3.1. XRD analysis
X-ray diffraction patterns of C-SiO2-Ni5%-625, C-SiO2-Ni5%-650 and C-SiO2-Ni5%-675 nanocomposites are illustrated in Fig. 1. Previous studies by Gouadria et al.  indicated that the composites are amorphous at different temperatures. However, the idea of the incorporation of the nickel is to improve the conductivity of the materials. Min Zhang et al.  have studied the synthesis of SiO2/C-PdNi composite. They have observed Ni particles on the surface of a sample with a size of 290 nm. On the other hand, this spectrum that Ni has improved the crystalline structure of these samples, the characteristic peaks were detected at 2θ = 44.5o, 51.9o and 76.5o which may correspond to crystalline planes of metallic Ni phases (JCPDS 01-1260) [24, 25], respectively at (111), (200) and (222). The appearance of two large peaks corresponding to the silica structure at approximately 21.7° (phase amorphous silica) and 12° (phase carbon) .
3.2. TEM micrograph
Microscopic electronic transmission (TEM) characterizations are performed on nanocomposites treated at different pyrolysis temperatures. TEM photograph is shown in Fig. 2. The samples (Fig. 2.a, b, c) show homogeneous interconnected nano-sized spherical particles in the range of 23–30 nm. The TEM micrograph of the sample processed at 675 exhibits the existence of a percolation phenomenon (Fig. 2.c). High resolution TEM images are presented at the right of each image. We can clearly see in these TEM images that the nanoparticles agglomerate by increasing the pyrolysis temperature, this agglomeration can be explained by the interaction between the matrix and the nanoparticle. Lei Ding et al.  studied the effects of the molar ratio of resorcinol to nickel ion on the SiO2/C-Ni composite and they found that this material showed a because of the good dispersion of Ni particle, the sample display excellent performance for protein adsorption.
Figure 3 shows Scanning Electron Microscopy (SEM) images and Energy-Dispersive X-ray (EDX) graph of SiO2/C-Ni nanocomposites samples. These analyses confirmed that the elements are only Ni, C, Si and O (Fig. 3). EDX confirmed purity of SiO2/C-Ni. Based on these results, it appears that silica particles can surround, carbon chains and become incorporated into pores, which can lead to changes in the material's textural and electrical properties. In addition to this, you need to know more about it. Previous studies done by Gouadria et al.  shows the same results of this agglomeration of nanoparticles but the addition of Nickel, in this work, favors the agglomerations of nanoparticles at low pyrolysis temperatures where Nickel increased the conductivity of these materials.
3.3. FTIR analysis
The FTIR spectrum of the nanocomposite processed at the 625 oC in 400–4000 cm− 1 range is shown in Fig. 4. Four major peaks were detected. A Ni–O bond was detected corresponding to 470 cm− 1, also an H-OH bond (water molecules) was observed at 1600 cm− 1 [29, 30]. We notice the appearance of the Si–O–Si, Si–C bands are appearing at 801 cm− 1 . In addition, a large band at approximately 1050 cm− 1 corresponds to the Si–O–Si or ethoxy groups attached to Si [32, 33]. Shahrokh Abadi et al.  studied the effects of the annealing temperature on the silica and they found that this material showed a band at 1110–1070 cm− 1 are interpreted with the Si-O-Si asymmetric bond.
3.4. V-I characterizes
The characteristics V(I) was studied according to the pyrolysis temperature and the measuring temperature for the C-SiO2-Ni5%-625, C-SiO2-Ni5%-650 and C-SiO2-Ni5%-675 nanocomposites are shown in Fig. 5. Two different characteristics appear: an ohmic region appears at low current and an NDR region exists at high voltage, the NDR behavior appears at low measuring temperature in the samples C-SiO2-Ni5%-650 (Fig. 5.b) in the range of 80 to 160 K. Also, the NDR appears only in the range 80 K − 120 K for (C-SiO2-Ni5%-675 (Fig. 5.c)) composite. However, the NDR persists even at room temperature in the sample treated at 625 (C-SiO2-Ni5%-650 (Fig. 5.a)). Many authors have been explained the existence of the NDR phenomenon by the model electro-thermal for the samples materials [35, 36]. We notice that the existence of the NDR is correlated with the conductivity of the material, in fact, for samples with high resistivity the NDR persists at ambient temperature, is the case for C-SiO2-Ni5%-625 which has a conductivity equal to σ = 1.63410− 5 (Ω.cm)−1 [37, 38]. While when conductivity increases the NDR does not appear at ambient temperature and only appears for low measuring temperatures, is the case for C-SiO2-Ni5%-650 ( σ = 1.54*10− 3(Ω.cm)−1 ) and for C-SiO2-Ni5%-675( σ = 4.1*10− 3 (Ω.cm)−1[38, 40]). As a conclusion of our work, we can explain that insulating materials have the advantage of the appearance of the NDR at ambient temperature. This is why we are interested in the sample processed at 625°C. Xia et al.  studied the SiO2/C materials composites and they found that enhanced electrochemical performance can be associated with their porous sample, which can improve electrical conductivity of the composite. The novelty in this article is that the NDR phase exists at room temperature and at low pyrolysis temperature, this allows to use in negatronic applications. The results of the analysis of conductance ac through the frequency for diverse samples are illustrated in Fig. 6. The ac conductance in the high range frequency obeys Jonscher’s law according to Eq. (1) :
G(ω) = Gdc + Aωs Eq. (1)
where Gdc shows dc conductance, A is a pre-exponent factor and s is the frequency exponent. In order to know the conduction mechanism in a material, several theoretical models have been used to show the temperature dependence with the exponent s. For CBH mechanism, s decreases by increasing the temperature [43, 44]. For Small Polaron Mechanism (SP), s increases with increasing the temperature [45, 46]. Following the OLPT (Overlapping large polaron tunnelling) mechanism, s decreases when the temperature increases, and shows a minimum for a precise temperature and then enlarges when the temperature increases [47, 48]. In quantum mechanical tunnel effect model (QMT), s does not vary with temperature and almost equal to 0.8 [49–51]. Previous studies done by Gouadria et al.  on nanocomposite based on a carbon matrix doped with 50% silica shows the existence of a quantum mechanical tunnel model (QMT) for the sample treated at 675°C whose the frequency exponent is independent of the temperature and almost equal to 0,87. Previous studies by Ben Mansour et al.  on nanocomposite based on pyrogallol and formaldehyde (PF) based carbon matrix doped by Nickel oxide (NiO) processed at 625 indicates the presence of a small polaron hopping model.
A linear adjustment of Eq. (1), gives the values of s are illustrated in Fig. 7. For the nanocomposite C-SiO2-Ni5%-650 (Fig. 7.a), the values of s vary between 0 and 1, indicating the presence of the hopping conduction mechanism. However, for the nanocomposite C-SiO2-Ni5%-675 (Fig. 7.b), s increases with the measurement temperature. It is clear, that the SPH model of this composite material can be dominant. The addition of nickel promotes percolation sites and allows the electron to jump from site to another so the conduction pattern in the samples changes as the pyrolysis temperature varied.