K. Takenaka 교수

We report the discovery of a large negative thermal expansion (NTE) up to α=−25×10−6K−1 (α: coefficient of linear thermal expansion) in Ge-doped anti-perovskite manganese nitrides Mn3AN (A=Cu,Zn,Ga). This gigantic NTE is several to ten times higher than that of commercially available NTE materials. The discontinuous lattice expansion seen in pure Mn3AN is broadened by Ge substitution over a wide temperature window, at widest ΔT∼100K, around room temperature. Such a large, isotropic and nonhyster

Most materials expand upon heating. However, although rare, some materials contract upon heating. Such negative thermal expansion (NTE) materials have enormous industrial merit because they can control the thermal expansion of materials. Recent progress in materials research enables us to obtain materials exhibiting negative coefficients of linear thermal expansion over -30 ppm K^-1. Such giant NTE is opening a new phase of control of thermal expansion in composites. Specifically exami

We have investigated the anisotropic resistivities of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$ using detwinned crystals with various oxygen contents (6.68 \ensuremath{\le}7-y\ensuremath{\le} 6.93). The out-of-plane resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathit{c}}$ shows a crossover from high-T metallic to low-T semiconducting behavior while the in-plane resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathit{a}}$ deviates in the low-T region fro

To meet strong demands for the control of thermal expansion necessary because of the advanced development of industrial technology, widely various giant negative thermal expansion (NTE) materials have been developed during the last decade. Discovery of large isotropic NTE in ZrW₂O₈ has greatly advanced research on NTE deriving from its characteristic crystal structure, which is now classified as <i>conventional</i> NTE. Materials classified in this category have increased r

Giant negative thermal expansion (NTE) recently discovered in antiperovskite manganese nitrides Mn3AN (A=Zn,Ga, etc.) is achieved by doping Ge on A as “relaxant” of the sharp volume change at the magnetic transition. To promote wider applications, we synthesized NTE antiperovskites without expensive Ge. We discovered that Sn broadens the volume change, though less effective than Ge. Simultaneous substitution of Sn for A and C for N expands the operation-temperature window of NTE almost as broad

Large negative thermal expansion (NTE) has been discovered during the last decade in materials of various kinds, particularly materials associated with a magnetic, ferroelectric or charge-transfer phase transition. Such NTE materials have attracted considerable attention for use as thermal-expansion compensators. Here, we report the discovery of giant NTE for reduced layered ruthenate. The total volume change related to NTE reaches 6.7% in dilatometry, a value twice as large as the largest volum

A zero thermal expansion material in a pure form is fabricated using an antiperovskite manganese nitride. The isotropic zero thermal expansion is achieved by optimizing the heat treatment and the chemical composition. The present study suggests that the heat treatment affects the thermal expansion mainly via the nitrogen content of the material. The obtained materials exhibit a low expansion of |α|&amp;lt;0.5×10−6 K−1 (α is the coefficient of linear thermal expansion) over a broad temperature ra

Magnetostructural correlations in antiperovskite manganese nitrides were investigated systematically for stoichiometric and solid solution Mn₃Cu_1-<i>x</i> A _<i>x</i> N (A = Co, Ni, Zn, Ga, Ge, Rh, Pd, Ag, In, Sn or Sb). This class of nitrides is attracting great attention because of their giant negative thermal expansion, which is achieved by doping Ge or Sn into the A site as a relaxant of the sharp volume contraction on heating (spontaneous volume magnetostrict

Polarized optical reflectivity spectra of untwinned ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ are reported. The spectra show a large ab-plane anisotropy in the infrared region with higher reflectivity along the chain direction. For the a polarization, the spectrum is similar to that of deoxygenated tetragonal ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathit{y}}$, with an optical energy gap of about 1.5 eV, indicating that it is difficult to

Electrical resistivity is systematically investigated in Mn3AgN and related compounds with an antiperovskite structure. Despite its overall metallic character, Mn3AgN features a broad maximum in the temperature-resistivity curve in the paramagnetic state and the temperature coefficient of resistance (TCR) is negative at higher temperatures. The resistivity-peak temperature was tuned to just room temperature by the partial substitution of Cu for Ag, and a TCR as low as 10−6 K−1 was achieved over

Metallic manganese nitrides Mn3AN (A=Zn, Ga, etc) are well-known for their large magnetovolume effect (MVE), i.e., a discontinuous volume expansion at the magnetic transition. However, MVE is exceptionally absent in Mn3CuN. We found that MVE is recovered by a small amount of Ge in the Cu site. This revival seems to coincide with recovery of the cubic structure. By further Ge doping, the volume expansion becomes gradual (ΔT∼100 K) and large negative thermal expansion (NTE) is exhibited around roo

We controlled thermal expansion of metal matrix composites (MMCs) that had been blended using antiperovskite manganese nitrides with giant negative thermal expansion (NTE). The NTE of the manganese nitrides, which is isotopic, is greater than −30 ppm K−1 in α (coefficient of linear thermal expansion), which is several or ten times as large as that of conventional NTE materials. These advantages of nitrides are desirable for practical application as a thermal-expansion compensator, which can supp

Optical reflectivity spectra of La(1-x)Sr(x)MnO(3) (0<=x<=0.30) were measured on cleavage surfaces of single crystals. The optical conductivity $\sigma(w) of ferromagnetic-metal La(0.70)Sr(0.30)MnO(3) is characterized by a Drude-like - but not simple Drude - component with large spectral weight below 1.6 eV, which yields a large effective carrier number N*_eff consistent with the results of Hall coefficient and specific heat measurements. The present result demonstrates that the previous result

We performed the systematic optical and transport experiment for ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ at high temperatures. The in-plane resistivity ${\ensuremath{\rho}}_{\mathrm{ab}}(T)$ saturates not at the classical Ioffe-Regel-Mott limit but at much higher value. The in-plane optical conductivity ${\ensuremath{\sigma}}_{\mathrm{ab}}(\ensuremath{\omega})$ exhibits at high temperatures a less characteristic, nearly flat spectrum over a wide energy range up to 1