Linear Thermal Expansion Behavior Research Articles

Thermal Expansion of Polypropylene Part II: Depression of In‐Plane Linear Thermal Expansion of Polypropylene by Elastomer Incorporation and Elucidation of Its Mechanism Through Temperature‐Variable X‐ray Scattering

AbstractIn this study, the in‐plane linear thermal expansion behaviors from −20 °C to 100 °C along the flow direction (FD) for injection molded isotactic homo polypropylene (hPP) and their elastomer blends are thoroughly investigated. The elastomers used in this study are two different poly(ethylene‐co‐octene)(EO) thermoplastic elastomers which have the same density but different melt flow rate (MFR), 5.0 and 30 (dg/10 min). Thus, the blends are designated as hPP‐EO5.0 having the EO with 5.0 of MFR and hPP‐EO30 having the EO with 30 of MFR. It is found that from −20 °C to 40 °C, the linear thermal expansion profiles for neat hPP, hPP‐EO5.0, and hPP‐EO30 overlapped, indicating that with or without the EO, their linear thermal expansions proceeded under the same mechanism. On the other hand, above 40 °C, their thermal expansion behaviors greatly changed; the linear thermal expansions of hPP‐EO5.0 and hPP‐EO30 are suppressed to be much lower than that of neat hPP. The morphological observation by a transmission electron microscope (TEM) reveals that the shapes of the EO domains in the blend systems are found to be elongated along the FD. Thus, it would be expected that the suppressed linear thermal expansions of the hPP‐EO blends are attributed to the suppression of the linear thermal expansions of the hPP amorphous layers in the lamellae due to the retraction of the elongated EO domains caused due to the instability of the interfacial tension at elevated temperatures. Yet, temperature variable small‐angle X‐ray scattering (SAXS) results reveal that the linear thermal expansions of the hPP amorphous layers in the EO blends, on the contrary, increase. One of the plausible mechanisms is proposed to elucidate seemingly contradictory phenomenon that the EO blends have their suppressed bulk linear thermal expansions while enhanced linear thermal expansions of the hPP amorphous layers. Finally, the suitable elastomer design for low coefficient of linear thermal expansion (CLTE) PP is proposed.

Effects of Nb and o Contents on the Negative Linear Thermal Expansion Behaviors of Cold Rolled Ti-34Nb Alloys

In this study, the effects of Nb and O contents on the negative thermal expansion behaviors including the coefficient of negative thermal expansion and matensite (α) stability of cold rolled (CR) Ti-34Nb alloys was investigated by optical microscopy, X-ray diffraction and Thermomechanical analysis. Results show that texture strength of CR-Ti-xNb-xO alloys is related to β stability, the addition of Nb and O (β stabilizers) enhances the texture strength. XRD results indicate that Nb and O inhibit the formation of α. The coefficient of negative thermal expansion of CR-Ti-xNb-xO alloys decrease with the increase of Nb and O contents. Moreover, cyclic thermal expansion results reveal that Nb and O promote the decomposition of α.

Linear thermal expansion behavior of compacted bentonite buffer materials

In a geological repository system, buffer is indispensable to ensure the safe disposal of high-level radioactive waste (HLW). Because heat generated from spent nuclear fuel in a canister is released to the surrounding buffers, thermal properties of such materials are fundamental in determining the overall disposal safety. Specifically, given that thermal expansion causes thermal stress to canisters and intact rock masses in the near-field location, it is imperative to evaluate the thermal expansion characteristics of the buffer, particularly when bentonite is used. This study investigates the linear thermal expansion properties of Kyeongju bentonite buffer, a type of Ca-bentonite produced in South Korea. The linear thermal expansion coefficient of dried bentonite was measured considering the heating rate, dry density, and temperature variation using dilatometer equipment. The linear thermal expansion coefficient values of the KJ bentonite buffers were found to be 4.0–6.2 × 10⁻⁶/°C. Based on test results, a numerical analysis was conducted, and the thermal strain values were similar between the test and numerical analysis. The overall linear thermal expansion coefficient of the KJ bentonite, considering radially confined or unconfined conditions and dried or saturated states, was predicted to be between 3.2 × 10⁻6/°C and 1.0 × 10⁻5/°C.

Research progress in thermal expansion behavior of polyimide films

<p indent=0mm>As one of the most important flexible substrates and interlayer dielectric materials, the thermal expansion performance of polyimide films has always attracted great interest. Especially with the development of advanced electronic and microelectronic devices, the linear and volumetric thermal expansion of polyimide films are facing more and more stringent requirements. In order to explore new generation polyimide films with low thermal expansion, it is necessary to clarify the thermal expansion behaviors and influence mechanism. For polymer films, the thermal expansion actually includes in-plane and out-of-plane linear as well as volumetric thermal expansion. Most researches on thermal expansion of polyimide films are focused on the in-plane thermal expansion, but there are few studies on the out-of-plane and volume thermal expansion of films. There is no systematic understanding of the mechanism of thermal expansion behavior. The research on linear and volume thermal expansion behavior of polyimide films were firstly summarized in this article. The testing methods were introduced in detail, including thermomechanical analysis (TMA), variable temperature wide-angle X-ray diffraction (VT-WAXD), capacitance technique, optical interferometry, <italic>etc</italic>. The influencing factors of thermal expansion behavior of polyimide films were discussed, such as molecular backbone structures, local molecular motions, chemical crosslinking, aggregation structures and film preparation process. The relationship between structures and processes on in-plane, out-of-plane linear and volumetric thermal expansion behavior of polyimide films was clarified. The problems and challenges in the future research on thermal expansion were prospected. This review can provide a necessary guidance for the development of high-performance polyimide films with low thermal expansion in the wide temperature range.

The mechanism of negative linear thermal expansion behavior of cold-rolled Ti-34Nb alloy

The mechanism of negative linear thermal expansion (NLTE) of Ti-34Nb (wt.%) alloy after 90% cold rolling is investigated by X-ray diffraction, thermal expansion and transmission electron microscopy. From the results, it is observed that 90% cold-rolled Ti-34Nb alloy is composed of β and α″ (Martensite) phases with the existence of β and α″ textures along rolling direction (RD). The cyclic thermal expansion, XRD and TEM studies show that when the thermal cycle temperature is at 100 °C, the RD of 90% cold-rolled Ti-34Nb alloy performs a reversible NLTE, which gradually weakens when thermal cycle temperature is at 300 °C, attributing to the gradual decomposition of α″-phase. When thermal cycle temperature rises to 380 °C, the reversible NLTE disappears and turns into positive linear thermal expansion, meanwhile, α″-phase decomposes completely. Based on the formation of β and α″ textures by cold rolling and α″ $$\leftrightarrow$$ β thermo-reversible transformation mechanism, the NLTE mechanism of 90% cold-rolled Ti-34Nb alloy is successfully explained. Moreover, according to the present results, a novel strategy is proposed to tailoring the negative coefficient of linear thermal expansion by changing α″ content, which improves the application potential of Ti alloys.

Influence of Si content on thermal expansion characteristic of Al–Si alloys

ABSTRACTThe objective of this study was to find out how Si precipitation affects the linear thermal expansion behaviors of Al–Si alloys with various Si content. These Al–Si alloys were manufactured by gravity casting using 99.98 wt-% pure Al and 98.5 wt-% Si pellets. Solution treatment was carried out at 530°C for 10 h for each specimen. As-quenched specimens were subjected to microstructure observation and linear thermal expansion analysis. Si precipitation and additional linear thermal expansion occurred at lower temperature when Si content was increased to 9.5 wt-%. The activation energy of Si precipitation was also lower when Si content was higher. Eutectic Si reduced diffusion distance and acted as a nucleation site of dissolved Si atoms in the Al matrix during aging. These Si phases decreased Si precipitation temperature and activation energy of Si precipitation.

Preparation and properties of LaMgAl11O19 thermal barrier coatings doped with Gd2O3

As a rare earth hexaaluminate, LaMgAl11O19 (LMA) has been one of the most promising materials used as thermal barrier coatings (TBCs). A large amount of amorphous phase, however, often exists in the plasma-sprayed LMA coating and significantly reduces the service lifetime of TBCs. In this study, La1-xGdxMgAl11O19 (x = 0, 0.2, 0.4, 0.6, and 0.8) ceramic powders are synthesised by solid-state reaction, and all of these powders are employed to prepare the corresponding coatings. The phase compositions and microstructures of samples are examined by X-ray diffraction and scanning electron microscopy, respectively. The linear thermal expansion behaviour and thermal cycling behaviour of the coatings are also analysed. The results show that the amorphous phase content is decreased and the thermal expansion behaviour is improved by doping the coatings with Gd2O3. The thermal cycling lifetime of the coating, however, basically remains unchanged.

The role of Co/Al ratio in glass-forming GdCoAl magnetocaloric metallic glasses

In this work, a prominent improvement of glass-forming ability is achieved through the adjustment of Co/Al ratio in the ternary GdCoAl system. The critical diameter of Gd55Co17.5Al27.5 bulk metallic glass (BMG) is up to 8 mm, which is larger than that of the Gd55Co22.5Al22.5 BMG with a critical diameter of 3 mm and most ternary Gd-based BMGs. Thermodynamic investigations in terms of specific heat capacity and Gibbs free energy difference indicate that the Gd55Co17.5Al27.5 BMG possesses a lower driving force for crystallization. Kinetic analyses based on viscosity and dynamic relaxation spectrum reveal a stronger liquid behavior and more sluggish kinetics of the Gd55Co17.5Al27.5 BMG compared with the Gd55Co22.5Al22.5 BMG. Through the combination of the linear thermal expansion behavior and relaxation, the distinct difference of the contraction in supercooled liquid region of Gd55Co17.5Al27.5 and Gd55Co22.5Al22.5 BMGs is well explained. In addition, the magnetic entropy change is further enhanced by appropriate annealing treatment through the formation of more complex structures comprising short-range order, medium-range order and nano-crystallized structure.

Reduced anisotropic properties of Cold Isostatic Pressed graphite matrix compacts

Matrix graphite (MG) compacts were produced by conventional uniaxial pressing (UP) and Cold Isostatic Pressing (CIP). An additional premolding procedure with UP was also carried out before CIP to manufacture MG compacts with a cylinderical shape which was not achievable using CIP alone. It was noted that the MG compacts made by CIP revealed more isotropic microstructural and thermal properties compared to the MG compacts made by UP. The MG compacts made by UP had graphite grains aligned in the transverse direction and severe anisotropic thermal expansion and conductivity according to direction of orientation (axial and transverse). On the other hand, the MG compacts made by CIP showed randomly aligned graphite grains, almost identical linear thermal expansion and shrinkage behaviors, and a reduced thermal conductivity difference between the axial and transverse directions compared to that of the UP MG compacts. The properties of the premolded CIP MG compacts also revealed considerable anisotropic properties. Finally, three-point bending experiments showed that the MG compacts made by CIP obtained 230% higher fracture toughness to the transverse direction compared to that of the UP MG compacts.

Solid solution limits and electrical properties of scheelite SryLa1-yNb1-xVxO4-δ materials for x = 0.25 and 0.30 as potential proton conducting ceramic electrolytes

The proton conductivity and solid solubility limits of acceptor strontium doped vanadium stabilised lanthanum niobate (SryLa1−yNb1-xVxO4-δ, x = 0.25, 0.30 and y = 0 to 0.10) were explored as potential proton conducting ceramic electrolytes. All samples were synthesized via a solid-state method. The phase purity, microstructure and thermal expansion behaviour of the materials were studied using powder X-ray diffraction, scanning electron microscopy and dilatometry, respectively. A maximum solid solution limit of 5% Sr in the A-site of SryLa1−yNb1-xVxO4-δ samples is observed for a vanadium content of x = 0.25, while further increases in the Sr or vanadium contents lead to the presence of Sr3(VO4)2 as a secondary phase. This acceptor dopant content of 5%Sr in the current scheelite material exceeds that possible in the parent vanadium-free fergusonite SryLa1−yNbO4-δ material by a factor of 5. All Sr doped scheelite materials show linear thermal expansion behaviour, successfully avoiding the scheelite to fergusonite structural phase change during thermal cycling. The average grain size is shown to be increased by increasing vanadium content. In humid conditions, all phase pure samples show predominantly proton conductivity at lower temperatures, while p-type conductivity is noted at higher temperatures under dry oxidising conditions. In the low temperature range, the Sr0.05La0.95Nb0.75V0.25O4-δ sample, containing the largest acceptor dopant concentration, exhibits slightly higher bulk and specific grain boundary conductivities in comparison to other phase pure compositions.

Effects of chain packing and structural isomerism on the anisotropic linear and volumetric thermal expansion behaviors of polyimide films

Anisotropic linear and volumetric thermal expansion behaviors of four types of isomeric polyimides (PIs) were investigated based on thermal mechanical analyses and optical interferometric measurements. Among PI films of poly (p-phenylene 3,3′,4,4′-diphenyldiimide) (sBPDA/PPD) with different chain packing densities, a film with a higher refractive index exhibited a smaller coefficient of volumetric thermal expansion (CVE), indicating that a smaller amount of free volume leads to a smaller CVE even at much lower temperatures than the glass transition temperature (Tg). The PI films containing quasi-linear sBPDA structures exhibited smaller CVEs than those containing bent 2,3′,3,4′-diphenyldiimide (aBPDA) structures, whereas the PI films with bent m-phenylene (MPD) linkages exhibited smaller CVEs than those containing linear p-phenylene (PPD) linkages. It is noteworthy that sBPDA/MPD exhibited a significantly smaller CVE (113.7 ppm/K) than that of sBPDA/PPD despite its lower packing coefficient. This value could be the smallest CVE among solid polymers. Based on the dynamic mechanical analyses and the consideration of stereochemical structures with conformational energy calculations, the local molecular motions of PIs containing MPD are more restricted in the solid state than those with PPD even below their Tgs because π-flip rotation at the m-phenylene linkage is sterically inhibited in the former, while that at the p-phenylene linkage is allowed in the latter.

Spin crossover-induced colossal positive and negative thermal expansion in a nanoporous coordination framework material

External control over the mechanical function of materials is paramount in the development of nanoscale machines. Yet, exploiting changes in atomic behaviour to produce controlled scalable motion is a formidable challenge. Here, we present an ultra-flexible coordination framework material in which a cooperative electronic transition induces an extreme abrupt change in the crystal lattice conformation. This arises due to a change in the preferred coordination character of Fe(II) sites at different spin states, generating scissor-type flexing of the crystal lattice. Diluting the framework with transition-inactive Ni(II) sites disrupts long-range communication of spin state through the lattice, producing a more gradual transition and continuous lattice movement, thus generating colossal positive and negative linear thermal expansion behaviour, with coefficients of thermal expansion an order of magnitude greater than previously reported. This study has wider implications in the development of advanced responsive structures, demonstrating electronic control over mechanical motion.

Anisotropic Linear and Volumetric Thermal‐Expansion Behaviors of Self‐Standing Polyimide Films Analyzed by Thermomechanical Analysis (TMA) and Optical Interferometry

AbstractThe relationships between the molecular structure, polymer chain orientation, β‐transition temperature, and the coefficients of in‐plane and out‐of‐plane linear thermal expansions (CTE, α), as well as the coefficients of volumetric thermal expansion (CVE, β), are extensively examined for ten types of self‐standing aromatic polyimide (PI) films via thermomechanical analysis, near‐infrared optical interferometry, and dynamic mechanical analysis. The out‐of‐plane CTEs (α⊥) of PI films having rigid‐rod structures are 20–40 times larger than their in‐plane CTEs (α//), whereas the α⊥ values of PI films having bent linkages in the main chains are comparable to their α// values. In addition, the values of normalized anisotropy in CTE for PIs (η), defined as (α⊥ −α//)/β, are proportional to the orientation coefficients (P200) estimated from the in‐plane/out‐of‐plane birefringence (Δn) and molecular polarizability tensor. Furthermore, larger CVE values are observed for PIs exhibiting lower β‐relaxation temperatures (Tβ), at which localized molecular motion occurs. In contrast, smaller CVE values are observed for PIs exhibiting higher Tβs because their local molecular motions are suppressed due to the structural rigidity and dense molecular packing.

Evaluation of free volume and anisotropic chain orientation of Tröger's base (TB)-based microporous polyimide/copolyimide membranes

Polarization-dependent refractive indices were utilized to probe the free volume and anisotropic chain orientation of several types of microporous polyimides (PIs)/copolyimides (co-PIs) based on Tröger's base (TB). The properties related to the free volume and chain orientation, such as dielectric constants and linear thermal expansion behaviors, as well as the out-of-plane thermal diffusivity (D⊥), were quantitatively investigated. The free volume fractions (Vf) of 0.183–0.228 were estimated for the membranes based on the Lorentz-Lorenz equation and molecular polarizability calculations. The TB-based PIs/co-PIs exhibited relatively low average refractive indices (1.5208–1.6061), low-to-moderate birefringence (0.0181–0.0505) at 1310 nm, and ultra-low D⊥ values ranging from 6.2 to 9.6 × 10−8 m2/s, depending on the chemical structures and the preparation conditions. Their excellent overall performance suggests their potential applications, for example, in protective sunshades, heat insulation tape, and thermal barrier coatings.

Sulfur tolerant redox stable layered perovskite SrLaFeO4 − δ as anode for solid oxide fuel cells

Redox stable K2NiF4 type layered perovskite SrLaFeO4−δ(SLFO4−δ) has been prepared and evaluated as anode for solid oxide fuel cell (SOFC). The SLFO4−δ shows linear thermal expansion behavior with TEC of 14.3×10−6K−1. It also demonstrates excellent catalytic activity for various fuels. A scandia stabilized zirconia (ScSZ, 180μm) electrolyte supported SOFC with the anode achieves maximum power densities (Pmax) of 0.93, 0.76, 0.63, and 0.46 Wcm−2 at 900–750°C, respectively, in wet H2. Pmaxs of cells supported by 250μm ScSZ reach 0.57, 0.60 and 0.50 Wcm−2 in H2, H2+50ppm H2S and propane, respectively, at 800°C. Moreover, the cells show stable power output during ~100h operation at 800°C under 0.7V in various fuels. The Pmax at 800°C in wet H2 even increases by ~11% in the subsequent two thermal cyclings, indicating that SLFO4−δ is a promising anode candidate for SOFC with good electro-catalytic activity, high stability and resistance to sulfur and coking.

Thermal expansion accompanying the glass-liquid transition and crystallization

We report the linear thermal expansion behaviors of a Zr-based (Vitreloy 1) bulk metallic glass in its as-cast, annealed and crystallized states. Accompanying the glass-liquid transition, the as-cast Vitreloy 1 shows a continuous decrease in the thermal expansivity, whereas the annealed glass shows a sudden increase. The crystallized Vitreloy 1 exhibits an almost unchanged thermal expansivity prior to its melting. Furthermore, it is demonstrated that the nucleation of crystalline phases can induce a significant thermal shrinkage of the supercooled liquid, but with the growth of these nuclei, the thermal expansion again dominates. These results are explained in the framework of the potential energy landscape, advocating that the configurational and vibrational contributions to the thermal expansion of the glass depend on both, structure and temperature.

Dependence of thermal expansion of LaNi0.6Fe0.4O3 − δ and La0.6Sr0.4Co0.2Fe0.8O3 − δ on oxygen partial pressure

The linear thermal expansion behavior of LaNi0.6Fe0.4O3−δ and La0.6Sr0.4Co0.2Fe0.8O3−δ, which have attracted much interest as cathode material for solid oxide fuel cell, was measured with dilatometer under various oxygen partial pressures, P(O2). Thermal expansion coefficient of LaNi0.6Fe0.4O3−δ is independent on P(O2) and between 1.20×10−5 K−1 and 1.35×10−5 K−1 in the measurement conditions of this study. Thermal expansion coefficient of La0.6Sr0.4Co0.2Fe0.8O3−δ is much larger than that of LaNi0.6Fe0.4O3−δ and increases with decreasing P(O2). The difference of P(O2) dependence of thermal expansion behavior between LaNi0.6Fe0.4O3−δ and La0.6Sr0.4Co0.2Fe0.8O3−δ could be ascribed to the difference of the dependence of oxygen nonstoichiometry on temperature and P(O2). Due to little variation of δ by temperature at constant P(O2) in LaNi0.6Fe0.4O3−δ, chemical expansion due to generation of δ scarcely contributes to total expansion, while chemical expansion largely contributes to total expansion of La0.6Sr0.4Co0.2Fe0.8O3−δ especially under low P(O2) owing to larger variation of δ by temperature at constant P(O2). By comparison with thermal expansion behavior of 8mol% yttria stabilized ZrO2, 8YSZ, under various P(O2), it was concluded that mechanical compatibility of LaNi0.6Fe0.4O3−δ against 8YSZ was better since La0.6Sr0.4Co0.2Fe0.8O3−δ showed not only much larger thermal expansion coefficient than 8YSZ but also large P(O2) dependence. Chemical expansion coefficients, ɛchem., of LNF64 and LSCF6428 were also evaluated. The ɛchem. of LNF64 was almost zero whereas those of LSCF6428 were order of 10−2, showing fair agreement with so far reported ɛchem. of LSCF and other oxide materials frequently used for SOFC.

Investigating the effect of thermal gradients on stress in solid oxide fuel cell anodes using combined synchrotron radiation and thermal imaging

Thermal gradients can arise within solid oxide fuel cells (SOFCs) due to start-up and shut-down, non-uniform gas distribution, fast cycling and operation under internal reforming conditions. Here, the effects of operationally relevant thermal gradients on Ni/YSZ SOFC anode half cells are investigated using combined synchrotron X-ray diffraction and thermal imaging. The combination of these techniques has identified significant deviation from linear thermal expansion behaviour in a sample exposed to a one dimensional thermal gradient. Stress gradients are identified along isothermal regions due to the presence of a proximate thermal gradient, with tensile stress deviations of up to 75 MPa being observed across the sample at a constant temperature. Significant strain is also observed due to the presence of thermal gradients when compared to work carried out at isothermal conditions.

Thermal Expansion in Alkane Diacids—Another Property Showing Alternation in an Odd–Even Series

The thermal expansion for a series of aliphatic diacids, HOOC(CH2)n−2COOH (n = 2–10), has been studied. Even (n = even) acids exhibit higher melting points and corresponding lower volumetric thermal expansions compared to the odd (n = odd) acids. This behavior seems to be in accordance to the Lindemann’s correlation of melting point with volumetric thermal expansion. The trend of alternation is also reflected in the linear thermal expansion behavior of the series. A trend of higher thermal expansion anisotropy is observed for the odd members in the series compared to the even members.

Glycine nitrate process로 합성된 La0.8Sr0.2Ga0.8Mg0.1Co0.1O3-δ전해질의 제조 및 특성평가

Conductivity of LSGMC materials were affected by secondary phase segregation, composition and synthetic route. (LSGMC) powders were prepared using the glycine nitrate process to produce high surface area and compositionally homogeneous powders. The powders were synthesized with different 0.5, 1, 1.5, 2, 2.5 of glycine/cation molar ratios. A single perovskite phase from the synthesized powders was characterized with X-ray diffraction patterns. The obtained sintered pellets showed the dense grain microstructure. In case of 1.5 molar ratio, its density was higher than the others. The electrical conductivity measured at was observed to be 0.131 . In addition, the linear thermal expansion behavior was indicated between and .