4. Loop heat pipe experiments

The next experiment was performed in frame scientific research of porous structures suitable for LHP and finding possibility of heat removal produced by IGBT. The knowledge gained from the IGBT cooling by LHP has given us the information necessary to know how much heat flux LHP is able remove from heat source. This information will be in the future useful in the design of cooling devices working with the LHP.

### 4.1. Characterization of sintered structures

According to above-mentioned experiences with sintered structures for LHP, we decided to make wick structures from nickel and copper powder. At first, we do analysis of several sintered structures depending on grain size, sintering temperature, and sintering time on porosity, pore size, and strength. In an electric furnace, etalons were sintered from copper powders with grain sizes 50 and 100 μm and nickel powders with grain sizes 10 and 25 μm. The copper powders were sintered at temperature 800 and 950�C for time 30 and 90 min, and nickel powders were sintered at temperature 600�C for time 30 and 90 min.

#### 4.1.1. Porosity measuring

The porosity of a wick structure describes the fraction of void space in the material, where the void may contain working fluid [34]. For the porosity measuring, the weight method was used. At first, the sample was weighed in dry state. Secondly, the sample was soaked with distilled water (r = 0.998 g cm�<sup>3</sup> at 20�C). The weight of absorbed water was estimated by the difference between both values, and then a deduction of the "empty space" (thus the total pore volume) and the porosity.

$$\varepsilon = \frac{M\_{\rm ss} - M\_{\rm ds}}{V\_{\rm total} - \rho\_w} \tag{14}$$

4.1.2. Microscopic analysis of pore size

Investigation of etalons sintered structures by microscopic analysis shown, how influent is the sintering temperature and time on the pore size and on the ratio of the grain size to pore size of each structures. Figures 3–8 of etalons were created by 100 time's zoom of porous structures sintered from copper powder grain size 50 and 100 μm. Figures 3 and 6 show that the structures sintered at temperature 800C have two times bigger pore than powder grain. Comparison of etalons sintered at temperatures 800 and 950C shows that the etalons sintered at temperature 800C have so much bigger pore size than at temperature 950C. It means that pore sizes have so much width to create capillary action in structure. Comparison of etalons sintered at same temperature and various time intervals observed that the time of sintering at temperature nearest the melting temperature of sintering material is not decisive. Comparison of etalons at the same sintering temperature and time interval observed that the grain size of sintered material has impact on pore size. According to microscopic analysis of sintered structures, which clarifies their shape and profile, it can be concluded that the main influencing factors of pore size are grain size, sintering temperature, and not so much sintering time.

Grain size (μm) 100 100 100 100 Sintering temperature (C) 800 800 950 950 Sintering time (min) 30 90 30 90 Porosity (%) 58 56 55 52

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Table 3. Porosity of sintered structures from copper powder with grain size 100 μm.

Table 4. Porosity of sintered structures from nickel powder with grain size 10 μm.

Table 5. Porosity of sintered structures from nickel powder with grain size 25 μm.

Grain size (μm) 10 10 Sintering temperature (C) 600 600 Sintering time (min) 30 90 Porosity (%) 69 67

Grain size (μm) 25 25 Sintering temperature (C) 600 600 Sintering time (min) 30 90 Porosity (%) 72 70

Next Figures 9–12 were created by 500 times zoom of porous structures sintered from nickel powder grain size 10 and 25 μm. Comparison of etalons sintered from nickel powder led to the

where ε is wick structure porosity, Mss is weight of porous soaked sample, Mds is weight of porous dry sample, Vtotal is pore volume of the porous sample, and r<sup>w</sup> is density of absorbed liquid (water).


The results of porosity measuring are shown in Tables 2–5.

Table 2. Porosity of sintered structures from copper powder with grain size 50 μm.

#### Porous Structures in Heat Pipes http://dx.doi.org/10.5772/intechopen.71763 151


Table 3. Porosity of sintered structures from copper powder with grain size 100 μm.


Table 4. Porosity of sintered structures from nickel powder with grain size 10 μm.


Table 5. Porosity of sintered structures from nickel powder with grain size 25 μm.

#### 4.1.2. Microscopic analysis of pore size

4. Loop heat pipe experiments

150 Porosity - Process, Technologies and Applications

design of cooling devices working with the LHP.

4.1. Characterization of sintered structures

4.1.1. Porosity measuring

volume) and the porosity.

liquid (water).

The next experiment was performed in frame scientific research of porous structures suitable for LHP and finding possibility of heat removal produced by IGBT. The knowledge gained from the IGBT cooling by LHP has given us the information necessary to know how much heat flux LHP is able remove from heat source. This information will be in the future useful in the

According to above-mentioned experiences with sintered structures for LHP, we decided to make wick structures from nickel and copper powder. At first, we do analysis of several sintered structures depending on grain size, sintering temperature, and sintering time on porosity, pore size, and strength. In an electric furnace, etalons were sintered from copper powders with grain sizes 50 and 100 μm and nickel powders with grain sizes 10 and 25 μm. The copper powders were sintered at temperature 800 and 950�C for time 30 and 90 min, and

The porosity of a wick structure describes the fraction of void space in the material, where the void may contain working fluid [34]. For the porosity measuring, the weight method was used. At first, the sample was weighed in dry state. Secondly, the sample was soaked with distilled water (r = 0.998 g cm�<sup>3</sup> at 20�C). The weight of absorbed water was estimated by the difference between both values, and then a deduction of the "empty space" (thus the total pore

> <sup>ε</sup> <sup>¼</sup> Mss � Mds Vtotal � r<sup>w</sup>

where ε is wick structure porosity, Mss is weight of porous soaked sample, Mds is weight of porous dry sample, Vtotal is pore volume of the porous sample, and r<sup>w</sup> is density of absorbed

Grain size (μm) 50 50 50 50 Sintering temperature (�C) 800 800 950 950 Sintering time (min) 30 90 30 90 Porosity (%) 55 54 52 50

(14)

nickel powders were sintered at temperature 600�C for time 30 and 90 min.

The results of porosity measuring are shown in Tables 2–5.

Table 2. Porosity of sintered structures from copper powder with grain size 50 μm.

Investigation of etalons sintered structures by microscopic analysis shown, how influent is the sintering temperature and time on the pore size and on the ratio of the grain size to pore size of each structures. Figures 3–8 of etalons were created by 100 time's zoom of porous structures sintered from copper powder grain size 50 and 100 μm. Figures 3 and 6 show that the structures sintered at temperature 800C have two times bigger pore than powder grain. Comparison of etalons sintered at temperatures 800 and 950C shows that the etalons sintered at temperature 800C have so much bigger pore size than at temperature 950C. It means that pore sizes have so much width to create capillary action in structure. Comparison of etalons sintered at same temperature and various time intervals observed that the time of sintering at temperature nearest the melting temperature of sintering material is not decisive. Comparison of etalons at the same sintering temperature and time interval observed that the grain size of sintered material has impact on pore size. According to microscopic analysis of sintered structures, which clarifies their shape and profile, it can be concluded that the main influencing factors of pore size are grain size, sintering temperature, and not so much sintering time.

Next Figures 9–12 were created by 500 times zoom of porous structures sintered from nickel powder grain size 10 and 25 μm. Comparison of etalons sintered from nickel powder led to the

Figure 3. Grain size 50 μm, sintering temperature 800C, sintering time 30 min.

Figure 4. Grain size 50 μm, sintering temperature 950C, sintering time 30 min.

Figure 8. Grain size 100 μm, sintering temperature 950C, sintering time 90 min.

Figure 6. Grain size 100 μm, sintering temperature 800C, sintering time 30 min.

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Figure 7. Grain size 100 μm, sintering temperature 950C, sintering time 30 min.

Figure 5. Grain size 50 μm, sintering temperature 950C, sintering time 90 min.

Figure 6. Grain size 100 μm, sintering temperature 800C, sintering time 30 min.

Figure 3. Grain size 50 μm, sintering temperature 800C, sintering time 30 min.

152 Porosity - Process, Technologies and Applications

Figure 4. Grain size 50 μm, sintering temperature 950C, sintering time 30 min.

Figure 5. Grain size 50 μm, sintering temperature 950C, sintering time 90 min.

Figure 7. Grain size 100 μm, sintering temperature 950C, sintering time 30 min.

Figure 8. Grain size 100 μm, sintering temperature 950C, sintering time 90 min.

Figure 9. Grain size 10 μm, sintering temperature 600C, sintering time 30 min.

same conclusion findings with etalons sintered copper powder. On the pore size, the formation

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From the results, porosity measurement and microscopic analysis were chosen for wick structure of LHP two copper etalons and two nickel etalons. The first structure was made of copper grain size 50 μm and sintered at temperature 950C for 30 min (Figure 13). Second structure was made of copper grain size 100 μm and sintered at temperature 950C for 30 min. Third structure was made of nickel grain size 10 μm and sintered at temperature 600C for 90 min (Figure 14). The fourth structure was made of nickel grain size 25 μm and sintered at temperature 600C for 90 min. The wick structures were sintered in send form (mold) and manufactured according to

The experiments' goal was to determine the influence of various dependencies such as kind of wick structure, kind of working fluid, and amount of working fluid on LHP cooling efficiency. Therefore, special experimental LHP was designed with aluminum block mounted on the evaporator part to fix insulated gate bipolar transistor (IGBT). All parts of LHP (evaporator,

of sintered structure does not affect sintering time but grain size.

Figure 12. Grain size 25 μm, sintering temperature 600C, sintering time 90 min.

4.2. Wick structure manufacture

model of required shape in muffle furnace.

Figure 13. Porous sintered wick structures: a—Copper, b—Nickel.

4.3. Loop heat pipe design

Figure 10. Grain size 25 μm, sintering temperature 600C, sintering time 30 min.

Figure 11. Grain size 10 μm, sintering temperature 600C, sintering time 90 min.

Figure 12. Grain size 25 μm, sintering temperature 600C, sintering time 90 min.

same conclusion findings with etalons sintered copper powder. On the pore size, the formation of sintered structure does not affect sintering time but grain size.
