MgB₂ superconductors were prepared by annealing of a mixture of B and gas-atomized Mg powders. The superconducting onset temperature of 40 K and Zero resistance temperature of 37 K were obtained for the samples annealed at 1073 K for 50 h or annealed at 1073 K for 12 h and then 1173 K for 24 h. The annealing at higher temperatures and/or for longer times causes a reduction in superconductivity. MgB₂ was also fabricated by using AZ31 or AZ91 alloy powders. The superconducting onset temperature of MgB₂ made from AZ91 alloy powder decreases to 37 K due to the addition of Al.

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Conductivities of undoped and 5mol% Sr-doped NdPO₄ were investigated with the dc four-probe method at temperatures from 500 to 925°C under various p(H₂O) and p(O₂). The conductivity of 5 mol% Sr-doped NdPO₄ was higher under wet than dry atmosphere though that of undoped NdPO₄ was almost independent of p(H₂O). The conductivity of 5 mol% Sr-doped NdPO₄ was lower under D₂O than H₂O containing atmosphere. These results indicated that NdPO₄ conducted protons under wet atmosphere by substituting Sr for Nd. Conductivity behavior of this material versus p(H₂O) or p(O₂) was discussed in terms of defect equilibrium. It was concluded that protons were dominant charge carriers at investigated temperatures though electron holes became to contribute slightly to the total conductivity with increasing temperature.

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Elemental Ti and Fe powders were mechanically alloyed using a high-energy ball mill and a conventional ball mill in an argon atmosphere. The morphology, structure, and hydriding behavior of the resultant powders were compared. The powders milled for 100h using a conventional ball mill consisted of Ti and Fe phases together with TiFe phase. On the other hand, those milled for 100 h using a high-energy ball mill consisted mostly of the TiFe phase together with a small amount of Ti and amorphous phases. The mechanically alloyed powders were easily absorbed hydrogen after one step activation with heating at 723 K in vacuum and subsequent hydrogenation at room temperature. The characteristics of the pressure-composition-temperature (PCT) curve were deeply dependent on the amounts of Ti and TiFe phases in the mechanically alloyed powders. The powders milled for 100 h using a high-energy ball mill had a lower plateau pressure than those milled for 100 h using a conventional ball mill. These mechanically alloyed Ti-Fe powders had a lower plateau pressure than the fully activated Ti-Fe alloy counterparts.

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Materials processing based on electromagnetic energies have recently been interested in syntheses of new bulk materials from powders. Among these electromagnetic processing, capabilities of millimeter-wave heating and pulsed high current energizing methods were reviewed by exemplifying several results on syntheses of bulk ceramics from powder compacts. From our results on millimeter-wave sintering of alumina, Si₃N₄ and A1N, it has indicated that millimeter-wave heating has various advantages of rapid densification, decrease of sintering temperature and improvement of properties of sintered bodies. For example, high bending strength over 800 GPa was attained in the high pure alumina without any additives, and high thermal conductivity over 180 W/(m⋅K) was obtained in the Yb₂O₃-added aluminum nitride irrespective of sintering in non-reducing atmosphere. These are attributed to improvements of grain boundary structure and distribution of pore size and so on, originated in so-called "microwave effect". Addition of Yb₂O₃was effective to obtain aluminum nitride with high thermal conductivity and possibility of aluminum nitride with more than 200 W/(m⋅K) was suggested. Subsequently, capability of pulsed high current method was indicated from the results on behaviors of densification without transformation to rutile and preferentially orientation of crystal planes in nano-powder anatase compact. Densification behavior of alumina was also compared with those by millimeter-wave heating method. KEYWORDS millimeter-wave heating, pulsed high current heating, densification, microwave effect, bending strength, thermal conductivity

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Authors have been investigating the sintering of ceramics by 2.45 GHz microwave heating since 1999. We developed an idea "isothermal barrier" to inhibit heat transfer through the surface of sample in our previous studies of microwave sintering of porcelain. The isothermal barrier consists of a thin most inner layer with the identical microwave characters to the sample and thick outer layers with low heat conductivity and low microwave absorption. The isothermal barrier was effective to create the uniform temperature distribution in the porcelain products.
This report explains the concept of the isothermal barrier and introduces our study on microwave sintering of alumina ceramics by using the isothermal barrier.

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Hydrothermal processing is genuinely one of low temperature techniques for ceramic materials. A recent innovation to hydrothermal synthesis of ceramic particles was the introduction of microwave at the temperature range of 100 to 200°C. The microwave hydrothermal technique leads to (a) rapid heating to reaction temperature, (b) enhancement of reaction kinetics by one to two orders of magnitudes which also save reaction time and energy, and (c) formation of smaller size of ceramic powders. In this study, the effect of microwave radiation on the hydrothermal synthesis of a-Fe₂O₃, NaY-type zeolite and hydroxyapatite crystals at 100 to 160°C and their properties are introduced.

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T. T. Meek proposed a model for a microwave ceramic sintering mechanism in his seminal paper. Meek's summary of the model states: "-the initial electric field intensity and power density between the ceramic grains will be greater than in the grains themselves, and thus heating will concentrate exactly where we want it to. Once dense regions begin to form, however, the microwave field will begin to decouple from these regions and couple more strongly to the regions of low density and low dielectric constant. Accordingly, the electromagnetic field will sweep through the composite coupling more strongly to the less sintered regions resulting in a driving force toward a uniformly sintered material, with a uniform dielectric constant."
To test the model, the authors prepared pairs of Al₂O₃ and Si₃N₄ samples whose densities were different. Each pair was heated in a uniform electromagnetic field. The temperature of the higher density samples increased faster than the temperature of the lower density samples in every case. This result indicates that Meek's model is incorrect. Meek assumed that the loss tangent of the lower density composite phase is approximately equal to that of the higher density phase. This assumption appears to be the source of the error.

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Dielectric constant of alumina was measured by free-space time-domain method in the W-band (75- 110 GHz) millimeter-wave region. By using a millimeter-wave reflection profile, the thickness of sample measured by millimeter-wave was determined from the time difference of the reflection peaks from the front and back surfaces of the sample. The refraction index was calculated from the thickness of sample measured by millimeter-wave and the apparent thickness measured by a caliper, and then the dielectric constant was calculated. Obtained values were almost the same as reported for alumina measured at low-frequency. Apparent dielectric constant rose with the increase of the sample density and equivalent dielectric constant decreased with the increase of the sample density. The slope for the former was larger than that for the later. The approximate lines coincided each other at the density of 100%.

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SrBi₂Ta₂O₉(SBT) films were prepared on Pt/Ti/SiO₂/Si substrates by the spin-coating technique, and they were annealed by 28 GHz millimeter-wave heating method. It was revealed that the annealed films were crystallized at a lower temperature than those by conventional electric furnace heating. For clarifying the difference in the interaction of the radiation with each materials of the substrate, transmission, reflection and absorption properties of various silicon substrates were measured by a low power millimeter-wave radiation. It was found that the carrier density of substrates exceedingly influenced millimeter-wave absorption. In the case of a low carrier density, the substrates were almost transparent for the millimeter-wave, while in the case of a high carrier density the reflection was dominant.

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The millimeter-wave sintering of aluminum nitride added with various rare-earth oxides was performed under nitrogen atmosphere. Sinterability of aluminum nitride depended on species of rare-earth oxide, and sintering temperature decreased with decreasing the ionic radius of rare-earth. Full densification over 96 %T.D. was attained at 1600°C for 20 min in the Yb₂O₃-added aluminum nitride sintered by millimeter-wave heating. The main factor for the decrease of sintering temperature was attributed to the species of complex oxide phases, formed between rareearth oxide and aluminum oxides on the outermost surface of aluminum nitride powders. It was also indicated that coexistence of two kinds of complex oxide phases was advantageous to the decrease of sintering temperature in millimeter-wave sintering. Thermal conductivity also changed with added sintering aid, though large dependence was not observed. The highest thermal conductivity of 190 W/(m⋅K) was obtained in the Dy₂O₃-added aluminum nitride sintered at 1800°C for 60 min. It is considered that the thermal conductivity depends on relative density of sintered aluminum nitride.

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Ti-10mass%Al and Ti-30mass%Al alloys were synthesized by mechanical alloying of elementary Ti powder and Al powder using a planetary ball milling for 360 ks. The obtained Ti-10mass%Al powder milled for 360 ks was identified as α-Ti phase including Al. Ti-30mass%Al powder milled for 360 ks was identified as amorphous state with fine crystals. The mixtures with the composition of Ti-34mass%Al, which were prepared by mixing of MA powders and Al powder, were shown endothermic reaction of Al melt at 930 K in DTA curves.
Ti-34mass%Al sample, which was prepared by mixing of mechanically alloyed Ti-30mass%Al powder and Al powder, was consolidated to a complicated shape by pulsed current sintering. After a large amount of shrinkage induced by Al melting was observed in sintering, Ti-Al intermetallic compounds were synthesized by the reaction between mechanically alloyed powder and Al. The sintered compact with a complicated shape cracked during cooling because of the difference of thermal expansion between graphite die and Ti-Al intermetallic compound. The existence of molten Al during the sintering was effective to fabricate a thin sintered compact with a simple shape.

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The present study deals with the fabrication of the zirconium nitride (ZrN) composite with a dispersion of hexagonal boron nitride (h-BN) and with the tribological behavior thereof. Mixture powders were milled for 180 ks or less by a planetary ball mill. The pulse current pressure sintering (PCPS) method was employed in order to consolidate the mixed powders. The sintered compact of ZrN-5 mass% h-BN powders milled for 180 ks shows high density consolidation having a relative density of 92%. Sliding friction tests between sintered compacts of composite ceramics and SUJ-2 steel ball were performed using a ball-on-disc apparatus, in order to estimate the triborogical characteristics of sintered compacts by the friction coefficient evolution. Only a 5% addition of h-BN into the ZrN matrix showed a marked decrease in the friction coefficient to about 0.2, preventing adhesive wear. The sintered compact of ZrN-5 mass% h-BN powders milled for 180 ks maintained low friction coefficient for two-times longer compared to the sintered compact of ZrN-5 mass% h-BN mixed powders. However, finer dispersion of h-BN particle under the range of nanometer size is essential to sustain the self-lubricant effect for longer time to be practical.

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We tried to fabricate self-lubricant composite ceramics consisting of transition metal nitride (TiN or ZrN) matrixes and hexagonal boron nitride (h-BN) dispersions. Mixture powders were milled with a planetary ball mill for not more than 180 ks. Xray-diffraction patterns for the mixed powder showed that h-BN powder was finely grounded smaller than a submicron level and dispersed by the milling for longer than 36 ks. The pulse current pressure sintering (PCPS) method was employed to consolidate the mixed powders. Sintered compacts consolidated under the pressure of 80 MPa and at 2073 K showed the highest density and hardness. The hardness of the sintered compacts decreased with the addition of BN of more than 5 mass%. This result suggests that the sintering of mixed powder could be strongly inhibited by the addition of h-BN. Relatively high density consolidation of more than 90% for single ZrN (or TiN) compact was obtained from the sintered compact of ZrN (or TiN) + 5mass% h-BN powders milled for 180 ks.

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An n-type thermoelectric film of Bi₂Te_2.7Se_0.3 was fabricated by radio frequency sputtering. The target for the sputtering was prepared by mechanical alloying (MA) and pulsed current sintering (PCS) process. The single phase of Bi₂Te₃ including Se was obtained by a planetary ball milling of elementary Bi powder, Te powder and Se powder for 18 ks. The obtained MA powder was consolidated by PCS into cylindrical target of 50 mm in diameter and 4 mm thickness at 643 K under a pressure of 13 MPa. The sintered Bi₂Te_2.7Se_0.3 was joined to Cu backing plate with In for an insertion by PCS process.
The thin film of Bi₂Te_2.7Se_0.3 was fabricated on a polyimide substrate by RF sputtering. This film was amorphous state and crystallized at about 623 K. The performance of the thermoelectric module with 5 thermoelectric film devices was 56 mV in voltage and 0.07 mA in current at 20 K of the difference between heating side temperature and cooling side temperature.

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β-FeSi₂ phase of high density (about 90%TD) was produced by two-steps spark plasma sintering using commercial FeSi₂ powder which consists of a-FeSi₂ and ε-FeSi. In order to advance the transformation to β-FeSi₂ enough, it was clarified that it is necessary to lengthen comparatively holding time (30 minutes) in the 2nd step sintering performed at temperatures ranging from 700 to 850°C. On the other hand, it was found that the holding time in the 1st step sintering performed at 1000°C hardly showed influence to the transformation to β-FeSi₂ phase.

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