresponding changes in the buoyancy of the member. The buoyancy changes induce vertical oscillations of the member relative to the float and known means are used for converting the relative movements of the float and submerged member to useful energy. Preferably, the float vertically oscillates in synchronization with the passing waves, and the vertical movements of the float increase the relative movements between the float and the submerged member.

Float dependent wave energy device

Systems and circuits embodying the invention include means for sensing at least one of the peaks, amplitude and frequency of an oscillatory input force impinging on a power generating device (22) for controlling the turn-on of a switch used to transfer energy collected by the power generating device to an associated load. The power generator may be a transducer such as a piezoelectric device or a wave energy converter (WEC). Sensors and sensing means embodying the invention are used to control the point in time at which power extracting circuits are switched in circuit with the power generating device. The power extracting circuit may include components which can resonate with the power generating circuit at a higher frequency than, and independent of, the frequency at which the power generating device is being operated. Thus, an electric power generator device operated and controlled by a slowly changing source of energy (e.g., ocean waves, wind, eddies of water) may develop energy at one frequency and may be operated to transfer the energy, at a selected point in time, at another frequency.

George W. Taylor

Papers

Ferroelectric thin films : synthesis and basic properties

Power generation from waves on the surface of bodies of water.

Apparatus and method for optimizing the power transfer produced by a wave energy converter (wec)

Float dependent wave energy device

Sensors for power conversion systems