Al2O3-Cu Substrate with Co-Continuous Phases Made by Powder Sintering Process

Ceramic-Al substrates with co-continuous ceramic and metal phases, which exhibit high thermal conductivity and compatible coefficient of thermal expansion (CTE), have been widely investigated through the process of die-casting. In this research, a kind of powder sintering process was proposed for fabricating ceramic-Cu composite substrates with co-continuous phases. Copper fiber (Cuf) has excellent thermal conductivity and large aspect ratio, making it an ideal material to form bridging network structures in the ceramic-Cu composite. To maintain the large aspect ratio of Cuf, and densify the composite substrate, ZnO-SiO2-CaO glass was introduced as a sintering additive. Both Al2O3/glass/Cuf and Al2O3/30glass/Cup composite substrates were hot-pressed at 850 °C under 25 MPa. Experimental results showed that the thermal conductivity of Al2O3/30glass/30Cuf composite substrate was as high as 38.9 W/mK, which was about 6 times that of Al2O3/30glass; in contrast, the thermal conductivity of Al2O3/30glass/30Cup composite substrate was only 25.9 W/mK. Microstructure observation showed that, influenced by hot press and corrosion of molten ZnO-SiO2-CaO glass, the copper fibers were deformed under hot-pressing, and some local melting-like phenomena occurred on the surface of copper fiber at 850 °C under 25 MPa. The molten phase originating from surface of Cuf welded the overlapping node of copper fibers during cooling process. Finally, the interconnecting metal bridging in ceramic matrix was formed and behaved as a rapid heat-dissipating channel, which is similar to substrates prepared through die-casting process by porous ceramic and melted Al.

The raw materials were milled and mixed homogeneously. Then, the powders were dried and melted in an alumina crucible at 1230 • C for 60 min. Next, the molten glass was poured into deionized water. Finally, the glass frits were milled in Al 2 O 3 jars with ZrO 2 balls at the speed of 80 r/min for 40 h.
Based on the volume content and shape of Cu in Al 2 O 3 /30glass/Cu composites, the experimental samples were designed as two contrasting groups, as shown in Table 2. In the traditional hot-press process, fibers or whiskers in the matrix are rotated under pressure and preferentially aligned parallel to the sample's surface. To avoid the preferential alignment of the copper fiber and form a three-dimensional (3D) network in the Al 2 O 3 /glass green matrix [16], a hot-press process with multi-stage pressure was employed, as shown in Figure 1. to the sample's surface. To avoid the preferential alignment of the copper fiber and form a threedimensional (3D) network in the Al2O3/glass green matrix [16], a hot-press process with multi-stage pressure was employed, as shown in Figure 1. Table 2. Composition of Al2O3/glass/Cu composites (in volume). The raw materials were mixed according to the proportions listed in Table 2 and milled in ethyl alcohol for 2 h. After drying and sieving, the mixture was placed in a graphite die with inner diameter of φ30 mm and height of 100 mm, and loaded in a hot-pressed sintering furnace under N2 atmosphere (FVPHP-R-10 FRET-40, Fujidempa Kogyo, Osaka, Japan). About five samples with size of φ30 mm × 3 mm could be fabricated within each furnace. To form a 3D net of Cuf in the composite, the pressure was applied in stages. The sintering process involved the following stages: (1) pre-sintering from room temperature to 650 °C in a stress-free manner; (2) sintering from 650 °C to 700 °C under 5 MPa (maintained for 15 min); (3) sintering from 800 °C to 850 °C under 25 MPa at a heating rate of 5 °C/min (maintained for 50 min), as shown in Figure 1. By a ceramic cutting machine (IsoMet 1000, Buehler, Lake Bluff, IL, USA), the sintered substrates with size of φ30 mm × 3 mm were cut into samples of 25 mm × 5 mm × 3 mm for CTE measuring or 10 mm × 10 mm × 1.5 mm for thermal conductivity.

Measurements
The relative density of the sintered specimens was measured by utilizing Archimedes' principle. The thermal conductivity of the composites was measured using a laser thermal conductance instrument (LFA457/2/G, Netzsch, Selb, Germany). The final measurement result was the average of 5 testing values. The CTE of the composites was measured by a thermal dilatometer (DIL402C, Netzsch, Selb, Germany). Phase identification of the composites was carried out by X-ray diffraction (XRD, D/max2000v/pc, Rigaku, Toyko, Japan). The X-ray source was Ni-filtered CuKα radiation, the X-ray source operating voltage was 40 kV. The microstructure and element distribution of the sintered composites was characterized by scanning electron microscopy (SEM, ULTRA PLUS-43-13, The raw materials were mixed according to the proportions listed in Table 2 and milled in ethyl alcohol for 2 h. After drying and sieving, the mixture was placed in a graphite die with inner diameter of φ30 mm and height of 100 mm, and loaded in a hot-pressed sintering furnace under N 2 atmosphere (FVPHP-R-10 FRET-40, Fujidempa Kogyo, Osaka, Japan). About five samples with size of φ30 mm × 3 mm could be fabricated within each furnace. To form a 3D net of Cu f in the composite, the pressure was applied in stages. The sintering process involved the following stages: (1) pre-sintering from room temperature to 650 • C in a stress-free manner; (2) sintering from 650 • C to 700 • C under 5 MPa (maintained for 15 min); (3) sintering from 800 • C to 850 • C under 25 MPa at a heating rate of 5 • C/min (maintained for 50 min), as shown in Figure 1. By a ceramic cutting machine (IsoMet 1000, Buehler, Lake Bluff, IL, USA), the sintered substrates with size of φ30 mm × 3 mm were cut into samples of 25 mm × 5 mm × 3 mm for CTE measuring or 10 mm × 10 mm × 1.5 mm for thermal conductivity.

Measurements
The relative density of the sintered specimens was measured by utilizing Archimedes' principle. The thermal conductivity of the composites was measured using a laser thermal conductance instrument (LFA457/2/G, Netzsch, Selb, Germany). The final measurement result was the average of 5 testing values. The CTE of the composites was measured by a thermal dilatometer (DIL402C, Netzsch, Selb, Germany). Phase identification of the composites was carried out by X-ray diffraction (XRD, D/max2000v/pc, Rigaku, Toyko, Japan). The X-ray source was Ni-filtered CuKα radiation, the X-ray source operating voltage was 40 kV. The microstructure and element distribution of the sintered composites was characterized by scanning electron microscopy (SEM, ULTRA PLUS-43-13, Zeiss Ultra Plus, Jena, Germany) and transmission electron microscopy (TEM, Talos F200, FEI, Hillsboro, OR, USA).

Relative Density of the Composites
Glass additives usually behave as a liquid phase to lower the sintering temperature of Al 2 O 3 composites. In this research, 30 vol % ZnO-SiO 2 -CaO glasses were added to the Al 2 O 3 /glass/Cu composite to ensure the sintering temperature was below 850 • C. Different contents of copper (either copper fibers material or copper particles material) from 0 to 30 vol % were added to the Al 2 O 3 /glass/Cu composites. The glass additives were expected to melt when sintered at 850 • C. As can be seen from Figure 2, the relative density of the Al 2 O 3 /30glass composite without Cu was only 87.6%. The relative density of Al 2 O 3 /30glass/Cu f and Al 2 O 3 /30glass/Cu p composites both increased dramatically when adding 10 vol % Cu f or Cu p into the Al 2 O 3 /30glass composite. This is because both copper phase and glass phase are deformable during hot-pressing at 850 • C, which is beneficial for the densification of Al 2 O 3 /glass/Cu composite. Because the volume of glass phase in these composites is constant, the greater the content of copper phase, the lower the content of Al 2 O 3 will be. Thus, relative density is improved with the increase of Cu phase content. It is worth noting that the relative density of Al 2 O 3 /30glass/Cu f is slightly higher than that of Al 2 O 3 /30glass/Cu p . This abnormal result is related to the presence of ZnO-SiO 2 -CaO glass phases. Generally, the ZnO-SiO 2 -CaO glass phase sintered under 850 • C behaves as if in liquid state. When the sintering temperature surpasses 600 • C (Tg of ZnO-SiO 2 -CaO glass), the glass begins to melt, and liquid glass still has high viscosity. It fills the intergranular pores of Al 2 O 3 particles, reducing the possibility of gas escaping, and some pores are enclosed in the Al 2 O 3 /30glass composite. The plastic deformation of Cu phase could significantly increase the densification of Al 2 O 3 /glass/Cu composites under hot-press conditions. Moreover, when adding some content of Cu f to the Al 2 O 3 /glass/Cu f composites, the liquid glass had a similar volume and viscosity to those of the Al 2 O 3 /30glass/Cu p composite. However, the liquid glass was able to infiltrate along the surface of the Cu fiber, accelerating the infiltrating speed and extending infiltrating range of the Al 2 O 3 /glass/Cu f composites, and providing a spatial structure for the inner gas to escape. Zeiss Ultra Plus, Jena, Germany) and transmission electron microscopy (TEM, Talos F200, FEI, Hillsboro, OR, USA).

Relative Density of the Composites
Glass additives usually behave as a liquid phase to lower the sintering temperature of Al2O3 composites. In this research, 30 vol % ZnO-SiO2-CaO glasses were added to the Al2O3/glass/Cu composite to ensure the sintering temperature was below 850 °C. Different contents of copper (either copper fibers material or copper particles material) from 0 to 30 vol % were added to the Al2O3/glass/Cu composites. The glass additives were expected to melt when sintered at 850 °C. As can be seen from Figure 2, the relative density of the Al2O3/30glass composite without Cu was only 87.6%. The relative density of Al2O3/30glass/Cuf and Al2O3/30glass/Cup composites both increased dramatically when adding 10 vol % Cuf or Cup into the Al2O3/30glass composite. This is because both copper phase and glass phase are deformable during hot-pressing at 850 °C, which is beneficial for the densification of Al2O3/glass/Cu composite. Because the volume of glass phase in these composites is constant, the greater the content of copper phase, the lower the content of Al2O3 will be. Thus, relative density is improved with the increase of Cu phase content. It is worth noting that the relative density of Al2O3/30glass/Cuf is slightly higher than that of Al2O3/30glass/Cup. This abnormal result is related to the presence of ZnO-SiO2-CaO glass phases. Generally, the ZnO-SiO2-CaO glass phase sintered under 850 °C behaves as if in liquid state. When the sintering temperature surpasses 600 °C (Tg of ZnO-SiO2-CaO glass), the glass begins to melt, and liquid glass still has high viscosity. It fills the intergranular pores of Al2O3 particles, reducing the possibility of gas escaping, and some pores are enclosed in the Al2O3/30glass composite. The plastic deformation of Cu phase could significantly increase the densification of Al2O3/glass/Cu composites under hot-press conditions. Moreover, when adding some content of Cuf to the Al2O3/glass/Cuf composites, the liquid glass had a similar volume and viscosity to those of the Al2O3/30glass/Cup composite. However, the liquid glass was able to infiltrate along the surface of the Cu fiber, accelerating the infiltrating speed and extending infiltrating range of the Al2O3/glass/Cuf composites, and providing a spatial structure for the inner gas to escape.   Figure 3 shows the morphology of the copper fiber materials. It can be seen that the diameters of the copper fibers were about 2 µm and there were some Cu particles present in the copper fiber raw material. The melting point of Cu is 1083 • C. When the powder mixture of Al 2 O 3 /glass/Cu f was hot-pressed from room temperature to 850 • C (which is far below the melting point of Cu) under a nitrogen atmosphere, the copper fibers would overlap with each other and form a 3D net. However, as shown in Figure 4, no straight fibers were observed in the fracture surface of the Al 2 O 3 /30glass/30Cu f composite. It can be seen that there existed some vermiform phase in the fracture surface of the Al 2 O 3 /30glass/30Cu composite parallel to the pressure direction. The XRD result showed that there were only peaks of copper phase present in the XRD pattern ( Figure 5).

Microstructure of the Composites
The distribution of Cu element on the fracture surface of the Al 2 O 3 /30glass/30Cu f composite parallel to the pressure direction was characterized by energy dispersive X-ray spectroscopy (EDX), as shown in Figure 6b. The trace of copper became continuous in the composite, which is highly consistent with the microstructure of the Al 2 O 3 /30glass/30Cu f composite ( Figure 6a). Figure 7b shows the distribution of Cu element on the polished surface of the Al 2 O 3 /30glass/30Cu f composite perpendicular to the pressure direction. It is worthy of note that most of the Cu element is distributed dotted on the polished surface, which means that most copper fibers are tilted at an angle with the surface of the substrate. Combined with Figures 6 and 7, it could be concluded that with this process, most copper fibers successfully avoid preferential orientation under hot-pressed conditions. The 3D connected net was formed by Cu fibers' bridging with each other.  Figure 3 shows the morphology of the copper fiber materials. It can be seen that the diameters of the copper fibers were about 2 µm and there were some Cu particles present in the copper fiber raw material. The melting point of Cu is 1083 °C. When the powder mixture of Al2O3/glass/Cuf was hot-pressed from room temperature to 850 °C (which is far below the melting point of Cu) under a nitrogen atmosphere, the copper fibers would overlap with each other and form a 3D net. However, as shown in Figure 4, no straight fibers were observed in the fracture surface of the Al2O3/30glass/30Cuf composite. It can be seen that there existed some vermiform phase in the fracture surface of the Al2O3/30glass/30Cu composite parallel to the pressure direction. The XRD result showed that there were only peaks of copper phase present in the XRD pattern ( Figure 5).

Microstructure of the Composites
The distribution of Cu element on the fracture surface of the Al2O3/30glass/30Cuf composite parallel to the pressure direction was characterized by energy dispersive X-ray spectroscopy (EDX), as shown in Figure 6b. The trace of copper became continuous in the composite, which is highly consistent with the microstructure of the Al2O3/30glass/30Cuf composite (Figure 6a). Figure 7b shows the distribution of Cu element on the polished surface of the Al2O3/30glass/30Cuf composite perpendicular to the pressure direction. It is worthy of note that most of the Cu element is distributed dotted on the polished surface, which means that most copper fibers are tilted at an angle with the surface of the substrate. Combined with Figures 6 and 7, it could be concluded that with this process, most copper fibers successfully avoid preferential orientation under hot-pressed conditions. The 3D connected net was formed by Cu fibers' bridging with each other.    Figure 3 shows the morphology of the copper fiber materials. It can be seen that the diameters of the copper fibers were about 2 µm and there were some Cu particles present in the copper fiber raw material. The melting point of Cu is 1083 °C. When the powder mixture of Al2O3/glass/Cuf was hot-pressed from room temperature to 850 °C (which is far below the melting point of Cu) under a nitrogen atmosphere, the copper fibers would overlap with each other and form a 3D net. However, as shown in Figure 4, no straight fibers were observed in the fracture surface of the Al2O3/30glass/30Cuf composite. It can be seen that there existed some vermiform phase in the fracture surface of the Al2O3/30glass/30Cu composite parallel to the pressure direction. The XRD result showed that there were only peaks of copper phase present in the XRD pattern ( Figure 5).

Microstructure of the Composites
The distribution of Cu element on the fracture surface of the Al2O3/30glass/30Cuf composite parallel to the pressure direction was characterized by energy dispersive X-ray spectroscopy (EDX), as shown in Figure 6b. The trace of copper became continuous in the composite, which is highly consistent with the microstructure of the Al2O3/30glass/30Cuf composite (Figure 6a). Figure 7b shows the distribution of Cu element on the polished surface of the Al2O3/30glass/30Cuf composite perpendicular to the pressure direction. It is worthy of note that most of the Cu element is distributed dotted on the polished surface, which means that most copper fibers are tilted at an angle with the surface of the substrate. Combined with Figures 6 and 7, it could be concluded that with this process, most copper fibers successfully avoid preferential orientation under hot-pressed conditions. The 3D connected net was formed by Cu fibers' bridging with each other.       Figure 8 shows the bright field TEM micrograph of the Al2O3/30glass/30Cuf composite. In this figure, it can be clearly seen that the Cu phases (dark) are deformed around the alumina (bright) and glass phases (gray). There are no pores enclosed in the interface between Cuf and alumina or the interface between Cuf and glass. The combination of these phases is very tight. A network structure    Figure 8 shows the bright field TEM micrograph of the Al2O3/30glass/30Cuf composite. In this figure, it can be clearly seen that the Cu phases (dark) are deformed around the alumina (bright) and glass phases (gray). There are no pores enclosed in the interface between Cuf and alumina or the interface between Cuf and glass. The combination of these phases is very tight. A network structure    Figure 8 shows the bright field TEM micrograph of the Al2O3/30glass/30Cuf composite. In this figure, it can be clearly seen that the Cu phases (dark) are deformed around the alumina (bright) and glass phases (gray). There are no pores enclosed in the interface between Cuf and alumina or the interface between Cuf and glass. The combination of these phases is very tight. A network structure  Figure 8 shows the bright field TEM micrograph of the Al 2 O 3 /30glass/30Cu f composite. In this figure, it can be clearly seen that the Cu phases (dark) are deformed around the alumina (bright) and glass phases (gray). There are no pores enclosed in the interface between Cu f and alumina or the interface between Cu f and glass. The combination of these phases is very tight. A network structure is apparent through the overlapping of the copper fibers. Figure 9a is a local enlarged HAADF-STEM micrograph of Figure 8. Please note that compared with the interface between alumina and copper fibers, the boundary of the copper fiber and glass is much rougher. In contact with the vermiform morphology of Cu characterized by SEM in Figures 6 and 7, it can be deduced that, influenced by melted glass etching and hot-press, both plastic deformation and chemical-mechanical interaction happened to the local surface of the copper fiber at 850 • C (which is far below the melting point of copper, 1083 • C). At the beginning of the hot-press, copper fibers were deformed under a pressure of 25 Mpa, and their surface oxide layers were destroyed by glass powders and Al 2 O 3 powders. Along with the increasing of sintering temperature, the copper fibers under high outside pressure were corroded by molten glass at high temperature. Then, the melting of the local surface in the overlapping node of Cu fibers can result in a welding effect, which would significantly promote the thermal conduction ability of the Al 2 O 3 /30glass/30Cu f composite. Figure 9b,c shows the distributions of Cu, Zn and Al elements in Al 2 O 3 /30glass/30Cu f composite by EDX. The distribution zone of Zn corresponds to glass phase. From Figure 9a, it can be seen that there is a rough gray layer existing between the copper fiber and the glass phase. is apparent through the overlapping of the copper fibers. Figure 9a is a local enlarged HAADF-STEM micrograph of Figure 8. Please note that compared with the interface between alumina and copper fibers, the boundary of the copper fiber and glass is much rougher. In contact with the vermiform morphology of Cu characterized by SEM in Figures 6 and 7, it can be deduced that, influenced by melted glass etching and hot-press, both plastic deformation and chemical-mechanical interaction happened to the local surface of the copper fiber at 850 °C (which is far below the melting point of copper, 1083 °C). At the beginning of the hot-press, copper fibers were deformed under a pressure of 25 Mpa, and their surface oxide layers were destroyed by glass powders and Al2O3 powders. Along with the increasing of sintering temperature, the copper fibers under high outside pressure were corroded by molten glass at high temperature. Then, the melting of the local surface in the overlapping node of Cu fibers can result in a welding effect, which would significantly promote the thermal conduction ability of the Al2O3/30glass/30Cuf composite. Figure 9b,c shows the distributions of Cu, Zn and Al elements in Al2O3/30glass/30Cuf composite by EDX. The distribution zone of Zn corresponds to glass phase. From Figure 9a, it can be seen that there is a rough gray layer existing between the copper fiber and the glass phase.  Therefore, it could be proved that the Cuf deformed and appeared as melting-like phenomena on the local surface contacting with glass when the composite was hot-pressed at 850 °C under nitrogen atmosphere. The local surface melting-like phenomenon of Cuf at 850 °C under hot-pressing may be caused by the combined influences of relative high pressure and erosion of the molten glass. is apparent through the overlapping of the copper fibers. Figure 9a is a local enlarged HAADF-STEM micrograph of Figure 8. Please note that compared with the interface between alumina and copper fibers, the boundary of the copper fiber and glass is much rougher. In contact with the vermiform morphology of Cu characterized by SEM in Figures 6 and 7, it can be deduced that, influenced by melted glass etching and hot-press, both plastic deformation and chemical-mechanical interaction happened to the local surface of the copper fiber at 850 °C (which is far below the melting point of copper, 1083 °C). At the beginning of the hot-press, copper fibers were deformed under a pressure of 25 Mpa, and their surface oxide layers were destroyed by glass powders and Al2O3 powders. Along with the increasing of sintering temperature, the copper fibers under high outside pressure were corroded by molten glass at high temperature. Then, the melting of the local surface in the overlapping node of Cu fibers can result in a welding effect, which would significantly promote the thermal conduction ability of the Al2O3/30glass/30Cuf composite. Figure 9b,c shows the distributions of Cu, Zn and Al elements in Al2O3/30glass/30Cuf composite by EDX. The distribution zone of Zn corresponds to glass phase. From Figure 9a, it can be seen that there is a rough gray layer existing between the copper fiber and the glass phase.  Therefore, it could be proved that the Cuf deformed and appeared as melting-like phenomena on the local surface contacting with glass when the composite was hot-pressed at 850 °C under nitrogen atmosphere. The local surface melting-like phenomenon of Cuf at 850 °C under hot-pressing may be caused by the combined influences of relative high pressure and erosion of the molten glass. Therefore, it could be proved that the Cu f deformed and appeared as melting-like phenomena on the local surface contacting with glass when the composite was hot-pressed at 850 • C under nitrogen atmosphere. The local surface melting-like phenomenon of Cu f at 850 • C under hot-pressing may be caused by the combined influences of relative high pressure and erosion of the molten glass. The surface and fracture microstructure of the Al 2 O 3 /30glass/30Cu f composite shows that the vermiform Cu is interconnected and forms a continuous 3D network, which is very similar to the ideal microstructure of the co-continuous ceramic-metal composites obtained from die-casting or reactive metal penetration process [15]. A synthesis of the conclusions drawn from the melting points of glass and copper fiber, as well as the final microstructure, the sintering procedure of Al 2 O 3 /glass/Cu f could be stated as follows: (1) At low temperature, the green body of Al 2 O 3 /glass/Cu f was formed, the Cu fiber deformed and the oxide layer on the surface of copper fiber was destroyed by glass and Al 2 O 3 particles under outside pressure. (2) ZnO-SiO 2 -CaO glass melted when the sintering temperature exceeded 600 • C.
(3) Melted glass infiltrated along the Cu fiber surface and inner gas escaped due to the spatial structure of fibers. The local surface of copper fibers corroded by the combined alternative influences of relative high pressure and erosion of the molten glass when the sintering temperature approached 850 • C. (4) The ceramic green body gradually finished densification during the hot-pressing process. (5) The deformed Cu fibers welded at the overlapping node during cooling process.
When using the die-casting process or the reactive metal penetration process, it is hard to guarantee the even distributing and continuous of metal phase. In addition, the fabricating efficiency of the die-casting process for composite substrates is limited [15,17]. Because fibers are easy to mix evenly with ceramic particles, the Al 2 O 3 /Cu ceramic composite with continuous Cu phase could be fabricated more simply and efficiently by the low-temperature hot-pressing process than by the die-casting or reactive metal penetration processes.

Thermal Performance
Ceramic-metal substrates with co-continuous ceramic-metal phases have both high thermal conductivity and compatible CTEs, but the die-casting infiltration process is too complicated to control composite quality and requires expensive equipment. The CTEs of Al 2 O 3 /30glass/Cu f composites at 20 • C are shown in Table 3. With the increase in the Cu f content, the CTE of the composites increased from 5.41 to 8.7 × 10 −6 /K, which is much lower than that of Cu (17.7 × 10 −6 /K) and matches well with that of the silicon chip (4.3 × 10 −6 /K).  Figure 10 shows the thermal conductivity of Al 2 O 3 /30glass/Cu as a function of the Cu content at room temperature. When the contents of copper fibers or copper powders were less than 10 vol %, the thermal conductivity of Al 2 O 3 /30glass/Cu f was similar to the thermal conductivity of Al 2 O 3 /30glass/Cu p , because at this content, the amount of copper fibers is too low to form the complete three-dimensional network structure in the composite. Similar to copper powders, copper fibers were distributed in a fragmented state in the composites. They were isolated by the glass and ceramic phases like Cu p in composites. When the contents of Cu f were more than 20 vol %, the enhancement of thermal conductivity by Cu f was more remarkably improved than that by the same volume of Cu p , because the amount of copper fibers was large enough to ensure the copper fibers overlapping each other and to form a continuous 3D network structure in Al 2 O 3 /30glass/Cu f . When 30 vol % Cu was added, the thermal conductivity of Al 2 O 3 /30glass/Cu f increased to 38.9 W/mK, while the thermal conductivity of Al 2 O 3 /30glass/30Cu p was only 25.9 W/mK. It is worthy of note that the thermal conductivity of the Al2O3/30glass/30Cuf composite substrate by low-temperature sintering was also higher than that of Al2O3/30glass/30Cf (28.98 W/mK) [16], although the thermal conductivity of C fiber is higher than that of Cu fiber. The thermal conduction enhancement mechanism of Al2O3/30glass/30Cuf composite substrate not only differs with that of Al2O3/30glass/30Cup composite substrate, but also differs from that of Al2O3/30glass/30Cf composite substrate. Similar to the carbon fiber 3D network in Al2O3/30glass/30Cf composite [16], Cuf can form a bridging 3D network in the Al2O3/30glass/30Cuf composite substrate, which would enhance the thermal conductivity effectively. When sintered at 850 °C under a pressure of 25 MPa, Cuf was etched by the melted glass at local surface, leading to welding bridging in the overlapped fibers. The morphology of Cu phase changed from a fiber type to molten-like vermiform type. The molten vermiform-like Cu metallurgically bonded with each other (Cf connected physically) and formed a continuous network for rapid thermal conduction, which is very similar to the structure of ceramicmetal substrates prepared by the die-casting or reactive metal penetration process [15,17]. This is the main reason why the thermal conductivity of Al2O3/30glass/30Cuf composite substrate is about 38.9 W/mK, but the thermal conductivity of Al2O3/30glass/30Cf is only 28.98 W/mK. Through the Cu fibers' bridging thermal conduction mechanism, it was possible to manufacture a low-cost substrate with excellent thermal conductivity and moderate CTE for LEDs, integrated circuits and other industry fields.
Combining the microstructure observation, the thermal conduction mechanism of Al2O3/30glass/30Cuf composite substrate could be deduced as follows: (1) At the beginning of hot-pressing, copper fibers deformed and connected physically, forming a 3D network like that in the Al2O3/30glass/30Cf substrate. The oxide layer on the surface of copper fibers was destroyed by glass and Al2O3 particles. (2) The surfaces of copper fibers tend to react with molten glass phase.
(3) Copper fibers achieved metallurgical bonding with each other and exhibited a molten-like vermiform shape. (4) A continuous metallurgy 3D network of copper fiber was established, similar to that of diecasting or reactive metal penetration processes.
The novelty of this process lies in the fact that it could not only fabricate ceramic-metal substrate It is worthy of note that the thermal conductivity of the Al 2 O 3 /30glass/30Cu f composite substrate by low-temperature sintering was also higher than that of Al 2 O 3 /30glass/30C f (28.98 W/mK) [16], although the thermal conductivity of C fiber is higher than that of Cu fiber. The thermal conduction enhancement mechanism of Al 2 O 3 /30glass/30Cu f composite substrate not only differs with that of Al 2 O 3 /30glass/30Cu p composite substrate, but also differs from that of Al 2 O 3 /30glass/30C f composite substrate. Similar to the carbon fiber 3D network in Al 2 O 3 /30glass/30C f composite [16], Cu f can form a bridging 3D network in the Al 2 O 3 /30glass/30Cu f composite substrate, which would enhance the thermal conductivity effectively. When sintered at 850 • C under a pressure of 25 MPa, Cu f was etched by the melted glass at local surface, leading to welding bridging in the overlapped fibers. The morphology of Cu phase changed from a fiber type to molten-like vermiform type. The molten vermiform-like Cu metallurgically bonded with each other (C f connected physically) and formed a continuous network for rapid thermal conduction, which is very similar to the structure of ceramic-metal substrates prepared by the die-casting or reactive metal penetration process [15,17]. This is the main reason why the thermal conductivity of Al 2 O 3 /30glass/30Cu f composite substrate is about 38.9 W/mK, but the thermal conductivity of Al 2 O 3 /30glass/30C f is only 28.98 W/mK. Through the Cu fibers' bridging thermal conduction mechanism, it was possible to manufacture a low-cost substrate with excellent thermal conductivity and moderate CTE for LEDs, integrated circuits and other industry fields.
Combining the microstructure observation, the thermal conduction mechanism of Al 2 O 3 /30glass/ 30Cu f composite substrate could be deduced as follows: (1) At the beginning of hot-pressing, copper fibers deformed and connected physically, forming a 3D network like that in the Al 2 O 3 /30glass/30C f substrate. The oxide layer on the surface of copper fibers was destroyed by glass and Al 2 O 3 particles. (2) The surfaces of copper fibers tend to react with molten glass phase.
(3) Copper fibers achieved metallurgical bonding with each other and exhibited a molten-like vermiform shape. (4) A continuous metallurgy 3D network of copper fiber was established, similar to that of die-casting or reactive metal penetration processes.
The novelty of this process lies in the fact that it could not only fabricate ceramic-metal substrate with excellent thermal properties, but also combines the advantages of the powder sintering process and the traditional die-casting penetration process for ceramic-based composite substrate with continuous metal phase. Cu f in the composite could be mixed homogeneously, overcoming the thickness limitation of infiltration and the uneven distribution of the metal phase by the melt infiltration process. This fabricating process of Al 2 O 3 /30glass/30Cu f provides a new way to mass-produce ceramic substrate with high thermal conductivity at low cost and high efficiency mode, which should be of much benefit to the industrial operations of the high-power LED and integrated circuits industry.

Conclusions
Al 2 O 3 /glass/Cu composite substrate with continuous metal phase was fabricated with copper fibers through a powder sintering process at low temperature. After hot-pressed sintering at 850 • C under a pressure of 25 MPa, the Cu f morphology changed to a molten-like vermiform shape. The molten-like vermiform copper interconnected metallurgically and formed a continuous 3D network for rapid thermal conduction. Owing to the interconnected 3D network of the molten-like vermiform copper, the thermal conductivity of the Al 2 O 3 /30glass was enhanced from 6.35 W/mK to 38.9 W/mK with the addition of 30 vol % Cu f . The novelty of this process lies in the fact that it could not only fabricate ceramic-metal substrate with excellent thermal properties, but also combines the advantages of the powder sintering process and the traditional die-casting penetration process for ceramic-based composite substrate with continuous metal phase.