Microstructure and enhanced dielectric properties of BaTiO 3 –SiO 2 nanocomposites using hydrogen treated nanoparticles

: BaTiO 3 (BT) nanoparticles treated in H 2 and then coated with a layer of SiO 2 were used as the starting material to synthesise BT–SiO 2 nanocomposites through the conventional ceramic process. A denser morphology was obtained in the BT– SiO 2 composites. The H 2 treatment induced not only the oxygen vacancies, but also other changes (i.e. ionic disorders) in the BT. These ionic disorders were more permanent and enhanced the reaction between the BT and SiO 2 during the sintering process. Comparing with the nanocomposites sintered from the untreated BT nanoparticles, the nanocomposites sintered from the H 2 treated BT nanoparticles exhibit an enhanced dielectric performance (i.e. a higher dielectric constant and a high electric breakdown field). Among the temperatures (i.e. 850, 900, 950 and 1000°C) used to treat the BT nanoparticles in H 2 , it is experimentally found that 950°C is the best. For example, for the composites with 20 wt% SiO 2 , the dielectric constant is enhanced more than four times and the breakdown field is enhanced by about 30%. It is believed that the enhancement in the dielectric response is due to the dipoles formed by the ionic disorders, especially the acceptor defects.


Introduction
Dielectric materials with a high dielectric constant (ε r ) and a high breakdown field (E b ) are desirable for the development of highperformance capacitors, insulators and energy storage devices [1][2][3]. Among the single dielectrics, the polar inorganics, especially ferroelectric ceramics, such as BaTiO 3 (BT), usually exhibit a high ε r (10 3 -10 4 ) but a low E b (∼10 0 MV/m) [4][5][6]. However, the ε r obtained in the ferroelectrics is strongly dependent on the temperature [7], which is undesirable for the dielectric applications. On the other side, organic materials, such as polypropylene, and non-polar inorganics, such as SiO 2 , exhibit a high E b (10 2 -10 3 MV/m) but a low ε r (mostly <5) [8,9]. In order to develop the materials with a relatively high ɛ r and E b , the composites especially 0-3 composites have been extensively studied [5,10]. A 0-3 composite is a composite in which the matrix (3-phase) is filled with particles (0-phase) of the other material. In general, dielectrics with a high E b are used as the matrix, while the particles are materials with a high ɛ r [5,10]. In the study of dielectric 0-3 composites, plenty of research has been focused on the development of composites using polymers as the matrix due to their high E b and good flexibility [5]. By using different polymer matrices, the composites with enhanced dielectric properties have been reported [11][12][13]. However, the working temperature of polymer-based composites is limited by the low decomposition temperature of polymers [10]. In addition, there is a general concern about the compatibility between the polymer matrix and inorganic fillers [13,14], which limits the E b and uniformity of the composites. Therefore, the development of composites using glass as the matrix (i.e. ceramic-glass 0-3 composites) became an interesting topic.
Regarding the polar inorganics used in the development of ceramic-glass composite, BT is an intensively studied ferroelectric since it is lead free and exhibit a high dielectric constant [1,5]. To form BT-based ceramic-glass composites, different glass compounds, such as SiO 2 , Al 2 O 3 and B 2 O 3 , have been studied as the matrix. These ceramic-glass composites are usually sintered using the conventional ceramic sintering process [15][16][17][18][19][20][21][22][23], in which the starting materials were either a mixture of BT and glass particles [17,18] or BT particles coated with a layer of glass [15,16,20]. In some of the studies, only a very small amount of glass (i.e. glass ceramics) was used [15]. For example, about 1.0 wt% of SiO 2 is usually used as the sintering agent in the fabrication of BT ceramic capacitor due to the good compatibility between the SiO 2 and BT [24]. Adding SiO 2 results in a denser morphology so that the E b is improved [23,24]. The composites with a small amount of glass usually exhibit a high ε r with a strong temperature dependence due to the ferroelectricity of BT [7,24]. To further increase the E b and weaken the temperature dependence of the dielectric constant, the amount of glass used was increased up to 65 wt% [19]. In the studies of BT-glass composites, SiO 2 is widely used as the glass compound due to the facts that SiO 2 has not only a good compatibility with the BT, but also a high E b and a very low dielectric loss [15]. In the study of the BT-SiO 2 composites, it was found that although adding SiO 2 results in a certain improvement on the E b , the ε r is significantly reduced. For example, the ε r and E b of the BT is about 4000 and 10 MV/m, respectively, while the ε r and E b of the BT-SiO 2 composites with 20.0 wt% SiO 2 is about 20 and 40 MV/m, respectively [15,24]. Therefore, it is interesting to increase the ε r of the high-SiO 2 -content BT-SiO 2 composites and, at the same time, improve or at least maintain the E b . It has to be mentioned that new compounds, such as Ba 2 TiSi 2 O 8 and BaTiSiO 5 [21,22], are usually formed in BT-SiO 2 ceramic-glass composites due to the reaction between the BT and SiO 2 during the sintering process.
It is well known that the oxygen vacancies have a strong influence on the electric/dielectric properties of the BT [25,26] and that the oxygen vacancies in the BT can be induced by the treatment of the BT at a high temperature in a reducing environment such as a low oxygen partial pressure environment, a reducing atmosphere like H 2 [27]. For example, the resistivity (ρ) of BT ceramics was reduced to about 10 −1 Ω·cm by treating the BT ceramics in H 2 at 1373 K [28]; an enhanced ε r of about 5000 was achieved by treating BT at 1300°C in the vacuum with an oxygen partial pressure of 10 −6 atm [29]. Therefore, one would expect that the BT-based composites using BT with oxygen vacancies would exhibit an enhanced dielectric constant. Actually, the BT particles that were pretreated in vacuum have been used as the filler in the development of the polymer-based composites and an enhanced dielectric constant was obtained in the composites using pretreated BT particles [30,31]. For example, the ε r of polyester-styrene reactive resin filled with 22 vol.% BT nanoparticles was increased from 10 to 25 by using BT particles that were treated in a vacuum at 1000°C [30]. However, to the best of our knowledge, there is no report on the BT-SiO 2 ceramic-glass composites sintered from the BT with oxygen vacancies. It should be mentioned that there is a great difference between the BT-polymer composites and BT-SiO 2 composites in terms of chemical reaction. For BT-polymer composites, there is no chemical reaction occurring between the BT and polymer to form new compounds. As mentioned above, the reaction between the BT and SiO 2 occurs during the sintering process of the ceramic-glass composites. Therefore, what happens in BT-SiO 2 ceramic-glass composites can be much more complicated.
In this work, the BT nanoparticles were first treated in H 2 at 850, 900, 950 and 1000°C, respectively. Multiple temperatures were used to create the BT nanoparticles with different concentrations of oxygen vacancies and to determine the optimised temperature for the treatment of BT in H 2 since the treatment effect is dependent on the temperature. The pretreated BT nanoparticles were then coated with a layer of SiO 2 using a wet chemical process. The conventional ceramic sintering process was used to prepare the BT-SiO 2 ceramic-glass nanocomposites by using SiO 2 coated BT nanoparticles as the raw materials. It is found that besides oxygen vacancies some other changes were induced into BT nanoparticles by the H 2 treatment and that these other changes (i.e. intrinsic/ionic disorders) had a strong influence on the reaction between the BT and SiO 2 during the sintering process of the composites. As a result, the composites sintered from the H 2 treated BT nanoparticles exhibit a better dielectric performance (i.e. a significantly higher ε r , a significantly higher E b and a similar, or even lower, dielectric loss) than the composites sintered from the untreated BT nanoparticles. Among all the temperatures used in the H 2 treatment, the BT nanoparticles treated at 950°C result in the nanocomposites with the best dielectric performance. It is believed that the high dielectric performance observed in the composites sintered from the H 2 treated BT nanoparticles originates from the ionic disorders created in the composites.

Experiments
(i) Treatment of BT nanoparticles in H 2 : BT nanoparticles with 99% purity in a diameter of about 200 nm were purchased from US Research Nanomaterials. The as-received BT nanoparticles were treated in a 25% hydrogen forming gas for 4 h at a temperature: 850, 900, 950 and 1000°C, respectively. It was expected that the oxygen vacancies were induced in the BT nanoparticles and that different treatment temperatures resulted in different concentrations of oxygen vacancies [25,26]. The H 2 treated BT nanoparticles were then used in the preparation of the BT-SiO 2 ceramic-glass nanocomposites. Here, the BT nanoparticles treated in H 2 at 850, 900, 950 and 1000°C, were labelled as BT(H850), BT(H900), BT(H950) and BT(H1000), respectively, while the untreated BT nanoparticles were labelled as BT(H0). (ii) Coating SiO 2 onto the BT nanoparticles: The H 2 treated BT nanoparticles were coated with a layer of SiO 2 by utilising the chemical reaction between tetraethylorthosilicate (TEOS) and H 2 O in suspension environment according to the process described in [32]. Different weight concentrations (2.5, 5, 10, 15, 20 wt%, respectively) of SiO 2 were coated on the BT by adjusting the addition of chemicals. First, the surface of BT nanoparticles was activated by acetic acid at 40°C with sonicated for 30 min. Then, TEOS with calculated amount was added to the suspension at the same condition. After that, the 28-30% ammonia-water solution purchased from VWR International was slowly dropped in during stirring and maintained for 30 min to complete the chemical reaction. The obtained mixture was washed with ethanol and deionised water, then heated to 80°C for 3 h and further heated to 120°C for 12 h to evaporate all the solvents. Finally, the SiO 2 coated BT nanoparticles were obtained and used for the preparation of BT-SiO 2 nanocomposites. (iii) Preparation of BaTiO 3 -xSiO 2 (BT-xSiO 2 ) ceramic-glass nanocomposites: The green body pellets with a diameter of 10 mm and a thickness of 1 mm were prepared by pressing the SiO 2 coated BT nanoparticles with 5 wt% PVA as the binder under a pressure of 20 MPa. All the BT-SiO 2 nanocomposites were sintered in air using a 46100 Barnstead Thermolyne furnace. During the sintering process, the furnace was first heated to 600°C with a heating rate of 2°C/min and maintained at 600°C for 1 h to remove the binder from the green body. Then, the furnace was heated to 1230°C with a heating rate of 2°C/min and maintained at 1230°C 5 h for sintering. Finally, the furnace was cooled down to 700°C with a cooling rate of 3°C/min and followed by a natural cooling process to room temperature. The detailed description of the process was given in [33].
In this work, the prepared BT-SiO 2 nanocomposites were labelled as BT(Hy)-xSiO 2 , where x (=0, 2.5, 5.0, 10.0, 15.0 and 20.0) is the weight percentage of SiO 2 in the composites (i.e. the weight percentage of SiO 2 coated onto BT nanoparticles) and y (=0, 850, 900, 950 and 1000) is the temperature used to treat the BT nanoparticles in H 2 . For example, BT(H950)-10SiO 2 is the nanocomposites containing 10.0 wt% of SiO 2 and was sintered from the BT nanoparticles treated in H 2 at 950°C. (iv) Characterisation and measurement: The SiO 2 coating layer on BT nanoparticles was examined using transmission electron microscope (TEM) -Hitachi H-7650 TEM. The morphology and uniformity of nanocomposites were characterised using scanning electron microscopy (SEM) -JEOL JSM 7000F FE-SEM. The crystalline structure of nanocomposites was characterised using Bruker D8 Discovery X-ray diffractometer (XRD). The XRD data was analysed using JADE-5 software. All the TEM, SEM and XRD were done at room temperature.
For the determination of the dielectric properties of the composites, both surfaces of nanocomposites were polished to obtain smooth surfaces, the final thickness of BT-SiO 2 nanocomposites was about 0.2-0.3 mm. Then, the gold electrodes in a diameter of 3.0 mm were sputtered on both surfaces of the polished nanocomposite disks. An Agilent 4294A impedance analyser was used to characterise the capacitance and loss over a frequency range from 100 Hz to 1 MHz by using the Cp-D function. The dielectric constant of the nanocomposites was then calculated using the parallel plate model. To determine the temperature dependence of the dielectric properties, the samples were placed in an ECT-2 temperature chamber, whose temperature was set at different temperatures from −50 to 140°C with a step of 10°C. At each temperature, the measurement was the same as it at room temperature and prior to the measurement, the temperature was stabilised for about 10 min. For the behaviour of the composites under a high electric field, the E b was first determined using a Trek 610D 10 kV High Voltage Supplier/Amplifier, then the polarisation-electric field (P-E) hysteresis loop at 10 Hz was characterised using a Precision-LC100 Ferroelectric Testers coupled with the 10 kV High Voltage Supplier/Amplifier.

Results and discussion
Based on the density of the SiO 2 and BT and the weight percentage of the SiO 2 coated onto BT nanoparticles, one can estimate the thickness of SiO 2 coating layer. Assuming that the SiO 2 is uniformly coated onto the surface of the BT nanoparticles and that all the BT nanoparticles are perfect spheres in a diameter of 200 nm, it was calculated that for the BT nanoparticles coated with 2.5, 5.0, 10.0, 15.0 and 20.0 wt% SiO 2 , the thickness of the SiO 2 layer is 2.3, 4.6, 9.2, 14.0 and 19.0 nm, respectively. In the calculation, the bulk density of SiO 2 (2.2 g/cm 3 ) and BT (6.0 g/cm 3 ) was used. Based on the TEM observation, the thickness of the SiO 2 layer is close to what was expected, as shown in Fig. 1a, for an example for the untreated BT coated with 5.0 wt% of SiO 2 .
The morphology and microstructure of the nanocomposites were examined using SEM as shown in Figs. 1b-d. For all the BT ceramics, the grain size was significantly larger than the size of the BT nanoparticles as shown in Fig. 1b as an example for BT(H950) ceramics. All the BT-SiO 2 ceramic-glass nanocomposites exhibit a similar morphology that is different from the BT ceramics. That is, the grain boundaries in the BT-SiO 2 nanocomposites are not clear and the nanocomposites have a denser microstructure structure than the BT ceramics, as shown in  (211) and (112) are two close peaks. Both (101) and (110) peaks have the same intensity, but the intensity of (200) and (211) peaks is two times of that of (002) and (112) peaks, respectively. It is well known that the XRD peak broadens as the size of particles decreases. When the size of BT particles is small enough, two close and broadened peaks may merge as a single peak. Considering the size of BT nanoparticles studied here, one would expect broadened peaks for the XRD results. If the BT particles are at cubic phase, all three peaks of (101)/(110), (200)/(002) and (211)/(112) will be symmetric. If the BT particles are at tetragonal phase, the merged (101)/(110) peak will be symmetric, but the merged (200)/(002) peak and merged (211)/(112) peak will be asymmetric. Therefore, based on the results shown in Fig. 2a1-a3, it is concluded that BT(H0) is at tetragonal phase which is consistent with the information provided by the vendor. The similar XRD patterns were observed in BT(H850) and BT(H900) nanoparticles, but the peaks shift to a slightly higher angle. Both of (002)/(200) and (112)/(211) peaks obtained in BT(H950) and BT(H1000) nanoparticles are splinted into two. All these indicate that the H 2 treatment induced some structural changes in the BT nanoparticles. Assuming the structure of all the nanoparticles was pure tetragonal, the lattice constants, a and c, of the tetragonal BT were determined. It was found that the volume of lattice unit decreases, while the ratio of lattice constant c to a increases, with increasing the temperature used to treat the BT nanoparticles in H 2 . In other words, the higher the temperature was used to treat the BT nanoparticles, the more was the changes in the BT nanoparticles. The nature of the structure change is unclear. Further study on this is needed.
Regarding the nanocomposites, it is found that both the content of SiO 2 and the temperature used to treat the BT nanoparticles in H 2 have a strong influence on the XRD results. First of all, for the composites sintered from the same BT nanoparticles, the intensity of XRD peaks associated with the tetragonal BT decreases with increasing SiO 2 content as shown in Fig. 2b, where the BT(H950)-xSiO 2 nanocomposites are presented as an example. Clearly, besides the peaks associated with the tetragonal BT, some new peaks are observed and the intensity of these new peaks increases with increasing SiO 2 content. That is, the new peaks reflect the compounds formed by the reaction between the BT and SiO 2 during the sintering process. Among all the BT-SiO 2 nanocomposites, the BT-2.5SiO 2 nanocomposites had the strongest XRD peaks associated with the BT phase. It was found that the BT in the nanocomposites was also at the tetragonal phase. The analysis of BT's tetragonal phase in the BT-2.5SiO 2 nanocomposites indicated that the volume of the lattice unit for all the composites sintered from the H 2 treated BT nanoparticles was almost the same, but smaller than that for the composites sintered from untreated BT nanoparticles. It was also found that the higher was the temperature used to treat the BT nanoparticles in H 2 , the higher was the ratio of the lattice constant c to a. However, compared with the BT nanoparticles, the change in the c/a ratio obtained in the nanocomposites is smaller. That is, the sintering process cannot completely recover/restore the changes induced in the BT by the H 2 treatment. If the H 2 treatment only induced oxygen vacancies, one would expect that the changes would be completely recovered since the sintering temperature was higher than the H 2 treatment temperature. In other words, the XRD results indicate: (i) besides the oxygen vacancies some other changes were induced by the H 2 treatment; (ii) these other changes are more permanent changes; and (iii) these other changes have a strong influence on the reaction between the BT and SiO 2 during the sintering process.
To compare the influence of the treatment temperature on the reaction between the BT and SiO 2 or the formation of new  Clearly, for the nanocomposites with the same SiO 2 content, the intensity of the peaks associated with tetragonal BT decreases slightly, while the intensity of new peaks increases slightly, with increasing temperature used to treat BT nanoparticles in H 2 , as shown in Fig. 3a for BT-15SiO 2 as an example.
For BT-20SiO 2 nanocomposites, the peaks associated with the tetragonal BT are almost disappeared for the nanocomposites sintered from BT(H950) and BT(H1000) nanoparticles, as shown in Fig. 3b. Since the new peaks observed reflect the new compounds formed due to the reaction between the BT and SiO 2 , the results shown in Fig. 3 indicate that the nanocomposites sintered from the BT nanoparticles treated in H 2 at a higher temperature have a higher amount of new compounds formed or a less amount of BT left from the reaction between the BT and SiO 2 .
Regarding the compounds formed in the BT-SiO 2 nanocomposites, both BaTiSiO 5 and Ba 2 TiSi 2 O 8 have been reported [21,22]. From the standard peaks, one can find that both compounds have the very similar XRD peaks, but Ba 2 TiSi 2 O 8 has a unique and weak (220) peak at 29.6° that is on the right side of the strongest (211) at 29.0°. Therefore, the XRD results shown in Fig. 3 indicate that Ba 2 TiSi 2 O 8 was certainly formed in the composites reported here due to the reaction between BT and SiO 2 . However, if Ba 2 TiSi 2 O 8 was the only compound formed, one would expect the formation of TiO 2 since the reaction would be 2BaTiO 3 + 2SiO 2 = Ba 2 TiSi 2 O 8 + TiO 2 .
However, no peak associated with crystalline TiO 2 was observed in the XRD results as shown in Fig. 3b, where the XRD results obtained in BT-20SiO 2 nanocomposites are presented since almost all BT in the composites was reacted with SiO 2 . That is, the new compounds formed in the nanocomposites may be more than Ba 2 TiSi 2 O 8 .
Regarding the dielectric properties of the composites, it is found that both the SiO 2 content and the temperature used to treat BT in H 2 have a strong influence. For the BT ceramics, the ceramics sintered from the H 2 treated BT nanoparticles exhibit a slightly lower dielectric constant, but a significantly higher dielectric loss, than the ceramics sintered from the untreated BT nanoparticles. For example, at room temperature, the dielectric loss at 1 kHz obtained in the BT(H0) ceramics was about 2%, while it was about 12, 20, 39 and 45% for BT(H850), BT(H900), BT(H950) and BT(H1000) ceramics, respectively. That is, the changes induced in the BT nanoparticles by the H 2 treatment cannot be completely recovered, which is consistent with the XRD results discussed above.
The influence of the H 2 treatment temperature on the dielectric properties of the BT-SiO 2 nanocomposites is summarised in Fig. 4a, where the dielectric constant at 1 kHz for all the composites are presented. The dielectric properties of BT(H0)-xSiO 2 studied here are the same as reported by others [15,17]. For example, the dielectric constant of the BT(H0)-20SiO 2 is about 20. For the nanocomposites sintered from the same H 2 treated BT nanoparticles, the ε r decreases with the increasing SiO 2 content. This is the same as reported for all the BT-SiO 2 systems [15,17]. However, for the nanocomposites with the same SiO 2 content, the nanocomposites sintered from the H 2 treated BT nanoparticles exhibit a higher dielectric constant than the nanocomposites sintered from the untreated BT nanoparticles. Among all the nanocomposites with the same SiO 2 content, BT(H950)-xSiO 2 exhibit the highest dielectric constant. Except BT-2.5SiO 2 composites, the difference in the dielectric constant between BT(H950)-xSiO 2 and BT(H0)-xSiO 2 nanocomposites decreases with increasing x. For example, the difference in the dielectric constant is more than 810 between BT(H950)-5SiO 2 and BT(H0)-5SiO 2 nanocomposites, while it is more than 530 between BT(H950)-10SiO 2 and BT(H0)-10SiO 2 nanocomposites, more than 250 between BT(H950)-15SiO 2 and BT(H0)-15SiO 2 nanocomposites and about 65 between BT(H950)-20SiO 2 and BT(H0)-20SiO 2 nanocomposites. However, the enhancement ratio in the dielectric constant increases with increasing x. For example, the dielectric constant of BT(H950)-20SiO 2 nanocomposites is more than 4.2 times of BT(H0)-20SiO 2 nanocomposites while the dielectric constant of BT(H950)-15SiO 2 nanocomposites is about 4.0 times of BT(H0)-15SiO 2 nanocomposites. As indicated by the XRD results, for the nanocomposites with the same SiO 2 content, the composites sintered from the H 2 treated BT nanoparticles have a less amount of the tetragonal BT than the composites sintered from untreated BT nanoparticles. Therefore, the increase in the dielectric constant obtained in the nanocomposites sintered from the H 2 treated BT nanoparticles is not due to the BT, but the new compounds formed in the composites. A higher value of x means more new compounds formed in the BT-xSiO 2 nanocomposites. However, as discussed above, the new compounds were also formed in the BT(H0)-xSiO 2 . Therefore, the results shown in Fig. 4a indicate that the new compounds formed in the nanocomposites sintered from the H 2 treated BT nanoparticles are very different from the compounds formed in the nanocomposites sintered from untreated BT nanoparticles. All the BT-20SiO 2 nanocomposites have very little amount of the BT left from the reaction between the BT and SiO 2 . Therefore, the dielectric response of BT-20SiO 2 nanocomposites is dominated by the new compounds formed during the sintering process. To determine the nature of the new compounds, the frequency dependence of the dielectric constant obtained in BT-20SiO 2 nanocomposites is shown in Fig. 4b. Clearly, the dielectric constant of these nanocomposites exhibits a very small frequency dispersion. Regarding the dielectric loss, all the nanocomposites exhibit a low loss. There is no clear trend except that BT(H950)-20SiO 2 nanocomposites exhibit the lowest dielectric loss.
Regarding the breakdown field (E b ), it was found that for the nanocomposites sintered from the same BT nanoparticles, the E b increased with increasing SiO 2 content. For example, the E b of BT(H0)-xSiO 2 increases from about 15 MV/m for BT(H0) ceramics to about 35 MV/m for BT(H0)-20SiO 2 , which are similar to the values reported by others [15,17]. For the nanocomposites with the same SiO 2 content, it was found that the nanocomposites sintered from the H 2 treated BT nanoparticles exhibited a higher E b , more than 10 MV/m higher than the nanocomposites sintered from the BT(H0) nanoparticles. For example, the E b of BT(H0)-20SiO 2 was about 35 MV/m, while the E b of BT(H950)-20SiO 2 was more than 45 MV/m. That is, the E b is enhanced by about 30%. The exact reason behind the high E b observed in the composites prepared using H 2 treated BT is not clear, but the following factors certainly contribute to the results. First of all, adding SiO 2 results in a dense microstructure as shown in Fig. 1. A denser microstructure is favourable for the high E b . Second, the new compounds formed by the reaction between the BT and SiO 2 are non-polar dielectric with a low dielectric constant. Usually, a non-polar dielectric with a lower dielectric constant exhibits a higher E b . It is well known that the E b is proportional to the reciprocal of the thickness. Considering the thickness of the composites studied here is about 0.2-0.3 mm, one would expect an E b of about 450 MV/m for these composites with a thickness of 20-30 μm.
In short, it is experimentally found that the nanocomposites sintered from the H 2 treated BT nanoparticles exhibit a better dielectric performance. Among all the temperatures used to treat the BT nanoparticles in H 2 , 950°C is the best. BT(H950)-xSiO 2 nanocomposites exhibit a significantly higher E b , a lower dielectric loss and a significantly higher dielectric constant than BT(H0)-xSiO 2 .
Regarding the changes in the BT nanoparticles during the H 2 treatment, it was expected that oxygen vacancies were induced [25][26][27]. It was reported that when the oxygen vacancy concentration is high, the intrinsic disorders (i.e. ionic disorders), including both Schottky and Frenkel defects, would be induced in the BT [34,35]. The structure of the BT can be assigned as ABO 3 perovskite structure, in which Ba and Ti atoms/ions take the lattice site A and B, respectively. That is, the intrinsic/ionic disorders in BT mean that Ba ions take either the lattice site B or the interstitial site and that Ti ions take either the lattice site A or the interstitial site. Considering the fact that the radii of Ba 2+ , Ti 4+ and O 2− ion is 0.134, 0.068 and 0.132 nm, respectively, it is difficult for a Ba ion to take an interstitial site in a normal BT crystal. However, when oxygen vacancies are introduced in a BT crystal, the Ba ions can take the interstitial sites next to the oxygen vacancies. When Ba and Ti ions take the interstitial sites, they can be treated as acceptor dopants/defects. When a Ba ion takes a lattice site B, it can be treated as a donor dopant/defect, while a Ti ion taking a lattice site A can be treated as a acceptor dopant/defect. The higher is the concentration of the oxygen vacancy, the easier is for both Ba and Ti ions to take interstitial sites since the high concentration of the oxygen vacancy results in more and bigger open space. Based on the structure and considering the size of these ions, one would expect that the concentration of the acceptor defects is much higher than that of the donor defects. When these ionic disorders were created in the BT nanoparticles, they would not be eliminated by simply sintering it in the oxygen environment. That is, these ionic disorders are more permanent. This is consistent with the XRD and dielectric results obtained in the BT ceramics sintered from the H 2 treated BT nanoparticles.
Regarding the reaction between the BT and SiO 2 , the pretreatment of the BT nanoparticles in H 2 would have a strong influence on the reaction through following mechanisms: (i) vacancies at lattice sites are available for Si ions to fill; (ii) the existence of the vacancies makes it is easier for Si ions to take interstitial sites. Therefore, the formation of new compounds in the BT-SiO 2 composites due to the reaction between the BT and SiO 2 would be influenced by the oxygen vacancies and ionic disorders in the BT. The higher is the concentration of the vacancies, the easier should be the reaction. This is consistent with the experimental results. For example, for the BT-SiO 2 nanocomposites with the same SiO 2 content, the nanocomposites sintered from the H 2 treated BT nanoparticles have less BT left from the reaction than the nanocomposites sintered from the untreated BT nanoparticles.
The dielectric response of the new compounds formed in the BT-SiO 2 nanocomposites should be affected by the existence of the defects. For the reaction between the untreated BT (i.e. regular BT) and SiO 2 , the new compounds formed exhibit a low dielectric constant with a low loss. That is why the dielectric constant of the BT(H0)-20SiO 2 nanocomposites is only about 20. For the BT-20SiO 2 nanocomposites, the reaction between the BT and SiO 2 would consume about 93% of the original BT if the reaction is completed. Therefore, the dielectric constant obtained in BT(H0)-20SiO 2 nanocomposites is close to the dielectric constant of the new compounds formed. As indicated by the XRD results, for the BT-20SiO 2 nanocomposites sintered from the H 2 treated BT nanoparticles, almost all the BT was consumed by the reaction. Therefore, one would expect a dielectric constant lower than 20 for the BT-20SiO 2 nanocomposites sintered from the H 2 treated BT nanoparticles, which is against the experimental results. Therefore, the high dielectric constant obtained in the nanocomposites sintered from the H 2 treated BT nanoparticles is related to the defects formed in the nanocomposites. As discussed above, the vacancies in BT results in the ionic disorders and also affects the reaction between the BT and SiO 2 . The reaction between the SiO 2 and the BT with oxygen vacancies and ionic disorders results in more ionic defects (i.e. Ba, Ti and Si ions at interstitial sites, Ba ions take the lattice site B and both Si and Ti ions take the lattice site A). That is, there are a large amount of acceptor/donor defects and the concentration of the acceptor defect is much higher than that of the donor defects. Electric dipoles will be formed in the materials when these defects exist [34,35]. Under an external electric field, the orientation of these dipoles will change, which contribute to the dielectric response. Additionally, due to the existence of the vacancies, the interstitial sites would be larger than the regular case. When an ion takes one of these large interstitial sites, the exact location of the ion in a large interstitial site would be changed by the external electric field, which will result in an additional dielectric response. That is, the ionic defects enhance the dielectric constant. Both mechanisms occur in the BT-SiO 2 nanocomposites sintered from the H 2 treated BT nanoparticles. That is why the composites sintered from the H 2 treated BT nanoparticles exhibit a higher dielectric constant than the composites sintered from the untreated BT nanoparticles. It was reported that the existence of dipoles in a dielectric can enhance its E b [36]. Therefore, the higher E b obtained in the nanocomposites sintered from the H 2 treated BT nanoparticles can also be attributed to the existence of the ionic defects.
For the polarisation response of the nanocomposites under a high electric field, the P-E hysteresis loops of all the BT-SiO 2 nanocomposites under an electric field, more than 5 MV/m lower than its E b , are shown in Fig. 5. First, the P-E loops of the nanocomposites sintered from the H 2 treated BT nanoparticles are shown in Figs. 5a-d. Each P-E loop is characterised by two important parameters: (i) the maximum polarisation (P s ) obtained in the nanocomposite at the highest electric field, (ii) the remnant polarisation (P r ) obtained at the zero electric field. For the nanocomposites sintered from the same H 2 treated BT nanoparticles, the P s decreases with increasing SiO 2 content, which is consistent with dielectric constant shown in Fig. 4a; similarly, the P r also decreases with increasing SiO 2 content.
For the nanocomposites with a high SiO 2 content, such as BT-15SiO 2 and BT-20SiO 2 nanocomposites, a nearly linear P-E loop is observed, which indicates that these nanocomposites are nopolar or close to no-polar dielectrics. In other words, the new compounds formed in the composites due to the reaction between the BT and SiO 2 are not polar dielectrics.
To determine the influence of the H 2 treatment temperature on the P-E loop, the loops obtained in the nanocomposites with the same SiO 2 content are shown in Figs. 5e and f,  exhibit the highest Ps, which is consistent with the dielectric constant shown in Fig. 4a.
The characteristics of the P-E loop for all the nanocomposites are summarised in Figs. 6a-c, where U charge is the charging energy density of the nanocomposite. One can find that for the nanocomposites with the same SiO 2 content, the composites sintered from the H 2 treated BT nanoparticles, especially the BT(H950) nanoparticles, exhibit a higher P s and a higher P s -P r and, thus, a higher U charge than that from the BT(H0) nanoparticles. At the same electric field, the U charge of BT(H950)-20SiO 2 is more than four times higher than the U charge of BT(H0)-20SiO 2 , which is constant with the dielectric constant difference discussed above.
Considering the E b of BT(H950)-20SiO 2 is about 30% higher than that of BT(H0)-20SiO 2 , one would expect that the maximum U charge of BT(H950)-20SiO 2 is almost seven times higher than that of BT(H0)-20SiO 2 .
Based on the XRD results, dielectric properties and P-E loops, one can conclude that the BT-20SiO 2 nanocomposites are nonpolar dielectrics, while BT-15SiO 2 nanocomposites are almost non-polar. The temperature dependence of the dielectric properties for both BT(H950)-15SiO 2 and BT(H950)-20SiO 2 nanocomposites are shown in Fig. 7. It is well known that the BT has a phase transition at about 0°C and another at about 120°C [24]. From the dielectric loss of BT(H950)-15SiO 2 nanocomposite as shown in Fig. 7a, a weak peak is observed at temperature around 0°C, which may indicate the existence of the BT in the nanocomposites. No sign of the phase transition was observed in BT(H950)-20SiO 2 over a temperature range from −50 to 140°C as shown in Fig. 7b, which confirms the XRD results that almost no BT left from the reaction between the BT and SiO 2 in BT(H950)-20SiO 2 nanocomposites. From the data shown in Fig. 7, it is found that the dielectric constant slightly increases with increasing temperature. However, the change of the dielectric constant with the temperature is very small. For BT(H950)-20SiO 2 nanocomposites, the change in the dielectric constant over the entire temperature range studied here (−50 to 140°C) is <3%, while it is <10% for BT(H950)-15SiO 2 nanocomposites. The data shown in Fig. 7 also indicates that both nanocomposites exhibit a very low loss over the entire temperature range. For example, the loss of BT(H950)-20SiO 2 nanocomposites at 1 kHz over the entire temperature range is <1%. A high dielectric constant and a very low dielectric loss with a very weak temperature dependence make the BT(H950)-20SiO 2 and BT(H950)-15SiO 2 nanocomposites strong candidates as non-polar dielectrics for dielectric applications.
The results shown in Fig. 7, especially Fig. 7b, are very favourable for the applications since the dielectric constant is almost independent of temperature and the dielectric loss is low. This temperature dependence indicates that the content of BT in the composites at room temperature is very low, which is consistent with XRD data shown in Fig. 3. The high performance observed in the composites is the results of the microstructure and new compounds formed in the composites. As mentioned above, the H 2 treatment induces oxygen vacancies and structure change in the BT nanoparticles, which in turn affect the reaction between the BT and SiO 2 . It is the reaction between the SiO 2 and the BT with defects, which results in the microstructure and new compounds that are responsible for the observed results.

Conclusion
The BT-SiO 2 ceramic-glass nanocomposites sintered from BT nanoparticles pretreated in H 2 at 850, 900, 950 and 1000°C, respectively, are studied. It is found that besides the oxygen vacancies, some changes (i.e. ionic disorders) in crystalline structure were induced in the BT nanoparticles by the H 2 treatment. The ionic disorders have a strong influence on the reaction between the BT and SiO 2 during the sintering process. All nanocomposites exhibit a low dielectric loss, but the nanocomposites sintered from H 2 treated BT nanoparticles exhibit a higher ε r and a higher E b . Among all the temperatures used to treat the BT nanoparticles, 950°C is the best. The nanocomposites sintered from BT(H950) nanoparticles exhibit the highest ε r , E b , P s and U charge . For example, the ε r and E b obtained in BT(H950)-20SiO 2 nanocomposites is about 4.2 times and 1.3 times, respectively, of that obtained in BT(H0)-20SiO 2 nanocomposites. It is believed that the high dielectric response observed in the nanocomposites sintered from the H 2 treated BT nanoparticles is due to the dipoles formed by the ionic disorders. BT(H950)-15SiO 2 and BT(H950)-20SiO 2 nanocomposites exhibit not only a high dielectric constant and a very low loss, but also a very weak temperature dependence of the dielectric constant over a temperature range of −50 to 140°C.

Acknowledgments
This work was financially supported by NASA through the grant nos. G00007275 and G00011592.