Analysis of the thermal expansion behavior of oriented polyoxymethylene sheets

The applicability of the molecular composite model is extended to include the analysis of the thermal expansion behavior of oriented (anisotropic) polyoxymethylene. The model accounts for changes in the morphological and structural aspects of polyoxymethylene during solid state-induced orientation. The results are reported and analyzed in terms of three-dimensional surfaces. Apart from the effect of temperature, it has been found that the crystalline phase orientation distribution, λ, is the most crucial parameter affecting the thermal expansion behavior of anisotropic polyoxymethylene followed by the volume fraction crystallinity. The size and the geometry of the crystalline phase have negligible effects on the thermal expansion behavior of oriented polyoxymethylene. Above the glass-transition temperature, a minimum in the transverse thermal expansion coefficient is exhibited in the vicinity of λ = 2 for volume fraction crystallinities ≤0.75. It is sufficient to align 75% of the crystallites with their c-axis at ±10° to the draw direction, to attain the ultimate degree of anisotropy in the thermal expansion behavior of oriented polyoxymethylene sheets.     

Tunable Linear Thermal Expansion Coefficient of Amorphous Zirconium Tungstate

Zirconium tungstate (α-ZrW₂O₈) exhibits isotropic negative thermal expansion over a wide range of temperatures. Under high pressure it transforms to a metastable amorphous phase with positive thermal expansion coefficient. α-ZrW₂O₈ can be fully recovered by annealing the amorphous phase at temperatures slightly above 600°C, at room pressure. In this work, we describe the dependency of the linear thermal expansion coefficient of amorphous ZrW₂O₈ with the annealing temperature, showing that it can be continuously tuned, in two stages, from about 7.5° to −9°C⁻¹. These two stages are related to the relaxation of the amorphous phase and the onset of recrystallization, respectively.     

Irradiation behavior of nuclear graphites at elevated temperatures

A number of anisotropic and near-isotropic graphites were irradiated at 300° to 1500°C to fluences of up to 1.5 × 10²²n/cm² (E > 0.18 MeV). The maximum volume contraction of the graphites ranged from 4 to 9 per cent at 4 to 6 × 10²¹ n/cm² at 1175° to 1280°C, and the maximum expansions ranged from 11 to 50 per cent at 1.5 × 10²² n/cm². Pores, located between crystallite clusters, were identified and are shown to be those eliminated by c-axis expansion during densification while intercrystallite pores formed. The rate of elimination of the pores located between crystallite clusters increases while the rate of formation of intercrystallite pores decreases with increasing temperature. During expansion new pores form between filler particles increasing the bulk volume expansion. The dimensional and microstructural changes are correlated with the distribution of apparent crystallite sizes in the graphites; a predominance of highly graphitic structure in each material causes maximum contraction strain and higher expansion rates after turnaround. The thermal expansivity of materials, composed predominantly of anisotropic and highly graphitic structure, increases by a factor of 2, whereas the thermal expansivity of materials predominant in isotropic structure decreases by a factor of 2. Thermal expansivity changes coincide with volume expansions. A graphite with evenly balanced volumes of isotropic and anisotropic structures shows no change in thermal expansivity with fluence. The thermal expansivity changes follow the same correlation with apparent crystallite size and distribution of crystallite sizes as does the maximum volume contraction and expansion rates of the graphites. Near-isotropic graphites that contain a narrow range of crystallite sizes are more stable under fast neutron irradiation at high temperatures than are anisotropic materials which have a wide range of crystallite sizes. The addition of semigraphitic isotropic carbon structure to an anisotropic material improves its stability and reduces the magnitude of the increase in thermal expansivity.     

Thermal conductivity and thermal expansivity of in situ composites of a liquid crystalline polymer and polycarbonate

The thermal conductivity and thermal expansivity of extruded blends of a liquid crystalline polymer (LCP) and polycarbonate (PC) with volume fraction (V_f) of LCP between 0.09 and 0.8 have been measured as functions of draw ratios λ ranging from 1.3 to 15. At V_f < 0.3, the LCP domains are dispersed in a PC matrix and the aspect ratio of the domains increases with increasing λ. At V_f > 0.55, phase inversion has occurred and the LCP becomes the continuous phase. The axial thermal conductivity K_∥ increases while the axial expansivity α_∥ decreases sharply with increasing λ, as a result of the higher aspect ratio of the LCP fibrils and the improved molecular orientation within the fibrils. Since the transverse thermal conductivity and expansivity are little affected by drawing, the blends exhibit strong anisotropy in the thermal conduction and expansion behavior at high λ. At V_f < 0.3, the behavior of K_∥ is reasonably modeled by the Halpin-Tsai equation for short fiber composites. At high draw ratio (λ = 15), all the blends behave like unidirectional continuous fiber composites, so K_∥ and α_∥ follow the rule of mixtures and the Schapery equation, respectively.     

均聚甲醛对共聚甲醛成核作用的研究

用示差扫描量热分析法、偏光显微镜法研究了均聚甲醛对共聚甲醛的成核作用 ,并对力学性能及线膨胀系数进行测定。结果表明 ,均聚甲醛的加入 ,使共聚甲醛的结晶温度升高 ,球晶细化 ,线膨胀系数降低 ,缺口冲击强度随均聚甲醛用量的增加出现峰值 ,采用母料法分散工艺使均聚甲醛分散更为均匀 ,成核效果更为显著。     

Dependence of Coefficient of volumetric thermal expansion (CVTE) of glass fiber reinforced (GFR) polymers on the glass fiber content

Summary In a Glass Fiber Reinforced (GFR) polymer, the coefficient of volumetric thermal expansion CVTE (determined as a sum of the coefficients of linear thermal expansion CLTE’s for the three principal directions) is sometimes much smaller than the value predictable on the basis of well acquainted models, such as Chow model, taking into account fibers anisotropy and aspect ratio. A detailed investigation of the CVTE of unfilled and GFR thermoplastics (polyethyleneterephthalate PET, polybuthyleneterephthalate PBT, polyamide 6 PA6, polyamide 4,6 PA46, polycarbonate PC) was performed through Pressure-Volume Temperature (PVT) measurements. In particular, it was found that CVTE is always much lower than the zeroth order “expected value”, defined according to the “rule of mixture”. The aspect ratio plays a major role, since in the case of polymers filled with glass spheres the rules of mixtures applies for the resulting CVTE. Finally, the nature of the matrix is of paramount importance: a GFR polymer with an amorphous matrix (PC) strictly follows the rule of mixture for CLVE even for highly anisotropic fillers exhibiting large aspect ratios (20 to 30).     

Thermal aging behavior of plasma sprayed LaMgAl11O19 thermal barrier coating

The crystallization behavior of the amorphous phase of the plasma sprayed LaMgAl₁₁O₁₉ (LaMA) coating during thermal aging processes has been investigated. Results indicate that LaMA coating exhibits much similar microstructure and thermal properties such as close coefficient of thermal expansion and specific heat capacity etc. to the sintered LaMA bulk after aging at 1673 K for 20 h. On the other hand, a solid state reaction seems to occur to reform the ideal magnetoplumbite-type LaMA phase coupled with the formations of the La-rich aluminate intermediate phases. When the aging temperature is held between 1273 K and 1473 K, nanosized platelet-like grains as well as sub-grains with high aspect ratios are present. The phase stability has been investigated through the chemical compositions and X-ray diffraction analysis. The recrystallization mechanism of the amorphous LaMA coating has been explored by tracing the microstructure evolutions during thermal aging process.     

Thermal expansion behavior of composites based on axisymmetric ellipsoidal particles

A new model for calculating the coefficients of thermal expansion, CTE, in all three coordinate directions is developed for composites containing aligned, axisymmetric elliptical particles, i.e., characterized by a single aspect ratio, that in the limit approximate the shapes of spheres, fibers and discs. This model is based on Elshelby's method but employs a somewhat different formulation than used in prior papers; a main advantage of the current approach is that it can be readily extended to composites based on ellipsoidal particles with no axes of symmetry, i.e., all three major axes are different, as recently demonstrated for modulus. CTE predictions for the simple case of axisymmetric particles are illustrated by calculations for glass particles in the shape of spheres, fibers and discs in an epoxy resin and are compared to those from the popular Chow theory. For spherical-shaped particles, the CTEs in all directions are the same and decrease modestly as the volume fraction of filler particles increases. As the particle aspect ratio increases from unity, the thermal expansion becomes anisotropic. The coefficient of longitudinal linear thermal expansion always decreases with increasing aspect ratio and filler loading due to the mechanical constraint of the filler. For aligned axisymmetric particles, the coefficients of linear thermal expansion are always the same in two directions. The values in the transverse direction may be higher or lower than that of the matrix depending on the values of aspect ratio and filler loading; these regions are mapped out for this particular set of matrix and filler properties. The two-dimensional constraints on matrix expansion caused by discs versus the one-dimensional effects of fibers cause quantitative differences in behavior for the two shapes.     

Thermal expansivity of oriented nylon-6 and nylon-6,6

Measurements on the thermal expansivities along (α_∥) and normal (α⊥) to the draw direction have been carried out from 120 to 380K for nylon-6 and nylon-6,6 with draw ratio λ between 1 and 3.6. The sharp drop in α∥ and the slight increase in α⊥ with increasing λ can be attributed to the gradual alignment of chain segments in both the crystalline and amorphous regions. Using the presently available expansivity data on nylon-6 crystals it is found that the observed orientation dependence below the glass transition can be quantitatively described by a two-phase aggregate model. Water acts as a plasticizer and so its absorption leads to a drop of about 80K in the glass transition temperature. At low temperature, however, water appears to form bridges between molecular chains. This gives rise to a stronger interchain interaction and hence a lowering of the thermal expansivity.     

The thermal expansion behaviour of highly oriented polyethylene

The thermal expansion behaviour of oriented polyethylene has been studied over the temperature range ?50°C to +20°C. In general the values for the thermal expansion coefficient in the orientation direction are negative and of a greater magnitude than the c-axis expansion for the crystal, which is approached at ?50°C. This remarkable effect has been quantitatively related to the influence of an internal shrinkage force, and the changes in expansivity with temperature shown to relate primarily to the change in modulus with temperature.     

Mechanical relaxations and moduli of oriented semicrystalline polymers

A systematic investigation was carried out on the mechanical relaxations and moduli of four drawn semicrystalline polymers: polyoxymethylene, polypropylene, polyvinylidene fluoride and polychlorotrifluoroethylene. Low-frequency tensile and torsional measuremnts were made between-140 and 140°C, and ultrasonic measurements of all five moduli were made by the water-tank method between 0 and 60°C. The patterns of relaxations remain essentially unchanged upon orientation, but there is a marked reduction of the height of relaxation peaks associated with the amorphous phase and, correspondingly, a smaller drop of moduli in the relaxation region. This reflects a lowering of molecular mobility in the amorphous phase due to the constraining effect of taut tie-molecules. The modulus C₃₃ increases sharply with draw ratio λ while the other moduli show little variation, which result from the alignment of molecular chain axes and the production of taut tie-molecules. The λ-dependence of the moduli is consistent with the aggregate model only when the polymer is glassy, that is, when its amorphous phase is comparable in stiffness to the crystalline phase and the polymer can reasonably be regarded as a one-phase material for which the aggregate model is valid.     

On the elastic constants of amorphous carbon nitride

Elastic and thermomechanical properties of amorphous carbon nitrite thin films as a function of nitrogen concentration are reported. The films were prepared by ion beam assisted deposition with nitrogen concentrations ranging from 0 to 33 at.%. By using a combination of the thermally induced bending technique and nano-indentation measurements it was possible to calculate independent values for the Young's modulus, the Poisson's ratio, as well as the thermal expansion coefficient of the films. The hardness and elastic recovery are discussed in terms of the Young's modulus and the Poisson's ratio.     

Phase transition and thermal stability of ceramics from Na-based geopolymers

Geopolymer ceramics undergo a series of thermal phase transitions, progressing from an amorphous geopolymer gel to a crystalline phase, and eventually to an amorphous glass phase as the temperature increases. However, there is a lack of mechanism understanding regarding to the crystallization process and the subsequent thermal degradation. Here, we fundamentally investigated the kinetics of nepheline formation in Na-based geopolymer systems and its thermal stability up to 1400°C. Nepheline crystallization is controlled by bulk nucleation and three-dimensional crystal growth based on the Avrami factor of 4.64, where the activation energy of nepheline formation is 350.59 kJ/mol. High thermal stability of geopolymer ceramics is achieved due to the appearance of nepheline up to 1400°C with the Si/Al ratio ranging from 1.40 to 1.94, while melting and amorphous structure are formed above a higher Si/Al ratio of 2.22. The nature of sintering for geopolymer ceramics consists of shrinkage, expansion and shrinkage corresponding to dehydroxylation, crystallization, and densification, leading to a thermal shrinkage of 21% at 1400°C.     

Thermal expansion coefficient tailoring of LAS glass-ceramic for anodic bondable low temperature co-fired ceramic application

The Li–Al–Si glass-ceramics were prepared by conventional glass-ceramic fabrication method. The influences of Na₂O content on the sintering property, microstructure, and coefficient of thermal expansion were investigated. The results show that the coefficient of thermal expansion of LAS glass-ceramics can be tailored to match that of silicon by the addition of Na₂O content. Na₂O has a remarkable influence on the crystallinity of Li–Al–Si glass-ceramic. The coefficient of thermal expansion of Li–Al–Si glass-ceramic is thus tunable between that of glass phase and crystal phase. The Si–O bond length change in stretch vibration modes introduced by Na₂O also contributes to the variation of coefficient of thermal expansion of the Li–Al–Si glass-ceramics. The coefficient of thermal expansion of the Li–Al–Si glass-ceramic with 1.5 wt% Na₂O addition is about +3.34 ppm/°C at 350 °C and shows a good compatibility to that of silicon in a wide temperature range, which makes it a promising candidate for anodic bondable low temperature co-fired ceramic substrate applications.     

Thermal behavior of reedmergnerite NaBSi<Subscript>3</Subscript>O<Subscript>8</Subscript> and searlesite NaBSi<Subscript>2</Subscript>O<Subscript>5</Subscript>(OH)<Subscript>2</Subscript>

The results of studying the thermal behavior of natural boron albite–reedmergnerite NaBSi₃O₈ and aqueous boricilicate–searlesite NaBSi₂O₅(OH)₂ obtained by the method of hydrothermal synthesis are presented. In the investigated temperature range, reedmergnerite (30–780°C) does not undergo phase transformations. Thermal expansion is sharply anisotropic, which is a characteristic feature of the expansion of feldspars. Aqueous borosilicate searlesite (25–750°C) is amorphized above 400°C as a result of dehydration and cristobalite crystallizes from the amorphous phase. The expansion of searlesite is relatively isotropic.     

Chemical Expansivity of Electrochemical Ceramics

To better understand thermal strain in electrochemical ceramics, the temperature and oxidation-state dependence of lattice volume in La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) were measured. Large values in the apparent thermal expansion coefficient at high temperature (>50 ppm/°C) were caused by changes in oxygen content, not increases in thermal expansivity. This material can be described using an improved thermodynamic formalism that incorporates a new physical property, the chemical expansivity . Our approach opens new avenues for modeling stress and strain in materials, probing defect structure, and analyzing transport and kinetic properties.     

Relationship between crystal structure and properties and irradiation behavior of reactor graphites

Nineteen graphites manufactured with a wide variety of raw materials and processes were characterized and correlations sought between structure and properties. The raw materials of the graphites ranged from highly isotropic cokes to anisotropic needle cokes and processing from conventional molding and extrusion to proprietary processes that produce high-strength isotropic materials. Strength, thermal expansivity, and the degree of anisotropy were found to correlate with crystallite size. The materials with relatively large mean apparent crystallite sizes $\text{L}\text{?}{}_{\mathrm{c}}$ were anisotropic and weak and had low thermal expansivity values, whereas those with smaller $\text{L}\text{?}{}_{\mathrm{c}}$ values were isotropic and strong and had large thermal expansivities. The graphites were irradiated at 625°–1625°C to fluences of up to 2 × 10²²n/cm². The only strong correlation found was between preirradiation strength and dimensional and volumetric changes. High-strength isotropic graphites with intermediate $\text{L}\text{?}{}_{\mathrm{c}}$ values were most stable with lowest initial contraction rates and lowest expansion rates after turnaround. Low-strength isotropic graphites were in the same range of stability with low-strength anisotropic graphites, except distortion was minimized due to their isotropic dimensional changes     

Anistropic thermal expansion of oriented crystalline polymers

The linear thermal expansitivity of oriented high-density polyethylene (HDPE) and polypropylene (PP) with draw ratio between 1 and 18 has been measured between 120 and 300 K. The expansivity perpendicular to the draw direction $\text{z}\text{?}\text{(α}{}_{\mathrm{\perp }}\text{)}$ increases with λ, because of the alignment of the crystallite chain axes along ?; the expansivity parallel to $\text{z}\text{?}\text{(α}{}_{\mathrm{}}\text{)}$ decreases very sharply with λ, becoming negative at about λ = 3 for HDPE and λ = 7 for PP — a consequence of the negative expansivity of the crystalline phase along its chain axis and the constraining effect of the stiff intercrystalline bridges. A model treating the drawn crystalline polymer as a composite consisting of partly aligned crystallites embedded in an isotropic amorphous matrix is adequate for explaining the behaviour of α_⊥, whereas a parallel-series model can give reasonable estimates of $\text{α}{}_{\mathrm{}}$ for ultra-oriented samples, which is largely independent of λ and decreases with increasing temperature. However, neither model can account for the behaviour of $\text{α}{}_{\mathrm{}}$ at low draw ratio.     

Characterization of spherical silica–titania mixed oxide particles synthesized by using a dry process

Silica–titania mixed oxides have excellent properties, such as a low thermal expansion coefficient and a refractive index that can be adjusted by changing the Ti content. However, when the Ti content increases, silica and titania phases in silica–titania mixed oxides can separate. This phase separation leads to the precipitation of the titania component as rutile or anatase crystals. When silica–titania mixed oxides undergo phase separation, their properties become unstable; for example, the refractive index of the particles becomes non-uniform. Therefore, it is preferable to synthesize silica–titania mixed oxides in an amorphous state without causing phase separation. Based on our previous studies on particle size control in silica synthesis, we employed a dry process using organosilicon compounds to synthesize silica–titania mixed oxides. In this study, spherical amorphous silica–titania mixed oxide particles were obtained via flame synthesis using organosilicon and organotitanium compounds. The purpose of this study was to characterize the obtained powder and explore the possibility of controlling particle size during synthesis. By studying the dry process synthesis of spherical silica–titania mixed oxide particles, we confirmed the relationships: between the Si/Ti molar ratio and the obtained crystal structure and between the adiabatic flame temperature and the particle size.     

高品质超低膨胀透明微晶玻璃实现批量化生产

锂铝硅系超低膨胀微晶玻璃是一种性能优良的低膨胀光学材料,热膨胀系数低、透明性好。国产高品质超低膨胀微晶玻璃一直受到技术制约,难以批量化生产。本研究采用熔融法结合搅拌工艺以及适宜的热处理制度,成功批量化生产出热膨胀系数接近于零的高品质超低膨胀透明微晶玻璃产品,产品外观完整,内部基本无气泡、条纹等缺陷。对优级产品进行热膨胀系数、应力双折射、抗弯强度测试,结果表明,优级产品热膨胀系数为1.6×10^-8 K^-1,应力双折射小于2 nm/cm,抗弯强度高达171 MPa,品质已达到ZERODUR零级。采用XRD、SEM、TEM等手段对该产品的物相组成、表面形貌、微观结构进行了分析研究,产品主晶相为β-石英固溶体,晶相含量较高且分布均匀,晶粒尺寸为几十纳米,保证了其极低的热膨胀系数和高透明性。