**6.3 Micro Reaction Sintering (MRS)**

This manufacturing techniques is developed for the production of SiC turbomachines rotors of 5÷10 millimetre of diameter. From graphite powders, α-SiC, silicon phenol resin is inserted in a stamp, derived from silicon wafer. The parts of the stamp are melted and all component is submitted to high temperature HIP. The technique has been calibrated in several conditions of pressure, temperature and composition of base powders and it has been verified that the microcrystalline structure of the final piece has an higher quality if the furnace temperatures is about 1500-1700°C, the pressure varies from 100 to 50 MPa and using α-SiC powders instead of silicon one. A microscopic analysis of the structure has demonstrated that melted stamp reacting with the graphite substrate forms a covering of β-SiC around a nucleus of α-SiC deriving from initial powders. But is not still available

Ultra Micro Gas Turbines 43

about 20-30%. So the lower performances could be compensated with the advantage of the redundancy, the lowest noisiness and the UMGTs compactness. Such levels are still the much far being reached. On the other hand, in the emergency applications, where the efficiency is not a limiting requirement regarding the compactness, succeeding to contain the costs, a UMGT would not have rivals. Three requirement that render this device a lot attractive are therefore, compactness, redundancy and lower noisiness thanks to the speed frequencies of hundred of kHz (beyond the audibility threshold) and to the length small scale (the exit flow is quickly stirred with the surrounding atmosphere). In propulsion system this device is considered as one of the more attractive relatively to little reconnaissance planes (100-1000 g of takeoff weight), thanks to the high thrust/weight ratio, which scale with the inverse of the length. The ICE motors currently under construction for these applications, are ten times greater, much noisy and with greater fuel consumptions. The possibility to use UMGT cluster, for the propulsion of great aircrafts is remote, as it seems improbable to succeed to obtain lower consumption to performance parity, without to count eventual difficulties of assembling process, due to the elevated number of necessary devices (at least 105). The determining factor in the diffusion of a UMGT system resides , however, in the production cost. In fact, the production of this devices presents high manufacturing cost, tied to the complex technologies to adopt, amortizable with the production in large series scale. According to an esteem of the MIT (a CMOS silicon wafer of 200 of millimetre of diameter can cost \$ 600-1000), if produced in large scale, a wafer of MEMS generators could produce several kW to the cost of several thousands of dollars for the fabrication. The result is a specific cost of 0,5-5 \$/W. Other considerations have to be done for civil application and for higher power rate (within the UMGT power rate). Here

The main advantage of these systems compared to the existing large scale devices resides in the highest power density. Although such systems have an overall efficiency still lower, the produced power for weight unit is the higher than which will be produced by the battery systems. In fact, at the moment, the battery package power density is about 100-150 Wh/kg. Even if an increase of the power density can be presupposed, the power developed by a UMGT is, undoubtedly, higher for weight and volume unit. Also with a lower efficiency the UMGTs have a power density that is, at least, equivalent to that theoretically produced by a lithium battery package It will be noticed, then, as in the field of the same micro turbines, there is a difference, in terms of performances, according to the used fuel: for instance, the combustion of the hydrogen, in fact, concurs to remarkably raise the value of the power density regarding the obtainable result with fuel like propane or kerosene. An ulterior advantage is the operating flexibility that offers such systems: while the battery package, once exhausted theirs energetic potential, need of a period, more or less long, to recharge

The main disadvantage, respect fuel cells or the battery package, is the high operational temperature. In fact, to achieve an higher output power is necessary to realize high TIT. Because of the small scale of the machine, this design constraint leads all the system to raise

them, the micro turbines allow the fuel change in a very short period.

below it is discussed a practical case study.

**8. User definition 8.1 UMGT advantages** 

**8.2 UMGT disadvantages** 

mechanical tests on the material. The adoption of this technique wants substantially to reduce the excessive long times of bonds production of the CVD or the DRIE on SiC wafer, caused by the speed of material deposition and recording, and to the lack of accuracy of the EDM (Electro Discharge Machining).

Fig. 28. Micro Reaction Sintering phases
