The Pore Structure of Hydrated Cementitious Compounds of Different Chemical Composition

The pore structure ofβ-C₂S, C₃S, and portland cement pastes was investigated using mercury porosimetry and H₂O and N₂ adsorption. The β-C₂S had more total macro- and mesoporosities than C₃S and portland cement pastes of a similar degree of hydration. C₃S and portland cement pastes had similar total porosities but differed in the porosity size distribution. In the mesopore range, the various test methods gave different results. These differences are discussed on the basis of the various models proposed for cement paste. It is shown that shrinkage could be correlated with the volume of pores <0.03 μm, but not with total porosity.     

Lognormal Simulation of Pore Size Distributions in Cementitious Materials

The pore size distributions in cement pastes and mortars, over the range of pore sizes determined by high-pressure mercury intrusion porosimetry (MIP), can be described in terms of a multimodal distribution by using lognormal simulation. The pore size distribution may be regarded as a mixture of lognormal distributions. Such a mixture is defined by a compound density function: p ( x ) =Σ f_i p ( x , μ _i , σ _i ), Σ f_i = 1, where x is the pore diameter, f_i , is the weighting factor of the i ^th lognormal subdistribution of pore sizes, p ( x , μ _i , σ _i ), and μ _i and σ _i are the location parameter and the shape parameter of the i ^th subdistribution, respectively. It may indicate that different origins and formation mechanisms exist for pores in different size ranges in cementitious materials. A graphical method is proposed to estimate the parameters for the compound distribution. Applications of this model to prediction of permeability are discussed.     

Fabrication of Mullite Ceramics With Ultrahigh Porosity by Gel Freeze Drying

Porous mullite (3Al₂O₃·2SiO₂) ceramics with an open porosity up to 92.9% were fabricated by a gel freeze-drying process. An alumina (Al₂O₃) gel mixed with ultrafine silica (SiO₂) was frozen and sublimation of ice crystals was carried out by drying the frozen body under a low pressure. Porous mullite ceramics were prepared in air at 1400°–1600°C due to the mullitization between Al₂O₃ and SiO₂. A complex and porous microstructure was formed, where large dentritic pores with a pore size of ∼100 μm contained small cellular pores of 1–10 μm on their internal walls. Owing to the complete mullitization, a relatively high-compressive strength of 1.52 MPa was obtained at an open porosity of 88.6%.     

Preparation of Monolithic Porous Titania-Silica Ceramics

Monolithic porous ceramics composed of TiO₂ (67 mol%) and SiO₂ (33 mol%) were prepared via casting a melt in the Na₂O-CaO-TiO₂-P₂O₅-SiO₂ system and subsequent acid leaching of the resultant ceramics which are constituted of NaCaPO₄, TiO₂, and amorphous SiO₂. The median pore diameter and specific surface area of the resulting porous ceramics are approximately 1 μm and 40 m², respectively. Amorphous silica surrounds the submicrometer-sized particles of TiO₂ acting as a binder and retaining monolithic forms. No significant shrinkage in the pore size occurred upon heating up to 1000°C.     

Suppression of Active Oxidation of Polycarbosilane-Derived Silicon Carbide Fibers by Preoxidation at High Oxygen Pressure

Three types of polycarbosilane-derived SiC fibers (Nicalon, Hi-Nicalon, and Hi-Nicalon S) with different SiO₂ film thicknesses ( b ) were subjected to exposure tests at 1773 K in an argon-oxygen gas mixture with an oxygen partial pressure of 1 Pa. The suppression effect of a SiO₂ coating on active oxidation was examined through TG, XRD analysis, SEM observation, and tensile tests. All the as-received fibers were oxidized in the active-oxidation regime. The mass gain and the SiO₂ film development showed a suppression of active oxidation at b values of ≧0.070 μm for Nicalon, ≧0.013 μm for Hi-Nicalon, and ≧0.010 μm for Hi-Nicalon S fibers. Considerable strength was retained in the SiO₂-coated fibers. For Hi-Nicalon fibers, the retained strength was 71%–90% of the strength in the as-received state (2.14–2.69 GPa).     

Anisotropic Abnormal Grain Growth in TiO2/SiO2-Doped Alumina

The effect of TiO₂/SiO₂ addition on the grain growth of alumina was reinvestigated. TiO₂ promoted the grain growth, but there was no abnormal grain growth. However, codoping of TiO₂ and SiO₂ resulted in a duplex microstructure consisting of large platelike grains, ∼800 μm long and ∼100 μm thick, and fine matrix grains. The observed anisotropic abnormal grain growth was explained in terms of liquid formation during heat treatment.     

Fabrication and Measurement of Micro Three-Dimensional Photonic Crystals of SiO2 Ceramic for Terahertz Wave Applications

Three-dimensional (3D) photonic crystals with a diamond structure made of a dense SiO₂ ceramic were successfully fabricated using a CAD/CAM micro-stereolithography and sintering process. The designed lattice constant of the diamond unit cell was 500 μm and the forming tolerance from 50 vol% SiO₂ paste (before sintering) was around 15 μm. After the SiO₂-resin photonic crystals were formed via micro-stereolithography, they were converted to pure SiO₂ ceramic photonic crystals of 99% theoretical density by sintering at 1400°C. The electromagnetic wave propagation in these dense SiO₂ photonic crystals was measured by terahertz-time-domain spectroscopy. The results showed that the band gap appeared between 470 and 580 GHz in the Γ– X 〈100〉 direction, between 490 and 630 GHz in the Γ– K 〈110〉 direction, and between 400 and 510 GHz in the Γ– L 〈111〉 direction, resulting in the formation of a common band gap in all directions between 490 and 510 GHz. These results agreed well with the band gaps calculated by the plane wave expansion method.     

Phase Composition of Hydrated DSP Cement Pastes

Cement pastes densified with small particles (DSP) containing up to 48% silica fume by weight of cement, and hydrated to up to 180 d at room temperature, have been analyzed using TMS-GPC, TGA, and ²⁹Si NMR to quantitatively estimate the amount of unreacted cement, Ca(OH)₂, and residual silica fume, respectively. Using a mass balance approach, the CaO/SiO₂ and H₂O/SiO₂ molar ratios of the C-S-H in the samples were calculated. For samples containing silica fume, the values of CaO/SiO₂ lie between 0.9 and 1.3, depending on the degree of hydration and silica fume content, whereas for samples without silica fume they were 1.6. Silicate polymerization analysis using TMS-GPC suggests that the molecular structure of the C-S-H is similar to that formed in conventional hydration. No cross-linking species were found, but the fraction of higher polymers (above octamer) increases as the CaO/SiO₂ ratio decreases.     

Effect of Additives on the Pressureless Sintering of Aluminum Nitride between 1500° and 1800°C

Combined oxide additives (Y₂O₃, CaO, La₂O₃, CeO₂, SiO₂, TiO₂, and Fe₂O₃) were investigated as AIN sintering aids. AIN can be fully sintered at 1600°C to substantial thermal conductivity (92 W/(m·K)) using a multiple sintering aid of Y₂O₃, CaO, SiO₂, La₂O₃, and CeO₂. This lowtemperature material has small grain size (1 to 3 μm).     

Design, Fabrication, and Characterization of Silicon Nitride Particle-Reinforced Silicon Nitride Matrix Composites by Chemical Vapor Infiltration

Silicon nitride particle-reinforced silicon nitride matrix composites were fabricated by chemical vapor infiltration (CVI). The particle preforms with a bimodal pore size distribution were favorable for the subsequent CVI process, which included intraagglomerate pores (0.1–4 μm) and interagglomerate pores (20–300 μm). X-ray fluorescence results showed that the main elements of the composites are Si, N, and O. The composite is composed of α-Si₃N₄, amorphous Si₃N₄, amorphous SiO₂, and a small amount of β-Si₃N₄ and free silicon. The α-Si₃N₄ transformed into β-Si₃N₄ after heat treatment at 1600°C for 2 h. The flexural strength, dielectric constant, and dielectric loss of the Si₃N_4(p)/Si₃N₄ composites increased with increasing infiltration time; however, the pore ratios decreased with increasing infiltration time. The maximum value of the flexural strength was 114.07 MPa. The dielectric constant and dielectric loss of the composites were 4.47 and 4.25 × 10⁻³, respectively. The present Si₃N_4(p)/Si₃N₄ composite is a good candidate for high-temperature radomes.     

Reactions of Silicon-Based Ceramics in Mixed Oxidation Chlorination Environments

The reaction of silicon-based ceramics with 2% Cl₂/Ar and 1% Cl₂/1% to 20% O₂/Ar at 950 °C was studied with thermogravimetric analysis and high-pressure mass spectrometry. Pure Si, SiO₂, several types of SiC, and Si₃N₄ were examined. The primary corrosion products were SiCl₄( g ) and SiO₂( s ) with smaller amounts of volatile silicon oxychlorides. The reactions appear to occur by chlorine penetration of the SiO₂ layer, and gas-phase diffusion of the silicon chlorides away from the sample appears to be rate limiting. Pure SiO₂ shows very little reaction with Cl₂. SiC with excess Si is more reactive than the other materials with Cl₂, whereas SiC with excess carbon is more reactive than the other materials with Cl₂/O₂. Si₃N₄ shows very little reaction with Cl₂. These diferences are explained on the basis of thermodynamic and microstructural factors.     

Infiltration-Inhibiting Reaction of Gadolinium Zirconate Thermal Barrier Coatings with CMAS Melts

The thermochemical interaction between a Gd₂Zr₂O₇ thermal barrier coating synthesized by electron-beam physical vapor deposition and a model 33CaO–9MgO–13AlO_3/2–45SiO₂ (CMAS) melt with a melting point of ∼1240°C was investigated. A dense, fine-grained, ∼6-μm thick reaction layer formed after 4 h of isothermal exposure to 1300°C. It consisted primarily of an apatite phase based on Gd₈Ca₂(SiO₄)₆O₂ and fluorite ZrO₂ with Gd and Ca in a solid solution. Remarkably, melt infiltration into the intercolumnar gaps was largely suppressed, with penetration rarely exceeding ∼30 μm below the original surface. The microstructural evidence suggests a mechanism in which CMAS infiltration is arrested by rapid filling of the gaps with crystalline reaction products, followed by slow attack of the column tips.     

Preparation of Porous Ca10(PO4)6(OH)2 and β-Ca3(PO4)2 Bioceramics

Submicrometer-sized, pure calcium hydroxyapatite (HA, (Ca₁₀(PO₄)₆(OH)₂)) and β-tricalcium phosphate (β-TCP, Ca₃(PO₄)₂) bioceramic powders, that have been synthesized via chemical precipitation techniques, were used in the preparation of aqueous slurries that contained methyl cellulose to manufacture porous (70%–95% porosity) HA or β-TCP ceramics. The pore sizes in HA bioceramics of this study were 200–400 μm, whereas those of β-TCP bioceramics were 100–300 μm. The pore morphology and total porosity of the HA and β-TCP samples were investigated via scanning electron microscopy, water absorption, and computerized tomography.     

Preparation of Ultrathin-Walled Carbon-Based Nanoporous Structures by Etching Pseudo-Amorphous Silicon Oxycarbide Ceramics

Etching polymer-derived silicon-oxycarbide ceramics with hydrofluoric acid creates nanoporous structures of specific surface areas as high as 600 m²/g. The change in composition upon etching shows the removal of silica, not carbon. The structure remaining after etching is postulated to consist of a scaffolding of graphene networks with their surfaces decorated with mixed bonds of tetrahedral silicon bonded to both oxygen and carbon (SiO _m C_{4− m} , where m =1, 2, or 3). The pores existing within such scaffoldings are presumed to have been filled with SiO₂ tetrahedra, which are removed by etching. The measurement of the average pore size and pore volumes permits us to estimate the width of the graphitic domain walls, δ_W, left behind by the etching process. The smallest value of δ_W, which corresponds to the specimen with the highest surface area, is approximately 1 nm, which is about equal to the total width of one graphene layer and two SiO _m C_{4– m} tetrahedra, one on either side of the graphene sheet. This highest surface area specimen is also believed to have the largest size of the silica domains in the unetched samples. In specimens with smaller domains, the etching is only partially successful in removing the silica, presumably because their small size hinders the access of the etchant to the silica tetrahedra. The above behavior is found for samples with low to moderate carbon content. In one sample with a very high carbon content, the etching process removes some carbon as well as silica.     

Ink Jet Printing of Microdot Arrays of Mesostructured Silica

We report a process for preparing microdot arrays of SiO₂ from a tetraethoxysilane precursor containing either a cationic (CTAB) or non-ionic surfactant. Controlling the ink jet deposition parameters and precursor hydrolysis made it possible to obtain mesoporous silica with a Pm 3 n cubic structure, using CTAB, or an Fmmm orthorhombic structure, using a non-ionic surfactant. The addition of hydrophobic organosilane F₃C(CF₂)₅CH₂CH₂Si(OC₂H₅)₃ leads to the most regular and best-defined three-dimensional microdot array with a constant diameter of 155 μm and a regular space of 250 μm.     

Processing of Microcellular Mullite

A new processing route for manufacturing partially interconnected open-cell, microcellular mullite ceramics has been developed. The strategy adopted for making microcellular mullite ceramics entailed the following steps: (i) fabricating a formed body from combining polysiloxane, Al₂O₃ (a reactive filler), polymer microbeads (used as sacrificial templates), and Y₂O₃ (a sintering additive); (ii) cross-linking the polysiloxane in the formed body; (iii) transforming the polysiloxane by pyrolysis into SiO₂; and (iv) synthesizing mullite by reacting SiO₂ and Al₂O₃. By controlling the sintering temperature and the microbead and additive contents, it was possible to adjust the porosity so that it ranged from 38% to 85%.
The compressive strengths of the microcellular ceramics with ∼40% and ∼70% porosities were ∼90 and ∼10 MPa, respectively. The superior compressive strengths were attributed to the homogeneous distribution of small (≤20 μm), spherical cells with dense struts in the microcellular ceramics.     

Joining SiAlON Ceramics Using Composite β-SiAlON–Glass Adhesives

Commercial β-SiAlON ceramics were joined using mixed Si₃N₄, Y₂O₃, Al₂O₃, and SiO₂ powders. At a joining temperature of 1600°C and a hold time in excess of 10 min, the adhesive was converted to an approximate 60:40 vol% composite of β-SiAlON–glass-ceramic. The grain size of the acicular β-SiAlON grains precipitated in the joint (submicrometer diameter, average aspect ratio of 10) was significantly smaller than those in the adherend ceramic (1–5 μm diameter). Intergrowth of β-SiAlON grains at the joint interface resulted in high bond strengths. The chemistry and microstructure of the ceramic adhesives used are described.     

Deposition and Sintering of Particle Films on a Rigid Substrate

Experimental techniques for the deposition and sintering of ceramic particle films on a rigid substrate have been investigated. Three representative systems (SiO₂, ZnO-Bi₂O₃, and Al₂O₃) were chosen to determine the validity of the conceptual processing model. Crack-free, sintered films 1 to 50 μm thick were produced using single or multiple deposition/sintering cycles.     

Rapid Oxidation Characterization of Ultra-High Temperature Ceramics

Here, a novel method for testing ultra-high-temperature ceramics (UHTC) at a high temperature, rapidly, at a low cost is introduced. A self-supported, self-heated ribbon specimen is used with a table-top apparatus to achieve the necessary high temperatures. This method enables a large temperature–time–composition parameter space to be covered by rapidly producing a large set of postoxidation samples for analysis. The complex oxide scale known to form during oxidation of UHTC materials is shown to be easily reproduced using this method. A ZrB₂–SiC (15 vol%) UHTC material was tested at 1700° C for 15 min. The oxide scale consists of a thin outermost silica (SiO₂) layer and a zirconia (ZrO₂) columnar layer with small amounts of SiO₂ embedded between the ZrO₂ columns. A region of SiC-depleted zone was observed between the unreacted core and the ZrO₂ layer. The measured thickness of the oxide scale was 102 μm and ∼120 μm for the SiC-depleted zone.     

Crystal Chemistry of Hydrous Calcium Silicates: II, Characterization of Interlayer Water

Examination of tobermorite, 4–SCaO.5SiO₂.-5H₂O, and xonotlite, 5CaO.5SiO₂.H₂O, by infrared absorption revealed striking structural similarities and differences in the two phases. In the 8- to 15- μ region, the absorption was the same for both; the differences arose from the manner in which water and hydroxyl ions were bonded. Tobermorite exhibited strong absorption at 6.2 μ , a band which is generally associated with interlayer water, and at 2.9 μ , a band generally attributed to bonded (OH). The mineral xonotlite did not show these two bands but contained the band at 2.75 μ generally associated with free (OH). Synthetic xonotlite prepared at 300°C. was essentially the same as the mineral, but samples prepared at progressively lower temperatures exhibited the 2.9- μ band in increasing intensity. The fibrous form of tobermorite showed a band at 6.5 to 7.0 μ which increased in intensity with increasing amount of CaO in the solid; this band was also found in the 14-a.u. 1.0 CaO: SiO₂ hydrate, but not in xonotlite. The great volume stability of xonotlite during drying and wetting is readily explained on the basis of the present results. Shrinkage of tobermorite during drying at temperatures up to 650°C. may be due to removal of both interlayer water and bonded (OH). The changes in absorption during drying at room temperature were too small, however, to permit drawing any conclusions. Similarities and differences between tobermorite and certain clays are discussed.