By tuning the structural phase transition in Zn2–xMgxP2O7, large negative thermal expansion (NTE) was achieved at room temperature. An earlier report described that Zn2P2O7 undergoes a structural phase transition at 405 K, accompanied by volume contraction of 1.8% on heating. Results showed that as Mg doping proceeds, the transition temperature decreases. Also, the volume change becomes gradual with respect to temperature. Particularly, Zn1.6Mg0.4P2O7 has a large negative coefficient of linear thermal expansion αL of −60 ppm/K at 280–350 K. Structural analysis using synchrotron radiation revealed that this dilatometric NTE is almost identical to that of crystallographic unit cells, indicating less dominant structural effects on NTE. We also verified thermal expansion compensation capabilities of powdered Zn1.6Mg0.4P2O7 by evaluating the thermal expansion of the epoxy resin matrix composites. The present phosphates are promising for use as practical thermal expansion compensators because they are free of toxic or expensive elements and can be fabricated in air using the simple solid-state reaction method.

Negative thermal expansion (NTE) is exhibited over the entire x range for Cu1.8Zn0.2V2–xPxO7. In particular, dilatometric measurements using epoxy resin matrix composites containing the spray-dried powder demonstrated that the thermal expansion suppressive capability was almost unchanged for x≤0.1. With increasing x, the x-ray diffraction peak position moves systematically, but some peaks are extremely broad and/or asymmetric, suggesting disorder in the internal structure. The crystallographic analysis confirmed NTE enhancement by microstructural effects at least for x=0.2. Preliminary measurements suggest higher resistivity and lower dielectric constant than that of pure vanadate, which is suitable for application to electronic devices.

Because of growing demands for thermal expansion control in a local region, typically inner components of electronic devices, there is currently a great deal of interest in negative thermal expansion (NTE) fine particles. The spray-drying method without grinding produced micrometer-scale fine particles of pyrovanadate Cu 1.8 Zn 0.2 V 2 O 7 showing a large negative coefficient of linear thermal expansion α L ~ -14 ppm/K equivalent to that of the bulk body. The solid-solution system Cu 1.8 Zn 0.2 V 2-x P x O 7 maintains the NTE functionality in a wide x compositional range. Phosphorus substitution contributes to cost reduction and to improvement of electrical insulation. Control of the structural phase transition in the phosphate analog Zn 2 P 2 O 7 by substituting Mg for Zn realized a huge negative α L exceeding -60 ppm/K over a wide temperature range including room temperature. Because small particles are obtainable even with the conventional solid-phase reaction, the 1 μm level fine particles were obtained successfully using the pulverization method without degrading the function. It has high practicality because it is composed only of inexpensive and environmentally friendly elements such as Zn, Mg, and P. These NTE fine particles are expected to support great progress in thermal expansion control engineering.

Discovery of giant negative thermal expansion (NTE) of Ti2O3 is reported herein. Ti2O3 undergoes a phase transition from a low-temperature (low-T) insulating state to a high-T metallic state gradually at temperatures of 400–600 K, accompanied by highly anisotropic thermal deformation of the crystallographic unit cell. This anisotropic deformation induces large bulk NTE in the sintered body, although the unit-cell volume estimated from diffraction experiments shows positive thermal expansion in this T range. Results of this study also demonstrate that partial replacement of Ti with Nb increases the total volume change related to bulk NTE and show that it lowers the operating-T range of NTE to include room temperature. The development of NTE materials particularly addressing such microstructural effects is effective and promising.

For Mg-doped Zn 2 P 2 O 7 , this systematic investigation of co-doping onto Zn sites has elucidated specific effects on negative thermal expansion (NTE). The low-cost and low-environmental-impact NTE material Zn 2¹x Mg x P 2 O 7 shows large NTE in a temperature range including room temperature for x = 0.4. Although Mg doping broadens the operating-temperature window, it remains several dozen degrees wide. Moreover, the total volume change related to NTE becomes less than that of the Zn 2 P 2 O 7 parent material. Findings obtained from this study demonstrate that co-doping of Mg and of another element onto the Zn site is effective for achieving simultaneous expansion of the operatingtemperature window and maintenance of the volume change related to NTE. One illustrative case is that Zn 1.64 Mg 0.30 Al 0.06 P 2 O 7 has a large negative coefficient of linear thermal expansion of about ¹65 ppm/K at temperatures of 300375 K. In fact, at temperatures high above room temperature, Zn 1.64 Mg 0.30 Al 0.06 P 2 O 7 powder shows better thermal expansion compensation capability than the composition without Al. The Aldoped phosphates are expected to have broad practical application because of their performance, cost, and environmental load characteristics.