This review examines the stages of drying, with the emphasis on the constant rate period (CRP), when the pores are full of liquid. It is during the CRP that most of the shrinkage occurs and the drying stresses rise to a maximum. We examine the forces that produce shrinkage and the mechanisms responsible for transport of liquid. By analyzing the interplay of fluid flow and shrinkage of the solid network, it is possible to calculate the pressure distribution in the liquid in the pores. The tension in the liquid is found to be greatest near the drying surface, resulting in greater compressive stresses on the network in that region. This produces differential shrinkage of the solid, which is the cause of cracking during drying. The probability of fracture is related to the size of the body, the rate of evaporation, and the strength of the network. A variety of strategies for avoiding fracture during drying are discussed.

Theory of Drying

This paper presents the results of a comprehensive experimental study of the desiccation of fine-grained soils. Air drying of initially saturated soil slabs in controlled conditions is investigated by performing three kinds of tests: free desiccation tests, constrained desiccation tests (prevented shrinkage), and crack pattern tests. Strains, suction, water con- tent, and crack geometry are investigated. Results reveal that unconstrained drying exhibits two stages: a domain with large, mostly irrecoverable deformations and degree of saturation close to 100%, followed by a domain with lower defor- mations at a decreasing degree of saturation. Homogeneous soil macroscopic cracking is possible only in the presence of boundary constraints and (or) moisture gradients, inducing the build-up of tensile stresses. Results also show that, for the initially saturated remoulded soils tested here, in the whole sample and near a crack initiation point, the degree of satura- tion remains very close to 100% until cracking, while cracking onset, the air-entry suction, and the shrinkage limit are close to each other. Cavitation nuclei and the formation of an irregular drying front at cracking initiation are commented upon in light of this observation. Finally, results suggest that the crack pattern geometry is the result of energy redistribu- tion. A quantification of the process is proposed. Resume´ : Cet article presente les resultats d'une etude experimentale complete de la dessiccation de sols fins. Le sechage de sols initialement satures est etudiedans des conditions controlees et atemperature ambiante. Trois types d'essais ont eteeffectues : essais de dessiccation libre, essais de dessiccation avec contraintes (le retrait est empechedans une direc- tion) et essais de plans de fissures. Les deformations, la succion, la teneur en eau et la geometrie des fissures ont eteinves- tiguees. Les resultats demontrent que le sechage sans contrainte presente deux etapes : un domaine durant lequel les deformations sont grandes et irreversibles et le degrede saturation reste pres de 100 %, suivi d'un domaine durant lequel les deformations sont plus faibles et le degrede saturation diminue. Pour un sol homogene, des fissures macroscopiques apparaissent seulement lorsqu'il y a des contraintes aux frontieres et (ou) des gradients d'humidite ´, ce qui induit des contraintes en tension. Les resultats montrent aussi que pour les silts et argiles remanies initialement satures utilises dans cette etude, le degrede saturation reste tres pres de 100 % jusqu'ala fissuration. De plus, la fissuration, l'entree d'air et la limite de retrait sont proches les unes des autres. Suite a ces observations, l'existence de germes de cavitation et la forma- tion d'un front de sechage irregulier lors de l'initiation des fissures sont discutees. Finalement, les resultats suggerent que la geometrie des fissures resulte d'une redistribution de l'energie a travers le corps. Une quantification du processus est proposee. Mots-cles: dessiccation, initiation de fissure, succion, succion a l'entree d'air, geometrie des fissures.

https://people.duke.edu/~hueckel/desiccationpapers/2009.can.fundam of cracking.pdf

Fundamentals of desiccation cracking of fine-grained soils: experimental characterisation and mechanisms identification

Negative thermal expansion (NTE) materials have become a rapidly growing area of research over the past two decades. The initial discovery of materials displaying NTE over a large temperature range, combined with elucidation of the mechanism behind this unusual property, was followed by predictions that these materials will find use in various applications through controlled thermal expansion composites. While some patents have been filed and devices built, a number of obstacles have prevented the widespread implementation of NTE materials to date. This paper reviews NTE materials that contract due to transverse atomic vibrations, their potential for use in controlled thermal expansion composites, and known problems that could interfere with such applications.

/pdf/two-decades-of-negative-thermal-expansion-research-where-do-ljrkvl3au7.pdf

Two Decades of Negative Thermal Expansion Research: Where Do We Stand?

https://hal.archives-ouvertes.fr/hal-00539624/document

Experiment evidence on the temperature dependence of desiccation cracking behavior of clayey soils

Using sol−gel technology, a porous glass matrix (xerogel) is formed in a capillary column and acts as a support for a stationary phase of chromatographic particles used in capillary electrochromatography. Preparation of the sol−gel matrix and immobilization of the octadecylsilica (ODS) stationary phase occur in a single step. The presence of the particles in the column greatly reduces matrix cracking caused by internal pressure differentials within the pores of the sol−gel matrix. Good electroosmotic flow is achieved in part because of the inherent negative charge of both the particles and the sol−gel matrix. The performance of these sol−gel/ODS capillary columns was evaluated with a mixture of aromatic and nonaromatic organic compounds. Efficiencies of up to 80 000 plates/m were observed in columns with immobilized 3-μm ODS particles. The efficiency and resolution are enhanced when 3-μm ODS particles are used in place of the 5-μm particles.

/pdf/preparation-and-characterization-of-monolithic-porous-42n952xs42.pdf

Preparation and Characterization of Monolithic Porous Capillary Columns Loaded with Chromatographic Particles

Conventional optical gratings are relatively temperature sensitive. This sensitivity is generally undesirable but can be reduced or eliminated by attaching the grating to a support member having a negative coefficient of thermal expansion. Exemplarily the member comprises Zr-tungstate and/or Hf-tungstate. The thermal expansion can be tailored by admixture of positive expansion coefficient material (e.g., Al2 O3, SiO2) to the negative expansion coefficient material (e.g., ZrW2 O8), or by a variety of other techniques.

Article comprising a temperature compensated optical fiber refractive index grating

Lowered cost fabrication including preparation of large bodies of void-free, high-silica glass, depends upon gellation of an aqueous sol of colloidal silica particles, followed by drying and firing of such gel. Freedom from cracks in the dried gel is the consequence of included polymeric material which wets the particles. The polymeric material is removed by thermal decomposition attended upon temperature attained in firing.

Manufacture of vitreous silica product via a sol-gel process using a polymer additive

A tunable fiber grating comprises a temperature-sensitive body secured to a fiber having a fiber grating region for transmitting thermally-induced strain to the grating. The amount of strain and hence the degree of wavelength tuning are controlled by adjusting the temperature of the temperature-sensitive body, wherein the extent of adjustment is preferably pre-determined according to feedback from a wavelength detector. Large thermal strains obtainable with the present invention allow a wide range of wavelength tuning with a relatively small and convenient temperature change near ambient temperature. In a preferred embodiment, the temperature-sensitive body is cylindrical and comprised of a nickel-titanium alloy bonded to the grating. In alternative arrangements, the thermal strain effect can be amplified. An add/drop multiplexer employing the tunable gratings is also described.

Device for tuning wavelength response of an optical fiber grating

The disclosed method of producing optical fiber comprises providing a silica-based glass rod of diameter D that comprises a core region of effective diameter d surrounded by cladding material (D/d typcially>2) and a silica-based porous jacketing tube, inserting the glass core rod into the porous tube, consolidating the porous material and sintering the jacketing tube onto the core rod such that a substantially pore-free optical fiber preform results. Fiber is then drawn from the preform in the conventional manner. Consolidation and sintering is carried out at a temperature below the softening temperature of the material, typically <1600° C. In a preferred embodiment, the porous tube is produced by a sol/gel process that comprises mixing colloidal silica (surface area 5-100 m2 /g) and water, adding quaternary ammonium hydroxide to the sol such that a pH in the range 11-14 results, lowering the pH into the range 4-11 by addition of methyl or ethyl formate, casting the sol into an appropriate mold and permitting it to gel. The relatively low processing temperature is an advantageous feature of the inventive process that, inter alia, makes it easier to maintain the fiber core concentric with the cladding.

Method of producing an optical fiber

The thermal fatigue performances of Sn98.5Ag1.0Cu0.5 (SAC105), Sn97.5Ag2.0Cu0.5 (SAC205), Sn96.5Ag3.0Cu0.5 (SAC305) and Sn95.5Ag4.0Cu0.5 (SAC405) solder alloys with Pb terminations were investigated by accelerated temperature cycling with and without thermal preconditioning. The performance of the SAC alloys was compared to eutectic SnPb and aged SAC alloys. The test vehicle consists of commercial 2512 ceramic chip resistors soldered to printed wiring boards using the different solder alloy compositions. The solder joints were monitored continuously during a thermal cycle of 0°C -100°C with a ramp rate of 9°C/min and a 30 min dwell between temperature extremes. Failures were defined in accordance with the IPC-9701 A industry test guidelines and failure data are reported as characteristic life η (number of cycles to 63.2% failure) from a two-parameter Weibull distribution. The microstructural evolution was characterized using metallographic techniques and back-scattered scanning electron microscopy. The findings show that the lifetime of the alloys can be ranked as follows: SAC 305 ~ SAC 405 >; SAC 205 >; SAC 105 >; SAC305 aged >; SAC 105 aged >; SnPb and to determine mechanisms of failure, electron microscopy analysis and fractography were performed on post-cycled components.

Debra Anne Fleming

Papers

Article comprising a temperature compensated optical fiber refractive index grating

Manufacture of vitreous silica product via a sol-gel process using a polymer additive

Device for tuning wavelength response of an optical fiber grating

Method of producing an optical fiber

Thermal Fatigue and Failure Analysis of SnAgCu Solder Alloys With Minor Pb Additions