Patent Application
20190375655
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2018-110035, filed on Jun. 8, 2018, the entire contents of which are incorporated herein by reference.
The present invention relates to negative thermal expansion materials.
In general, it is known that substances thermally expand as the temperature rises. However, advanced development of industrial technology in recent years requires control over even thermal expansion to which solid materials can be considered to be destined. Even a rate of change of about 10 ppm (10−5) in length, which is small in the general sense, is a big problem in the field of semiconductor device manufacturing where high accuracy at a nanometer level is required, precision instruments whose functions are greatly affected by slight distortion of parts, and so on. Further, in a device in which a plurality of materials are combined, other problems such as interface peeling and disconnection may also occur due to differences in thermal expansion of the respective constituent materials.
On the other hand, negative thermal expansion materials (with a negative coefficient of thermal expansion) are also known whose lattice volume decreases as the temperature rises. For example, a composite material is known that suppresses thermal expansion by mixing α-Cu2V2O7 having a negative coefficient of thermal expansion and Al having a positive coefficient of thermal expansion (Non-patent Document 1).
It is known that α-Cu2V2O7 exhibits negative thermal expansion of −5 to −6 ppm/° C. in a linear expansion coefficient in a temperature range from room temperature to 200° C. However, there is room for improvement in the magnitude of the linear expansion coefficient of α-Cu2V2O7 and the temperature range in which negative thermal expansion is exhibited.
In this background, a purpose of the present disclosure is to provide a new material that exhibits large negative thermal expansion in a wide temperature range.
A negative thermal expansion material according to one embodiment of the present disclosure is represented by a general formula (1): Cu2-xRxV2O7 (R is at least one element selected from Zn, Ga, and Fe) and includes an oxide sintered compact whose linear expansion coefficient is −10 ppm/K or less.
Embodiments will now be described, by way of example only, with reference to the accompanying drawings that are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several figures, in which:
The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention.
The present inventors have focused on a Cu2V2O7 system as a candidate for a substance that exhibits negative thermal expansion. Although α-Cu2V2O7 which has an orthorhombic crystal structure has drawn interest as a multiferroic substance in which both a ferroelectric property and a weak paramagnetic property coexist, anisotropic thermal deformation of the crystal lattice can be seen, which is believed to be due to dielectric instability, in a relatively wide temperature range including room temperature and temperature higher than the room temperature. As a result, negative thermal expansion appears where the unit cell volume contracts as the temperature rises in a wide temperature range.
By substituting Cu2V2O7 with various elements, a monoclinic β phase and a triclinic y phase in addition to an orthorhombic α phase can be realized. Accordingly, the present inventors have found that when a part of the Cu site or the V site is substituted with another element, negative thermal expansion properties that cannot be realized in the conventional α-Cu2V2O7 system are expressed and have devised a negative thermal expansion material illustrated in the following.
A negative thermal expansion material according to an embodiment of the present disclosure is represented by a general formula (1): Cu2-xRxV2O7 (R is at least one element selected from Zn, Ga, and Fe) and includes an oxide sintered compact whose linear expansion coefficient is −10 (ppm/K) or less.
According to this embodiment, a negative linear expansion coefficient can be realized whose absolute value is larger than that of the linear expansion coefficient of a-Cu2V2O7 in which Cu is not substituted with R.
In the general formula (1), x may be 0.1 to 1. Thereby, a linear expansion coefficient can be realized whose absolute value is larger than that of the linear expansion coefficient of α-Cu2V2O7 in which Cu is not substituted with R.
R may be Zn. This allows a p-phase (monoclinic phase) crystal structure to be obtained that is stable at room temperature.
In the general formula (1), x may be 0.15 to 1. Thereby, a linear expansion coefficient can be realized whose absolute value is larger than that of the linear expansion coefficient of α-Cu2V2O7 in which Cu is not substituted with R.
Another embodiment of the present disclosure also relates to a negative thermal expansion material. This negative thermal expansion material is represented by a general formula (2): Cu2V2-xMoxO7, and includes an oxide sintered compact whose linear expansion coefficient is −10 ppm/K or less.
According to this embodiment, a negative linear expansion coefficient can be realized whose absolute value is larger than that of the linear expansion coefficient of a-Cu2V2O7 in which Cu is not substituted with R.
X may be 0.1 to 0.3. Thereby, a linear expansion coefficient can be realized whose absolute value is larger than that of the linear expansion coefficient of α-Cu2V2O7 in which Cu is not substituted with R.
The oxide sintered compact may be in a monoclinic β phase.
The linear expansion coefficient may be −10 ppm/K or less in a temperature range of 100 to 700K.
Another embodiment of the present disclosure relates to a composite material. This composite material includes a negative thermal expansion material and a positive thermal expansion material having a positive linear expansion coefficient. This allows for the realization of the composite material in which volume change with respect to temperature change is suppressed.
Hereinafter, an embodiment for carrying out the present disclosure will be described in detail with reference to the accompanying drawing and the like.
(Method for Preparing Samples)
First, a polycrystalline sintered compact (ceramics) sample of α-Cu2V2O7 and a polycrystalline sintered compact (ceramics) sample of β-Cu1.8Zn0.2V2O7 were prepared using a solid phase reaction method. More specifically, CuO, ZnO, and V2O5, which were weighed at a stoichiometric molar ratio, were mixed in a mortar and heated in the atmosphere at a temperature of 873 to 953K for 10 hours. The powder that was obtained was sintered using a spark plasma sintering (SPS) furnace (manufactured by SPS SYNTEX INC.) so as to obtain an oxide sintered compact. The sintering was performed for 5 minutes at 723K using a graphite die under vacuum (<10−1 Pa).
Thereafter, the crystal structure of each sample was evaluated using powder X-ray diffraction (XRD) method (measurement temperature of 295K, CuKα characteristic X-ray: wavelength λ=0.15418 nm) and a radiation light temperature change X-ray diffraction method (wavelength A=0.06521 nm).
As shown in
As shown in Table 1, in β-Cu1.8Zn0.2V2O7, the absolute value of the volumetric expansion coefficient β (=3a) is 2.5 or more times the absolute value of the volumetric expansion coefficient of α-Cu2V2O7. Further, the total volume change amount ΔV/V of β-Cu1.8Zn0.2V2O7 is 2.6%, which is three or more times the total volume change amount of α-Cu2V2O7, and it can be found that the material exhibits large negative thermal expansion. Further, while the absolute value of the linear expansion coefficient starts to decrease around when the temperature exceeds 600K in α-Cu2V2O7, the linear expansion coefficient is almost constant even at 700K in β-Cu1.8Zn0.2V2O7.
Next, the influence of a substitution element on negative thermal expansion will be described.
As shown in Table 2, in the general formula (1) or (2), even when the substitution element was Ga, Fe, or Mo and the substitution amount x was 0.1 to 0.2, negative thermal expansion larger than that of α-Cu2V2O7 was observed at least in the temperature range of 100 to 500K. More specifically, all the samples have a linear expansion coefficient of −10 ppm/K or less and can realize a negative linear expansion coefficient whose absolute value is larger than that of the linear expansion coefficient of α-Cu2V2O7 in which Cu is not substituted with R. Therefore, in the general formula (1) or (2), when the substitution element is Ga, Fe, or Mo, the substitution amount x is 0.05 or more, preferably 0.1 or more, and the substitution amount x is 0.3 or less, preferably 0.2 or less.
Next, the influence of the substitution amount x of the substitution element will be described.
As shown in Table 3, even when the substitution amount x of the substitution element Zn of Cu2-xZnxV2O7 was 0.15 to 1, negative thermal expansion larger than that of a-Cu2V2O7 was observed at least in the temperature range of 100 to 500K. Note that β-Cu2-xZnxV2O7 may have a linear expansion coefficient of −10 ppm/K or less and preferably −14 ppm/K or less in a temperature range of 100 to 700 K. More specifically, the substitution amount x of the substitution element Zn of Cu2-xZnxV2O7 is preferably 0.15 or more and 0.5 or less and more preferably 0.2 or more and 0.3 or less.
Next, a composite material will be described that includes a negative thermal expansion material composed of an oxide sintered compact represented by the general formula (1): Cu2-xRxV2O7 (R is at least one element selected from Zn, Ga, and Fe), or the general formula (2): Cu2V2-xMoxO7, and a positive thermal expansion material having a positive linear expansion coefficient such as a resin, a metal, or the like.
The line described as ROM (Rule of Mixture) in
In β-Cu1.8Zn0.2V2O7 explained in the present embodiment, ΔV/V (unit cell) associated with a temperature rise in terms of a unit cell of the crystal is much smaller than ΔV/V (bulk) associated with a temperature rise in terms of the whole oxide sintered compact. More specifically, when the temperature of the sintered compact of β-Cu1.8Zn0.2V2O7 is raised from 200K to 700K, the lattice constant of a monoclinic crystal (β phase) changes by −1.6% in the a axis, 1.1% in the b axis, and −0.3% in the c axis, and −0.1% in the angle β, and ΔV/V (unit cell) is −0.8%. Therefore, ΔV/V (unit cell) is only about one third of ΔV/V (bulk), which is −2.6%, shown in Table 1.
As described above, in the negative thermal expansion material according to the embodiment of the present disclosure, the linear expansion coefficient is substantially constant under temperature change in a wide temperature range of about 100 to 700K, and material function designing is thus easy. Further, there are industrial merits such as being composed mainly of inexpensive elements such as Cu, Zn, and V and being oxides having low synthesis temperature that allows for easy manufacturing.
Described above is an explanation of the present disclosure based on the embodiments. These embodiments are intended to be illustrative only, and it will be obvious to those skilled in the art that various modifications to constituting elements and processes could be developed and that such modifications are also within the scope of the present disclosure.
The oxide sintered compact represented by the general formula (1): Cu2-xRxV2O7 (R is at least one element selected from Zn, Ga, and Fe) or the general formula (2): Cu2V2-xMoxO7 of the present disclosure can be used as a thermal expansion suppressor for canceling out and suppressing thermal expansion usually exhibited by a material. Further, zero thermal expansion materials can be also made that do not expand positively or negatively in a particular temperature range.
More specifically, the oxide sintered compact can be used for precision optical components and mechanical components, process equipment and tools, temperature compensation materials for fiber gratings, printed circuit boards, encapsulants for electronic components, thermal switches, refrigerator parts, satellite parts, and the like that disfavor changes in shape and/or dimensions due to temperature. In particular, by using a composite material in which a negative thermal expansion material is dispersed in a matrix phase of a resin having a large positive thermal expansion coefficient, thermal expansion can be suppressed and controlled even in a resin material, and thus usage in various applications can be possible.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2018-110035 | Jun 2018 | JP | national |
