Low Expansion Materials Research Articles

Negative thermal expansion materials

The recent discovery of negative thermal expansion over an unprecedented temperature range in ZrW 2O 8 (which contracts continuously on warming from below 2 K to above 1000 K) has stimulated considerable interest in this unusual phenomenon. Negative and low thermal expansion materials have a number of important potential uses in ceramic, optical and electronic applications. We have now found negative thermal expansion in a large new family of materials with the general formula A 2(MO 4) 3. Chemical substitution dramatically influences the thermal expansion properties of these materials allowing the production of ceramics with negative, positive or zero coefficients of thermal expansion, with the potential to control other important materials properties such as refractive index and dielectric constant. The mechanism of negative thermal expansion and the phase transitions exhibited by this important new class of low-expansion materials will be discussed.

Passive frequency and intensity stabilization of extended-cavity diode lasers

Studies are reported on frequency drifts in extended-cavity diode lasers caused by external effects, such as changes in temperature and air pressure. A laser system operating at 780 nm has been constructed utilizing low expansion materials and such mechanical structures that compensate for the external effects. By placing the laser system in a pressure-proof and temperature-controlled housing, a relative frequency stability of better than 10−10 (40 kHz) is obtained for integration times of 10 μs to 10 s. The drift of the laser frequency caused by spectral aging of the diode laser is about 3 MHz/h. As a consequence of high passive stability, the variations of the laser intensity are also greatly reduced to a relative drift value of 4×10−5/h.

Thermal expansion behaviour of M′Ti2P3O12 (M′=Li, Na, K, Cs) and M″Ti4P6O24 (M″=Mg, Ca, Sr, Ba) compounds

The [NZP] family has been attracting considerable attention because of its potential in thermal shock-resistant applications. The compounds M′Ti2P3O12 (M″=Li, Na, K, Cs) and M″Ti4P6O24 (M″=Mg, Ca, Sr, Ba) belong to this new family of low-expansion materials. The results of a systematic study undertaken to investigate the thermal expansion behaviour of these materials are reported. A correlation between the ionic size and lattice expansion was also attempted, and compounds possessing lowest bulk thermal and low anisotropy were identified.

Input/output fiber configuration in a laser package design: optimization for lower stresses

Mechanically and thermally induced stresses in the input/output (I/O) fiber of a laser package design are evaluated for different configurations of this fiber. It is found that if the fiber exhibits bending deformations, mechanical stresses can be minimized if a proper end offset is applied and thermal stresses can be reduced if the fiber is mechanically prestressed. It is found also that if the optical device can be rotated by a small angle around the transverse axis, this rotation can be used effectively to minimize the stresses in both categories. It is shown that the smallest fiber span can be obtained if one makes the end planes of the device perpendicular to the package axis, i.e., by making the fiber straight. Clearly in this case the fiber should be made short enough to avoid buckling under the compressive action of thermally induced stresses. Such a configuration is the most feasible because it results in the shortest fiber span (length) and in minimal optical losses. Such a configuration should be employed in all cases when rotation of the optical device is possible, when the fiber ends can be easily aligned, and if the support structures are strong enough to withstand the higher thermally induced forces from the compressed fiber. Thermal stresses can be brought down by the use of low-expansion materials such as Kovar or Invar for the package enclosure. It should be pointed out that although the results of this analysis provide designers with a useful theoretical guide for optimizing the I/O fiber configuration, the final configuration can be selected only after the allowable stress and the achievable alignment (in the case of straight fiber) are evaluated experimentally.

Input/output fiber configuration in a laser package design

The mechanical and thermally induced stresses are evaluated for different configurations of the input/output (I/O) fibers in a laser package design. We show that, if the fibers experience bending deformations, the mechanical stresses can be minimized by applying a proper ends off-set and that the thermal stresses can be reduced, if necessary, by the mechanical prestressing of the fibers. We show also that if the optical device can be rotated by a small angle around the transverse axis, this rotation can be effectively used for minimizing the stresses. We found that the smallest fiber span can be achieved by making the end planes of the optical device perpendicular to the package's axis. In this case the I/O fibers become straight, and should be made short enough to avoid buckling under the action of the thermally induced compressive stresses. We suggest that such a configuration is employed, when the appropriate rotation of the optical device is possible, the fiber ends can be easily aligned, and the support structures are sufficiently strong to withstand a relatively high thermally induced force from the compressed fiber. This force can be brought down, if necessary, by application of low expansion materials for the package enclosure (such as, say, Kovar or Invar). Although the results of our analysis enable a designer to evaluate the mechanical behavior of the I/O fiber of different configurations, the final selection of the I/O fiber configuration can be made only after the allowable stresses and the achievable alignment (in the case of straight fiber) are determined experimentally.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

NZP: A new family of low-thermal expansion materials

A new structural family of low-expansion materials known as NZP has been recently discovered and has generated great interest for wide-ranging applications such as fast ionic conductors, devices requiring good thermal shock resistance, hosts for nuclear wastes, catalyst supports in automobiles, etc. This family is derived from the prototype composition NaZr2P2O12 in which various ionic substitutions can be made leading to numerous new compositions. The bulk thermal expansion of these materials varies from low negative to low positive values and can be controlled and tailored to suit the needs for specific applications. In general, most of the NZP members demonstrate an anisotropy in their lattice thermal expansions, which is the main cause of the low-thermal expansion behavior of these materials. In CaZr4P6O24 and SrZr4P6O24 an opposite anisotropy has been observed which has led to the development of near-zero expansion crystalline solution composition. On the basis of the coupled rotations of the polyhedral network formed by ZrO6 octahedra and PO4 tetrahedra, a crystal structure model to interpret and explain the thermal expansion behavior has been discussed.

Chemistry and structure of glass-ceramic materials for high precision optical applications

Li2O–Al2O3–SiO2-based glass-ceramics are materials that are used in large volumes in applications in which low thermal expansion is the property of prime importance. Although these glass-ceramics are low-expansion materials in a broad sense, the very low-expansion characteristic for which they are famous is achieved only in certain temperature ranges. The deviation of the coefficient of thermal expansion from the ideal zero-value and the temperature ranges in which close to zero-values are achieved are dependent on the chemical composition of the glass-ceramic base glass and the ceramization conditions. Some principal rules by which the desired expansion characteristics can be adjusted are illustrated. For materials that are used in precision optical applications representative data of commercially available products are given. These data include the expansion characteristics as well as homogeneity. Recently the development of the microstructure in glass-ceramics with Zerodur-type composition has been further elucidated. It has been shown that phase separation precedes nucleation. In addition, some specially developed microstructures suggest the existence of an epitaxial relationship between the nuclei and the crystalline phase of the glass-ceramic. Examples of applications of Zerodur-type glass-ceramics together with some of the requirements for these applications are discussed.

A dilatometer for low-expansion materials

Materials with low coefficients of linear expansion are of considerable interest. The following are the basic specifications for dilatometers to be used with them: stability in the displacement sensor and small contributions from the components to the measured thermal expansion. Only interference dilatometers meet those requirements strictly, but new dilatometers have been described [i] based on the measurement of electrical quantities whose sensitivity is greater than that of an interference dilatometer, while not completely meeting those requirements. Here we describe a capacitance dilatometer for the range 300-5 K, which combines high stability in the displacement sensor with almost complete absence of thermal expansion in the body. The means of excluding the thermal expansion in the body is derived from [2], but we used a radio-frequency oscillator because of the lack of good Soviet capacitance bridges. Also, to improve specimen installation for routine measurements, the temperature monitoring system was mounted in a special unit, which was in thermal contact with the specimen. The dilatometer cell (Fig. I) is novel. The capacitance change is determined from the frequency of the oscillator present in the cryostat along with the cell. The cryostat is not subject to any particular specifications, as it can be a circulation one or have a system of vessels containing coolants linked to the cell by a cold finger. The cell and oscillator are linked by a common copper body; copper provides good thermostatic control (!3 x 10 .4 K), and thus stability in size during the measurement in spite of the high temperature coefficient of expansion for copper. The specimen 1 is heated independently of the body 2, which is provided by the support 3 with specimen resting on the insulators 4. The measurement thermometer 5 and control thermometer 6 work with the heater 7 to provide a given temperature in the support. Thermal contact between the support and the specimen occurs over a fairly large area, and in addition, the specimen is enclosed in the foil 9, which also has thermal contact with the support. We used cylinders i00  0.i mm long and 15-25 mm in diameter with ground flat ends. The large diameter was chosen to give better temperature equalization for materials with low thermal conductivity. A difference from [2] was that the support enabled one to mount the specimen on the thermal insulators together with the heating and thermostatic control system and simplified the specimen mounting. The support should not participate in the thermal expansion, which was a task handled as in [3]. The support consisted of two mating copper parts held by the bolts 8. The plane of contact between the upper disk is the plane of support containing the insulators, while the plane of contact for the lower part is the plane of contact with the specimen. When the temperature varies, the position of the plane of contact in space does not alter, so the expansion of the substrate does not influence the measured expansion of the specimen.

ChemInform Abstract: Low‐Temperature Sol‐Gel Synthesis of NaZr2P3O12.

The NZP family of new low-expansion materials has attracted wide interest for its potential in advanced technological applications. NaZr{sub 2}P{sub 3}O{sub 12}, which is the parent composition of this family, has been synthesized by the solution sol-gel method using special precursor solutions, which led to its formation (although poorly crystalline) at temperatures as low as 120{degrees}C. The lowest temperature of formation of a single phase of NaZr{sub 2}P{sub 3}O{sub 12} with a high degree of crystallinity was found to be 600{degrees}C.

Measurement of thermal expansivity of low-expansion glasses by interferometric methods: Results of an interlaboratory comparison

An interlaboratory testing program on the measurement of the linear thermal expansion coefficient of low-expansion glasses has been carried out. Three different types of interferometric dilatometers, each located at three different organizations, and two kinds of low-expansion glass materials were selected for the experiments. As a result of the comparison, a reasonable agreement among the different measuring instruments was confirmed, and it was determined that the thermal expansion coefficient for low-expansion glasses can be measured with an accuracy of ±4×10−8 K−1 by using commercially available interferometric dilatometers.

Very Low Thermal Expansion Coefficient Materials

Materials science treats the relations of composition and structure, on the one hand, to properties. On the other hand, the task of materials engineer­ ing is to design, evaluate, and optimize potential applications of a (new) material. In the last four decades dramatic advances have demonstrated how far our understanding of materials science and engineering has enabled us to design and bring into use a whole range of new devices based on materials in the field of monatomic and compound semiconductors. Unfortunately, this level of detailed knowledge and of control of the synthesis capacity is unique to the semiconductor field. In most of the materials fields our knowledge is not sufficient to develop a new material. Nevertheless, alJ such understanding is helpful in trying to develop a rational theory for the next generation of materials. In this review we treat a much neglected property of most materials­ the coefficient of thermal expansion-where the level of materials science is much more primitive. The relation of the property of thermal expansion to composition and structure eludes us, especiaIJy in the anomalously low thermal expansion materials in which there is the greatest interest. Very low expansion materials are not only important for practical uses but are also interesting scientifically. In the materials research and ceramic indus­ try the importance of the low thermal expansion materials is manifested in their widespread use in high anti thermal shock applications (cookware for oven to freezer use), electronic devices, heat-engine components, auto-

Fibre reinforced glass and ceramic composites

Reinforced glass and ceramic materials are discussed with particular reference to continuous fibre reinforced structures. The advantages of these composites are described with respect to their high strength, high temperature withstand and low mass to high strength ratio. The effects of fibre inclusion and orientation of the tensile and flexural strength of these materials are included Fabrication techniques are outlined for powder-derived glass and ceramic matrix containing composites. In addition, a novel procedure, developed by the GEC Engineering Research Centre, Stafford, which involves the use of sol-gel technology, is described Properties of these materials are described with reference to particular applications including mass sensitive items, for example engines and aerospace structures. The design of very low expansion materials, suitable for metrological applications, is outlined

Development of a new laser interferometric dilatometer and its application to low expansion materials

The newly developed laser interferometric dilatometer permits measurement of absolute expansion of low expansion materials with high accuracy and high reproducibility, which has been difficult to measure by the conventional rod type dilatometer.

A laser interferometric dilatometer for low-expansion materials

The development of a high-precision dilatometer based on interferometry presents a number of challenges. This paper describes a laser interferometric dilatometer developed for high-precision thermal expansion measurements in the materials laboratory. The Sandia dilatometer uses dual-beam laser interferometry with a computer-controlled optical alignment feature to achieve high-precision length change measurements. This device has been modified to expand its measurement capabilities in a number of areas. A new thermal chamber has been incorporated which provides a range of measurement from 90 to 500 K while minimizing thermal effects on the optical portion of the instrument. A new specimen holder has been developed to cover this temperature range while accommodating a wider variety of specimen types. In particular, thin (1-mm) composities may be employed in the holder, which uses a single specimen, with only modest shape and preparation requirements, to provide absolute length change measurements. Tilt errors limit the overall performance of dilatometers based on dual-beam interferometry. Significant improvements in precision were demonstrated by incorporating a unique optical system which independently measures specimen holder tilt; tilt errors were corrected and a length change resolution near 0.4μstrain was achieved. Expansion coefficient data obtained with the device agreed with established results on fused silica and stainless steel. New expansion data were obtained from 90 to 293 K on stainless steel, NBS Standard Reference Material 738.

High-sensitivity dilatometer for quality-control use

A simple-to-operate dilatometer, intended for rapid quality-control testing of low-expansion materials in a single temperature interval, is described. The instrument employs a thermoelectric heat/cool element to supply the thermal environment for the sample and a high-sensitivity linear variable differential transformer (LVDT) for displacement measurement. The mechanical configuration is made so as to eliminate the need for quartz correction and to provide a 4∶1 mechanical advantage to the displacement signal for improved accuracy. Operating between 0 and 100°C, the machine proved to give consistently good results with materials having expansion coefficients as low as 0.5×10−6 °C−1. Representative data on some carbonaceous materials and fused silica (quartz) are given.

Hydrothermal preparation of the low-expansion NZP family of materials

This paper reports on the synthesis of low thermal expansion NaZr 2(PO 4) 3 (NZP) and CaZr 4(PO 4) 6 (CZP) families of materials and their analogues for each of the alkali and alkaline earth metals by the hydrothermal method. Starting gels were prepared by mixing stoichiometric proportions of alkali and alkaline earth metal chlorides of nitrates with a 0·5 m ZrOCl 2 solution and by titrating this mixture with a stoichiometric amount of 1·0 m H 3PO 4 while the solution was constantly stirred. The gels were dried to remove the HCl produced during the reaction between ZrOCl 2 and H 3PO 4, and were then heated with water in teflon-lined Parr bombs or gold capsules. In general, the hydrothermal method yielded 100–300 nm sized rhombohedral single crystals of different NZP materials at 175–200°C under saturated steam pressure conditions. The initial pH and the temperature of treatment are the most critical factors in the hydrothermal synthesis of these technologically important low expansion materials.

Ultra low thermal expansion phases: Substituted ‘PMN’ perovskites

Abstract Among different principles which can be utilized in a structure to exhibit anomalously low thermal expansion, a ‘distributed’ phase transition is a candidate. Pb(Mg 1 3 Nb 2 3 ) 3 is a well known relaxor ferroelectric material in which such a transition occurs. The change in thermal expansion of PMN ceramics caused by partially substituting various ions at the B-site has been studied in order to see any new low expansion materials with good electrostriction properties exist. Partial ionic substitution of Ni 2+ , Co 2+ , Zn 2+ , Mn 2+ , Cd 2+ , Fe 3+ , Sc 3+ , Ti 4+ , Sn 4+ , Hf 4+ , Zr 4+ , and W 6+ was made at B-site. The sintering temperature was kept at 1250° for 1 hour. Thermal expansion data and their interpretations are presented.

A new dilatometer based on an electro-optical measurement technique

An electro-optical sensor developed for microdisplacement measurements has been used for linear thermal-expansion measurements. The non-interferometric system gives a precision of 0.2 mu m in the range of 25-200 degrees C for an alumina tube specimen. The simple sensor capable of giving resolution up to 0.01 mu m thus offers an easy method for measuring the thermal expansion of low expansion materials with small specimen size requirements.

State primary standard and test scheme for means of measuring coefficients of linear expansion for solids in the range 90?1800�K

It has been necessary to revise GOST 8.018-75 and to devise a new state primary standard (SPS) for the unit of linear expansion coefficient (LEC) on account of the extending temperature ranges used with materials, the general use of low-expansion materials, and tightening requirements on accuracy in measuring LEC. The new test scheme of [I] covers the range 0.05-25'10 -6 K -: and temperatures of 901800~ and it contains four grades of standard means of measurement. The new standard has the following changes relative to GOST 8-08-75: the upper temperature limit of SPS has been raised to 1800~ the range in LEC measured with the SPS has been extended at the lower end; the accuracy of the SPS has been improved by a factor five in the range 300-450~ and by 1.5 in the range 450-II00~ comparison standards and working standards have been introduced; and the number of grades in the standard facilities has been increased. The latter change has occurred because the accuracy of third-class standard measures according to GOST 8.018-75 is insufficient for checking precision dilatometers of various types. In order to relieve the load on the SPS, the new test scheme had introduced first-class standard dilatometers for certifying LEC standard measures for checking precision dilatometers. The SPS heads the test scheme, as it provides for reproducing the unit of LEC with a standard deviation (SD) for the average LED in a i00 ~ range with three independent measurements of 0.20-1.5"i0 -e K -l with a residual systematic error (RSE) not exceeding 0.03-30' I0 -e K -l, in accordance with the temperature and LEC. The primary standard is a standard suite of means of measurement serving to reproduce and store the LEC unit and to transmit it to secondary standards [2]. The LEC unit is reproduced on standard dilatometers by using standards for the units of length and temperature and is transmitted to secondary standards (LEC measures) by absolute joint measurement of the extension of the measure and change in temperature causing it, with subsequent calculation of the LEC. The joint measurements are processed on an algorithm for defining the parameters in the approximating function and the variances of them, as well as the LEC (average and differential) and the variances of these [3]. The SPS consists of the following means of measurement: dilatometers type DIS-7, DI-8, and DIV-6 correspondingly for the ranges 90-450~ 300-1100~ and 600-1800~ together with LEC measures for monitoring the stability of the standard dilatometers. An interference principle is used to measure the extension in all the dilatometers, which enables one to measure this in the secondary standards at the wavelengths of :*aCd and He--Ne lasers. The measure is placed between interference plates, which form a Fizeau interTABLE i

A laser-interferometric dilatometer for thermal-expansion measurements of composites

A high-precision, Fizeau-type laser-interferometric dilatometer system has been developed for low-expansion composite materials. The strain resolution is about one microstrain. The system is automated to operate over a large temperature range and record data during the test. A technique has been developed to reduce the data in real time. The dilatometer system is described and thermal-expansion measurements for several fiber-reinforced and particle-filled composites are presented.