Low Coefficient Of Thermal Expansion Research Articles

Design of Unconventional Polymers with Thermal Contraction Units for High-Performance Dental Adhesives.

The coefficient of thermal expansion (CTE) of adhesives is considered to be a vital parameter for dental restoration due to the oral temperature fluctuation induced by hot and cold drinks in daily life. Even more challenging, the adhesives need to bond various materials with different CTE values, and mismatched thermal expansion at the interface of two materials will produce thermal stress and cause cracking, leading to bond failure. In this study, we designed and synthesized a divinyl monomer containing a dibenzocyclooctadiene (DBCOD) unit, which was incorporated into a commercial adhesive, Single Bond Universal (SBU) to prepare low CTE adhesives. The CTE value of the adhesives can be adjusted low to 6.5 ppm/K, which is much lower than that of pure SBU. Mimicking the real applying conditions, the composite resin columns were bonded to the zirconia ceramics as a dental crown with our designed adhesives, and the shear bond strength test was carried out before and after 10 000 thermal cycles between 5 and 55 °C. The shear bond strength of pure SBU retains 44.7% of its original value after 10 000 thermal cycles, while those low CTE adhesives retain 74.6% and 61.9% of shear bond strength due to less deformation and interfacial stress during thermocycling. The newly designed adhesives provide a persistent way to enhance the shear bond strength and achieve a long lifetime in tooth restoration.

High performance ultrasonic vibration assisted Wire-arc directed energy deposition of Invar alloy

As the utilization of composite materials flourishes in commercial aircraft, the use of Invar alloy with low coefficient of thermal expansion (CTE) for fabricating composite molds has gained prominence. The large-size composite molds can be fabricated by Wire-arc Directed Energy Deposition (Wire-arc DED), due to its high deposition efficiency and ability to form optimized topologies. However, the Wire-arc DED Invar components still exhibit heterogeneous microstructure, low strength and non-tunable CTE. To address these challenges, this study explores the integration of an ultrasonic energy field during the deposition process to systematically investigate its effects on microstructural evolution, mechanical properties and CTE of Invar alloy. The results reveal that the ultrasonic vibration-assisted deposition significantly refines the grain structure, resulting in a decrease of 81.04 % in grain size compared to the reference state. Moreover, the components fabricated by ultrasonic vibration assisted Wire-arc DED exhibit exceptional properties, with a yield strength (YS) of 408 ± 11.37 MPa, ultimate tensile strength (UTS) of 645 ± 7.61 MPa, and elongation (EL) of 31.3 %. Additionally, the correlation between grain size and CTE was established. The effectiveness of introducing an ultrasonic energy field in improving the mechanical properties and modulating the CTE of the components is further validated by rigorous theoretical calculations. This research provides a promising way to fabricate high performance Invar alloy composite molds.

Design, synthesis, and characterization of intrinsic black polyimide with complete absorption of visible light and low coefficient of thermal expansion

Black polyimides (BPIs) show increasing demand in the microelectronics and optoelectronics fields. However, the existing BPIs have drawbacks of poor masking ability, dimensional stability, thermal and electrical properties. 3,6-Bis(thiophen-2-yl)diketopyrrolopyrroles (TDPP) is a typical dye that possesses a high molar extinction coefficient. In the work, a new diamine (DPPTOMePDA) bearing TDPP connected with methoxyl-substituted benzene rings was synthesized. Based on such diamine and pyromellitic dianhydride (PMDA), a high-performance intrinsic BPI (DPPTOMePPI) was obtained. The resulting DPPTOMePPI exhibited a red-shifted and broadened absorption spectrum, thus achieving complete absorption of visible light and highly black color. Its cut-off wavelength (λcut) was up to 734 nm, and the Commission International de l’Eclairage (CIE)-Lab coordinate L* decreased to 0.53. Furthermore, DPPTOMePPI exhibited excellent thermal and electrical performances with a low coefficient of thermal expansion (CTE). Density functional theory calculation was executed to analyze the underlying electron transitions of DPPTOMePPI. The results showed that the strong light absorption of DPPTOMePPI was primarily ascribed to the electronic transition of HOMO to LUMO+1 that occurred in the chromophores units. The flexible copper clad laminates (FCCLs) derived from DPPTOMePPI film showed high peel strength and resistance to solder heat. The work offers a theoretical guide for the construction of intrinsic black PI possessing complete visible absorption and low CTE.

Heat transfer characteristic of an alumina oscillating heat pipe

Oscillating heat pipes (OHP) are a promising and highly efficient heat transfer technology especially in the field of power electronics thermal management. However, conventional metal OHP do not meet the need for insulation or low coefficients of thermal expansion. In order to expand the application field and break the application limitation of the OHP technology, alumina ceramic oscillating heat pipes was designed and investigated in this paper. The alumina OHPs combine the excellent insulating, oxidation-resistant, corrosion-resistant and low thermal expansion properties of alumina ceramic with the efficient heat exchange mechanism of oscillating heat pipe technology, exhibiting great application potential. An alumina flat-plate oscillating heat pipe was prepared, 6 internal microchannels were examined using micro-CT, and the microstructure was characterized using scanning electron microscopy. The heat transfer characteristics of the OHP charged with water were investigated using a heat transfer test system. Increasing heat loads were employed to the OHP, and the operating temperature of 20 °C and 60 °C as well as the orientation varied between 0° and 90° were also introduced. The oscillation motion of the alumina OHPs were investigated, including temperature oscillations before and after startup of the OHPs, and a comparison effective thermal conductivities at various tilt angles and operating temperatures was carried out. The experimental results revealed that the heat transfer performance of the alumina oscillating heat pipe significantly increased with the increased heat loads and reached to 861.7 W/(m∙K) at 56 W. The effective thermal conductivity of the OHP was 6 times higher than that of the empty OHP at heat load of 16 W. When the orientation was 30°, the temperature oscillation instability increased, the oscillation amplitude increased, and the heat transfer performance of the oscillating heat pipe decreased. As the operating temperature increased, the stability of temperature oscillations improved, the amplitude decreased, and heat transfer performance was enhanced.

Influence of reinforcement with multimodal distribution on thermophysical properties and fracture behavior in Al-Si/SiC composites infiltrated under super-gravity field

Super-gravity field assisted infiltration is reported as an effective method to obtain the nearly fully-dense Al-Si/SiC composites at relatively lower temperature, which inhibit the interfacial reaction and result in the enhancement of thermal conductivity (TC) and bending strength (BS). Currently, SiC reinforcement with multimodal distribution was designed to further enhance the TC and BS with lower coefficient of thermal expansion (CTE) of Al-Si/SiC composites. For the composites with monomodal SiC distribution, enlarging the size of SiC results in an enhancement of TC with the sacrifice of BS. Interestingly, for the composites with multimodal SiC distribution, the TC can be stabilized within an extended range (175-187W/m·K at 373K) with reduced CTE and higher BS. When the large to small size ratio was 1:2, the optimal comprehensive properties (CTE: 8.66×10-6K-1 ranged from 323 to 373K, TC: 179.2W/m·K at 373K, BS: 309 ± 9MPa) can be obtained. Hasselman-Johnson, Turner, and Kerner models are established to better understand the thermophysical properties, while the fracture behavior is discussed based on the track of crack propagation.

Sm2Gd0.25Dy0.25Er0.25Yb0.25TaO7: A novel candidate thermal insulation oxide for high-temperature coatings

A novel high-entropy oxide, Sm2Gd0.25Dy0.25Er0.25Yb0.25TaO7, was synthesized via Sol-Gel and high-temperature calcining method. The powder and bulk specimen for Sm2Gd0.25Dy0.25Er0.25Yb0.25TaO7 were characterized, and its thermophysical performances were measured. The researching conclusions indicate that the synthesized nano-powder is very pure, and the block specimen has a sole pyrochlore structure, densified microstructure, and uniform distribution of elements. The low thermal conductivity is ascribed to phonon scattering stemmed from the ionic size and atomic mass disorders derived from multi-component addition. The high lattice-energy leads to a relatively low thermal expansion coefficient, which satisfies the demand of thermal barrier coating. The Sm2Gd0.25Dy0.25Er0.25Yb0.25TaO7 oxide also maintains outstanding phase steadiness up to 1200 °C.

Reinforcement mechanisms and current research status of silicon carbide whisker-reinforced composites: A comprehensive review

Abstract In recent decades, with the advancement of micro-electro-mechanical systems technology, traditional materials have become insufficient to meet the demands of cutting-edge technology for various material properties. Composites have attracted widespread attention as an effective and viable solution. Silicon carbide whiskers (SiCw) have emerged as excellent reinforcements due to their high thermal conductivity, low thermal expansion coefficient, high melting point, superior mechanical properties, and high chemical stability. This article provides a comprehensive review of the reinforcement mechanisms and current research state of SiCw-reinforced composites. The reinforcement mechanisms include mainly grain refinement, load transfer, and crack bridging. The composites are categorized based on the type of the matrix: ceramic-based, metal-based, and polymer-based composites. The influence and parameter performance of the reinforcement mechanism on SiCw-reinforced composite materials with different matrices vary. However, the key to improving SiCw-reinforced composites lies in understanding the interplay of properties between the matrix and the reinforcement, as well as the ordered and regular distribution and binding at the interface. Finally, the current state and limitations of SiCw-reinforced composites are summarized, and future development trends are discussed. This article represents a great contribution to the future applications of SiCw-reinforced composite materials.

Enhanced thermophysical and mechanical properties of gadolinium zirconate ceramics via non-stoichiometric design

Rare-earth zirconate ceramics are potential materials for thermal barrier coatings due to their low thermal conductivity and excellent structural stability. However, the low thermal expansion coefficient and fracture toughness limit their application and further improvements are needed. In this work, non-stoichiometric gadolinium zirconate ceramics with fluorite phase structure were successfully prepared using solid-state reaction method. HRTEM, iDPC-STEM, and in-situ XRD were employed to investigate nanostructure, lattice defects, and phase stability. The non-stoichiometric ratio introduces more defects and forms special nanostructures, enhancing the thermophysical properties of gadolinium zirconate ceramics. The non-stoichiometric gadolinium zirconate ceramics have lower phonon thermal conductivity with higher hardness and fracture toughness compared to stoichiometric ratio ceramics. In addition, the non-stoichiometric gadolinium zirconate ceramics exhibit excellent structural stability in a wide temperature range. These performances provide further applications for non-stoichiometric gadolinium zirconate ceramics as thermal barrier coatings.

High-frequency 5G substrate: Low dielectric biphenyl polyimide with low CTE and high thermal stability

Polyimide (PI) faces new challenges in meeting the requirements of low coefficient of thermal expansion (CTE), low dielectric content/loss, and high thermal stability to be utilized effectively as a substrate for flexible electronic materials. At present research, the CTE of PI films can be reduced to around 5 ppm/K, with Tg > 420 °C. However, it is difficult to balance low CTE and high Tg at the same time, and reducing dielectric performance is even more difficult. This study focused on synthesizing four types of PI films (using BPDA as the dianhydride) by employing biphenyl structure diamines (PDA, BZD, DPT, DMP) with an increasing number of benzene rings. The improvement in CTE, dielectric properties, and thermal stability was observed with an increase in the number of benzene rings and a decrease in the number of imine rings in the PI horizontal structure. The Tetraphenylene PI-DMP exhibited values of ε and tanθ as 3.31 and 4.73 ‰ respectively under 10 GHz. Additionally, the biphenyl PI films demonstrated commendable thermal stability (Tg = 442°C), outstanding mechanical properties (elastic modulus >10 GPa), and a low CTE (2 ppm/K within the temperature range of 50–300 °C, and close to the 3.6 ppm/K CTE of monocrystalline silicon). The low CTE, high thermal stability, and low dielectric properties that 5G flexible substrate materials need to be qualified are simultaneously reflected in this study. MS theoretical calculations were used to analyze the results. The excellent consistency with experimental results can promote the feasibility of MS theory in PI dielectric properties at high-frequency (10 Ghz). This innovative approach is anticipated to provide foldable chip fields with intrinsic biphenyl PI materials possessing low dielectric, low CTE, and high thermal stability.

Review of recent research on thermal management of diamond/metal composites

Diamond/metal composites have attracted much attention due to their excellent thermal conductivity (TC) and low coefficient of thermal expansion (CTE). This paper introduces two significant interface modification means which can improve interface bonding between diamond and metal. And the key factors affecting the TC of composite are analysed, making the thermal management of diamond/metal composites more efficient.

Research on the microstructure and properties of SiCp-SiO2/6061Al composites based on ball milling time

In this study, SiCp-SiO2/6061Al composites were synthesized by integrating SiC and SiO2 particulates into a 6061Al matrix, followed by ball milling and hot isostatic pressing sintering. The impact of ball milling duration on microstructure, mechanical properties, corrosion resistance, and thermal properties was evaluated. After 10 h of ball milling, reinforcement particles were uniformly dispersed, achieving a material density of over 98 %. Extending milling to 15 h enhanced tensile strength to 253.53 MPa and flexural strength to 433.56 MPa, with a maximum elongation at break of 6.4 %. Uniform SiO2 distribution improved corrosion resistance, evidenced by a self-corrosion potential of −1209 mV and a self-corrosion current density of 53.02 µA·cm−2. After 20 h, the lowest thermal expansion coefficient, 19.87×10−6·K−1, was achieved. However, agglomerated particles after 5 h resulted in inferior thermal properties, including a thermal expansion coefficient of 20.45×10−6·K−1 and thermal conductivity of 94 W·m−1·K−1. The optimal ball milling duration for these composites was determined to be 10 h, balancing performance and cost-effectiveness.

Influence of varying matrix/fiber concentration on mechanical properties of bi-directional carbon fiber reinforced polymer composite

The polymer composites comprising bi-directional carbon fabric incorporated in the epoxy matrix have been developed for various advanced applications in the field of automobile and aerospace sectors. The distinct advantages of using epoxy material with hardener are good adhesion, high tensile, compression and bending strengths, good corrosion, and heat resistance properties. Using carbon fiber enhances the stiffness, reduces the weight, and improves the chemical resistance apart from possessing high thermal conductivity and low coefficient of thermal expansion. Considering all the above aspects, the present work proposes to investigate the mechanical properties of carbon-reinforced epoxy composites with fiber matrix ratios of 40:60, 50:50, and 60:40 by wt% as these particular combinations have been limitedly addressed so far. The well-known hand lay-up technique has produced the laminates and the test samples prepared from the laminate are subjected to tensile, flexural, hardness, and impact strength properties. Based on the experimental work of the composites made in the laboratory, the fiber matrix ratio of 60:40 has revealed the best attributes in terms of higher mechanical strengths compared to the composites with fiber matrix ratio of 40:60 and 50:50 and the data obtained have been substantiated using scanning electron microscope analysis.

Discovery of a Ni-based superalloy with low thermal expansion via machine learning

Traditional Ni-based superalloys are widely used due to their excellent high-temperature performance; however, their high thermal expansion at elevated temperatures limits their further application in the modern aerospace industry. To discover Ni-based superalloys with low thermal expansion for high-temperature environments, such as 900 °C. In this study, we developed a Support Vector Regression (SVR) model with a high coefficient of determination (R2 = 0.84) to predict the coefficient of thermal expansion (CTE) in Ni-based superalloys. Ten samples were selected and produced from a pool of candidates generated via genetic algorithm and the SVR model, revealing that seven of them exhibited superior CTE performance. Notably, the discovered alloy with the lowest CTE (11.0 e-6 °C-1 from RT to 900 °C) presented a remarkable 16.7 % reduction compared to the best performance in existing database. The finally selected alloy demonstrated both low thermal expansion and superior mechanical properties compared to traditional superalloys with low thermal expansion.

A cost-effective way to utilize low-grade Ti-rich clay: Preparing cordierite-mullite composite ceramics for thermal storage

Cordierite-mullite composite ceramics were prepared by solid-state method using Panzhihua low-grade Ti-rich clay, talc, and alumina. Influences of cordierite/mullite ratio, sintering temperature, and the chemical composition of clay on the phase composition, microstructure, physical properties, and thermal properties were investigated in detail. The results showed that the impurity oxides inside the Ti-rich clay had a great ability of forming liquid. Above 1300 °C, the reaction between TiO2 of the clay and Al2O3 produced stable Al2TiO5, which was conducive to decreasing the coefficient of thermal expansion (CTE). At 1300–1350 °C, the secondary mullitization caused the volumetric expansion, decreasing the bending strength and increasing the CTE. The optimum ratio of cordierite to mullite ranged from 90:10 to 80:20, and the sample fired at 1400 °C exhibited the lowest water absorption and porosity (0.64 % and 1.56 %, respectively), optimal bulk density, and bending strength (2.59 g cm−3 and 87.79 MPa), as well as better CTE, thermal conductivity, and thermal storage density (2.91 × 10−6 °C−1, 4.11 W/(m⋅K) (30 °C), and 1181 kJ kg−1, respectively). Compared with the reported values of cordierite-based thermal storage ceramics prepared by using the high-grade clay or minerals, the obtained samples showed the advantages such as lower costs for raw materials, higher bending strength, higher thermal conductivity (1.63–2 times higher), low CTE, and high thermal storage density. This study demonstrated that the low-grade Ti/Fe-rich clay has excellent cost-effective potential for the preparation of thermal storage ceramics.

Using carbon fiber tape to tailor the coefficient of thermal expansion in 3D-Printed composite tooling

In this work, we investigated the effect of plain woven carbon fiber tape embedded in each layer of an additively manufactured part on the coefficient of thermal expansion (CTE) and compared it to conventionally printed parts. Current advancements in Additive Manufacturing enable cost-efficient 3D printing of composite tools. However, these tools do not yet offer a low CTE comparable to Invar, necessary for producing aerospace-quality composite parts. Using the novel Advanced Tape Layer Additive Manufacturing process, the tape is placed on top of the bead immediately after extruding the short fiber-reinforced material. The samples are compared to Material Extrusion specimens from a Large Format Additive Manufacturing System. A lower CTE was achieved within the printing plane. Micro-computed tomography images correlate the preferential orientation of short fibers with measured CTE values. The CTE modification can match the part CTE to the tool CTE and therefore optimize the quality of manufactured parts.

Spodumene glass-ceramics with low thermal expansion coefficient prepared using gold tailings

Spodumene glass-ceramics with low thermal expansion coefficient prepared using gold tailings

Poly(ester imide)s with Low Linear Coefficients of Thermal Expansion and Low Water Uptake (VIII): Structure-Flame Retardancy Relationship.

A series of ester-linked tetracarboxylic dianhydrides containing multiple para-phenylene units (TA-pPhs) was synthesized to obtain novel modified polyimides, namely poly(ester imide)s (PEsIs). The flame retardancy and film toughness of PEsIs tended to deteriorate with the structural extension of the repeating units (or monomers) via ester groups. To identify the structural factors necessary for achieving the highest flame retardancy rank (UL-94, V-0), we systematically investigated the structure-property relationships of a series of TA-pPh-based PEsIs. Among them, a PEsI derived from para-quaterphenylene-containing TA-pPh (TA-DPQP) and p-phenylenediamine (p-PDA) exhibited the best property combination, featuring an extremely high glass transition temperature (Tg), very low linear coefficient of thermal expansion (CTE), low water uptake (WA), ultralow linear coefficient of humidity (hygroscopic) expansion (CHE), unexpectedly high film toughness, and excellent flame retardancy (V-0 rank). Moreover, we examined the effects of substituents of TA-pPh and discussed the mode of action for the increased film toughness. This study also investigated the structure-property relationship for a series of PEsIs derived from isomeric naphthalene-containing tetracarboxylic dianhydrides. Some of the PEsIs obtained in this study, such as the TA-DPQP/p-PDA system, hold promise as novel high-temperature dielectric substrates for use in flexible printed circuits.

An experimental and numerical study on the evolution of pores and its effect on the tensile properties of LPBF Invar36 with different energy density

Laser powder bed fusion (LPBF) Invar 36 alloy has attracted plenty of attention in cryogenic components or precision instruments due to its low coefficient of thermal expansion in a wide range of temperature. However, the LPBF Invar 36 has a significant anisotropic mechanical property due to the unique columnar grain morphology and pore feature. In this study, the influence of scanning speed on the anisotropic tensile properties were investigated, and the melt pool dynamics were analyzed by the numerical simulation. In this respect, tensile tests of specimens parallel and perpendicular to the building direction with a scanning speed from 200 to 1400 mm/s of LPBF Invar 36 alloy were conducted, and the microstructural influential factors on the anisotropic tensile properties were analyzed. Results show that as the volumetric energy density (VED) increases, the inhomogeneity of grain morphology decreases, while the pore feature changes from the lack-of-fusion (LOF) pore perpendicular to the building direction to the bubble-shaped spherical pore, which is resulted from the collapse of the key-holes. In this case, the grain orientation and size, as well as the dislocation density in the vicinity of pores are influenced by the pore morphology due to different thermal history. On the whole, the combined morphology of dislocation and stress concentration affected by the pore morphology are the main reason of the increased level of anisotropy in tensile properties.

Multi-optimization for thermal deformation of gravitational wave telescope based on CFRP characteristics

Gravitational wave telescope place extremely high demands on structural thermal deformation, making material selection a critical issue. Carbon fiber reinforced polymer (CFRP) is an ideal choice for the support structure of telescope due to its low coefficient of thermal expansion (CTE) and designable properties. However, current research on the optimization of the CTE of CFRP is scarce, and conventional methods struggle to find layups that meet the requirements. In this paper, an unconventional layup optimization method is proposed to solve this problem. Initially defining the characteristics of the telescope structure and using different layup material for the main and side support rods to minimize thermal deformation. Subsequently, the NSGA-II algorithm is used to optimize the layups which are divided into conventional and unconventional layups. Specimens are then produced from these results and tested to assess the impact of processing errors on practical applications. The results demonstrate that the optimized CFRP meet the CTE requirements and, when applied to the structure, significantly reduces the thermal deformation in the eccentric direction compared to conventional designs. Additionally, a numerical analysis evaluates the effect of ply orientation errors on the performance of unconventional layups, discussing the method's limitations within these contexts.

Fabrication of matrix graphite with a high degree of graphitization for spherical fuel elements by using natural microcrystalline graphite fillers

Matrix graphite is used as a structural material, thermal conductor, moderator, and secondary fission product barrier for fuel elements in high-temperature gas-cooled reactors (HTRs). Due to its high graphitization degree and compressibility, natural flake graphite (NFG) is used as the main filler in traditional A3-3 matrix graphite, whereas artificial graphite (AG), with a lower graphitization degree than NFG, serves as an additive for toughness and gas permeability. Matrix graphite could be improved in terms of thermal conductivity, oxidation resistance, and irradiation performance by increasing the degree of graphitization. However, reports on the development of new matrix graphite formulations are scarce. In this study, MG-20 matrix graphite was prepared by mixing 60 wt% NFG, 20 wt% natural microcrystalline graphite (MG), and 20 wt% phenolic resin. Due to the high graphitization degree (higher than AG) and low coefficient of thermal expansion (CTE) of MG, MG-20 exhibited higher thermal conductivity (∼6%) and lower CTE (∼2.4%) than A3-3. Thus, MG-20 with higher graphitization degree and better thermal properties than A3-3 could improve the performance of HTR fuel elements in the future.