Elastic properties, hardness, and anisotropy in baddeleyite IVTMO2 (M=Ti, Zr, Hf)

Abstract

and HfO 2 thin films. Other researchers have investigated the transformation from monoclinic phase to tetragonal phase in HfO 2 [5], as well as its mechanical [11], optical, and elastic properties [12]. Even with this range of study, some properties of IVTMO 2 , particularly their mechanical properties, are not well understood . For example, elastic anisotropy in thin films usually leads to microcracks, and differences in thermal expansion between a film and matrix might induce a dropping of the film from the matrix.Zhao et al. [10] reported that bulk ZrO 2 has three crystalline structures at normal pressure: cubic (2300−2680°C), tetragonal (1170−2300°C), and monoclinic (<1170°C). Bulk HfO 2 has only two crystalline structures: tetragonal (1750−2800°C) and monoclinic (<1750°C). The most abundant polymorph of TiO 2 found in nature is rutile, and at room temperature it is stable up to 12 GPa, where it directly converts to the baddeleyite-type (monoclinic) phase [13]. Because m-HfO 2 has a larger bulk modulus than the other two metal dioxides [12], it was expected to have high hardness. The bulk modulus was once thought to be good predictor of hardness. Although it is true that a hard material must have a high modulus [14], it is a common misconception that an incompressible material is a hard one. For example, osmium has only a twentieth the hardness of diamond, though its bulk modulus is comparable to diamond, making it one of the most incompressible materials known.It is now well recognized that high bulk and shear moduli do not guarantee high hardness. Other factors-including high electron density, short bond length, high degree of covalent bonding, and near-isotropic deformation-usually dominate a material's intrinsic hardness [15]. Thus, it is important to thoroughly investigate baddeleyite-type IVTMO 2 (we henceforth refer to these m-MO 2 materials as m-TiO 2 , m-ZrO 2 , and m-HfO 2 ), especially their hardness, elasticity, and anisotropy. To our best knowledge, there are no systemic studies of monoclinic metal dioxides, which are stable at room temperature. In the past decade,In this article, we used plane-wave density functional theory to investigate the elasticity, anisotropy, and minimum thermal conductivities of baddeleyite type IVTMO2 (m-TiO2, m-ZrO2, and m-HfO2). The elastic constants and modulus, Poisson's ratio, hardness, sound speed, Debye temperature, and minimum thermal conductivities at high temperature were calculated. These calculations show that m-MO2 is not superhard, with a hardness range of about 8-13 GPa. Among these materials, m-TiO2 is the hardest, while m-HfO2 is the least hard. Their elastic and plastic anisotropy are given in detail. Moreover, the m-HfO2 thin film is the most likely to develop microcracks during preparation because it has the highest elastic anisotropy. Among the three dioxides, m-HfO2 is the best thermal barrier because it has the lowest thermal conductivity.

Y1/6Yb5/6TaO4/8YSZ composite ceramics with enhanced mechanical and thermal properties

“…The hardness and fracture toughness of the YYT (1− x ) /8YSZ x composite ceramics are shown in Figure 3. The inherent high hardness of 8YSZ contributes to a gradual increase in the hardness of the composite, which is in good agreement with the results of previous studies 36,37 . The fracture toughness of the composite initially increased with increasing 8YSZ content, reaching its maximum value (3.57 MPa m 1/2 ) at 50% 8YSZ.…”

“…The inherent high hardness of 8YSZ contributes to a gradual increase in the hardness of the composite, which is in good agreement with the results of previous studies. 36,37 The fracture toughness of the composite initially increased with increasing 8YSZ content, reaching its maximum value (3.57 MPa m 1/2 ) at 50% 8YSZ. Further increasing the 8YSZ content results in a slight decrease in the toughness of the composite (Figure 3A).…”

Study of Electronic Structure, Thermal Conductivity, Elastic and Optical Properties of α, β, γ-Graphyne

“…The heat transfer of 2D material diverges as the distance increases [ 53 ], and the thermal conductivity decreases with increasing temperature [ 54 ]. Knowledge on the factors affecting the thermal conductivity, the minimum values under high temperature are important for their applications.…”

New insight into the formation and oxygen barrier mechanism of carbonaceous oxide interlayer in a multicomponent carbide

“…A mathematical model presented by Guo et al indicates a considerable temperature gradient (ie, a 500‐μm distance is about 900°C) in the ZrO 2 scale 42 . Given the lower thermal conductivity of HfO 2 than that of ZrO 2 , the temperature variation among the HfO 2 ‐dominant scale (ie, Hf–Zr–Ti–O) with a thickness of about 500 μm (see Figure 3A and Figure 8B) would not be less than 900°C 43 . Therefore, 900°C can be deemed as the critical value of the temperature gradient in the present work; as the test temperature reached about 2500°C, the thermodynamic temperature of the volatility diagram could be fixed at 1626.85°C (1900 K) for the convenience of calculation.…”

“…42 Given the lower thermal conductivity of HfO 2 than that of ZrO 2 , the temperature variation among the HfO 2dominant scale (ie, Hf-Zr-Ti-O) with a thickness of about 500 μm (see Figure 3A and Figure 8B) would not be less than 900°C. 43 Therefore, 900°C can be deemed as the critical value of the temperature gradient in the present work; as the test temperature reached about 2500°C, the thermodynamic temperature of the volatility diagram could be fixed at 1626.85°C (1900 K) for the convenience of calculation. Meanwhile due to a possibly larger temperature gradient resulting from lower thermal conductivity of HfO 2 compared to ZrO 2 , the volatility diagrams at 1326.85°C (1600 K)/1426.85°C (1700 K) /1526.85°C (1800 K) were calculated.…”