Research on the Structure and Mechanical Properties of Mesh Powder Composite Copper Microporous Materials

Abstract

With the proliferation of flexible electronics, the advancement of mechanically compliant thermal management systems, notably flexible heat pipes, is imperative to address evolving demands for adaptive thermal regulation in deformable device architectures. The wicks of heat pipes commonly utilize porous copper. In this study, three types of porous copper materials were fabricated: sintered pure copper powder, sintered copper powder with a copper mesh (as a reinforcing network), and sintered copper powder with NaCl (as a pore-forming agent). Their pore structure characteristics, tensile, and compressive mechanical properties were systematically investigated. Results demonstrated that incorporating NaCl into copper powder significantly increased porosity and enlarged pore size, thereby enhancing permeability. For instance, compared to sintered pure copper powder, the addition of NaCl increased the average pore diameter from 0.31 μm to 2.44 μm and improved permeability from 1.908 × 10⁻¹⁴ m² to 2.832 × 10⁻¹² m², effectively reducing fluid flow resistance. The introduction of copper mesh notably improved mechanical performance: under a sintering temperature of 900 °C, tensile strength increased from 121.6 MPa to 132.2 MPa, and compressive strength rose from 443.5 MPa to 458.4 MPa. However, NaCl-added porous copper exhibited a drastic decline in tensile strength. Consequently, NaCl-modified porous copper is unsuitable for flexible wick applications, whereas copper mesh-reinforced porous copper shows potential as a flexible wick, though further investigation is required to enhance its permeability mechanisms.

1. Introduction

Electronic devices are constantly being updated and iterated, becoming faster, lighter and smarter. However, this rapid development has led to microchips producing more heat, and how to dissipate this heat efficiently has become a serious challenge [1,2,3]. As a result, there is an increasing demand for cooling elements that can operate within heat transfer limits and effectively reduce temperature differences [4,5]. Ultra-thin (≤2 mm) heat pipes are widely used in micro electronic devices owing to their high heat dissipation efficiency [6,7].

According to statistics, about 80% of traditional heat pipes use sintered powder as the wick structure [8], and ultra-thin heat pipes are prepared directly by flattening traditional cylindrical heat pipes. It is worth noting that, due to the thick powder layer, the ultra-thin heat pipe is easy to crack or even fall off after flattening, resulting in reduced mechanical properties of the material, which will also affect the use stability of the heat pipe to varying degrees [9]. Aiming at research into the influence of sintering conditions on mechanical properties of materials, Zhang et al. [10] fabricated monolithic copper honeycombs by a plasticizing powder extruding-sintering technology. The effect of sintering conditions on volume shrinkage, apparent density, microstructure, mechanical properties and heat conductivity of copper honeycombs were studied. With increasing sintering temperature and time, the metal particles form sintering necks and gradually coalesce into grains, volume shrinkage, apparent density and strength increase, and the optimum sintering parameters are 950 °C for 2 h. When sintering temperature rises from 800 to 1000 °C, the volume shrinkage ranges from 15 to 30%, and the apparent density ranges from 1.49 to 1.74 g/cm³. When sintering time increases from 1 to 2.5 h, the volume shrinkage ranges from 18 to 27%, and the apparent density ranges from 1.52 to 1.70 g/cm³. Under axial compression, the yield strength ranges from 7.2 to 20.4 MPa. Under radial compression, the yield strength ranges from 2.1 to 3.5 MPa. To improve the disadvantage of relatively low porosity or strength that is compromised by stress-concentrating interparticle bonds in powder metallurgy processes. Kun et al. [11] utilized a Cu–CuO metal matrix composite powder to produce additional microscale porosity within the particles by oxide reduction. These Cu–CuO powders were pressed at 1, 2, or 3 GPa, and made porous at 600, 800, or 1000 °C to investigate the effects of pressing and sintering parameters on the overall strength and density. It was found that the formation of porosity is weakly dependent on compaction pressure (maximum 6% difference from 1 GPa to 3 GPa), while the final porosity varied by ~16% overall (~40% for 1 GPa and 600 °C to 24% for 3 GPa and 1000 °C). The strength of the porous Cu was highest after being reduced at 600 °C but also exhibited some flaking at the edges at high strain. The 1 GPa, 600 °C samples have a higher specific strength than wrought Cu annealed at the same temperature, as was demonstrated under uniaxial quasi-static compression, as well as split Hopkinson pressure bar impact.

In addition, as the thickness of the ultra-thin heat pipe decreases, the wick structure gradually changes, and thin and flexible sintered powder/mesh has been applied in actual production. Chen et al. [12] utilized copper powder and copper mesh as raw materials and produced porous copper powder/mesh plates by pressing, rolling, and vacuum sintering. The results showed that the copper powder/mesh porous plate had a uniform pore distribution and a porosity range of 10% to 30%; the higher the porosity and lower the powder concentration [13], the better the performance was in terms of permeability. Tensile mechanical properties improved with decreasing porosity and increasing powder content. Zou et al. [14] developed a novel porous metal fiber/powder sintered composite sheet by sintering a mixture of a porous metal fiber sintered sheet and copper powders with particles of a spherical shape. Experimental results show that the tensile strength of the porous metal fiber/powder sintered composite sheet is determined by a reticulated skeleton of fibers and reinforcement of copper powders. With an increase in the porosity of the porous metal fiber sintered sheet, the tensile strength of the porous metal fiber/powder sintered composite sheet decreases, whereas the reinforcement of copper powders increases.

In summary, the ultra-thin heat pipe phase transformation flattening process requires excellent mechanical strength in the final thickness. The single powder-type porous structure has poor plasticity and easy pore collapse. The mesh can effectively retain the pores during the flattening process and has a strong compression flattening ability and a stable structure. Therefore, the composite porous structure of copper based powder and mesh needs to be designed and developed to fill the gap in the research into porous structure.

2. Preparation and Structural Characterization of Copper-Based Porous Materials

2.1. Materials

In this experiment, copper was used to prepare the porous structure. Industrial pure copper has good electrical and thermal conductivity properties, second only to silver. Copper is low in price, has good ductility, and copper impurity content is less than 0.05%, it does not react with water in the heat transfer process, and is often used to prepare cooling elements. Li et al. [15] investigated the heat transfer performance of copper-based porous materials with different particle sizes and found that copper powder with a particle size of 0.165 mm has the best heat transfer performance among experimental materials. This study selected 100-mesh spherical copper powder as the experimental material and configured a 100-mesh copper mesh and NaCl crystal as the pore-forming agent. The 100-mesh copper mesh came from the manufacturer. The total weight of copper powder + copper mesh porous structure billet is 100 g, pressed under different pressures according to the ratio of copper mesh content to 4%. The total weight of copper powder + NaCl porous structure billet is 50 g, and the content ratio of copper powder to pore-making agent NaCl is 14:1 to press. The billet of copper powder + copper mesh porous structure is fed by a layering method. The copper powder and copper mesh are filled into the mold layer by layer and laid into the cylindrical mold with layers of copper powder and layer by layer of copper mesh. The copper mesh is of a circular shape, and the porosity can be changed by rotating an angle between the copper mesh to change the pore shape, so as to achieve the random and disordered distribution of porous pores. The copper powder and mesh are as shown in Figure 1.

2.2. Preparation of Copper-Based Porous Materials

Figure 2 shows the preparation process of sintered copper powder type porous materials, sintered copper powder + copper mesh type porous materials and sintered copper powder + NaCl type porous materials, which are divided into batching, cold pressing and solid-phase sintering processes. The cold pressing pressure is set to 350 MPa~550 MPa. The solid-phase sintering temperature of copper powder type and copper powder + copper mesh type porous materials is 700 °C~900 °C, and the sintering temperature of copper powder + NaCl type porous materials is 800 °C~1000 °C. The temperature gradient is 50 °C. The heat maintaining time is set to 60 min during the sintering process.

In the sintering process, the cold-pressed copper-based raw material is placed on the graphite in the sintering furnace and sealed. When the vacuum degree reaches about 0.01 Pa~0.1 Pa, nitrogen gas is introduced to remove the air in the furnace, and then the sintering temperature is set through the programmable temperature control platform. Figure 3 illustrates the main equipment used in the preparation of copper-based porous materials. In the heating stage, the temperature rise rate is 300 °C/h until 700 °C to enter the sintering stage. During the sintering stage, the heating rate is reduced to 200 °C/h and heated to the set maximum sintering temperature. Subsequently, the sample enters the heat preservation stage for 1h. After that, the material is naturally cooled to room temperature and then taken out. Figure 4 illustrates the three copper-based sintered porous materials.

2.3. Microstructural Characterization and Mechanical Property Measurements

2.3.1. Aperture Testing

The experiment tested the aperture size of porous materials through the bubble point method. As shown in Figure 5, the porous sample is immersed in a liquid with good wettability until complete infiltration, and then the gas pressure is slowly increased until the infiltration liquid is replaced. The corresponding pressure difference when bubbles appear is the bubble point pressure. The experimental sample has a diameter of 10 mm and a thickness of 4 mm and is cut into circles using a wire cutting machine (AgieCharmilles 2000S, Xinyihui CNC Machinery Co., Ltd., Changzhou China).

The porosity of the material is

$$\rho =1-{\displaystyle \frac{{\rho }_m}{{\rho }_s}}$$

(1)

where ${\rho }_m$ is the density of the porous body, and ${\rho }_s$ is the density of the copper solid material.

The relationship between specimen porosity and bubble point pressure is shown in Equation (2) [16]:

$$D={\displaystyle \frac{4\gamma cos\theta }{∆p}}$$

(2)

In the formula, D is the equivalent diameter of the aperture, Δp is the pressure difference on both sides of the sample, γ is the surface tension of the wetting liquid, and θ is the contact angle between the wetting liquid and the sample surface. Fluorine was used as the wetting fluid during the test, and its surface tension coefficient was 16 mN/m. Since the wetting solution has good effect, the contact angle between the wetting solution and the sample surface is approximately 0°.

2.3.2. Tensile and Compression Tests

In the experiment, a DNS electronic universal testing machine (China Machinery Testing Equipment Co., Ltd., Beijing, China) was used to examine the room temperature static tensile and compression performance of the sintered porous materials. The test temperature is 25 °C, and the tensile and compression speeds are both 0.1 mm/min. The experiment used a wire cutting method for sample processing to ensure the accuracy of sample size. The overall size of the rectangle tensile specimen is 60 mm × 4 mm × 13 mm. The diameter of the cylindrical compression specimen is 10 mm and the height is 15 mm. At the same time, the fracture morphology of the porous material after tensile fracture was also observed through a scanning electron microscope (Fina Scientific Instruments (Shanghai) China Co., Ltd., Shanghai, China).

Figure 6a shows the mold used for compacting the copper powder + copper mesh porous body. Figure 6b presents the sintered copper powder + copper mesh porous body after compaction and sintering. Figure 6c illustrates the machining of tensile and compressive specimens from the cross-section of the porous body using wire-cutting. Figure 6d,e indicates the tensile and compressive directions during the tensile and compressive tests.

The stress–strain curves of the porous materials were obtained through tensile experiments, from which the parameters of tensile strength, yield strength and elongation after breaking were derived, and then the mechanical properties of the materials were analyzed and compared.

3. Experimental Results and Analysis

3.1. Aperture Size Distribution of Porous Materials

Table 1. Aperture structure parameters of three porous specimens determined by the bubble point method.

Structure Type	Porosity	Minimum-Value Aperture (μm)	Maximum-Value Aperture (μm)	Average-Value Aperture (μm)	The Most Probable Aperture (μm)
copper powder	9.3%	0.17	0.46	0.31	0.18
copper powder + copper mesh	10.7%	0.18	0.80	0.48	0.39
copper powder + Nacl	21.3%	0.20	5.44	2.44	1.60

lists the aperture size structure parameters of copper powder, copper powder + copper mesh and copper powder + NaCl measured by the bubble point method. The results show that the average aperture of the three porous materials are as follows: copper powder + NaCl type > copper powder + copper mesh type > copper powder type.

Figure 7 shows the dry, semi-dry and wet curves of the material measured by the aperture size analyzer. It can be seen that the dry and wet curves overlap well, indicating that the aperture size distribution of the prepared sintered samples is more uniform.

Figure 8 shows the aperture size distribution curves of three porous materials. It can be seen that the uniformity of aperture size distribution of the three porous materials is as follows: copper powder + NaCl type > copper powder + copper mesh type > copper powder type.

The minimum aperture pressure of the pure copper powder porous material sample is 290.96 KPa, the maximum aperture pressure is 118.4 KPa, and the medium flow pressure is 148.3 KPa, indicating that it has the smallest aperture size. The minimum aperture pressure of the copper powder + copper mesh type porous sample is 288.4 KPa, the maximum aperture pressure is 60.8 KPa, the medium flow pressure is 83.69 KPa, and the aperture size is medium. The copper powder + copper mesh structure has a low degree of freedom, and the surface is regular and uniform after solid-phase sintering, which is consistent with the aperture size data measured by the bubble point method. The minimum aperture pressure of the copper powder + NaCl type porous sample is 277.3 KPa, the maximum aperture pressure is 12.9 KPa, and the medium flow pressure is 18.8 KPa, indicating that it has the largest aperture size and the smallest fluid penetration resistance.

3.2. Permeability of Porous Materials

Permeability is one of the important parameters of the wick. It is related to the aperture size, aperture distribution, curvature and other factors of the wick. The Kozeny–Carman equation can be used to solve the permeability of porous materials. The formula is shown in Equation (3) [17]:

$$K={\displaystyle \frac{d^2{\epsilon }^3}{C{\left(1-\epsilon \right)}^2}}$$

(3)

In the formula, K is the permeability, $\epsilon$ is the porosity, C is the constant and d is the particle diameter of the wick. For different porous structures, such as sintered copper powder and wire mesh, the constant C in the equation take different parameter values [18].

The empirical formula for calculating the permeability of porous materials is shown in Equation (4) [19]:

$$K={\displaystyle \frac{d^2{\epsilon }^3}{150{\left(1-\epsilon \right)}^2}}$$

(4)

Based on the experimentally detected porosity of the three porous materials, the permeability of the porous materials was calculated as shown in

Table 2. Permeability of three porous materials.

Structure Type	Porosity	Permeability/m2
copper powder	9.3%	1.908 × 10−14
copper powder + copper mesh	10.7%	2.788 × 10−13
copper powder + NaCl	21.3%	2.832 × 10−12

.

In Table 2, it can be seen that the permeability increases with the increase in porosity. Among the three porous materials, copper powder + NaCl has one order of magnitude more permeability than the copper powder + copper mesh materials, and two orders of magnitude more permeability than the copper powder materials. The addition of NaCl makes the aperture size of porous materials become larger. The fluidity of the working fluid is enhanced, which can significantly improve the permeability of porous materials. Adding metal mesh to the copper powder + copper mesh material increases the permeability of the porous material, but the effect is worse than that of the copper powder + NaCl porous material.

3.3. Effect of Sintering Temperature on Aperture Size Distribution of Porous Materials

The experiment further investigated the relationship between different sintering temperatures and aperture size distribution.

Table 3. Aperture size parameters of copper powder + NaCl type porous sample.

Sintering Temperature/°C	Porosity	Minimum-Value Aperture (μm)	Maximum-Value Aperture (μm)	Average-Value Aperture (μm)	The Most Probable Aperture (μm)
800	22.3%	0.25	5.92	2.58	1.90
900	21.3%	0.20	5.44	2.44	1.60
1000	20.0%	0.17	5.03	2.19	1.2

lists the aperture size structural parameters of copper powder + NaCl type porous materials prepared at 800 °C, 900 °C and 1000 °C sintering temperatures measured by the bubble point method. It can be seen that both the porosity and aperture size of porous materials decrease with the increase in sintering temperature. NaCl is used as a pore-forming agent in copper powder + NaCl type porous material. The melting point of NaCl is 801 °C. When the sintering temperature is 800 °C, NaCl can play a greater role in pore-forming during the sintering process. When the sintering temperature is 900 °C and 1000 °C, NaCl melts and volatilizes in the early stage of the sintering process, which fails to maximize the pore-forming effect. The porosity and aperture size of the sample obtained by sintering are low. Figure 9 shows the dry curve, semi-dry curve and wet curve of the material measured by the aperture size analyzer. It can be seen that the dry curve and the wet curve coincide well, indicating that the aperture size distribution of the prepared sintered sample is relatively uniform.

Figure 10 shows the aperture size distribution curves of porous materials prepared at different sintering temperatures. It can be seen that the aperture size distribution of porous materials prepared at different sintering temperatures is relatively uniform. Among them, the maximum aperture diameter pressure of the porous material sintered at 800 °C is 9.8 KPa, the medium flow pressure is 11.6 KPa, and the minimum aperture diameter pressure is 270.1 KPa, indicating that it has the largest internal pores and the smallest fluid penetration resistance. The maximum aperture diameter pressure of the porous material sintered at 1000 °C is 18.3 KPa, the medium flow pressure is 25.2 KPa, and the minimum aperture diameter pressure is 288.5 KPa, indicating that its internal pores are the smallest and the fluid penetration resistance is the highest. Since the aperture diameter changes slightly at 900 °C–1000 °C, and 1000 °C is close to the melting point of copper, i.e., 1080 °C, the suitable sintering temperature is between 900 °C and 1000 °C.

3.4. Effect of Sintering Temperature on the Tensile Mechanical Properties of Copper-Based Porous Materials

3.4.1. Porous Materials Made of Pure Copper Powder

The experiments were carried out using a wire cutter to cut and prepare tensile test samples of copper powder porous materials prepared at sintering temperatures of 700 °C, 750 °C, 800 °C, 850 °C and 900 °C, and the stress–strain curves were obtained, as shown in Figure 11. The tensile strength and elongation at break data of samples at different sintering temperatures are shown in

Table 4. Tensile mechanical property parameters of copper powder porous materials at different sintering temperatures.

Sintering Temperature/°C	$\mathbf{Tensile} \mathbf{Strength} \mathit{\sigma }$m/MPa	$\mathbf{Elongation} \mathbf{After} \mathbf{Breaking} {\mathit{\sigma }}_{\mathit{h}}$/%
700	59.0	7.0
750	62.6	7.0
800	70.7	11.2
850	92.9	14.1
900	121.6	19.5

.

From the tensile behavior curve and tensile property data, we can clearly draw a conclusion that the tensile strength and elongation increase with the increase in sintering temperature. When the sintering temperature is 700 °C, the tensile strength is 59 MPa, and when the sintering temperature is 900 °C, the tensile strength increases to 121.6 MPa; the elongation also increased from 7% to 19.5%.

The forming quality and quantity of sintered necks play a key role in the mechanical properties of copper-based porous materials [20]. Based on the new sintering model proposed by Huang et al. [21], the pre-melting of particle surface can be explained by the thermally less stable surface atoms due to the lower coordination number than with bulk ones. The melting process of a particle could be separated into two quite different stages, the gradual surface pre-melting and the homogeneous core melting. In the thermal processing, with the increase of temperature, the particles will melt layer by layer from the outmost part of the surface. The melted thickness permits the shrinkage of distance between two adjacent particles. The temperature will determine the melted thickness of the surface, thus enhancing the mechanical strength of the product and simultaneously avoiding structure collapse of the product. The increase in the sintering temperature will lead to the generation and expansion of a large number of sintering necks inside the specimen material, and the specimens prepared at the sintering temperature of 900 °C have better mechanical properties.

3.4.2. Copper Powder + Copper Mesh Porous Material

In order to investigate the effect of sintering temperature on the copper powder + copper mesh porous materials, the experiments were carried out on the copper powder + copper mesh porous materials (copper mesh content of 4%) prepared at sintering temperatures of 700 °C, 750 °C, 800 °C, 850 °C and 900 °C, which were cut and the tensile test specimens were prepared.

Figure 12 shows the stress–strain curves of the specimens prepared at different sintering temperatures. It can be seen that in the elastic deformation stage, the five stress–strain curves roughly coincide, indicating that the elastic properties are similar. However, the strain value required for the yielding phenomenon is positively correlated with the sintering temperature. With increasing tension, the specimens deformed plastically. The sintering temperature has a greater effect on the tensile strength of copper powder + copper mesh porous materials. At the same time, the higher the sintering temperature, the greater the effect of the tensile strength of the specimen, and work hardening shows an increasing trend. In the tensile process, the degree of stress change of the specimens at 700 °C and 750 °C sintering temperature is small, while the degree of stress change of the specimens at 800 °C, 850 °C and 900 °C sintering temperature is large, and the stress required for fracture of the specimens prepared at high sintering temperature is higher. This indicates that the sintered neck between copper mesh and copper powder is well formed at 800 °C sintering temperature, thus obtaining copper powder + copper mesh porous materials with good mechanical properties.

Table 5. Parameters of tensile mechanical properties of copper powder + copper mesh porous materials at different sintering temperatures.

Sintering Temperature/°C	$\mathbf{Tensile} \mathbf{Strength} \mathit{\sigma }$m/MPa	$\mathbf{Elongation} \mathbf{After} \mathbf{Breaking} {\mathit{\sigma }}_{\mathit{h}}$/%
700	63.5	10.8
750	68.2	10.8
800	89.1	14.2
850	112.5	14.4
900	132.2	14.6

lists the tensile strength and elongation after breaking of copper powder + copper mesh porous samples prepared at different sintering temperatures. It can be seen that. compared to the 700 °C sintering temperature, the tensile strength of the sample prepared at the 900 °C sintering temperature is increased by 68.7 MPa, an increase of about 108.3%, and the elongation after breaking is increased by 3.8%, improved by about 35%.

3.4.3. Copper Powder + NaCl Porous Material

In order to explore the effect of sintering temperature on copper powder + NaCl porous materials, experiments were performed on copper powder + NaCl porous materials prepared at sintering temperatures of 800 °C, 900 °C, and 1000 °C (the content ratio of copper powder to NaCl is 14:1). Tensile test specimens were prepared.

Figure 13 shows the stress–strain curves of copper powder + NaCl type porous samples prepared at different sintering temperatures. It can be seen that the stress–strain curves of copper powder + NaCl porous materials prepared at sintering temperatures of 800 °C and 900 °C are relatively similar. Compared with the sintering temperatures of 800 °C and 900 °C, the elastic deformation stage of the copper powder + NaCl porous material prepared at the sintering temperature of 1000 °C is weaker, but the maximum stress required in the yield stage, plastic deformation stage and fracture is significantly higher.

Table 6. Tensile mechanical parameters of copper powder + NaCl porous materials at different sintering temperatures.

Sintering Temperature/°C	$\mathbf{Tensile} \mathbf{Strength} \mathit{\sigma }$m/MPa	$\mathbf{Elongation} \mathbf{After} \mathbf{Breaking} {\mathit{\sigma }}_{\mathit{h}}$/%
800	6.2	3.2
900	6.3	3.7
1000	8.3	4.3

lists the tensile strength and elongation after breaking of copper powder + NaCl porous samples prepared at different sintering temperatures. It can be seen that increasing the sintering temperature is beneficial of improving the mechanical properties of copper powder + NaCl porous materials. Compared with the 800 °C sintering temperature, the tensile strength of the specimen prepared at the 1000 °C sintering temperature increased by 2.1 MPa, and the elongation after breaking increased by 1.1%.

3.4.4. Comparing the Mechanical Properties of the Three Structures

Experimental comprehensive and comparative analysis took place of the mechanical properties of copper powder type, copper powder + copper mesh type and copper powder + NaCl porous materials prepared at sintering temperatures of 800 °C and 900 °C, as shown in Figure 13.

It can be seen from Figure 14 that the tensile strength of the copper powder + copper mesh sintered body is the highest no matter whether the sintering temperature is 800 °C or 900 °C. The strengthening mechanism of copper mesh is analyzed. The main points are as follows: in the cold pressing stage, the composite structure of copper mesh and copper powder is strengthened by mechanical interlocking; During sintering, the shrinkage of copper powder particles and the stability of copper mesh form a dense and coherent reinforcement network. The copper mesh is embedded in the copper powder matrix as a continuous metal skeleton, and the load is preferentially borne in the tensile process. The high strength of copper mesh directly bears most of the stress and significantly improves the overall tensile strength. The toughness of copper mesh can hinder the crack propagation in the copper powder matrix. When the crack meets the copper mesh, it needs to bypass or tear the copper wire, which needs to consume more energy, so as to improve the fracture toughness of the material.

3.4.5. Tensile Fracture Mechanisms of Sintered Copper-Based Porous Materials

According to the plasticity of the material and brittle deformation, the elongation after breaking is greater than 5% for toughness fracture, and less than 5% for brittle fracture, so the metal tensile fracture is divided into brittle fracture and toughness fracture [22]. Copper is a metal with a face-centered cubic crystal structure, and no isotropic transformation. It features 12 slip systems during plastic deformation, and good plastic deformation ability [23]. The fracture of the sintered copper powder structure occurred at the sintered neck, the powder particles did not undergo obvious plastic deformation, the transverse has a slight crack, the brittle fracture of the sintered copper powder structure is obvious, the sintered neck is the weak link in resisting tensile deformation, and the stress concentration leads to the fracture damage of the sintered neck, as shown in Figure 15.

As can be seen from Figure 16, the fracture of copper powder + copper mesh occurs in the sintered neck region between the copper powder particles as well as on the filaments, and it can be observed that the fracture of the sintered neck between the powder and the powder is still an obvious brittle fracture, but the filaments have obvious neck shrinkage in the axial direction, and there is an obvious toughness nest on the end face, which is typical of ductile fracture, indicating that there are brittle and ductile fractures in the composite structure when it is a tensile fracture. The cracks first arise from the sintered neck between the copper particles and then extend to the weak region. With the fracture of the sintered necks, the stresses are gradually concentrated to the copper wire lines, leading to necking fracture. The addition of wire mesh can significantly improve the tensile strength.

Copper powder + NaCl type porous materials are similar to copper powder type porous materials. The addition of pore-forming agent NaCl increases the pores of copper powder porous materials, and the number of sintering necks formed between powders is greatly reduced, resulting in a sharp decrease in tensile strength, as shown in Figure 17.

3.5. Effect of Sintering Temperature on the Compressive Mechanical Properties of Copper-Based Porous Materials

3.5.1. Porous Materials Made of Pure Copper Powder

Figure 18 shows the stress–strain curves of pure copper powder type porous specimens in axial compression at different sintering temperatures. It can be seen that the deformation of the pure copper powder porous specimen is divided into an elastic deformation stage and plastic deformation stage during axial compression [10]. In the elastic deformation stage, the pores mainly undergo elastic compression and return to their original shape when the pressure is reduced [24]. In the elastic deformation stage, the pore mainly undergoes elastic compression, to return to the original shape when the pressure is reduced; the strain range of this stage is about 2%. In the plastic deformation stage, the pore undergoes plastic deformation and, with the increase in pressure, the surface of the pore wall is pressed out of the cracks, the pore is gradually ruptured by the extrusion, and the pore is gradually crushed by the pressure [25,26].The stress–strain curve rises gently as the pores inside the specimen are gradually compacted. The internal pores of the sample are gradually compacted, the stress–strain curve rises gently, the stress is almost constant, and the strain increases, so the copper-based porous material has good energy-absorbing properties [22,27,28,29]. Finally, the stress–strain curve rises sharply, indicating that the porous material is fully compacted, the strain remains constant and the stress increases. Finally, the stress–strain curve rises sharply, indicating that the porous material is fully compacted, the strain remains constant and the stress increases sharply.

Huang et al. [21] pointed out that, with the increase in sintering temperature, the volume shrinkage of materials depends on the melting degree of particles. The shrinkage of the volume will increase gradually with the increase in the sintering temperature. This can advance the compression strength of the materials.

The compression strength of the pure copper powder porous specimens increased with the increase in sintering temperature. The porous specimens prepared at 700 °C and 800 °C sintering temperature had similar elastic stage curves. When the sintering temperature increased to 900 °C, the elastic and plastic phases appeared to be obviously different, and the compression breaking stress was obviously increased.

Table 7. Compression performance parameters of copper powder type porous specimens.

Sintering Temperature/°C	Compression Strength/MPa	Compression Ratio/%
700 °C	270.2	17.6
800 °C	375.1	26.2
900 °C	443.5	38.8

shows the compression performance of copper powder type porous samples. It can be seen that, compared with the 700 °C sintering temperature, the compression strength of the sample prepared at 900 °C sintering temperature increases by 173.3 MPa, which is about 64.1% higher, and the compression ratio increases by 21.2%, which is about 120.8% higher. As the sintering temperature increases, the compression performance of the copper powder-type porous samples is significantly improved, indicating that the sintering necks between copper powder particles are formed more firmly at high sintering temperatures and the mechanical properties are significantly improved.

3.5.2. Copper Powder + Copper Mesh Porous Material

Figure 19 shows the compression performance stress–strain curves of copper powder + copper mesh type (copper mesh content 4%) porous samples prepared at sintering temperatures of 700 °C, 800 °C and 900 °C. It can be seen that the compression deformation process of the copper powder + copper mesh porous sample is divided into two stages, the elastic deformation stage and the plastic deformation stage. The stress required for compression to break the ring increases as the sintering temperature increases. When the sintering temperatures are 700 °C and 800 °C, the elastic deformation stages of the copper powder + copper mesh porous samples are relatively close. When the sintering temperature increases to 900 °C, the influence of the elastic deformation stage and the plastic deformation stage is more significant, and the stress required for compression breakage increases significantly.

Table 8. Compression performance parameters of copper powder + copper mesh type porous specimens.

Sintering Temperature/°C	Compression Strength/MPa	Compression Ratio/%
700 °C	321.5	26.9
800 °C	401.8	36.2
900 °C	458.4	48.7

shows the compression performance of copper powder + copper mesh type porous samples. It can be seen that, compared with the 700 °C sintering temperature, the compression strength of the sample prepared at the 900 °C sintering temperature increases by 136.86 MPa, which is about 42.57%, and the compression ratio increases by 21.825%.

3.5.3. Copper Powder + NaCl Porous Material

Figure 20 shows the compression performance stress–strain curves of copper powder + NaCl type (copper powder: NaCl = 16:1) porous samples prepared at sintering temperatures of 800 °C, 900 °C and 1000 °C. It can be seen that the compression deformation process of the copper powder + NaCl type porous sample is also the elastic deformation stage and the plastic deformation stage, and its compression performance is enhanced as the sintering temperature increases. In the elastic deformation stage, the curves at the three sintering temperatures almost overlap, indicating that the addition of the pore-forming agent NaCl does not significantly enhance the performance of the copper-based porous sample. In the plastic deformation stage, as the sintering temperature increases, the compressive resistance of the copper powder + NaCl type porous sample is enhanced.

Table 9. Compression performance parameters of copper powder + NaCl type porous samples.

Sintering Temperature/°C	Compression Strength/MPa	Compression Ratio/%
800 °C	351.968	44.425
900 °C	400.631	46.667
1000 °C	518.521	48.125

shows the compression performance of copper powder + NaCl type porous samples. It can be seen that compared with the 800 °C sintering temperature, the compression strength of the sample prepared at 1000 °C sintering temperature increases by 166.6 MPa, which is about 47.3%, and the compression ratio increases by 3.7%.

3.5.4. Compressive Mechanical Properties of Different Porous Materials

The experiments comprehensively and comparatively analyzed the compression mechanical properties of copper powder type, copper powder + copper mesh type and copper powder + NaCl porous materials prepared at sintering temperatures of 800 °C and 900 °C, as shown in Figure 21.

It can be seen from the figure that the compressive strength of copper powder + copper mesh sintered body is the highest no matter whether the sintering temperature is 800 °C or 900 °C. However, compared with the tensile strength, the increase in compressive strength is limited. This is mainly because the copper mesh forms a continuous metal skeleton during the sintering process, providing a rigid support network. Under compression load, this network can transfer stress more evenly, reduce local stress concentration, and provide a beneficial effect from improving the compressive strength of materials. The compressive strength mainly depends on the density of the material, the bonding strength between matrix particles and the uniformity of pore distribution. Different from the tensile strength, the material under compressive load is more likely to disperse stress through plastic deformation and pore closure, rather than directly produce crack propagation. As a continuous reinforcement, copper mesh can significantly improve strength by preventing crack propagation in tension, but its role in compression is to provide rigid support rather than directly resist fracture. The densification and pore collapse of the matrix (sintered copper powder) during compression may dominate the deformation process, and the reinforcement effect of copper mesh is relatively weak.

4. Conclusions

In this study, three types of copper-based porous materials were prepared via solid-phase sintering, and their structural characteristics and mechanical properties were systematically analyzed. The main conclusions are as follows:

• The increase of sintering temperature promotes the diffusion between particles and metallurgical combination so, with the increase of sintering temperature, the tensile strength of pure copper powder sinter, copper powder + copper mesh sinter and copper powder + NaCl sinter will increase. However, for the materials sintered at the same temperature, the tensile strength and compressive strength of copper powder + copper mesh sintered body are the highest, which indicates that the copper mesh, as a continuous metal skeleton embedded in the copper powder matrix, directly bears most of the stress and significantly improves the overall mechanical properties.
• Adding NaCl into pure copper powder as pore forming agent can significantly improve the porosity of the material, and can significantly increase the pore diameter of the material and improve the permeability of the material. However, from the point of view of the material mechanical properties test, adding the pore forming agent significantly weakened the mechanical properties of the material, including tensile strength, compressive strength and plastic properties. This material is suitable for filtration, but it is not suitable for ultra-thin heat pipes, especially flexible heat pipes. The strength and plastic properties of the porous material with copper mesh have been significantly improved, which can be expected to be used in flexible heat pipes.
• Although adding copper mesh to copper powder can effectively improve the mechanical properties of this material, the improvement in pore size and permeability is not obvious, so we should focus on this direction in subsequent research.