Coefficient of thermal expansion of bioactive glasses: Available literature data and analytical equation estimates

Bioactive glasses are able to develop a tenacious bond with human bone tissues and therefore they are largely used in orthopaedic and dental implants. However, due to their brittleness, they are mainly applied as coatings on tough substrates, such as titanium, alumina and zirconia. The reliability of bioactive glass coatings is deeply influenced by their thermodilatometric compatibility with the substrate, which may govern the development of dangerous thermal stresses at the interface. In spite of the technological relevance of the coefficient of thermal expansion (CTE) of bioactive glasses, few papers are specifically dedicated to such topic. In the present contribution, more than 70 bioactive glasses were reviewed in the literature, in order to investigate the relation existing between their composition and their CTE. Then four analytical models were applied to estimate the CTE of the same glasses and the calculated values were compared to the experimental results, in order to assess the reliability of the models and define an effective tool to predict the CTE. In particular, on the basis of the literature data and calculated values, the effect of modifier oxides and intermediate oxides, such as K₂O and MgO, on the CTE was discussed.     

Statistical data on thermal expansion in dinas refractories for coking ovens

Conclusions Statistical data have been obtained on the temperature dependence of the thermal expansion for dinas refractories for coking ovens in the range from 20 to 1400°C. The temperature coefficient of linear expansion does not vary monotonically as the temperature rises and has peaks near the temperatures of the phase transformations in tridimite, cristobalite, and quartz, at which the values attained are 50·10^–6, 80·10^–6 and 10·10^–6 K^–1 correspondingly. The averaged TCLE should be considered as standard reference ones. The thermalexpansion curves for dinas brick show hysteresis due to the phase transitions in silica.Translated from Ogneupory, No. 5, pp. 30–33, May, 1987.     

Algorithm for calculating the thermal expansion tensor and constructing the thermal expansion diagram for crystals

A new program package based on the Microsoft Windows operational systems is proposed for thermal investigations of crystals. The package consists of two programs, one of which provides a means for calculating the thermal expansion tensor for crystals of any symmetry and the second program is intended for drawing three-dimensional thermal expansion diagrams. The calculation and drawing procedures are described.     

Control of coefficient of thermal expansion in elastomers using boron nitride

The thermal expansion properties of three commercial elastomers; Pebax®, Estane® and Hytrel® modified with 2.5–10 wt % boron nitride were investigated. The glass transition temperatures of the filled materials were relatively unaffected; however boron nitride did effectively reinforce all the three elastomers as seen by dynamic mechanical analysis and tensile tests. The coefficients of thermal expansion of the composite materials do not obey the rule of mixtures and show a large decrease without the loss of ductility typically associated with filled elastomers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5153–5161, 2006     

Determination of non-spherical particle terminal velocity using particulate expansion data

A new method is proposed for the determination of the terminal velocity of non-spherical particles and compared with experimental data. The method is based on particulate expansion data of fluidized bed and variational model for calculating fluid–particle interphase drag coefficient. Other methods require knowledge of the particle shape, a parameter that is not easy to obtain for real materials. We use pressure drops data in packed bed for indirect determination of particle shape factor which depends on the reliability of coefficients in the Ergun equation. Our data, however, show that these coefficients are system-specific. The proposed method for the determination of non-spherical particle settling velocity in liquid as well as extrapolation to system gas-particles gives results which are in good agreement with experimental data. The method is restricted to particles which can be fluidized particulately by liquid.     

Transverse and longitudinal coefficient of thermal expansion of carbon fibers at high temperatures (300-2500 K)

The transverse and longitudinal coefficients of thermal expansion (CTE) of carbon fibers are important parameters in C/C composites behaviour. In this study, the measurements of CTE are presented at very high temperatures (up to 2500 K). Experimental validations are first realised on isotropic metallic (tungsten) fiber. Then, measurements are performed on four carbon fibers (rayon-based, PAN-based and pitch-based). The results show that the mean transverse CTE varies from 5 × 10⁻⁶ K⁻¹ to 10 × 10⁻⁶ K⁻¹ and the longitudinal one from 1.6 × 10⁻⁶ K⁻¹ to 2.1 × 10⁻⁶ K⁻¹ depending on the fiber structural properties. For that, the relationships between the structural properties and the heat treatment temperature of the carbon fibers are discussed in relation to thermal expansion behaviour. A calculation of the ratio between the transverse and the longitudinal measured CTE as a function of the temperature allows us to classify fibers from the most isotropic to the most anisotropic in relation with the structural properties measured at room temperature.     

Analysis of thermal degradation for plastic optical fibers

Attenuation loss of plastic optical fibers (POFs) gradually increases with long use at high temperatures. We separated attenuation loss of the POFs before and after heating at 150°C in air into four loss factors, and identified the main factor for attenuation loss increase as electronic transition absorption loss (α_e). The increase of α_e was caused by a thermal oxidation reaction of the core polymer in the POF. But, according to NMR and FT-IR spectra, elementary analysis, etc., hardly any reaction products were detected. We assume that the reaction products were only a small amount of the conjugated carbonyl groups. So we fabricated POFs containing model compounds with conjugated carbonyl groups, and measured their attenuation loss. The attenuation loss spectra of these POFs were similar to those of thermally degraded POFs. Consequently, a very small amount of conjugated carbonyl groups were formed by the thermal oxidation reaction of core polymer so that α_e increased to an unacceptable level.     

Low thermal expansion porous SiC–WC composite ceramics

Low thermal expansion porous SiC–WC composite ceramics were prepared by solid state reaction of Si and WC at 1560 °C, with NH₄HCO₃ as a pore generating agent. Phase composition, thermal expansion, flexural strength, and microstructure of the carbide ceramics were examined. Presence of the SiC, WC and WC_1−X phases were detected in the carbide ceramics. As Si content increased from 2 to 14 wt%, the coefficient of thermal expansion first decreased and then increased, with a minimum of 4.11 × 10⁻⁶ °C at 8 wt% Si, whereas the flexural strength decreased gradually, from 143.9 to 82.7 MPa. Pores of SiC–WC ceramics were less than 2 μm in diameter, because of the stacking interstice of carbide particles and volatilization of silicon. However in the presence of NH₄HCO₃, pores of SiC–WC ceramics were bimodally distributed, the stacking interstice of carbide particles loosened from 1 to 4 μm and pores larger than 5 μm were also formed.     

Rubber‐toughened PLA blends with low thermal expansion

In this study, poly (lactic acid) (PLA) blended with various rubber components, i.e., poly (ethylene‐glycidyl methacrylate) (EGMA), maleic anhydride grafted poly(styrene‐ethylene/butylene‐styrene) triblock elastomer (m‐SEBS), and poly(ethylene‐co‐octene) (EOR), was investigated. It was observed that EGMA is highly compatible due to its reaction with PLA. m‐SEBS is less compatible with PLA and EOR is incompatible with PLA. Electron microscopy (SEM and TEM) revealed that a fine co‐continuous microlayer structure is formed in the injection‐molded PLA/EGMA blends. This leads to polymer blends with high toughness and very low linear thermal expansion both in the flow direction and in the transverse direction. The microlayer thickness of rubber in PLA blends was found to play key roles in reducing the linear thermal expansion and achieving high toughness of the blends. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Graphite thermal expansion relationship for different temperature ranges

For practical reasons, the coefficient of thermal expansion (CTE) has to be measured over a particular temperature range, for example 20-120 °C, 20-400 °C or 500-700 °C. However, in many cases, engineers or scientists involved in the assessment of graphite components such as nuclear reactor moderator bricks, electrodes or moulds require CTE over temperature ranges other than that of the original measurement. This paper compares three different techniques used to convert CTE from one temperature range to another. The method used by the UK nuclear industry is compared with techniques proposed by two international companies. There was close agreement between two of the methods. However there was some divergence in the case of the third method. This may be related to the type of graphite (fine-grain) for which the third method was developed.     

Understanding thermal expansion of pressurized silica glass using topological pruning of ring structures

We investigate the structural origin of the large thermal expansion coefficient of hot-compressed silica glass upon heating to a threshold temperature using molecular dynamics (MD) simulations. While the simulated thermal expansion at low temperature correlates well with the elongation of the average interatomic separation distance of the Si–O or Si–Si pair, the excess thermal expansion due to hot compression mainly results from change in medium range order, including a decrease in the population of large rings upon heating, without significant modification of the small ring population and without changing the short range ordering, such as the Si-O coordination number. The reduction in the characteristic ring size can be connected to the high thermal expansion through the topological pruning mechanism. Such large thermal expansion is suppressed when the silica glasses are held at their respective quench pressures. The suppression of this excess thermal expansion is consistent with the pressure stabilization of the large ring structures.     

Solution‐processable colorless polyimides with ultralow coefficients of thermal expansion for optoelectronic applications

This work reports colorless polyimides (PIs) that are applicable as plastic substrates in image display devices, which produce an ultralow coefficient of thermal expansion (CTE) by solution casting without thermal imidization and mechanical stretching. An effective monomer molecular design is proposed for this purpose. Chemical imidization (CI) process compatibility is the key factor in attaining the target properties. We focused on a PI system derived from 1,2,3,4‐cyclobutanetetracarboxylic dianhydride (CBDA) and a novel para‐amide‐linked diamine (AB‐TFMB) with CF₃ groups as it has great potential as an ultralow CTE material, although it offers no CI process compatibility because of its poor solubility. The CBDA/AB‐TFMB system was modified by copolymerizing with 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride. This approach drastically improved CI process compatibility. The CTE of the PI films linearly decreased with increasing CBDA content. At a CBDA content of 70 mol%, the PI achieved an ultralow CTE of 7.3 ppm K^?1, non‐coloration/non‐turbidity, a very high glass transition temperature of 329 °C and sufficient ductility. The ultralow CTE results from the highly oriented main chains along the X–Y direction during the casting process as supported by the very high birefringence exceeding 0.1. Thus, our materials almost achieved the target properties required for novel coating‐type high‐temperature plastic substrate materials. © 2016 Society of Chemical Industry     

Study of the thermal behavior of alkali‐treated jute fibers

Jute fibers were treated with 5% NaOH solution for 2, 4, 6, and 8 h to study the performance of the fibers as a reinforcing material in the composites. Thermal analysis of the fibers was done by the DTG and DSC technique. The moisture desorption was observed at a lower temperature in the case of all the treated fibers, which might be a result of the increased fineness of the fibers, which provides more surface area for moisture evaporation. The decrease in percentage moisture loss for the fibers treated with alkali for 6 and 8 h could be the result of the increased crystallinity of the fibers. The percentage degradation of the hemicellulose decreased considerably in all the treated fibers, conforming to the fact that the hemicellulose content was lowered on alkali treatment. The decomposition temperature for α‐cellulose was lowered to 348°C from 362.2°C for all the treated fibers, and the residual char formation increased to a significant extent. The enthalpy for the thermal degradation of α‐cellulose showed a decreasing trend for the fibers treated for 2 and 4 h, which could be caused by the initial loosening of the structure, followed by an increase in the enthalpy value in the case of the 6‐ and 8‐h‐alkali‐treated fibers resulting from increased crystallinity, as evident from the X‐ray diffraction. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2594–2599, 2002     

Thickness-direction thermal expansion of polyimide films

The coefficient of thermal expansion (CTE) for thin polymer films is a property of considerable practical importance. In contrast to the abundance of CTE data in the in-plane or film direction, data in the thickness or z-direction is rarely available, in part due to the high sensitivity required for measurement of the small thickness changes involved. In this paper, we will describe both a capacitance change and a Fabry-Perot laser interferometric method for measuring z-direction CTE's. Results are presented for a variety of commercially available polyimide films of thickness 50 to 130 μm. Upilex and Kapton were found to have z-direction CTE's of about 55 and 81 ppm/°C, respectively. These CTE's for the thickness direction are many times higher than the corresponding values in the in-plane direction. This anisotropy is attributed to the preferential in-plane molecular orientation of the polymer chains.     

Opto‐thermal properties of fibers: XVII. Structure characterization of cold drawn boiled viscose fibers

A two‐beam interferometric method is used to study the change of optical parameters of cold drawn boiled viscose fibers at different times. A stress–strain device conjugated to the Pluta polarizing interference microscope is used to investigate the dynamical behavior of opto‐mechanical properties at room temperature. Some structural parameters such as the number of molecules per unit volume, the virtual and isotropic refractive indices, the optical orientation factor and angle, the strain and the stress optical coefficients are evaluated. Some mechanical parameters such as, Young's modules, elastic shear modules and the compressibility are calculated over different strain values. Also the number of network chain per unit volume N_c, work per unit volume W, average work per chains W′, reduction in entropy ΔS, and the optical configuration parameter Δα are determined. Also calculation of the constants of Moony–Rivilin equation are given. A systematic study over different wavelengths range extending from 405 to 590 nm was carried out to obtain the dielectric constant at infinity. The obtained results clarify that new reorientations occurred due to cold drawing at different conditions. Empirical formula is suggested to correlate the change in Δn, f(θ), W, W′, A, θ, and N_c with different draw ratio, and its constants were determined. Microinterferograms and curves are given for illustrations. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 1185–1201, 1999     

Tuning stoichiometry of high-entropy oxides for tailorable thermal expansion coefficients and low thermal conductivity

Thermal barrier coating materials with proper thermal expansion coefficient (TEC), low thermal conductivity, and good high-temperature stability are of great significance for their applications in next-generation turbine engines. Herein, we report a new class of high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x with different Ce⁴⁺ contents synthesized by a solid-state reaction method. They exhibit different crystal structures at different Ce⁴⁺ content, including a bixbyite single phase without Ce⁴⁺ doping (x = 0), bixbyite-fluorite dual-phase in the RE₂O₃-rich region (0 < x < 2), and fluorite single phase in the stoichiometric (x = 2) and CeO₂-rich region (x > 2). The high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x exhibit tailorable TECs at a large range of 9.04 × 10^–6–13.12 × 10^–6 °C^–1 and engineered low thermal conductivity of 1.79–2.63 W·m^–1·K^–1. They also possess good sintering resistance and high-temperature phase stability. These results reveal that the high-entropy (La_0.2Sm_0.2Er_0.2Yb_0.2Y_0.2)₂Ce_xO_3+2x are promising candidates for thermal barrier coating materials as well as thermally insulating materials and refractories.     

Preparation of form-stable silica/polyethylene glycol composites using flash-drying for large-scale melt-spun fibers with thermal management property

The combination of phase change materials (PCMs) with fibers can afford smart fibers with thermal management properties. However, the issues of easy leakage and poor thermal stability of PCMs often limit their use in high-temperature spinning. Herein, we report a form-stable PCM of spherical SiO₂/PEG composite that was prepared through flash-drying using inorganic dendritic silica (D-SiO₂) as the core skeleton to support organic polyethylene glycol (PEG). The SiO₂/PEG composite not only presents high crystallization enthalpy (101.35 J/g), but also maintains a superior phase change stability. Meanwhile, it exhibits a significant temperature hysteresis effect during heating and cooling, and the endothermic and exothermic time are 381.95 and 293.57 s, respectively. Because the degradation temperature of 300°C for SiO₂/PEG is higher than the melt processing temperature of 240–270°C for the preparation of polyamide 6 (PA6) fibers, PA6/SiO₂/PEG fibers were prepared using melt spinning. The prepared PA6/SiO₂/PEG fibers exhibit high latent heat (17.14 J/g), outstanding thermal cycling stability and satisfactory temperature adjustment properties, and the temperature-adjustment time of 458.97 s and temperature difference of 10.68°C under the thermal environment. Moreover, the tensile strength of PCFs-20% reached 1.97 cN/dtex after drawing, which make PCFs meet the requirements of uses in textile industries.     

Thermal expansion of polyurethane reinforced with continuous glass fibers

This paper is concerned with the study of the thermal expansion properties of polyurethanes which are reinforced with randomly oriented continuous glass fibers. The thermal expansion of reinforced polymers is of prime importance since it has a significiant effect on the overall dimensional stability of molded products. In this work the coefficient of linear thermal expansion (CLTE) is measured for both soft and rigid polyurethane containing various amounts of glass fibers. The experimental results show that: (1) there exists a maximum in the CLTE-temperature curves, the position of which depends on the type of resin and not on the glass content; (2) over the range of 0 to +70°C for rigid polyurethane, and –30 to +70°C for soft polyurethane, the CLTE decreases with temperature. The existence of a maximum in the above findings is confirmed with computations from a theoretical expression which was modified by the introduction of an efficiency factor. In applying this expression, parameters such as the Poison's ratios, elastic moduli, and the CLTe's of the resins had to be evaluated at various temperatures. Based on these results, the CLTE-temperature curves were obtained and found to be in agreement with the experimental observations.