Negative thermal expansion (NTE) is an intriguing physical property of solids, which is a consequence of a complex interplay among the lattice, phonons, and electrons. Interestingly, a large number of NTE materials have been found in various types of functional materials. In the last two decades good progress has been achieved to discover new phenomena and mechanisms of NTE. In the present review article, NTE is reviewed in functional materials of ferroelectrics, magnetics, multiferroics, superconductors, temperature-induced electron configuration change and so on. Zero thermal expansion (ZTE) of functional materials is emphasized due to the importance for practical applications. The NTE functional materials present a general physical picture to reveal a strong coupling role between physical properties and NTE. There is a general nature of NTE for both ferroelectrics and magnetics, in which NTE is determined by either ferroelectric order or magnetic one. In NTE functional materials, a multi-way to control thermal expansion can be established through the coupling roles of ferroelectricity-NTE, magnetism-NTE, change of electron configuration-NTE, open-framework-NTE, and so on. Chemical modification has been proved to be an effective method to control thermal expansion. Finally, challenges and questions are discussed for the development of NTE materials. There remains a challenge to discover a “perfect” NTE material for each specific application for chemists. The future studies on NTE functional materials will definitely promote the development of NTE materials.

Negative thermal expansion in functional materials: controllable thermal expansion by chemical modifications

Nasicons (sodium super ion conductors) are a class of solid electrolytes. Their structure, compositional diversity, evolution, and applications are reviewed. A wide range of materials is considered based on crystalline and glassy Nasicon compositions.

A wide-ranging review on Nasicon type materials

As possible electrolyte materials for all-solid-state Na-ion batteries (NIBs), scandium-substituted Na3Zr2(SiO4)2(PO4) in the structure of NASICONs (Na superionic conductors) has received hardly any attention so far, although among all the trivalent cations, Sc3+ might be the most suitable substitution ion for Na3Zr2(SiO4)2(PO4) because the ionic radius of Sc3+ (74.5 pm) is the closest to that of Zr4+ (72.0 pm). In this study, a solution-assisted solid-state reaction (SASSR) method is described, and a series of scandium-substituted Na3Zr2(SiO4)2(PO4) with the formula of Na3+xScxZr2-x(SiO4)2(PO4) (NSZSPx, 0 ≤ x ≤ 0.6) have been prepared. This synthesis route can be applied for powder preparation on a large scale and at low cost. With increasing degrees of scandium substitution, the total conductivity of the samples also increases. An optimum total Na-ion conductivity of 4.0 × 10–3 S cm–1 at 25 °C is achieved by Na3.4Sc0.4Zr1.6(SiO4)2(PO4) (NSZSP0.4), which is the best value of all reported polycrystalline ...

Scandium-Substituted Na3Zr2(SiO4)2(PO4) Prepared by a Solution-Assisted Solid-State Reaction Method as Sodium-Ion Conductors

Tailored thermal expansion alloys

The decrease in remnant polarization, P r , and increase in coercive field, E c , of ferroelectric ceramics subjected to a large a.c. electric field is termed electric fatigue. In transparent hot-pressed lanthanum doped lead zirconate titanate (PLZT) ceramics, electric fatigue was not detected after 10 9 cycles when grain size is small (∼ 5 μ m) but became increasingly severe as the grain size increases to 10 μm and beyond. The electric field causes large strains (⩾ 10 −3 ) due to the anisotropic piezoelectric and electro-strictive coefficients of the material, which results in intergranular microcracking due to incompatible deformations. The effective field on a grain in a cracked sample is less than the applied field, limiting domain switching and polarization. The microcracking occurs only in the electroded, fatigued region of the sample and is initiated on the high voltage terminal side. The microscopic and acoustic emission data support the proposed mechanism of electric fatigue. An estimate of the critical grain size for microcracking due to electric stress, based on energy criterion, is in good agreement with experiment. The process of crack growth during electric fatigue is also elucidated.

Grain size dependence of electric fatigue behavior of hot pressed PLZT ferroelectric ceramics

Most members of the NaZr{sub 2}P{sub 3}O{sub 12} (NZP) family possess low, near zero, overall thermal expansion coefficients. However, they also exhibit anisotropy of axial thermal expansion. Some compounds have opposite anisotropy; for example, the a parameter of CaZr{sub 4}P{sub 6}O{sub 24} contracts on heating and that of SrZr{sub 4}P{sub 6}O{sub 24} expands, while the c parameter expands for the Ca compound and contracts for the Sr compound. The anisotropy of the axial thermal expansion of these materials is believed to induce microcracking. The acoustic emission method was employed here to detect microcracking in ceramics due to the axial thermal expansion anisotropy. Acoustic signals were observed during cooling of the Ca and Sr compounds from 500 {degrees} C, and Na and K compounds from 600 {degrees} C. On the other hand, no acoustic emission signal is detected in Ca{sub 0.5}Sr{sub 0.5}Zr{sub 4}P{sub 6}O{sub 24} ceramics, in which the lattice parameters a and c remain nearly unchanged in the temperature range of room temperature to 500 {degrees}C. Thus, a direct correlation between microcracking of ceramics and their anisotropic axial thermal expansion coefficients was established by employing acoustic emission monitoring techniques.

Chi‐Yuen ‐Y Huang

Papers

Thermal Expansion Anisotropy and Acoustic Emission of NaZr2P3O12 Family Ceramics