Near-zero Thermal Expansion Research Articles

Temperature-independent ytterbium valence in YbGaGe

We have determined the ytterbium valence as a function of temperature in the reported near-zero thermal expansion material YbGaGe using x-ray photoemission at various incident photon energies. The Yb $3d$, $4d$, and $4f$ levels, which directly yield the Yb valence, have been measured. Careful analysis enabled the clear separation of surface and bulk contributions. Resonant photoemission at the $4d\text{\ensuremath{-}}4f$ absorption edge was used to enhance the low contribution of the ${\mathrm{Yb}}^{3+}$ component. Contrary to the initially proposed Yb valence transition, we find no change in the valence from room temperature down to $115\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Ytterbium divalency and lattice disorder in near-zero thermal expansion YbGaGe

While near-zero thermal expansion (NZTE) in YbGaGe is sensitive to stoichiometry and defect concentration, the NZTE mechanism remains elusive. We present x-ray absorption spectra that show unequivocally that Yb is nearly divalent in YbGaGe and the valence does not change with temperature or with nominally 1% B or 5% C impurities, ruling out a valence-fluctuation mechanism. Moreover, substantial changes occur in the local structure around Yb with B and C inclusion. Together with inelastic neutron scattering measurements, these data indicate a strong tendency for the lattice to disorder, providing a possible explanation for NZTE in YbGaGe.

Addition of GeO2to Reduce the Viscosity of Parent Glasses for Low-Expansion, Transparent Glass?Ceramics Containing High-Quartz Solid Solutions

Parent glasses for fabricating glass–ceramics with nanometer-sized crystals usually have high viscosities, resulting in high processing temperatures. In this study, GeO2 was added to a transparent, near-zero thermal-expansion Li2O–Al2O3–SiO2 glass–ceramic to reduce the viscosity of the parent glass. The effects of this compositional modification on the viscosity and crystal-nucleation rate of the parent glasses, and on the crystal size, thermal expansion, and optical transparency of the resulting glass–ceramics were investigated. It was found that addition of GeO2 was useful in reducing the glass viscosity. Owing to the reduced nucleating rate with the increase in the GeO2 content, the nucleating times required for reaching the smallest crystal size, the lowest coefficient of thermal expansion, and the highest transparency were all increased. With increasing GeO2 content, the lowest coefficient of thermal expansion that can be reached for glass–ceramics increased (0.14–2.9 × 10−6 K−1). The highest transparency of the GeO2-containing glass–ceramics is almost as good as that of the GeO2-free glass–ceramic and is almost independent of GeO2 content when the crystal size is smaller than about 65 nm.

Search for a novel zero thermal expansion material: dilatometry of the AgI-CuI system

AgI is a well-known superionic conductor possessing a negative thermal expansion (NTE) coefficient while CuI is a p-type semiconductor possessing a positive thermal expansion coefficient. Pellets of X-Ray Diffraction (XRD) characterized compositions in the AgI–CuI system namely, β AgI, γ AgI, Ag0.5Cu0.5I, Ag0.25Cu0.75I, Ag0.10Cu0.90I, Ag0.05Cu0.95I and γ CuI have been examined by quartz pushrod dilatometry measurements in order to look for a zero thermal expansion material. It is found that the systematic displacement of Ag by Cu gradually reduces the NTE anomaly in AgI. The composition Ag0.25Cu0.75I apparently exhibits near-zero thermal expansion. The results are discussed qualitatively in terms of relevant models.

Development of a Near Zero Thermal Expansion Porous Material

A porous material having near zero thermal expansion (NZTE) around room temperature was developed. This material was prepared by sintering a mixture of SiC, vitrified bonding material (VBM) and LiAlSiO4 at 850°C. LiAlSiO4 was added in different weight proportions, and it was found that a change in the amount of LiAlSiO4 leads to a material with NZTE at room temperature, porosity around 50%, and Young's modulus around 14 GPa. Through X-ray diffraction (XRD) and scanning electron microscopy (SEM) it was determined that no reaction occurred between SiC and LiAlSiO4 during sintering, and they are joined by the VBM, which melted at the sintering temperature. In conclusion, the addition of LiAlSiO4 and VBM to SiC, allowed the development of porous SiC materials having NZTE at room temperature.

Optical Property of Li2O-Al2O3-SiO2 Glass-Ceramic Crystallized under High Pressure

The optical property of Li2O-Al2O3-SiO2 glass-ceramic, with a near-zero thermal expansion coefficient, crystallized under high pressure has been studied. A hydrostatic pressure of 196 MPa was applied by a hot isostatic pressing (HIP) technique during the nucleation process and the crystal growth process. The temperature coefficient of the optical path length, 1/l•dS/dT, decreased by 0.6×10-6 K-1, because the temperature coefficient of the refractive index, dn/dT, was decreased by the HIP treatment. The decrease in dn/dT was ascribed to the suppression of the temperature coefficient of the molar polarizability, φ, of the glass-ceramic crystallized by the HIP treatment. It was concluded that the suppression of φ occurs in the crystalline phase during the HIP treatment, based on the fact that the reduced 1/l•dS/dT was maintained even after the relaxation of the densified structure in the glass phase upon thermal annealing. The decrease in the number of lattice defects in the crystalline phase was hypothesized to be a cause of the reduced φ.

Fabrication of near-zero thermal expansion (Fe xSc 1− x) 2Mo 3O 12–MoO 3 ceramic composite using the reaction sintering process

(Fe x Sc 1− x ) 2Mo 3O 12–MoO 3 ceramic composites ( x=0, 0.1, 0.2, 0.3) with negative thermal expansion (Fe x Sc 1− x ) 2Mo 3O 12 particles dispersed in a matrix of MoO 3 were fabricated from oxides using a melt reaction process at 960°C. Microscopy and Energy Dispersive X-ray spectroscopy show particles of iron/scandium molybdate uniformly distributed in the MoO 3 matrix. The matrix has a layered plate-form shape with cleavage structure. The electrical resistivity of the composites is >10 6 Ω·cm. The mean coefficient of linear thermal expansion of the composites increased from −3.5×10 −6 K −1 when x=0 to near-zero values (−0.2 to +0.2×10 −6 K −1) when x=0.2. This near-zero thermal expansion is nearly constant over the temperature range 25–500°C.

Structural mechanisms underlying near-zero thermal expansion in β-eucryptite: A combined synchrotron x-ray and neutron Rietveld analysis

The structures of ordered and disordered β-eucryptite have been determined from Rietveld analysis of powder synchrotron x-ray and neutron diffraction data over a temperature range of 20 to 873 K. On heating, both materials show an expansion within the (001) plane and a contraction along thecaxis. However, the anisotropic character of the thermal behavior of ordered β-eucryptite is much more pronounced than that of the disordered compound; the linear expansion coefficients of the ordered and disordered phases are αa= 7.26 × 10−6K−1; αc= −16.35 × 10−6K−1, and αa= 5.98 × 10−6K−1; αc= −3.82 × 10−6K−1, respectively. The thermal behavior of β-eucryptite can be attributed to three interdependent processes that all cause an increase inabut a decrease incwith increasing temperature: (i) Si/Al tetrahedral deformation, (ii) Li positional disordering, and (iii) tetrahedral tilting. Because disordered β-eucryptite does not exhibit tetrahedral tilting, the absolute values of its axial thermal coefficients are smaller than those for the ordered sample. At low temperatures, both ordered and disordered β-eucryptite exhibit a continuous expansion parallel to thecaxis with decreasing temperature, whereasaremains approximately unchanged. Our difference Fourier synthesis reveals localization of Li ions below room temperature, and we suggest that repulsion between Li and Al/Si inhibits contraction along theaaxes.

Coated carbon fibres and their composites with tin-lead matrix

The white metal, or Babbitt, bearing metal, based on lead or tin, has excellent frictional properties. It also possesses a number of other important properties such as embeddabitity, conformability and corrosion resistance. However, it is mechanically weak. Tin-lead alloys are most commonly used for solders and bearings. Solder preforms are used to joint various parts of an electronic package. Because the thermal expansion coefficient of a solder is in general much higher than that of a substrate, the solder joint suffers from poor resistance to thermal fatigue. Thus, there is a need for a solder with a low thermal expansion. On the other hand, if tin-lead alloys can be made stronger, they can carry greater bearing loads and hence offer the possibility of reduction in overall bearing size for a given application. Composite materials can be tailored to exhibit a chosen thermal expansion coefficient, as the filler species and the filler volume fraction can be suitably chosen. For a low thermal expansion composite, the filler must have a low thermal expansion coefficient. Moreover, the filler should be a good electrical conductor because the soldered joint then serves as a good electrical connection as well as mechanical connection. It is also required to be a good thermal conductor to enable heat dissipation from the electronic package. In addition, the filler should have good mechanical properties in order to strengthen the tin-lead alloy. One of the suggested means of strengthening bearings involves the incorporation of carbon fibres, thus using the fibres as a reinforceing agent and possibly also as a lubricating agent by a virtue of its graphite nature. Carbon fibres also have near-zero thermal expansion coefficient. Furthermore, carbon fibres are widely available in continuous form, which makes them more effective in lowering the thermal expansion coefficient of the composite and which prevents fibre distribution from becoming nonuniform after remelting and solidification [2]. Carbon fibre reinforced tin-lead alloys have been fabricated by liquid metal infiltration [1, 2] and by investment casting for fundamental process studies [3, 4]. It is well known that carbon fibres are not wetted by liquid lead or tin. In order to produce good wetting and prevent chemical reaction between carbon fibres and metal matrix during synthesis of composites, fibres have often been coated with refractory or metal materials [4-7]. In the previous studies [1-4], carbon fibres were electroplated with copper or nickel in order to ensure good wetting of the fibres by the alloy, but the weak bonding

Grain Size Dependence of Thermal Shock Resistance in KZr<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> Ceramic

Thermal shock fracture behavior of KZr2(PO4)3 ceramic, which has a near-zero thermal expansion coefficient, was evaluated by the water-quenching test. The specimens having almost the same density, strength, Young's modulus, and thermal expansion coefficient, but different grain sizes, were prepared by adjusting the sintering conditions. The maximum temperature difference (ΔTmax), to which the specimens were subjected without failure in the thermal quench test, increased with decreasing grain size. KZr2(PO4)3 ceramic composed of fine grains<3μm withstood the test without lowering of strength even when quenching from 1300°C into water was repeated 20 times. The grain size dependence of ΔTmax has been attributed to residual stress caused by the thermal expansion anisotropy. As a result, grain size and thermal expansion anisotropy were incorporated into the equation for the thermal shock resistance.

Ultralow expansion ceramics in the HfO2-TiO2 system synthesized by an hydrolysis and polycondensation technique

Ultra-refractory ceramics from the HfO2-TiO2 system in the range 30–40 mol% TiO2, with a near-zero thermal expansion, have been synthesized by hydrolysis and polycondensation of titanium alkoxide and hafnium dichloride alcoholic solutions and sintered at moderate temperature. Thermal stability, crystallization, density and microstructure of these materials have been examined. The as-prepared powder, amorphous at room temperature, crystallized quickly when heated at 500 ° C. Entire crystallization occurred after treatment at 1000 °C. Sintering at 1500 °C on cold-pressed samples led to ceramics with weak porosity (⩽7%), low expansion coefficient <1×10−6 °C− with a minimum for 30 mol% TiO2 content. SEM examination on sintered materials at 1500 °C reveals a grain size from 2–6 μm, increasing with TiO2 content.

A second ceramic age—A new materials frontier

A second ceramic age started in the mid-twentieth century as a new, exciting materials frontier. Electroceramics with phenomenally wide range of electrical resistivity (spread over 30 orders of magnitude) span insulators, semiconductors, metal-like conductors, ionic conductors, and, recently, superconductors. They also include ferroelectrics, piezoelectrics, pyroelectrics and electro-optics beside ferrites. Advances in electroceramics have been fascinating and rapid, leading to unprecedented rates of industrial growth. Age-old limitations of poor mechanical strength and brittleness of ceramics are being overcome by outstanding toughness and strength achieved in zirconiabased ceramics exploiting the martensitic transformation at the tetragonal-monoclinic phase change. The dimensional changes at this transition which prevented the use of zirconia earlier has now been turned into a mechanism for toughening ceramics to significant levels. Ceramics with near-zero overall thermal expansion coefficient offer new opportunities to science and industry.

Heat capacity and thermal expansion of Zerodur and Zerodur M at low temperatures

Measurements are reported of the thermal expansion, α, from 2 to 100 K and heat capacity from 1.5 to 20 K for a sample of Zerodur M and two samples of Zerodur for comparison. Zerodur M is a new glass - ceramic produced by Schott Glaswerke (Mainz), which has a near-zero thermal expansion coefficient at ambient temperature and improved dimensional stability on thermal cycling compared with the earlier Zerodur. The heat capacities of Zerodur M and Zerodur are shown to be similar to each other, whereas the α values for Zerodur M are significantly different between 50 to 150 K compared with those for Zerodur.

Ultralighltweight Mirror Blanks

The background and historical development at Corning Glass Works of lightweight mirror blanks are presented. Design considerations using a new low-expansion glass are given. The combination of the new material with near-zero thermal expansion and a fused monolithic core technology allows lightweight mirrors to be manufactured with weight savings approaching 80 percent of the equivalent solid and, at the same time, provides exceptional thermal and dimensional stability. Illustrations of lightweight mirror structures and core configurations are also presented.

A Proposal for a Reflecting Astrolabe

A proposal is made for the construction of a 25 cm aperture “reflecting astrolabe”, using a CER-VIT prism of near-zero thermal expansion. Such an instrument could be expected to have an advantage over the Danjon astrolabe as regards accuracy, declination-range and limiting magnitude.