**5. Materials**

22 Efficiency, Performance and Robustness of Gas Turbines

reduce the properties of the same materials, have pushed to consider, in first analysis, the adoption of electrical devices. The power density scale with the square of the force of the electric field, with the frequency and the rotational speed. Numerous possible configurations for an electrical motor-generator exist; the first choice made by the MIT researchers is an induction machine, because this type of machine does not require a direct contact between the electrical device and the rotor neither the exact knowledge of the position of the rotor [Epstein 2003]. The rotor is composed by a layer of 5-20 µm of good insulator covered by a thin layer of a low conductor (high superficial resistance), while the stator is composed by a series of radial electrodes conductors supported by an insulating layer. The rotating electrical potential is imposed by the external electronics on the stator electrodes and the rotating electric field generates a distribution of charges on the rotor, which is mechanically driven. According to the relative phase between the motion of the charges on the rotor and the statoric field, adjustable from the outside, the device will operate as generator, motor or brake. The torque increases with the square of the force of the electric field and the frequency, but the maximum allowable force is regulated by the dimension of the gap. For the air the maximum is obtained for interstitial values of little micron, so that such machine can theoretically achieve an higher power density than the conventional device with analogous configuration. The frequency is regulated by the external electronic systems and by manufacturing device of the electrodes. Currently 300 volts and a frequency of 1-2 MHz is the maximum value obtained with a 6 mm rotor to producing 10 W with a gap of 3µm and a number of poles much high (beyond 100). To maximize the output power is necessary that the space between rotor and stator is of the same order of the statoric electrodes pitch, that means few µm, but with this type of gap the losses for viscous resistance, considering the machine rotational speed, are extremely high and represent the main source of losses. It is necessary to find a compromise between power density and efficiency. The viscous losses unfortunately represent more than the half of the total ones and limit the efficiency to 40-50%. A magnetic induction machine has the advantage of having less poles and optimal distances rotor-stator very wider (≈10 µm), that produce higher efficiency, around 60%. An ulterior advantage of these devices is that they operate at low frequencies and voltages, and are simpler to build. Currently the greater problems is the thermal resistance of the materials (actually 500 K, but material to working to 800 K have been currently studied) and the rotational speed in conflict with the necessity to insert on the rotor surface a iron layer of about 100 µm. Recently, thanks to new technologies of implantation of the ferromagnetic materials on silicon wafer, a good magnetic induction machine to with interesting efficiency and performances has been realized. As already said, to succeed to adopt a magnetic induction machine respect to analogous electrical devices, implies a greater efficiency, tolerances less limiting and higher manufacturing simplicity. An example of a magnetic induction machine is shown in the figure. The rotor diameter is 10 mm. The machine consists of a two phases stator and 8 poles and of a annular rotor [Epstein 2003]. The electromechanical conversion of the energy is achieved by the interaction of the magnetic field that evolve in the interstice rotor-stator with the current induced in the rotor the displacement of the magnetic wave. The stator consists of two phases composed by planar copper spirals, insert in three-dimensional blocks of vertically laminated ferromagnetic material, all supported by a silicon chassis. The ferromagnetic nucleus has an "onion" configuration, the sheets forms concentric rings, approximately 30 µm thick. This particular shape serves to reduce the induced current losses. The rotor is composed by 2 ferromagnetic ring with a thickness of 250 µm and 2 mm wide covered by a copper layer of 20 µm. The copper is extended over the external beam of the rotor for an ulterior millimetre, to

exalt the induced current generation and to increase the maximum torque.

The problems connected to the choice of the materials for a UMGT are referred, partially, to those of conventional machine, in terms of mechanical stresses constraints, operational temperature and manufacturing processes. The materials, generally, satisfy some characteristics penalizing some others, therefore, it is indispensable in the preliminary design to find a good compromise, and not always, the better choice is univocal. It is considered, moreover, that according to manufacturing, the same material can introduce different characteristics. The properties of greater interest for the studied machine are the material absolute specific strength and its resistance to the thermal shocks at high temperatures, the creep and the oxidation, and its derangement characteristics under fatigue cycle. Considering the manufacturing processes and the device dimensions, the first choice for a device MEMS falls on the silicon, thanks the great maturity of the material productive technologies. The silicon also having of the good properties of specific strength and to thermal shock, presents a ductility and a strong "flauge" over the 550°C. This means that I can not use this material in the turbine blade manufacturing. On the contrary, the Silicon alloys appear more interesting, but the manufacturing of these materials is not still mature as well as the silicon ones. The metallic alloys are not able to operate at the demanded high temperatures without cooling or covering, that could create constructive complications. Moreover, the metallic manufacturing does not have a good accuracy. Other advanced ceramic materials, characterized by higher operational temperatures, have insufficient mechanical characteristics to the high temperatures. Adding the fragile behaviour of the ceramic materials, the manufacturing process results problematic and, at the moment, MEMS techniques are accurate only for Silicon and its alloy. Figure 13 put in evidence that the silicon alloys, in this case alumina, can be used at the high temperature, but the Al2O3 alloy has lower thermal conductivity and an higher thermal expansion. These characteristics renders the alloy particularly subject to thermal shocks and deformations. Also regarding hardness and elasticity modulus the SiC and Si3N4 have higher resistance performances, in particular to bending, and it can be noticed that, also having lower values to low temperatures respect to, for example, the zirconium, such value is maintained approximately constant to the high temperatures and is higher than the other materials. In figure 14 the SiC mechanical properties are maintained, substantially, unchanged with the temperature. The weak of the silicon alloy, like all ceramic materials, is the fragile behaviour, that to large-scale has delayed theirs uses. In large scale components, in fact, the inner imperfections can be in such amounts and largeness that is sufficient the effect of a small extra-solicitations to "prime" the propagation of crack, especially in these materials where the reticulum plans sliding is extremely reduced. For the UMGT order of magnitude, a single piece is composed by a low grains number, so the problem of the structure inner defects is more controllable. The piece surface-volume ratio grows, and will be, therefore, necessary to put greater attention to the superficial defects, prime points for the propagation of crack. From this first comparison, SiC and Si3N4, seem to be the more appropriate materials, thanks to the better behaviour at the high temperatures. There is Recently, adding some elements which boron, aluminium, yttrium and/or relative oxides, meaningful improvements of the mechanical properties have been verified. As previously said, the characteristics of these materials depend on the manufacturing process.

### **5.1 Silicon carbide (SIC)**

SiC presents an optimal material specific strength, a good resistance to the thermal shocks, thanks to its relative high thermal conductivity. In Table 3 the characteristics of the SiC

Ultra Micro Gas Turbines 25

**Creep Rate Exponent** 1.6 1.1 - 0.9 **Density [g/cm3]** 2.9-3.1 3.05-3.17 3.21 3.1-3.3 **Elastic Modulus [GPa]** 275-390 372-450 434-476 380-451 **Flexural Strength [MPa]** 190-400 359-511 468-575 400-500 **Fracture Toughness [MPa m1/2]** 4 2.6-4 2.7-3.5 3.9 **Hardness Vickers [GPa]** 22-26 23-26.7 27 21 **Max use Temperature °C** 1350-1600 1400-1600 1400-1600 1400-1600 **Tensile Strength [MPa]** 77-310 234-310 220-310 200-310 **Thermal Conductivity [W/m K]** 110-200 31.3-116 63-300 50-120 **Thermal Expansion From 0°C [ 10-6 K-1]** 4.3-4.6 4.2-5.9 4.0-4.6 4.3-4.6

**Percentage in parentheses denote estimated combined relative standard uncertaints of the propriety. For example, 3.0 (5%) is equivalent to 3.0** ± **0.15. Property values in parentheses are extrapolated Property [unit] 20°C 500°C 1000°C 1200°C 1400°C Bulk modulus [GPa]** 203 (3%) 197 191 188 186 **Creep rate [10-9s-1] at 300 Mpa** 0 0 0 0.004 (17%) 0.27 **Density [g/cm3]** 3.16 (1%) 3.14 3.11 3.10 3.09 **Elastic modulus [GPa]** 415 (3%) 404 392 387 383 **Flexural strength [MPa]** 359 (15%) 359 397 437 446 **Fracture toughness [MPa m-2]** 3.1 (10%) 3.1 3.1 4.1 4.1

**Hardness (Vickers,1 kg)[GPa]** 32 (15%) 17 8.9 (6.9) (5.3) **Lattice parameter a (polytype 6H) [ ]** 3.0815 (0.01%) 3.0874 3.0950 (3.0894) (3.1021) **Lattice parameter c (polytype 6H) [ ]** 15.117 (0.02%) 15.1440 15.179 (15.194) (15.210) **Poisson ratio []** 0.16 (25%) 0.159 0.157 0.157 0.156 **Shear modulus [GPa]** 179 (3%) 174 169 167 166 **Sound velocity, longitudinal [km/s]** 11.82 (2%) 11.69 11.57 11.52 11.47 **Sound velocity, shear [km/s]** 7.52 (2%) 7.45 7.38 7.35 7.32 **Specific heat [J/kg K]** 715 (5%) 1086 1240 1282 1318 **Tensile strength** 250 (6%) 250 250 250 250 **Thermal conductivity [W/m K]** 114 (8%) 55.1 35.7 31.8 27.8 **Thermal diffusivity [cm2/s]** 0.50 (12%) 0.16 0.092 0.079 0.068 **Thermal expansion from 0°C [10-6 K-1]** 1.1 (10%) 4.4 5.0 5.2 5.4 **Wear coefficient (log10) [0.2 m/s, 5N]** - 4.0 (5%) -3.6 -3.6 **… … Weibull modulus []** 11 (27 %) 11 11 11 11

The silicone nitrite is a material much industrially diffusing. In Table 5 the main characteristics of the Si3N4 are reported, according to the manufacturing process, for a temperature within 20°C and 1400°C. Data, relative to CVD operations, are not available, because for such material is only used to generate protecting films, not for solid pieces

**Bonding Sintering CVD Hot** 

**Pressing** 

**SiC Reaction** 

Table 3. SiC proprieties at varying of manufacturing process

**Friction coefficient[], 0.2 m/s, 5N** 0.7 (21%) 0.4 0.4

A °

A °

Table 4. Sic proprieties [NIST}

**5.2 Silicon nitrate (Si3N4)** 

according to the adopted manufacturing process are listed. The data refer to temperatures between the 20°C and 1400°C and the sources include the main manufacturers companies and some scientific organs, including the NIST (National Institute for Standards and Technologies, USA). Data are, sometimes, discordant depending on the manufacturing process and operational temperature, or because the test and production techniques are not perfectly standardized. In general the worse properties obtained with the Reaction Bonding, the more economic process, and the best ones with the CVD, most expensive one. The Sintering and the Hot Pressing show analogous intermediate characteristics and not too much distant from the CVD. The numerous and reliable data in literature are reported in Table 4. A sintered SiC has been considered, and the result have been completely validated by the NIST.

Fig. 13. Materials characteristics

Fig. 14. Materials proprieties at varying of operational temperature [NIST]

according to the adopted manufacturing process are listed. The data refer to temperatures between the 20°C and 1400°C and the sources include the main manufacturers companies and some scientific organs, including the NIST (National Institute for Standards and Technologies, USA). Data are, sometimes, discordant depending on the manufacturing process and operational temperature, or because the test and production techniques are not perfectly standardized. In general the worse properties obtained with the Reaction Bonding, the more economic process, and the best ones with the CVD, most expensive one. The Sintering and the Hot Pressing show analogous intermediate characteristics and not too much distant from the CVD. The numerous and reliable data in literature are reported in Table 4. A sintered SiC has been considered, and the result have been completely validated by the NIST.

Fig. 13. Materials characteristics

Fig. 14. Materials proprieties at varying of operational temperature [NIST]


Table 3. SiC proprieties at varying of manufacturing process


Table 4. Sic proprieties [NIST}
