Alumina ceramics prepared at various processing conditions were subjected to hot isostatic pressing (HIP) to remove processing-originated voids. Transmission optical microscope was used for characterizing voids in specimens before and after HIP, and for understanding the effect of HIP condition on the removal of void. The strengths of all specimens were measured to discuss the relation between void and strength. Voids of large size and high concentration were present in all sintered alumina ceramics. The size and concentration of void increased with decreasing pressure used in powder compaction process. HIP was found to be very effective for removing these voids which are the major processing defects in ceramics. The strength of alumina ceramics increased markedly with decreasing size and concentration of voids. The strength distribution of alumina ceramics was explained in terms of the measured void size distribution.

Carbon/Carbon composite is densified by Hipping infiltrated pitch. The pore size in matrix decreased as processing pressure increases. The porosity decreasing rate is defined as (Vi−Vi+ 1)/Vi, and is proportional to the applied gas pressure, where Vi and Vi+1 are the pore volume before and after hipping respectively. The bonding strength also increases with the applied gas pressure and with densification cycle. The density of a 67% porosity fiber preform can reach up to 1.9 g/cm3 in less than 6 hipping cycles with maximum gas pressure of 103 Mpa.

We have found that pressurized carbonization cause the increase of carbon yield and the improvement of micro structure In the case of pitch resin. While the effects of pressurized carbonization on phenolic resin are not obvious.

In this study, carbonization and graphitization characteristics of phenolic resin were investigated experimentally. Phenolic resin specimens with and without iron powder addition were carbonized at the temperature of 650 °C under the pressure of atmospheric or 100MPa. Then each of them was heat-treated at the temperature of 1200, 1500 and 1900 °C respectively. Measurement of yield, density, x-ray diffraction and optical observation were carried out in the experiments. It is indicated from those results that pressurized carbonization of phenolic resin makes graphite crystallization progress and this effect is promoted by addition of iron powder.

Dense titanium diboride (TiB2) ceramics were fabricated by near-net processing based on containerless hot isostatic pressing. Resultant materials displayed good mechanical strength and high fracture toughness.

We have developed a new type of near net shape sintering process using nitridation of Si and other inorganic powders. In this report, the sintering property of powder mixture compacts of TiC (10μm or 0.9μm) and Si (0.9μm) in a nitrogen atmosphere at 1350°C was investigated.

A large portion of the Si formed Si3N4 on reaction with nitrogen, while TiC formed TiN and free carbon. A small portion of this free carbon was converted into β-SiC on reaction with Si and about 98% of the free carbon remained in the sintered body. Thus, composite ceramics having a composition of Si3N4·TiC·TiN·C·β-SiC were obtained. The porosity of this composite was much smaller than that of ordinary Si3N4 bonded ceramics, because of the large volume expansion for formation of free carbon. The pore size under 5μm can be got using TiC powder with an avarage size of 0.9μm.

The relationship between sintering conditions and mechanical properties were examined for fully densified Si3N4 with different, β phase fractions (0.2 to 1). The samples were sintered by a capsule-HIP method with Y2O3 and Al2O3 additives (6 mol% in total) at temperatures between 1823 to 2023 K, under 60 MPa for 1 hour. The elastic deformation was calculated by Vickers indentation recovery. The Vickers hardness reached the maximum value for a β phase fraction of about 0.5, while fracture toughness, elastic deformation, and Young's modulus increased as the β phase content augmented.

The use of various gas pressure sintering (GPS) techniques for densifying ceramics are reviewed for both oxides and non-oxides. Variations of the process are discussed with respect to process parameters selected, process sequence, and microstructural development. Theoretical considerations underlying the technique are presented. GPS and hot isostatic pressing are compared and the advantages and disadvantages of each are briefly discussed.

In this paper, we consider certain technological and economic advantages associated with rate controlled sintering (RCS) as the critical first step in a two-step hybrid densification cycle: RCS-plus-HIP. With proper combinations of(a) powder processing (to obtain narrow initial particle size distribution and high green density) and (b) carefully optimized RCS sintering profile (to assure effective pore closure with minimal grain growth), one can readily produce technically dense (gas-tight) sintered ceramic components that can then be rather easily HIPed to full density with very little additional grain growth.

The experimental background and underlying concepts for RCS are briefly reviewed, and are related to the broader issue of establishing and maintaining control of microstructural evolution during densification processes, including those that are best concluded in the high pressure gas atmospheres typically employed for hot isostatic pressing (HIP). To illustrate these points, some examples of RCS-plus-HIP processed oxide ceramics – dense, optically translucent and/or transparent, yet also quite uniformly fine-grained – are shown and discussed.

It is a very well known fact that there exists a narrow relationship between the structure and the properties of materials.

Structure can be modified by changing the composition of the material and for a selected composition by adapting its elaboration process. Powder metallurgy (P/M) and hot isostatic pressing (HIP) can be part of the elaboration process and permit to realize mono or multi-materials with or without complex shapes which can eventually be characterized by specific properties which are not possible to be obtained through the conventional metallurgy road.

The objective of this presentation is to illustrate this through a few examples.

The influence of high oxygen partial pressures on the PTC behavior of non acceptor doped BaTiO3 is studied by using oxygen-hot-isostatic-press (O2-HIP). Annealing in high oxygen partial pressures increased the maximum resistivity with a factor 3, also the minimum resistivity and the gradient in the Arrhenius plot, i.e. resistivity versus the reciprocal of the temperature, increased. The results are analyzed using the well accepted Heywang model. The changes after O2-HIPping can be explained with an increase in acceptor density at the grain boundaries.

MnZn ferrites and PLZT with fine and uniform microstructures were prepared by hot isostatic pressing.

Initial permeability of MnZn ferrites with grain size of 2.0 and 1.2 μ m was 600 at 10 MHz.

An optical shutter array was made using PLZT with grain size of 2μ m. Optical transmittance was dependent upon the amount of PbO in HIP'ed PLZT. A slight deficiency of PbO in PLZT increased the transmittance at short wave length (400 nm).

Control of HIP of Mn-Zn ferrite through the composition of the ambient gas was investigated as an alternative to the powder embedding method. Optimized conditions for this approach are described.

Optical transparent MgAl2O4 spinel is obtained by processing with high gas pressure to eliminate or reduce the residual pores. Both hot–pressed and sintered specimens are post–treated by the high argon hipping process here. The densities of all the specimens before hipping are over 99% theoretical density. In the high gas pressure process, the pressure ranges from 2000 to 30000psi at 1500°C for 2hrs. The transmittance from visible to IR are measured before and after gas pressure treatment. The transparency from visible to IR increases significantly with the HIP process. When applied pressure less than 6000psi, bulk density is proportional to the logarithm of the pressure and the logarithm of the absorption coefficient is proportional to the porosity. Bimodal grain–size distribution exists in the sintered and hipped specimens but is not observed in the uniaxial–hot–pressed–only samples. XRD analysis indicates the width of the diffraction peaks in the HIP specimens is less than that in UAHP specimens. The result illustrates the HIP process improves the spinel crystallinity.

The development of high performance piezoelectric and dielectric ceramics needs a close control of density and microstructure. Hot isostatic pressing technique using glass encapsulation method allows to produce dense and complex shape ceramics with controlled grain size. However, diffusion phenomena occur at glass ceramic interface during HIP treatment.

In this study, the glass diffusion into PZT ceramics was examined for different glass types. We noted silicon species diffusion into ceramic and also lead migration into glass resulting from PZT decomposition.

HIP carried out with diffusion barrier surrounding ceramic samples allows to prevent such a diffusion.

Densification of glass under high pressure and high temperature are shortly reviewed from the points of view of glass structure and densification mechanism. Some of characteristic properties of the densified state are promising for use in the fields of optics and ultra-sonic waves.

X-ray induced optical absorption has been studied on silica glasses densified by hot isostatic pressing. Density of the glass increases up to about 1% in proportion to the operating pressure. Two main absorption bands centered at 5.8eV(215nm) and 4.8eV(260nm) are induced both in the densified and undensified glasses under the irradiation of X-rays at room temperature. The 5.8eV band is enhanced with the increase in density for the glass with low-OH content(<˜200ppm), but is decreased slightly for the glass with high-OH content(>˜200ppm). On the other hand, the 4.8eV band decreases with the increase in density irrespective of OH content, and reaches at a constant value above the densification of about 0.5%. It is found that these absorption bands originate in point defects in the glass and that the densification improves the optical durability of the glass against X-rays when hydroxyl impurities are contained in the glass.

A review of the phase diagram studies of the YBaCuO and GaN systems under high active gas (O2 or N2) pressure is given. The P,T equilibrium conditions for three superconducting phases existing in YBaCuO system vary as a function of Cu content. Application of high O2 pressure allows crystal growth of the 124 and 247 compounds and high N2 pressure alow crystal growth of GaN. We discuss also indications, that other YBaCuO compounds may exist.

The oxygen partial pressure - temperature phase diagram of YBCO type superconductors reported before by us showed that the YBa2Cu4Ox phase formation from Y2Ba4Cu7Oz with CuO, as well as from YBa2Cu3Oy, with CuO are oxidation reactions. The total gas pressure - temperature phase diagram shows that the YBa2Cu4Ox, phase formation temperature from Y2Ba4Cu7Oz with CuO increases with increasing the total gas pressure. On this phase boundary line, the fugacity of oxygen increases with increasing the total gas pressure even at constant oxygen partial pressure.

Superconducting La2−xSrxCaCu2O6−δ ceramics (x=0−0.6) were prepared with application of the oxygen-hot-isostatic pressing (O2-HIP) technique. The Tc of various samples was found to depend on both the Sr content and the applied oxygen partial pressure in the HIP treatment. The maximum Tc onset of 50K was observed for samples of the composition x=0.3 and 0.4 HIPped at 1050°C under 60atm oxygen partial pressure, and x=0.1 and 0.2 HIPped at 1050°C under 400atm oxygen partial pressure, respectively. From the analysis of x-ray diffraction patterns for superconducting substances, it was found that there existed a strong correlation between superconducting properties and the lattice parameters.

We investigate the crystal growth of YBa2Cu4O8 (124) and Y2Ba4Cu7O15 (247) in Al2O3 crucibles at an oxygen partial pressure of 20MPa employing an O2- HIP apparatus in a mixed gas environment of Ar-20%O2. Various melts compositions, rich in Ba and Cu, are explored to optimize crystal growth of 124. Large 124 single crystals up to a size of 1×0.5×0.05mm3 are obtained from compositions with about 65˜67%CuO. 247 single crystals having a maximum size of 3×1.5×0.05mm3 are grown from the same composition of melts. 124 crystals exhibit superconductivity at 75K. 247 crystals show Tc of 20K.