The phase transformation of hydroxyapatite (HAP, Ca 10(PO 4) 6(OH) 2) to the beta tricalcium phosphate phase ( β-TCP, β-Ca 3(PO 4) 2) at 1100°C is well known. However, in the case of human tooth, the HAP phase transformation is still an open area. For example, the CaO phase has sometimes been reported in the set of phases that make up the teeth. In this study, physical changes of human teeth when subjected to heat treatment in inert atmosphere (argon) were studied. The results were compared with those obtained in air atmosphere, from room temperature (25 °C) up to 1200 °C. Morphological changes were analyzed by light and scanning electron microscopy (SEM). The HAP to β-TCP phase transformation was followed in powder samples by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). Heating of teeth results in the removal of organic material and structural water before the HAP to β-TCP phase transformation, the increment in hardness and the induced crystal growth. The percentage of the phases, crystal growth and lattice parameter variations as a function of temperature was quantified by Rietveld analysis. The black color was observed in dentin heated under argon atmosphere. Differences in expansivity produce fractures in dentin at 300°C in argon and at 400°C in air. In dentin, the coexistence of the HAP and β-TCP phases was observed after 800°C in argon and after 600°C in air; in enamel it was observed at 600°C in argon compared with 400°C in air. In general, the role played by the argon atmosphere during the thermal treatment of the teeth is to retard the processes observed in air.
Enamel has a prismatic structure composed in 96% by weight of hydroxyapatite (HAP, Ca10(PO4)6(OH)2) nanometric-sized crystals and 4% of organic material and water [
The phase transformation of HAP is well stablished, and it is done in agreement with the CaO-P2O5 phase diagram. It is well known that HAP transforms to the beta tricalcium phosphate phase (β-TCP, β-Ca3(PO4)2) at 1100˚C [
Temperature effect on the structure of human tooth has been analyzed in air by different techniques since long ago. It is well known that when teeth are submitted to temperature in air, structural changes can be evidenced by color changes and fractures [
The aim of this paper is to observe the phases and to find the differences produced by the heating treatments of the teeth done in argon atmosphere compare the results with those observed in air. Therefore, in this work bulk and powder tooth samples were subjected to temperatures in the range from room temperature (25˚C) to 1200˚C in argon and in air atmospheres. The macro and microstructure, color, and mechanical changes in enamel and dentin were analyzed as a function of temperature. The color, mechanical and structural changes of the bulk samples were analyzed through hardness variation via the micro-indentation, and the light (LM) and scanning electron (SEM) microscopies. The structural and phase changes were also analyzed in powder samples via the X-ray diffraction (XRD), thermogravimetric analysis (TGA) and its differential (dTGA) and Fourier transform infrared spectroscopy (FTIR) analyses which were used to determine lattice parameters of crystal size, phase ratios and other crystallographic aspects using the Rietveld method.
It is worth to mention that similar studies have also been carried out in human and animal bone samples in the range 20˚C to 1200˚C to investigate the HAP structure change by heating to design materials with potential applications in odontology and medicine. For example, Rogers and Daniels [
The effects of heating on the structure of the human tooth, mainly in enamel, have been minimized due to its impractical use in the dental clinic. However, exists several clinical scenarios where the temperature is increased of dental treatments (lasers, dental burrs, etc.) that authorize this study. For example, the use of the hand-piece tool in the treatment of carious teeth involves a temperature close to 200˚C [
Dentin and enamel of thirty permanent human molars extracted for orthodontic reasons from adult patients (between 25 and 30 years old) were the working material (Institutional Review Board (IRB) approval FMED/CI/SPR/083/2015 for use of human teeth approved by the University of Mexico). This IRB approval, in accordance with the Committee on Publication Ethics (COPE) guidelines, includes written informed consent from patients about this type of study. Children's teeth were not used.
After extraction, dental pieces were rinsed with distilled water and subjected to a careful visual review using a Carl Zeiss Stemi 2000-C light stereo microscope to select only healthy pieces. They were then cut into four parts on a Buehler IsoMet 1000 diamond disk cutter. Each of them was set at temperatures ranging from 25˚C to 1200˚C at 100˚C intervals and with one hour of permanence in a Lindberg 54,233 tubular type furnace. Heating and cooling were performed slowly (10˚C/min). The experiment was carried out in argon and in air atmospheres. For heating under the argon atmosphere, the samples were heated immersed in a constant argon gas flow of 0.1 ml/min that get into through one end of the tube and get out through the other. The temperature was measured with a Pt-Pt/Rh thermocouple.
For microstructure, bulk samples were observed with the Carl Zeiss Stemi 2000-C light stereo-microscope in normal reflection mode, and a FESEM JSM-6701F field emission SEM microscope with image resolution of secondary electrons of 1.0 nm for the acceleration voltage of 15 kV and of 2.2 nm for the acceleration voltage of 1 kV. For microhardness, a Matsuzawa equipment model MHT2 with a squared diamond indenter and an angle of 136˚ was used. The indentations were made using 25-gram force for 20 s. The number of indentations was 10 in each sample.
For the crystalline structure analysis, dentin and enamel were carefully mechanically separated, avoiding the zone of the amelodentinal junction to eliminate the mixing of materials. The samples were powdered using a hand dental drill and a Lynx EM-II milling cutter.
The X-ray diffraction characterization of powders was performed on a Bruker equipment model D8-Advanced with monochromatic Cu (Kα) radiation (λ = 0.154 nm). The diffractograms were obtained in the 2θ range from 5 to 60˚ with a step of 0.02˚/s, and 25 s of counting time per step using the Bragg-Brentano geometry. Computer structural analysis of diffractograms was performed by the Rietveld method using the software Profex-BGMN-Bundle-3.6.0.
The TGA analysis of the powders was performed on a TA Instrument SDT Q600 V20.9 Build 20 calorimeter from room temperature (25˚C) to 1200˚C with a heating rate of 10˚C/min, both in argon and in air. FT-IR analysis was done in the mid-infrared range 400 to 4000 cm−1 using a Perkin Elmer infrared spectrometer coupled with a total attenuated reflectance (ATR) of germanium.
In dentin, from 25˚C to 250˚C, the loss of adsorbed water and some organic material produces the weight loss of 8% in argon and 10% in air. Between 250˚C to 650˚C in argon and from 250˚C to 550˚C in air, the weight losses of 20% and 22%, respectively, are due to the loss of structural water and the total organic material removal. From 650˚C in argon and from 550˚C in air and up to 1200˚C, the weight loss of 4% in both atmospheres is due to decarbonization and dehydroxylation.
In enamel, from 25˚C to 250˚C, the weight loss of 5% is produced both in argon
and in air by the loss of adsorbed water and some organic material. From 250˚C to 450˚C in argon and from 250˚C to 500˚C in air, the weight losses of 11% and 13% respectively are produced by the elimination of structural water and the total elimination of organic material. The last weight loss registered in air is of 4% from 500˚C to 1200˚C; but in argon there are two losses: one of 6% from 450˚C to 900˚C and other of 2% from 900˚C to 1200˚C. These losses are due to decarbonization and dihydroxylation reactions.
The color changes in enamel are in function of temperature.
For comparison,
At 400˚C, the amelodentinal junction shows the olive-green color, which spreads towards enamel, while dentin becomes light gray. In addition, some cracks appear in dentin, and enamel and dentin begin to separate. At 600˚C, dentin presents fractures and its color is dark gray. Enamel is dark gray, which changes to light gray towards the outside. At 800˚C, all enamel is dark gray, while dentin is light gray. At 900˚C both dentin and enamel show the same color, light gray. At 1000˚C, enamel and dentin are white. Finally, at 1200˚C, both structures display a gray white color.
Therefore, enamel follows the color sequence:
[ Natural Color ] → [ Light Gray ] → [ Dark Gray ] → [ Light Gray ] → [ White ]
In air, the dark gray in enamel is observed at 800˚C, while in argon it is observed at around 300˚C. The darkening of the color is due to the removal of the organic material that occurs between 200˚C and 600˚C.
The color changes in dentin in air as a function of temperature follow the sequence:
[ Natural Color ] → [ Brown ] → [ Gray ] → [ Black ] → [ Gray ] → [ White ]
Dentin is observed in black color only in argon atmosphere, at around 300˚C. This sequence is consistent with that reported in the literature under air atmosphere [
Temperature (˚C) | Air | Argon | ||
---|---|---|---|---|
Enamel (HVN) | Dentin (HVN) | Enamel (HVN) | Dentin (HVN) | |
25 | 306 ± 20 | 103 ± 16 | 306 ± 20 | 103 ± 16 |
200 | 346 ± 52 | 101 ± 9 | 490 ± 62 | 84 ± 10 |
400 | 352 ± 79 | 65 ± 19 | 331 ± 46 | 80 ± 6 |
600 | 286 ± 23 | 69 ± 7 | 305 ± 56 | 107 ± 22 |
800 | 229 ± 49 | 128 ±15 | 459 ± 64 | 120 ±11 |
1000 | 371 ± 66 | 375 ± 52 | 409 ± 79 | 207 ± 29 |
1200 | 373 ± 83 | 525 ± 80 | 299 ± 101 | 322 ± 57 |
As shown in
In dentin, the increment in hardness is observed after 200˚C in argon while it decreases in air. This is, after the elimination of the organic material. After 800˚C, the hardness increases both in air and in argon. Moreover, the hardness in dentin in air is higher than in enamel at 1200˚C.
Mezahi et al. [
At room temperature, HAP is the phase identified, although in dentin it shows an “amorphous-type” spectrum when the temperature is below 400˚C
produced by the smaller crystal size of HAP (see
The presence of the β-TCP phase was observed after the elimination of water and organic material, and after the crystal growth of HAP. The coexistence of HAP and β-TCP, in enamel was registered at 400˚C in air and at 600˚C in argon. In dentin, it was observed at 600˚C in air and at 800˚C in argon.
Temperature (˚C) | Air | Argon | |||||
---|---|---|---|---|---|---|---|
HAp | β-TCP | HAp | β-TCP | ||||
(110) (nm) | (002) (nm) | (002) (nm) | (110) (nm) | (002) (nm) | (002) (nm) | ||
Dentin | 25 | 4.8 ± 0.08 | 11 ± 0.5 | -- | 4.8 ± 0.1 | 10.8 ± 0.5 | -- |
200 | 6.5 ± 0.1 | 15 ± 0.6 | -- | 6.8 ± 0.2 | 9.3 ± 0.3 | -- | |
400 | 4 ± 0.1 | 3.4 ± 0.1 | -- | 7.7 ± 0.1 | 18.5 ± 0.8 | -- | |
600 | 6 ± 0.3 | 5.6 ± 0.3 | 55 ± 2 | 12 ± 0.1 | 19.2 ± 0.6 | -- | |
800 | 56 ± 3 | 53 ± 4 | 64 ± 1 | 36 ± 0.9 | 57 ± 2 | 87 ± 8 | |
1000 | 98 ± 3 | 92 ± 4 | 108 ± 7 | 136 ± 3 | 145 ± 5 | 139 ± 18 | |
1200 | 155 ± 7 | 161 ± 9 | 98 ± 6 | 121 ± 5 | 118 ± 7 | 84 ± 4 | |
Enamel | 25 | 26 ± 0.3 | 28 ± 0.8 | -- | 26 ± 0.3 | 28 ± 0.8 | -- |
200 | 22 ± 0.4 | 23 ± 0.8 | -- | 22 ± 0.5 | 23 ± 0.8 | -- | |
400 | 53 ± 2 | 56 ± 3 | 23 ± 4 | 61 ± 2 | 68 ± 3 | -- | |
600 | 58 ± 1 | 69 ± 3 | 98 ± 11 | 58 ± 1 | 69 ± 3 | 98 ± 11 | |
800 | 77 ± 4 | 90 ± 6 | 57 ± 4 | 62 ± 2 | 69 ± 3 | 100 ± 17 | |
1000 | 129 ± 7 | 144 ± 11 | 147 ± 21 | 126 ± 6 | 150 ± 10 | 134 ± 12 | |
1200 | 38 ± 3 | 31 ± 3 | 54 ± 25 | 134 ± 7 | 154 ± 13 | 125 ± 12 |
Temperature (˚C) | Air | Argon | |||||
---|---|---|---|---|---|---|---|
% Hap | % β-TCP | % RWP | % HAp | % β-TCP | % Rwp | ||
Dentin | 25 | 100 | -- | 4.45 | 100 | -- | 4.45 |
200 | 100 | -- | 4.73 | 100 | -- | 5.25 | |
400 | 100 | -- | 4.88 | 100 | -- | 5.41 | |
600 | 98.04 | 1.96 | 5.65 | 100 | -- | 5.93 | |
800 | 90.71 | 9.29 | 7.95 | 90.91 | 9.09 | 5.99 | |
1000 | 77.98 | 22.02 | 7.4 | 89.94 | 10.52 | 7.63 | |
1200 | 84.15 | 15.85 | 6.42 | 72.58 | 27.42 | 7.31 | |
Enamel | 25 | 100 | -- | 5.72 | 100 | -- | 5.72 |
200 | 100 | -- | 5.52 | 100 | -- | 4.71 | |
400 | 99.39 | 0.61 | 5.89 | 100 | -- | 5.97 | |
600 | 94.88 | 5.12 | 6.35 | 93.67 | 6.33 | 5.72 | |
800 | 88.93 | 11.07 | 6.55 | 96.12 | 3.88 | 5.78 | |
1000 | 92.13 | 7.087 | 7.15 | 92.59 | 7.41 | 7.33 | |
1200 | 94.9 | 5.1 | 8.35 | 91.83 | 8.17 | 7.02 |
The crystal-size values were also obtained by the Rietveld analysis.
The lattice parameters were also obtained by the Rietveld analysis.
% = Repoted Value − Calculated Value Reported Value × 100
where the “reported values” (nominal values) were a = 0.94 nm and c = 0.688 nm for HAP and a = 10.43 nm and c = 37.375 nm for β-TCP; the “calculated value” is the one calculated by Rietveld.
Temperature (˚C) | Air | Argon | |||||||
---|---|---|---|---|---|---|---|---|---|
HAp | β-TCP | HAp | β-TCP | ||||||
Δa (%) | Δc (%) | Δa (%) | Δc (%) | Δa (%) | Δc (%) | Δa (%) | Δc (%) | ||
Dentin | 25 | 5.9 | 6.3 | -- | -- | 5.9 | 6.3 | -- | |
200 | 1.8 | 2.0 | -- | -- | 3.5 | 3.8 | -- | ||
400 | 2.2 | 1.2 | -- | -- | 1.4 | 1.5 | -- | ||
600 | −0.1 | −0.1 | 5.6 | −0.6 | 0.2 | 0.5 | -- | ||
800 | −0.1 | 0.0 | 0.9 | 0.7 | 0.3 | 0.3 | 1.0 | 1.0 | |
1000 | 0.2 | 0.2 | 0.7 | 0.8 | 0.1 | 0.1 | 0.6 | 0.5 | |
1200 | 0.2 | 0.1 | 0.5 | 0.5 | 0.2 | 0.1 | 0.6 | 0.6 | |
Enamel | 25 | −0.1 | 0.2 | -- | -- | −0.1 | 0.2 | -- | |
200 | −0.1 | 0.2 | -- | -- | −0.1 | 0.2 | -- | ||
400 | −0.2 | 0.0 | 0.8 | 0.5 | −0.1 | 0.1 | -- | ||
600 | 0.0 | 0.2 | 0.9 | 0.9 | 0.0 | 0.2 | 0.9 | 0.9 | |
800 | −0.1 | 0.2 | 0.7 | 0.7 | 0.0 | 0.2 | 1.0 | 0.7 | |
1000 | −0.1 | 0.1 | 0.6 | 0.5 | 0.0 | 0.1 | 0.5 | 0.5 | |
1200 | −0.2 | 0.2 | 0.8 | 0.7 | −0.1 | 0.0 | 0.4 | 0.4 |
In dentin, for HAP in argon, Δ a and Δ c are highly expanded at 25˚C. They slowly decrease after 200˚C reaching 0.1% at 1000˚C. For β-TCP in dentin and in argon, Δ a and Δ c are expanded (around 1.0%) at 600˚C, and after 1000˚C both parameter present contraction (0.6%).
In dentin in air, for HAP Δ a is highly expanded (5.9%) at 25˚C. It rapidly decreases after 200˚C reaching −0.1% at 600˚C, and it is expanded again but only 0.2% at 1200˚C. For β-TCP in dentin in argon, Δ a presents a high variation in contraction from 5.6% at 600˚C to 0.9% at 800˚C. After this, remains in contraction until 0.5% at 1200˚C. Δ c is expanded from −0.6% at 600˚C to 0.8% at 1000˚C.
Enamel in argon, the variations from their nominal values are very small. For HAP in enamel and in argon, Δ a , varies from −0.1% (contraction) at 25˚C to 0.0% at 600˚C and remains there till 1200˚C. Δ c varies from 0.2% (expansion) to 0.0% (contraction) at 1200˚C. For β-TCP in enamel and in argon, Δ a and Δ c are expanded (around 0.9%) at 600˚C, and after 1000˚C both parameter present contraction.
In enamel in air, for HAP, ∆a, varies from −0.1% (contraction) at 25˚C to 0.0% (increase) at 600˚C and back to contraction (−0.2%), while ∆c varies from 0.2% (expansion) at 25˚C to 0.0% (contraction) at 400˚C, and back to expansion (0.2%). For β-TCP in enamel in air, ∆a and ∆c remains expanded (around 0.7%).
Therefore, in dentin, the lattice parameters of HAP and β-TCP phases show contraction during heating but in argon these variations are slower than in air. In enamel, the lattice parameters show very small variations during heating, and they are very similar in argon and in air. Both materials reach their nominal lattice parameter values during heating.
The carbonate CO 3 2 − ions may occupy two sites within the apatite structure. When they occupy the hydroxyl groups OH − sites, HAP is a type-A carbonated apatite and an expansion in the a-axis and the contraction in the c-axis are registered [
~1384, 1420, 1455 cm−1, corresponding to the ν3 stretch vibration, are observed [
In dentin, amount of CO 3 2 − eliminated in air is bigger than in argon.
For the phosphate group PO 4 3 − , the bands at 1010 and 1026 cm−1 and at 1080 and 1088 cm−1, corresponding to the ν3 triple asymmetric degeneration vibration mode, and at 958 to 968 cm−1, corresponding to the ν1 symmetric vibration mode, are always observed both in detin and enamel and both air and argon. Also, the degenerate double bond of the ν4 mode is in the bands at 598 and 601
Vibration | Band position (cm−1) | |||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Dentin | Enamel | |||||||||||||||
Air | Argon | Air | Argon | |||||||||||||
25˚C | 400˚C | 800˚C | 1000˚C | 25˚C | 400˚C | 800˚C | 1000˚C | 25˚C | 400˚C | 800˚C | 1000˚C | 25˚C | 400˚C | 800˚C | 1000˚C | |
O-P-O ν2 | 469 | 467 | 475 | 470 | 470 | 468 | 470 | 472 | 468 | 471 | 470 | 475 | 476 | 473 | 470 | 471 |
O-P-O ν4 | 557 559 | 559 601 | 548 562 598 | 547, 564 599 | 557 599 | 559 601 | 564 599 | 545, 561 599 | 561 600 | 561 599 | 547 563 599 | 546, 564 599 | 560 600 | 560 599 | 561 599 | 561, 599 |
OH | - | - | 630 | 630 | - | - | 631 | 630 | - | 629 | 629 | 630 | - | 629 | 629 | 629 |
O-C-O (B-type) | 872 | 872 | - | - | 873 | 873 | - | - | 872 | 872 | 872 | - | 873 | 872 | -D | - |
O-C-O (A-type) | 877 | 878 | 879 | 879 | 876 | 877 | 879 | 879 | 879 | 880 | 879 | 877 | -D | 879 | 876 | 878 |
P-O (β-TCP) | - | - | 946 | 946 | - | - | 947 | 945 | - | - | - | - | - | - | - | - |
P-O | 961 | 962 | 968 | 962 | 959 | 960 | 962 | 960 | 959 | 959 | 959 | 958 | 958 | 959 | 958 | 958 |
P-O ν3 (β-TCP) | - | - | 982 | 982 | - | - | 985 | 982 | - | - | 982 | 982 | - | - | 982 | 980 |
P-O ν3 (β-TCP) | - | - | 1014 | 1014 | - | - | -D | 1012 | - | -D | 1015 | 1015 | - | 1015 | 1012 | 1013 |
P-O ν3 | 1010 | 1014 10,180 | 1024 1087 | 1025 1088 | 1013 | -D | 1026 1088 | 1017 1088 | 1019 1081 | 1020 1085 | 1024 1085 | 1026, 1085 | 1022 1082 | 1020 1086 | 1020 1085 | 1017 1085 |
P-O ν3 (β-TCP) | - | - | 1120 | 1120 | - | - | -D | -D | - | - | - | - | - | - | - | - |
C-O | (B)1411 (A)1444 (V)1467 | (B)1413 (B)1415 (V)1470 | D- | - | (B)1412 (A)1441 (V)1468 | (B)1412 (A)1447 1501 1538 | (A)1460 (A)1541 | - | (B)1411 (A)1452 | (B)1413 (A)1454 | (B)1411 (A)1456 (A)1547 | (A)1456 (A)1545 | (B)1409 (A)1454 | (B)1411 (A)1456 | (B)1411 (A)1456 | (B)1413 (A)1459 (A)1545 |
and 544 to 564 cm−1 of the O-P-O bond, and the bands located between 463 and 475 cm−1 of the ν2 vibration mode, are always shown.
Therefore, at room temperature, HAP in dentin and enamel is a carbonated-HAP, with carbonate at sites A and B. As temperature increases, carbonates move from sites B to sites A, producing the contraction of the a-axis. At the same time,
The differences observed under argon and under air are better described if the heating process is divided into two parts: before and after the elimination of the organic material that takes place around 600˚C. In the first part, during the eliminating the water and organic material, all the combustion reactions are favored by the oxidizing atmosphere in air, and, at the same time, the resulted heat is dissipated. In argon, the chemical reactions are completely entrained by the argon environment and the heat dissipation is reduced [
The argon atmosphere produces slightly different color changes as compared to those in air due to argon does not react with the organic material and the residues are trapped. This explain the black color of dentin that was observed in only under argon atmosphere. Cracking and fracture of dentin are the result of the difference in expansibility, that is more severe in argon than in air.
The variation in the lattice parameters at the beginning and during heating, both in air and in argon, is greater in dentin than in enamel. In dentin, the a- and c-axes decrease 6% approximately. In enamel, the variation ranges from 0.2% to −0.1% during all heating. A simple explanation of these variations is not easy, although, they should be associated with the movement of CO3 ions within the crystal structure of the HAP [
After the combustion of the organic components, removal of water and carbonates, the increment of hardness in dentin must have to do with the crystal growth. The particle size is reduced by the nucleation and growth of the β-TCP phase, producing a kind of composite material increasing hardness.
Note that CaO was not observed. Analyzing the decomposition of HAP, Savino et al. [
The chemical reactions in air for the Ca-deficient HAP by dehydroxylation are:
Ca 10 ( PO 4 ) 6 ( H 2 O ) 2 → Ca 10 ( PO 4 ) ( OH ) 2 − 2 x O x □ x + x H 2 O
Ca 10 ( PO 4 ) 6 ( OH ) 2 − 2 x O x □ x → Ca 10 ( PO 4 ) 6 O + ( 1 − x ) H 2 O
where □ are hydrogen vacancies, C a 10 ( PO 4 ) 6 ( OH ) 2 − 2 x O x □ x is oxyhydroxyapatite (OHA), and C a 10 ( PO 4 ) 6 O is oxyapatite (OA). For the Ca-deficient HAP to β-TCP occurring between 700˚C and 800˚C the reaction is:
Ca 10 − x ( HPO 4 ) x ( PO 4 ) 6 − x ( H 2 O ) 2 − x → ( 1 − x ) Ca 10 ( PO 4 ) ( H 2 O ) 2 + 3 x β -Ca 3 ( PO 4 ) 2 + x H 2 O
The argon atmosphere affects the temperature at which the β-TCP phase is observed because in air, by being an oxidizing atmosphere, it interacts with the material forming oxides and releasing the β-TCP phase. The decomposition reactions need a greater amount of heat.
The content of β-TCP phase is higher in dentin. This could be since the crystal size in dentin is smaller than in enamel, resulting in the increase in surface energy which favors the decrease of the activation energy that the formation of this phase needs, even when this transformation starts after 600˚C.
The temperature effect on the crystal size and morphology of the enamel and dentin grains during the heat treatment of human tooth will be analyzed by electron transmission microscopy. We are working on it.
Heating of teeth in air and in argon atmospheres presents significant structural and chemical changes; the CaO was not observed. The heating analysis becomes simple if the process is divided in two parts: those occurring before and after the water and organic removal (600˚C, approximately). During the removal of the organic material and structural water the air atmosphere play an important role in the reaction of the combustion products. The appearance of the β-TCP phase depends on the temperature, the atmosphere and the chemical composition. During the phase transformation to β-TCP, the air atmosphere is important in the heat dissipation process, both in dentin and in enamel. In addition to this, the removal of organic material and of the structural water content modifies significantly the lattice parameters, mainly in the dentin. The lattice parameter variations are slower in argon than in air. The phase transformation induces hardness, mainly in dentin, and the hardness increment is accompanied by crystal growth. Therefore, there is difference between the results obtained in the heating treatment of the teeth done in air than those obtained from the heating done in argon atmosphere. In general, the argon atmosphere delays the process observed in air. Because oxidation is important, similar behavior as those in argon can be expected in vacuum.
We thank to J. Barreto Rentería, S. Tehuacanero Nuñez, R. Trejo Luna, A. Gómez Cortés, A. Morales Espino, M. Aguilar Franco, D. Quiterio, C. Zorrilla Cangas, M. Moreno Ríos and S. Tehuacanero Cuapa for the technical support. We also thank Dr. Ivet Gil-Chavarría for the Institutional Review Board (IRB) approval FMED/CI/SPR/083/2015 for use of human teeth approved by the University of Mexico (Red Conacyt Ciencia Forense). We also thank to DGAPA-UNAM for financial support through the project PAPIIT No. IN-109516. NVB thanks the CONACYT for the economic support to perform a postdoctoral stay at the Instituto de Física, UNAM.
Vargas-Becerril, N., García-García, R. and Reyes-Gasga, J. (2018) Structural Changes in Human Teeth after Heating up to 1200˚C in Argon Atmosphere. Materials Sciences and Applications, 9, 637-656. https://doi.org/10.4236/msa.2018.97046